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1
Melling, David R., Charles E. Blackburn, David H. Watkinson e Jack R. Parker. "Geological setting of gold, western Wabigoon Subprovince, Canadian Shield: exploration targets in mixed volcanic successions". Canadian Journal of Earth Sciences 25, n. 12 (1 dicembre 1988): 2075–88. http://dx.doi.org/10.1139/e88-192.
Testo completoAbstract (sommario):
The Archean volcanic rocks in the Cameron–Rowan lakes area may be divided into three distinct successions based on field mapping, petrographic studies, and lithogeochemical characteristics. The lowermost Rowan Lake Volcanics are tholeiitic pillowed basalts. These rocks are unconformably overlain by the Cameron Lake Volcanics, a mixed succession of tholeiitic massive and pillowed basalts and intermediate to felsic volcaniclastic rocks. The Brooks Lake Volcanics consist of tholeiitic basalts and represent the youngest volcanic rocks at the top of the preserved succession.Most of the gold concentrations in the Cameron–Rowan lakes area are confined to the mixed Cameron Lake Volcanics. The majority of these, including the Cameron Lake deposit, occur within shear zones near lithologic contacts. In the Eagle–Wabigoon and Manitou lakes areas there are similar stratigraphic subdivisions of the supracrustal rocks and many of the gold concentrations also occur in deformation zones within the mixed volcanic successions. The contrasting competencies among the basalts, the intermediate to felsic volcaniclastic rocks, and the intrusive rocks, which are characteristic of the mixed volcanic successions, localized stress during deformation, forming shear zones into which gold-bearing fluids gained access. The potential for successfully delineating economic gold concentrations appears greatest in the mixed volcanic successions within these areas and elsewhere in the western Wabigoon Subprovince of the Canadian Shield.
2
Monjoie, Philippe, Henriette Lapierre, Artan Tashko, Georges H. Mascle, Aline Dechamp, Bardhyl Muceku e Pierre Brunet. "Nature and origin of the Triassic volcanism in Albania and Othrys: a key to understanding the Neotethys opening?" Bulletin de la Société Géologique de France 179, n. 4 (1 luglio 2008): 411–25. http://dx.doi.org/10.2113/gssgfbull.179.4.411.
Testo completoAbstract (sommario):
AbstractTriassic volcanic rocks, stratigraphically associated with pelagic or reef limestones, are tectonically juxtaposed with Mesozoic ophiolites in the Tethyan realm. From the central (Dinarides, Hellenides) and eastern Mediterranean (Antalya, Troodos, Baër Bassit) to the Semail nappes (Oman), they occur either associated to the tectonic sole of the ophiolitic nappes or as a distinct tectonic pile intercalated between the ophiolites and other underthrust units. In the Dinaro-Hellenic belt, the Pelagonian units represent the lower plate, which is underthrust beneath the ophiolites. Middle to Late Triassic volcanic sequences are interpreted as the eastern flank of the Pelagonian platform and are therefore considered as a distal, deep-water part of the Pelagonian margin.The Triassic volcanics from Albania and Othrys are made up of basaltic pillowed and massive flows, associated locally with dolerites and trachytes. New elemental, Nd and Pb isotopic data allow to recognize four types of volcanic suites: (1) intra-oceanic alkaline and tholeiitic basalts, (2) intra-oceanic arc-tholeiites, (3) back-arc basin basalts, (4) calc-alkaline mafic to felsic rocks. Nd and Pb isotopic initial ratios suggest that the within-plate volcanic rocks were derived from an enriched oceanic island basalt type mantle source, devoid of any continental crustal component. The lower εNd value of the trachyte could be due to assimilation of oceanic altered crust or sediments in a shallow magma chamber. Island arc tholeiites and back-arc basin basalts have a similar wide range of εNd. The absence of Nb negative anomalies in the back-arc basin basalts suggests that the basin floored by these basalts was wide and mature. The high Th contents of the island arc tholeiites suggest that the arc volcanoes were located not far away from the continental margin.Albania and Othrys volcanics contrast with the Late Triassic volcanism from eastern Mediterranean (SW Cyprus, SW Turkey), which displays solely features of oceanic within plate suites. The presence of back-arc basin basalts associated with arc-related volcanics in Central Mediterranean indicates that they were close to a still active subduction during the Upper Triassic, while back-arc basins developed, associated with within-plate volcanism, leading to the NeoTethys opening.
3
Tripathi, C. "Volcanism in Gondwanas". Journal of Palaeosciences 36 (31 dicembre 1987): 285–89. http://dx.doi.org/10.54991/jop.1987.1587.
Testo completoAbstract (sommario):
In India the Lower Permian event is marked by a major volcanic episode in the Himalayan belt and rift faulting in the Peninsula which gave rise to various Gondwana basins. The Lower Cretaceous major volcanic episode represented by the Rajmahal Trap represents the termination of Gondwana sedimentation. Lower Permian volcanism is represented by the Panjal Volcanics in Kashmir Basin and its equivalent, the Volcanics in Spiti-Zanskar Basin and Rotung Volcanics (Abor Volcanics) in Arunachal Pradesh. In Karakarom Basin of Ladakh, volcanism is associated with Changtash and Aqtash formations of Permian age.
The Agglomeratic Slates in Kashmir are supposed to have originated as explosive volcanism in the form of pyroclastic which was followed later by flows of the Panjal Volcanics represented by subaqueous and subaerial tholeiitic basalt with occasional basaltic, andesitic and rhyolitic volcanics. The Agglomeratic slates are divided into two divisions, the Lower Diamicites and the Upper Pyroclastic. At the base of the Pyroclastic division and at the top of the Diamictite division, we get Eurydesma-Deltopecten Fauna of Lower Permian age. It is thus established that volcanism in Kashmir, Spiti-Zanskar and Ladakh is restricted to Lower Permian only. The sills and dykes associated with the underlying sequence in Syringothyris Limestone and Fenestella Shale in Kashmir, in Lipak and Po Formations in Spiti are related to this volcanism.
4
Smith, Alan D. "Geochemistry and tectonic setting of volcanics from the Anyox mining camp, British Columbia". Canadian Journal of Earth Sciences 30, n. 1 (1 gennaio 1993): 48–59. http://dx.doi.org/10.1139/e93-005.
Testo completoAbstract (sommario):
Volcanogenic massive sulphide deposits at Anyox in the Tracy Arm terrane of the Canadian Cordillera are associated with a sequence of tholeiitic basalts with minor intercalated basaltic andesite tuffs and siliceous sediments. Sm–Nd and Pb–Pb systematics indicate an Early to Middle Jurassic age. The tholeiites are characterized by normal mid-ocean-ridge basalt to weak island-arc tholeiite trace element signatures with slight enrichment in large-ion lithophile elements and depletion in high-field-strength elements, high 207Pb/204Pb, and εNd(170 Ma) values of +8.2 to +8.4. The mineralized sequence is conformably overlain by argillaceous sediments and minor limestones. These features, combined with the location of the strata and similarities with the Spider Peak Formation of the Methow terrane, indicate an origin in a narrowing marginal basin that once separated superterranes I and II.
5
Rouer, O., H. Lapierre, C. Coulon e A. Michard. "New petrological and geochemical data on mid-Paleozoic island-arc volcanics of northern Sierra Nevada, California: evidence for a continent-based island arc". Canadian Journal of Earth Sciences 26, n. 12 (1 dicembre 1989): 2465–78. http://dx.doi.org/10.1139/e89-210.
Testo completoAbstract (sommario):
The mid-Paleozoic volcanics of northern Sierra Nevada consist of the Sierra Buttes rhyolites, the Taylor basalts and andesites, and the Keddie Ridge basalt–latite–rhyolite suite. The Sierra Buttes calc-alkaline rhyolites display strong light rare-earth element enrichment and negative εNd values. The Taylor basalts and andesites in the northern Hough and Genesee blocks exhibit calc-alkaline affinities (REE rare-earth element patterns highly enriched in LREE), whereas in the southern Hough block they are tholeiitic (flat rare-earth element patterns). The abundance of silicic lavas, the low εNd values of both the Sierra Buttes and Taylor volcanics and the δ18O values of the Sierra Buttes rhyolite and Bowman Lake trondjhemite provide evidence that the northern Sierra Nevada island arc was continent based. The Keddie Ridge differentiated volcanics, characterized by high Zr, Y, Nb, K, and light rare-earth elements, are geochemically similar to a shoshonite suite. Their eruption at the end of the mid-Paleozoic volcanic episode suggests a reversal of subduction, uplift, and block faulting in the island arc.The mid-Paleozoic volcanics of the northern Sierra Nevada are thought to represent the remnant of a mature island arc because calc-alkaline rocks predominate over tholeiitic ones, the lavas display a K enrichment with time, and the volcanics are evolved in their isotopes, compared with rocks erupted in young or primitive island arcs.
6
Ludden, John, Claude Hubert e Clement Gariépy. "The tectonic evolution of the Abitibi greenstone belt of Canada". Geological Magazine 123, n. 2 (marzo 1986): 153–66. http://dx.doi.org/10.1017/s0016756800029800.
Testo completoAbstract (sommario):
AbstractBased on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics.The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma.The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.
7
Norman, David I., Kent C. Condie, Robert W. Smith e W. F. Thomann. "Geochemical and Sr and Nd isotopic constraints on the origin of late Proterozoic volcanics and associated tin-bearing granites from the Franklin Mountains, west Texas". Canadian Journal of Earth Sciences 24, n. 4 (1 aprile 1987): 830–39. http://dx.doi.org/10.1139/e87-081.
Testo completoAbstract (sommario):
Late Proterozoic volcanic and granitic rocks from the Franklin Mountains in west Texas define a continuous tholeiitic trend from rhyolite (granite) to trachyte with a small compositional gap between trachyte and basalt. Volcanics and granitic rocks have immobile-element geochemical affinities with volcanics from continental-margin-arc systems and associated back-arc basins. Felsic volcanics and granitic rocks appear to have formed by fractional crystallization of basalt during which REE and high-field-strength elements were stabilized as fluoride complexes and concentrated in late or post-magmatic fluids.A Rb–Sr isochron from nonhematized samples (1064 ± 5 Ma, i = 0.7034) falls within the error of reported U–Pb zircon dates and probably dates a period of hydrothermal alteration. εNd values (0.7–2.5 at 1065 Ma) fall below a depleted mantle growth curve and probably reflect contamination of the fractionating magmas with older continental crust. Changes in the isotopic composition of Nd during hydrothermal alteration may also contribute to the spread in εNd values.
8
Chandra, Rakesh, Rajeev Upadhyay e Anshu K. Sinha. "Subduction and collision related magmatism in the Shyok Suture and eastern Karakoram". Journal of Palaeosciences 48, n. (1-3) (31 dicembre 1999): 183–209. http://dx.doi.org/10.54991/jop.1999.1303.
Testo completoAbstract (sommario):
The Shyok Suture is represented by distinct sets of volcano-plutonic rock assemblages. The high-Mg tholeiitic basalt and calc-alkaline andesites of the Shyok Volcanics have a subduction zone chemical signature. The REE data on tholeiitic basalt suggest a chemical affinity between primitive N-MORB to E-MORB. The calc-alkaline andesites, however, resembles to transitional nature of basalt between E-MORB to OIB. The geochemical data and regional tectonic setting suggest a close similarity between the Shyok Volcanics of northern Ladakh and the Chalt Volcanics of Kohistan.
The mildly deformed trondhjemite-tonalite-granodiorite of the Tirit Granitoids are composite plutons located south of the Shyok Suture melange. These granitoids are subalkaline, I-type and represented by volcanic arc chemical signatures. The regional tectonic setting, the nature of occurrence and the composition of Tirit Granitoids are similar to the plutonic suites of northern Kohistan (Gindai, Matum Das and Nomal plutons).
The eastern Karakoram Batholith is dominated by quartz monzonite-tonalite-granodiorite and granite. The subalkaline to calc-alkaline Karakoram Batholith is constituted by both I-and S-type granitoids with volcanic arc and syn-collision chemical signatures. REE data suggest that the I-type granitoids of eastern Karakoram are calc-alkaline magmatism of a subduction zone environment. In contrast, most of the S-type granitoids are crust-derived peraluminous granitoids. New Rb/Sr isotopic whole rock age data indicates that an S-type intrusive phase was active in the eastern Karakoram region during 83±9 Ma. The syn-collision nature of these granitoids are similar to those of north Sost pluton and Karambar pluton of northern Kohistan.
This indicates that the collision between Kohistan-Ladakh arc and Karakoram block was active during 83±9 Ma.
9
Chauvet, François, Henriette Lapierre, Delphine Bosch, Stéphane Guillot, Georges Mascle, Jean-Claude Vannay, Jo Cotten, Pierre Brunet e Francine Keller. "Geochemistry of the Panjal Traps basalts (NW Himalaya): records of the Pangea Permian break-up". Bulletin de la Société Géologique de France 179, n. 4 (1 luglio 2008): 383–95. http://dx.doi.org/10.2113/gssgfbull.179.4.383.
Testo completoAbstract (sommario):
AbstractThe late Lower to Middle Permian Panjal Traps (NW Himalaya, India-Pakistan) represent the greatest magmatic province erupted on the northern Indian platform during the Neotethys opening. New geochemical and isotopic analyses were performed on basalts from the eastern borders of the traps (SE Zanskar-NW Spiti area) in order to characterize this volcanism, to discuss its compositional variations in comparison to Panjal counterparts and its relationships with the opening of Neotethys. Lavas show features of tholeiitic low-Ti (< 1.6%) continental flood basalts with LREE, Th enrichments and Nb-Ta negative anomalies. Trace element ratios combined with εNdi values (−3.6 to +0.9) and high Pb isotopic ratios suggest that these tholeiitic basalts were derived from an OIB-like mantle contaminated at various degrees by a continental crust component. Previous geochemical features are broadly similar to those of the coeval Panjal volcanic sequences identified westwards (Ladakh, Kashmir and Pakistan). Present geochemical constraints obtained for the Panjal Traps basalts suggest they originated from rapid effusion of tholeiitic melts during opening of the Neotethys Ocean. Similar magmatism implying an OIB-type reservoir is contemporaneously recognized on and along the adjacent Arabian platform. Both Indian and Arabian Permian volcanics were emplaced during coeval syn-rift to post rift transition. These Lower to Middle Permian south Neotethyan continental flood magmatism are regarded as associated to a passive rifting. In this scheme, OIB-type isotopic signature would be related either to a melting episode of syn-rift up-welling mantle plumes or to a melting of a regional abnormally hot and enriched mantle.
10
Petterson, M. G. "The plutonic crust of Kohistan and volcanic crust of Kohistan–Ladakh, north Pakistan/India: lessons learned for deep and shallow arc processes". Geological Society, London, Special Publications 483, n. 1 (30 luglio 2018): 123–64. http://dx.doi.org/10.1144/sp483.4.
Testo completoAbstract (sommario):
AbstractThe Kohistan–Ladakh terrane, northern Pakistan/India, offers a unique insight into whole-arc processes. This research review presents summaries of fundamental crustal genesis and evolution models. Earlier work focused on arc sequence definition. Later work focused on holistic petrogenesis. A new model emerges of an unusually thick (c. 55 km) arc with a c. 30 km-thick batholith. Volatile-rich, hornblende ± garnet ± sediment assimilation-controlled magmatism is predominant. The thick batholith has a complementary mafic–ultramafic residue. Kohistan crustal SiO2 contents are estimated at >56%. The new-Kohistan, silicic-crust model contrasts with previous lower SiO2 estimates (c. 51% SiO2 crust) and modern arcs that imply <35 km crustal thicknesses and arc batholith thicknesses of c. 7 km. A synthetic overview of Kohistan–Ladakh volcanic rocks presents a model of an older, cleaved/deformed Cretaceous volcanic system at least 800 km across strike. The Jaglot–Chalt–Dras–Shyok volcanics exhibit predominant tholeiitic-calc-alkaline signatures, with a range of arc-related facies/tectonic settings. A younger, post-collisional, Tertiary silicic volcanic system (the Shamran–Dir–Dras-2–Khardung volcanics) lie unconformably upon Cretaceous basement, and erupted within an intra-continental tectonic setting. Kohistan–Ladakh tectonic model controversies remain. In essence, isotope-focused researchers prefer later (Tertiary) collisions, whilst structural field-geology-orientated researchers prefer an older (Cretaceous) age for the Northern/Shyok Suture.
11
Singh, M. M., e Vinod K. Singh. "Geochemistry and tectonic setting of the supracrustal rocks from the central part of the Bundelkhand craton, India". Journal of Geoscience, Engineering, Environment, and Technology 4, n. 2-2 (25 luglio 2019): 3. http://dx.doi.org/10.25299/jgeet.2019.4.2-2.2175.
Testo completoAbstract (sommario):
Supracrustal rocks (mafics and ultramafics) occurs along with banded iron formation, and felsic volcanics around Babina, Dhaura, and Mauranipur linear east-west trends in central part of the Bundelkhand craton represent Archean crust. The mafic and ultramafic rocks geochemically classified into Komatiite and Basaltic Komatiite and have high-Fe Tholeiitic in composition which may relate with the primitive mantle. The major and trace element geochemistry of mafic and ultramafic rocks correspond to hydrated mantle with wedge tectonic sources and ocean ridge geological characteristics.
12
Turek, A., R. Keller e W. R. Van Schmus. "U–Pb zircon ages of volcanism and plutonism in the Mishibishu greenstone belt near Wawa, Ontario". Canadian Journal of Earth Sciences 27, n. 5 (1 maggio 1990): 649–56. http://dx.doi.org/10.1139/e90-062.
Testo completoAbstract (sommario):
The Mishibishu greenstone belt, located 40 km west of Wawa, is a typical Archean greenstone belt and is probably an extension of the Michipicoten belt. This belt is composed of basic to felsic metavolcanic rocks of tholeiitic to calc-alkaline affinity and of metasedimentary rocks ranging from conglomerate to argillite. Granitoids, diorites, and gabbros intrude and embay supracrustal rocks as internal and external plutons.Six U–Pb zircon ages have been obtained on rocks in this area. The oldest is 2721 ± 4 Ma for the Jostle Lake tonalite. The bulk of the volcanic rocks formed by 2696 ± 17 Ma, which is the age of the Chimney Point porphyry at the top of the volcanic pile. The Pilot Harbour granite has a similar age of 2693 ± 7 Ma. The age of the Tee Lake tonalite is 2673 ± 12 Ma, and the age of the Iron. Lake gabbro is 2671 ± 4 Ma. The youngest age for volcanics in this part of the Superior Province is 2677 ± 7 Ma, obtained from, the David Lakes pyroclastic breccia. these ages agree with those reported for the adjacent Michipicoten and Gamitagama belts.
13
Ueng, W. C., T. P. Fox, D. K. Larue e J. T. Wilband. "Geochemistry and petrogenesis of the early Proterozoic Hemlock volcanic rocks and the Kiernan sills, southern Lake Superior region". Canadian Journal of Earth Sciences 25, n. 4 (1 aprile 1988): 528–46. http://dx.doi.org/10.1139/e88-052.
Testo completoAbstract (sommario):
During the early Proterozoic, the 2 km thick differentiated gabbroic Kiernan sills were emplaced into a thick accumulation of pillow basalt and associated deep-water strata, the Hemlock Formation, in the southern Lake Superior region. On the basis of major elements and trace elements (including rare-earth-element data), the Kiernan sills and the hosting volcanic rocks of the Hemlock Formation were determined to be comagmatic in origin, and both evolved from assimilation – crystal fractionation processes. The major assimilated components in these igneous rocks are identified as terrigenous sedimentary rocks. Assimilation affected the abundance of Nb, Ta, light rare-earth elements, and most likely P, Rb, Th, and K in the magma. The effect of chemical contamination from wall-rock assimilation accumulates with increasing differentiation.With wall-rock contamination carefully evaluated, a series of tectonic discriminating methods utilizing immobile trace elements indicates that the source magma was a high-Ti tholeiitic basalt similar to present-day mid-ocean-ridge basalts (MORB). It is suggested from this study that most of the enriched large-ion lithophile elements and LREE of the magma were not inherited from the mantle but from assimilation of supracrustal rocks. Chemical signatures of these rocks are distinctively different from those of arc-related volcanics. A rifting tectonic regime analogous to the opening of the North Atlantic Ocean and extrusion of North Atlantic Tertiary volcanics best fits the criteria revealed by this study.
14
Hald, N., e J. G. Larsen. "Early Tertiary, low-potassium tholeiites from exploration wells on the West Greenland shelf". Rapport Grønlands Geologiske Undersøgelse 136 (31 dicembre 1987): 1–25. http://dx.doi.org/10.34194/rapggu.v136.8008.
Testo completoAbstract (sommario):
Data on the Tertiary basalts in the Davis Strait region are reported from two exploration wells drilled by Arco and Mobil on the West Greenland shelf. Hellefisk 1 (67°53 'N, 56°44'W), situated only 60 km east of the mid-line in Davis Strait, penetrated the upper 690 m of a subaeriallava sequence continuous with the onshore volcanics of Disko and situated beneath 2.3 km of Paleocene to Quaternary sediments. The lavas are feldspar microporphyritic tholeiites and mostly unmetamorphosed despite the presence of laumontite and prehnite in the vesicular top zones. Nukik 2 (65°38'N, 54°46'W) penetrated 150 m of hyaloclastites and tholeiitic olivine dolerite sheets, presumably sills, some 200 km further to the south. These vo1canics are also deeply buried and are of unknown extension. The drilled rocks, except for the much altered hyaloclastites in the Nukik 2 well, have low contents of Ti02 (0.99-2.03%), K2O (0.09-0.18%) and P2O5 (0.08-0.21%), La/Sm ratios less than one and 87Sr/86Sr ratios of 0.7032 to 0.7044. Chemically they are related to the MORB-like picrites of Baffin Island rather than the less depleted tholeiites of West Greenland. In both areas the MORB affinity may be related to eruptions through a strongly attenuated lithosphere associated with the opening of Baffin Bay and Davis Strait.
15
Wirth, Karl R., Zachary J. Naiman e Jeffrey D. Vervoort. "The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift". Canadian Journal of Earth Sciences 34, n. 4 (1 aprile 1997): 536–48. http://dx.doi.org/10.1139/e17-043.
Testo completoAbstract (sommario):
The southernmost exposed rocks of the North American Midcontinent rift system (1100 Ma) consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the St. Croix River in Minnesota and Wisconsin. The flows are mostly high-Fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene. Abundant secondary chlorite, epidote, and actinolite indicate the group was metamorphosed to greenschist facies (~350 °C). Low sodium (M4 site) and tetrahedral aluminum (AlIV) contents of actinolite indicate low-pressure metamorphism (0.25 GPa) and imply a geothermal gradient of 45 – 50 °C/km. Low magnesium (Mg# = 0.37–0.58) and Ni contents (36–185 ppm) indicate the basalts have undergone significant fractionation and are not primary mantle melts. Incompatible element abundances are inversely correlated with Mg#, and most samples plot within either high or low trace element groups (e.g., Ti, P, Zr). The basalts are enriched in the light rare earth elements and Th, and are variably depleted in Ta and Nb relative to La and Th. Initial 143Nd/144Nd compositions of the group range from 0.51099 to 0.51122 (initial εNd = −4.5 to +0.1). Most flows have isotopic compositions within a relatively limited range (initial εNd = −2.5 to −1.6), but exhibit variable trace element abundances. Flows with the highest and lowest initial 143Nd/144Nd ratios have isotopic compositions that are inversely correlated with trace element abundances and ratios (e.g., La/Yb, Th/La, Th/Ta). The combined geochemical data suggest that the Chengwatana basalts originated from plume-derived melts and underwent variable fractional crystallization and crustal contamination. These melts may have interacted with lithospheric mantle enriched during Penokean subduction.
16
Ewart, A., R. W. Schon e B. W. Chappell. "The Cretaceous volcanic-plutonic province of the central Queensland (Australia) coast—a rift related ‘calc-alkaline’ province". Earth and Environmental Science Transactions of the Royal Society of Edinburgh 83, n. 1-2 (1992): 327–45. http://dx.doi.org/10.1017/s0263593300008002.
Testo completoAbstract (sommario):
ABSTRACTSilicic and minor intermediate and mafic pyroclastics, lavas, and dykes occupy a NW-trending zone through the Whitsunday, Cumberland and Northumberland Island groups, and locally areas on the adjacent mainland, over a distance of more than 300 km along the central Queensland coast. K-Ar and Rb-Sr data indicate an age range of 95–132 Ma, with the main activity approximately between 105–120 Ma; there is, however, evidence for easterly increasing ages. Comagmatic granites, some clearly intrusive into the volcanics, occur together with two localised areas of Triassic potassic granites (229 Ma), that form the immediate basement.The volcanics are dominantly rhyolitic to dacitic lithic ignimbrites, with intercalated surge and bedded tuffs, accretionary lapilli tuffs, and lag deposits. Associated rock types include isolated rhyolitic and dacitic domes, and volumetrically minor andesite and rare basalt flows. The sequence is cut by abundant dykes, especially in the northern region and adjacent mainland, ranging from dolerite through andesite, dacite and rhyolite. Dyke orientations show maxima between NW-NNE. Isotope data, similarities in petrography and mineralogy, and alteration patterns all suggest dyke intrusion to be broadly contemporaneous with volcanism. The thickness of the volcanics is unconstrained, although in the Whitsunday area, minimum thicknesses of >1 km are inferred. Eruptive centres are believed to occur throughout the region, and include at least two areas of caldera-style collapse. The sequences are thus considered as predominantly intracaldera.The phenocryst mineralogy is similar to modern “orogenic” volcanics. Phases include plagioclase, augite, hypersthene (uralitised), magnetite, ilmenite, with less common hornblende, and even rarer quartz, sanidine, and biotite. Fe-enriched compositions only develop in some high-silica rhyolites. The granites range from quartz diorite to granite s.s., and some contain spectacular concentrations of partially disaggregated dioritic inclusions.Chemically, the suite ranges continuously from basalt to high-silica rhyolite, with calc-alkali to high-K affinities, and geochemical signatures similar to modern subduction-related magmas. Only the high-silica rhyolites and granites exhibit evidence of extensive fractional crystallisation (e.g. pronounced Eu anomalies). Variation within the suite can only satisfactorily be modelled in terms of two component mixing, with superimposed crystal fractionation. Nd and Sr isotope compositions are relatively coherent, with εNd + 2·2 to +7·3, and ISr (calculated at 110 and 115 Ma) 0·7031-0·7044. These are relatively primitive, and imply mantle and/or newly accreted crustal magma sources.The two end-members proposed are within-plate tholeiitic melt, and ?low-silica rhyolitic melts generated by partial fusion of Permian (to ?Carboniferous) arc and arc basement. The arc-like geochemistry is thus considered to be source inherited. The tectonic setting for Cretaceous volcanism is correlated with updoming and basin rifting during the early stages of continental breakup, culminating in the opening of the Tasman Basin. Cretaceous volcanism is also recognised in the Maryborough Basin (S Queensland), the Lord Howe Rise, and New Caledonia, indicating the regional extent of volcanism associated with the complex breakup of the eastern Australasian continent margin.
17
Piccirillo, E. M., G. Bellieni, G. Cavazzini, P. Comin-Chiaramonti, R. Petrini, A. J. Melfi, J. P. P. Pinese, P. Zantedeschi e A. De Min. "Tholeiitic dyke swarm from the Ponta Grossa Arch and genetic relationships to the Paraná flood volcanics (Brazil)". Boletim IG-USP. Série Científica 20 (1 gennaio 1989): 65. http://dx.doi.org/10.11606/issn.2316-8986.v20i0p65-66.
Testo completo
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Legault, F., D. Francis, A. Hynes e P. Budkewitsch. "Proterozoic continental volcanism in the Belcher Islands: implications for the evolution of the Circum Ungava Fold Belt". Canadian Journal of Earth Sciences 31, n. 10 (1 ottobre 1994): 1536–49. http://dx.doi.org/10.1139/e94-136.
Testo completoAbstract (sommario):
The Belcher Islands of eastern Hudson Bay expose a continuous sequence of Early Proterozoic volcanics, and continental and shallow-marine sedimentary rocks. The volcanic rocks comprise two sequences of continental basalts: the older Eskimo Formation and the younger Flaherty Formation. The flows of both formations are composed of tholeiitic basalts (MgO < 9 wt.%) which, in each formation, are divided into two groups based on the concentrations of high field strength elements. Both groups of Eskimo flows have fractionated rare earth element (REE) patterns, with negative Nb anomalies. Relative to Eskimo flows, the Flaherty flows have flatter REE profiles and higher Nb contents. The chemical variations observed in the Eskimo flows are consistent with an assimilation – fractional crystallization process, involving a lower continental crust contaminant. The major and most trace element trends of the Flaherty flows are attributed to gabbroic crystal fractionation of two parental liquids, which are interpreted to represent varying degrees of partial melting of a common mantle source. An increase in La/Sm with increasing La, however, requires that the Flaherty flows evolved under open-system conditions, possibly in a replenished magma chamber or by selective contamination. The interbedding of low- and high-Zr flows in the upper portion of the Flaherty Formation combined with the possibility that their parental magmas originated from a common source require a complex melt segregation process in a partially molten ascending mantle.The chemical compositions of coeval basalts exposed along the eastern Hudson Bay coast and in the Cape Smith Fold Belt (western and eastern Povungnituk Group) indicate that they can be divided into two groups that are the chemical equivalents of the Eskimo and Flaherty Formations. Presently, the chemical stratigraphy of the Belcher Islands offers the most reliable means of correlating the isolated segments of Proterozoic supracrustal volcanics that rim the Superior Province of northern Quebec.
19
Znamensky, S. E., e N. M. Znamenskaya. "Voznesenskoe gold ore deposit (Southern Urals): Geological structure, ore-bearing rock geochemistry, geodynamic formation conditions". LITHOSPHERE (Russia) 22, n. 3 (2 luglio 2022): 391–403. http://dx.doi.org/10.24930/1681-9004-2022-22-3-391-403.
Testo completoAbstract (sommario):
Research subject. The geological structure, petro-geochem ical features of the ore-bearing rocks of the Voznesenskoe gold ore deposit (Southern Urals) and the geodynamic conditions of its formation. Methods. The content of petrogenic oxides was determined using silicate analysis; rare elements were determ ined using ICP-MS methods by an EIAH 9000 mass spectrometer and ICP-AES by an ICPE-9000 mass spectrometer. Results. The Voznesenskoe deposit is confined to a fragment of the crustal part of the section of harzburgite-type island-arc ophiolites. The ophiolite section is composed of taxite gabbroids, a sequence of dolerites, basalts, basaltic andesites and their tuffs, in places separated from gabbroids by lenses of serpentinites, and a package of subparallel dikes of porphyry gabbro-dolerites, gabbro-diorites, and diorites. Volcanics and dyke rocks with normal alkalinity and tholeiitic composition exhibit the geochemical characteristics of suprasubduction formations. In terms of chemical composition, they are comparable to the volcanic rocks of the pyritebearing complexes of the Baimak-Buribaevskaya Formation (D1e2). At the same time, the Voznesensky rocks have a number of distinct features, which are likely to be related to the geodynamic setting of their formation. In particular, ore-bearing effusive rocks and dykes differ from volcanic rocks of pyrite-bearing complexes in terms of a higher titanium content, the absence of boninite and silicic volcanic rocks, as well as the predominance of porphyry rock types. Conclusion. The conducted analysis of geochemical data using the V-Ti/1000 and LaN/SmN-TiO2 diagrams suggests that the association of mafic volcanic rocks and ore-bearing dykes of gabbro-dolerites, gabbro-diorites and diorites of the Voznesensky deposit was formed in the back-arc basin of the Late Ems frontal island arc.
20
Pedersen, Svend, e Jens Konnerup Madsen. "Geology of the Setesdalen area, South Norway: Implications for the Sveconorwegian evolution of South Norway". Bulletin of the Geological Society of Denmark 46 (22 giugno 1999): 181–201. http://dx.doi.org/10.37570/bgsd-1999-46-15.
Testo completoAbstract (sommario):
Crust forming processes in the Setesdalen area in the central part of southern Norway are dominated by the development of pre-Sveconorwegian supracrustal rocks (immature clastic sediments and bimodal volcanics) with an assumed depositional age of 1150–1100 Ma and by the emplacement of scattered infracrustal granitoids. The supracrustal rocks were deposited on a partly older than 1300 Ma gneissic basement including rocks which may have suffered one or more pre-Sveconorwegian orogeneses. During the Sveconorwegian orogeny, with the main upper greenschist to middle amphibolite facies high-temperature metamorphism and deformational phases in the period 1060–970 Ma, igneous activity comprising K-rich rocks high in elements such as P, Ti, La, Sr, Zr, and LREE and Ba was dominant. Significantly younger than this activity is the development of many REE pegmatites which are so characteristic for the region. The Precambrian geological activity terminated (at about 830 Ma?) with the development of E-W trending tholeiitic dolerites.
21
Friedmann, S. Julio, e John P. Grotzinger. "Sedimentology, stratigraphy, and tectonic implications of a paleo-Proterozoic continental extensional basin: the El Sherana – Edith River groups, Northern Territory, Australia". Canadian Journal of Earth Sciences 31, n. 4 (1 aprile 1994): 748–64. http://dx.doi.org/10.1139/e94-068.
Testo completoAbstract (sommario):
The ca. 1.83 Ga El Sherana – Edith River basin of Northern Territory, Australia, contains terrestrial sedimentary and volcanic rocks deposited during continental extension or transtension. Braid-plain sandstones and conglomerates, turbiditic sediments, and interbedded mafic and felsic volcanics, including ignimbrites, filled the basin. Alluvial fans and rock avalanche breccias are locally developed. Prebasinal structure controlled antecedent topography and influenced drainage patterns, thickness changes, and facies distribution.Unconformities bound all formations of the El Sherana and Edith River groups, revealed by beveled and incised strata, reversals in paleocurrent trends, sharp discordance in juxtaposed facies, and paleovalleys filled with unique sediments. Scarp-derived sediments are preserved only in a small, transfer-related strike-slip basin within the larger basin environment. Unconformities are regional and are interpreted as time lines throughout the basin. These unconformities developed during regrading of slopes forced by active tectonism, accompanied by syntectonic sedimentation and basin depocenter migration.The El Sherana – Edith River strata overlie older sediments (~2.1–1.88 Ga) of the Pine Creek orogen, which formed during a period of global orogeny related to continental assembly. Development of the El Sherana – Edith River basin began substantially (40–50 Ma) after denudation of the Pine Creek orogen belt, and is related to regional postcollisional extension. This extension is recorded by basin formation, bimodal tholeiitic and alkalic magmatism, and elevated geotherms. Other paleo-Proterozoic basins of northern Australia show a similar history of rift deposition above a fossil compressional belt.
22
Pardiarto, Bambang. "EVALUATION OF COPPER DEPOSITS PROSPECT IN WAI WAJO AREA OF SIKKA REGENCY, EAST NUSA TENGGARA PROVINCE". Buletin Sumber Daya Geologi 9, n. 2 (8 agosto 2014): 46–61. http://dx.doi.org/10.47599/bsdg.v9i2.127.
Testo completoAbstract (sommario):
Lowo Deba prospect in Sikka Regency, East Nusa Tenggara Province shows significant evidences for copper deposit. The prospect is discovered by joint cooperation activity between Directorate of Mineral Resources Inventory (DMRI) and Korea Resources Corporation (KORES) in the systematic exploration program. The evaluation based on the quantitative analysis of rock and mineral characteristics as well as geologic mapping, petrography, mineragraphy, fluid inclusion, spectra analysis (PIMA), geochemical and geophysical data. The geology of the prospect area consists of Miocene volcanics of Kiro Formation and Tanahau Formation, intrusion of granodiorite and Quaternary volcanics. The volcanic rock shows the characteristic of tholeiitic magma. The predominant system of lineaments in the prospect area tends to be NE-SW trend. This fault structure appears to have closely relationship with the mineralization in Lowo Deba prospect. The mineralization and alteration outcrops appear to be structural controlled to form epithermal deposit type. Most of the mineralizations are hosted by phyllic – argillic altered andesitic to dacitic tuff which is intruded by granodiorite. Rock samples indicate the mineralization type is quartz vein containing chalcopyrite, galena, sphalerite, covellite and pyrite. The best grade revealed from these altered rocks of 6,980 ppm Cu and 50 ppb Au, and from quartz vein of 4,868 ppm Cu and 57 ppb Au. Mineralization stages evolved from initial higher temperatures (> 320° C) to later lower temperatures (near 170° C). Soil geochemical analysis identify two zones of combined anomaly i.e. Au-Cu-Mo and AgPb-Zn. Those anomalies are concentrated in the phyllic and argillic altered andesitic tuff. These soil anomaly coincide with IP anomalies which are found in electrode separation index of n=5 and n=7 in line WA7 with chargeability value up to 405.7 Msec and resistivity value of 37.7 Ohm-m. In general high chargeability and low resistivity anomalies are developed in the direction of southwest to northeast and still open to the northeast. The high chargeability value allows to predict the occurrence of copper deposits potential. Some bore holes are proposed for the next survey to confirm the presence of new copper deposits in the prospect area.
23
Milner, Simon C., Anton P. Le Roex e Ronald T. Watkins. "Rb-Sr age determinations of rocks from the Okenyenya igneous complex, northwestern Namibia". Geological Magazine 130, n. 3 (maggio 1993): 335–43. http://dx.doi.org/10.1017/s001675680002001x.
Testo completoAbstract (sommario):
AbstractThe Okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast-trending linear feature in Damaraland, northwestern Namibia. Precise Rb–Sr internal isochron ages range from 128.6 ± 1 to 123.4 ± 1.4 Ma for the major phases of intrusion identified within the complex. The tholeiitic gabbros forming the outer rings of the complex, and the later alkali gabbros which form the central hills, cannot be distinguished in terms of Rb–Sr ages, although field relations clearly indicate the younger age of the latter. The intrusionsof nepheline-syenite and essexite comprising the mountain of Okenyenya Bergon the northern edge of the complex give ages of 123.4 ± 1.4 and 126.3 ± 1 Ma, respectively, and form the final major phase of intrusion. The ages obtained for early and late intrusive phases define a minimum magmatic ‘life-span’ of approximately 5 Ma for the complex. The determined age of the Okenyenya igneous complex (129–123 Ma), when taken together with the few reliable published ages for other Damaraland complexes (130–134 Ma), suggests that these sub-volcanic complexes were emplaced contemporaneously with the widespread Etendeka volcanics (˜ 130 Ma), and relate to magmatism associated with the breakup of southern Africa and South America with the opening of the South Atlantic Ocean. The linear distributionof intrusions in Damaraland is interpreted to be due to magmatism resultingfrom the upwelling Tristan plume being focused along a structural discontinuity between the Pan-African, Damaran terrain to the south, and Proterozoiccratonic basement to the north.
24
Camiré, G. E., J. N. Ludden, M. R. La Flèche e J. P. Burg. "Mafic and ultramafic amphibolites from the northwestern Pontiac Subprovince: chemical characterization and implications for tectonic setting". Canadian Journal of Earth Sciences 30, n. 6 (1 giugno 1993): 1110–22. http://dx.doi.org/10.1139/e93-094.
Testo completoAbstract (sommario):
In the northwestern Pontiac Subprovince, metavolcanic rocks are exposed within a metagraywacke sequence that is intruded by metamorphosed mafic dykes. The metavolcanics are Al-undepleted komatiites ([La/Sm]N = 0.3, [Tb/Yb]N = 0.9) and tholeiitic Fe-basalts ([La/Sm]N = 0.8 and [Tb/Yb]N = 0.8). The nearly flat chondrite-normalized distributions of high field strength elements (HFSE), Ti and P, the constant Zr/Y, Nb/Th, Ti/Zr, and Ti/P ratios, and the lack of depletion of HFSE relative to rare-earth elements (REE) in both ultramafic and mafic metavolcanics, imply that crustal assimilation and magma mixing with crustal melts were not significant during differentiation and argue against the presence of subduction-related magmatic components. Contemporaneous volcanism and sedimentation in the northwestern Pontiac Subprovince are unlikely. The metavolcanics do not show any evidence of crustal contamination and likely represent a structurally emplaced, disrupted assemblage, chemically similar to early volcanics of the adjacent southern Abitibi Subprovince.Metamorphosed mafic dykes intruding the metagraywackes are not genetically related to the metavolcanics. The dykes have high CaO, P2O5, K2O, Ba, Rb, and Sr, intermediate Cr and Ni contents, and strongly fractionated REE patterns ([La/Yb]N = 10.8). Normalized to the primitive mantle, they display pronounced negative Nb, Ta, Ti, Zr, and Hf anomalies. These amphibolites are metamorphosed equivalents of Mg-rich calc-alkaline lamprophyre dykes, most likely derived from a hybridized mantle source. Mantle metasomatism was probably related to a subduction event prior to the peak of compressional Kenoran deformation in the Pontiac Subprovince.
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Mortimer, N. "The Nicola Group: Late Triassic and Early Jurassic subduction-related volcanism in British Columbia". Canadian Journal of Earth Sciences 24, n. 12 (1 dicembre 1987): 2521–36. http://dx.doi.org/10.1139/e87-236.
Testo completoAbstract (sommario):
Mafic lavas of the Nicola Group are divided into three distinct petrographic and geochemical types: type 1 lavas are strongly augite-porphyritic picrites, basalts, and andesites that belong to a high-potassium to shoshonitic rock series; type 2 lavas are augite- and plagioclase-porphyritic basalts and andesites that belong to a low-potassium calc-alkaline series; and type 3 lavas are petrographically variable tholeiitic to transitional basalts and andesites.Low concentrations of Ti, Zr, Y, and Nb and moderate to high concentrations of K, Rb, Ba, and Sr in type 1 and 2 lavas clearly indicate a subduction-related tectonic setting of eruption. Type 3 lavas show chemical affinities intermediate between modern-day island-arc and intraplate volcanics. Type 1 (shoshonitic) lavas generally lie east of and are younger than type 2 (calc-alkaline) lavas, a relationship that implies an east-dipping early Mesozoic subduction zone beneath the Nicola arc. These interpretations resolve previous uncertainties regarding the tectonic setting of eruption of the Nicola Group.Several major 205–220 Ma plutons that intrude the Nicola Group crystallized from type 1 and 2 magmas and represent the final stages of Late Triassic to Early Jurassic arc-related igneous activity in southern Quesnellia.
26
Barrett, T. J., S. Cattalani e W. H. MacLean. "Massive sulfide deposits of the Noranda area, Quebec. I. The Horne mine". Canadian Journal of Earth Sciences 28, n. 4 (1 aprile 1991): 465–88. http://dx.doi.org/10.1139/e91-042.
Testo completoAbstract (sommario):
The Horne massive sulfide deposits occur within volcanic rocks of the Blake River Group of the Archean Abitibi greenstone belt. The orebodies dip subvertically within rhyolitic flows, breccias, and tuffs that are bounded by the Andesite and the Horne Creek faults. Least-altered rhyolites have low K2O contents and other geochemical features that place them within the FII tholeiitic series. Graded volcaniclastic beds, metal zoning in the orebodies, and locations of chloritized–mineralized rhyolites indicate that the volcanic sequence youngs to the north. The volcanics in the fault wedge are variably silicified and sericitized, and local zones in the orebody sidewalls and footwall are chloritized.The H orebodies formed podiform masses up to 120 m wide, 100 m thick, and 300 m in downplunge extent, consisting of chalcopyrite–pyrrhotite–pyrite Au ore. Between 1927 and 1976, 54 × 106 t of ore were recovered, grading 2.2% Cu, 6.1 g/t Au, and 13.0 g/t Ag (Zn and Pb are <0.1% and <0.01%, respectively). A semicontinuous Cu-rich base (up to ~15 m thick) exists above the footwall and adjacent to the sidewalls of the orebodies. The ore changes stratigraphically upwards from a chalcopyrite-rich base, through middle pyrrhotite–pyrite-rich zones, to upper pyrite-rich zones. Au enrichments occur in some of the Cu-rich ores but also in overlying pyritic ores and in adjacent host volcanics. Cu–Au-bearing chloritized rhyolites occur mainly in the western and eastern sidewalls and at downplunge terminations of the H orebodies.The No. 5 zone occurs at lower mine levels and consists of numerous, partly overlapping Zn-bearing pyritic lenses up to 30 m thick, within mineralized rhyolitic breccias and tuffs. The No. 5 zone extends up to 750 m along strike and at least 1500 m downdip, with high-pyrite reserves of ~22 × 106 t between the 21st and 39th levels, grading 1.2% Zn, 0.15% Cu, and 1.4 g/t Au. Massive pyritic lenses are richer in Zn (> 50 ×) and Pb, Ag, As, Cd, and Sb relative to the H orebodies but are low in Cu and Au.The restored stratigraphic level of the H orebodies and No. 5 zone was dominated from south to north by rhyolite flows and breccias, then rhyolite breccias and tuffs. The volcanic rocks are interpreted as proximal to distal facies on a volcanic edifice that was affected by widespread silicification and sericitization. A graben system on the flank of the edifice became the depositional site of the H orebodies. High-temperature fluid discharge occurred along the fault-bounded graben margins, producing zones of chloritization and stringer-type Cu mineralization ± Au in rhyolites, and infilling the grabens with Cu-bearing massive sulfides. Lower on the edifice, in the No. 5 zone, Zn-bearing pyritic sulfide lenses accumulated within broader, breccia-based depressions roughly on strike with the H orebodies. Mineralization in the No. 5 zone may reflect lower temperature, more diffuse fluid discharge through a permeable sequence of volcaniclastic rocks.
27
Findlay, Jon M., Randall R. Parrish, Tyson C. Birkett e Donald H. Watanabe. "U–Pb ages from the Nimish Formation and Montagnais glomeroporphyritic gabbro of the central New Québec Orogen, Canada". Canadian Journal of Earth Sciences 32, n. 8 (1 agosto 1995): 1208–20. http://dx.doi.org/10.1139/e95-099.
Testo completoAbstract (sommario):
Three U–Pb zircon dates were obtained from two igneous suites associated with cycle 2 sedimentation in the central New Québec Orogen (Labrador Trough). In the Dyke Lake area of the western part of the orogen (Schefferville zone), the Nimish Formation includes a polymictic conglomerate containing quartz syenite cobbles that crystallized at 1877.8 ± 1.3 Ma. These cobbles are petrographically and geochemically linked to the Nimish volcanics, which are intercalated with the Sokoman iron formation. Consequently, the syenite date gives an approximate age for both Nimish magmatism and ironstone deposition in the Schefferville zone. A date of ca. 2.65 Ga obtained from a Nimish trachyte in the same area indicates that zircons in the sample are xenocrysts. Farther east, at Howse Lake (Howse zone), a Montagnais plagioclase-glomeroporphyritic gabbro sill crystallized at 1884.0 ± 1.6 Ma. The Howse Lake sill, which intrudes turbidites of the Menihek Formation, is considered comagmatic with the basalts that cap the formation, and with the Willbob basalts to the east (Doublet Terrane). Consequently, the Menihek turbidites, as well as the underlying Sokoman Formation, were deposited prior to 1884 Ma in the Howse zone. In contrast, the syenite date indicates that deposition of the Menihek Formation in the Schefferville zone did not commence until after 1878 Ma. The 1884 Ma date from the Howse Lake sill also provides an estimate for the timing of cycle 2 tholeiitic volcanism in the eastern part of the orogen. The correlation between the upper Menihek and Willbob basalts suggests that the Murdoch and Thompson Lake formations, which underlie the Willbob basalts in the Doublet Terrane, are the stratigraphie equivalents of the lower and middle portions of the Menihek Formation, respectively.
28
Piccirillo, E. M., G. Bellieni, G. Cavazzini, P. Comin-Chiaramonti, R. Petrini, A. J. Melfi, J. P. P. Pinese, P. Zantadeschi e A. De Min. "Lower Cretaceous tholeiitic dyke swarms from the Ponta Grossa Arch (southeast Brazil): Petrology, Sr-Nd isotopes and genetic relationships with the Paraná flood volcanics". Chemical Geology 89, n. 1-2 (dicembre 1990): 19–48. http://dx.doi.org/10.1016/0009-2541(90)90058-f.
Testo completo
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Smellie, John L., Kurt S. Panter e Jenna Reindel. "Chapter 5.3a Mount Early and Sheridan Bluff: volcanology". Geological Society, London, Memoirs 55, n. 1 (2021): 491–98. http://dx.doi.org/10.1144/m55-2018-61.
Testo completoAbstract (sommario):
AbstractTwo small monogenetic volcanoes are exposed at Mount Early and Sheridan Bluff, in the upper reaches of Scott Glacier. In addition, the presence of abundant fresh volcanic detritus in moraines at two other localities suggests further associated volcanism, now obscured by the modern Antarctic ice sheet. One of those occurrences has been attributed to a small subglacial volcano onlyc.200 km from South Pole, making it the southernmost volcano in the world. All of the volcanic outcrops in the Scott Glacier region are grouped in a newly defined Upper Scott Glacier Volcanic Field, which is part of the McMurdo Volcanic Group (Western Ross Supergroup). The volcanism is early Miocene in age (c.25–16 Ma), and the combination of tholeiitic and alkaline mafic compositions differs from the more voluminous alkaline volcanism in the West Antarctic Rift System. The Mount Early volcano was erupted subglacially, when the contemporary ice was considerably thicker than present. By contrast, lithologies associated with the southernmost volcano, currently covered by 1.5 km of modern ice, indicate that it was erupted when any associated ice was either much thinner or absent. The eruptive setting for Sheridan Bluff is uncertain and is still being investigated.
30
Isachsen, Clark E. "Geology, geochemistry, and cooling history of the Westcoast Crystalline Complex and related rocks, Meares Island and vicinity, Vancouver Island, British Columbia". Canadian Journal of Earth Sciences 24, n. 10 (1 ottobre 1987): 2047–64. http://dx.doi.org/10.1139/e87-194.
Testo completoAbstract (sommario):
The Westcoast Crystalline Complex is a belt of plutonic rocks along the west coast of Vancouver Island. It is composed mainly of heterogeneous amphibolitic country rock (Westcoast amphibolite), granitoids of trondhjemitic to gabbroic composition (Westcoast diorite), and variable mixtures of these two components (Westcoast migmatite).Although the protolith of some deformed enclaves may be Paleozoic, most of these rocks were generated in a magmatic-arc setting and intruded in Jurassic time. Major- and trace-element chemistry of the Westcoast Crystalline Complex shows a sub-alkaline tholeiitic to calc-alkaline trend.The exponential cooling curves derived for Westcoast diorites are not consistent with in situ crustal magma genesis but instead indicate that these rocks intruded relatively cool country rock.Based on age and chemistry, the Westcoast Crystalline Complex can be interpreted as the deeper crustal equivalent of the more differentiated Island Intrusions and Bonanza Volcanics. Taken together, these rocks provide a disrupted and perhaps incomplete cross section of the magmatic arc of Vancouver Island.Reconnaissance of the Wark–Colquitz Complex of southern Vancouver Island shows it to be essentially indistinguishable in petrography, chemistry, and age from the Westcoast Crystalline Complex, and a similar history is inferred.A calc-alkaline chemistry and rapid initial cooling also characterize a Catface Intrusion dated at 41 Ma. This is again compatible with arc magmatism, but its proximity to the coeval trench is enigmatic.
31
Smolkin, V. F. "The Paleoproterozoic (2.5–1.7 Ga) Midcontinent rift system of the northeastern Fennoscandian Shield". Canadian Journal of Earth Sciences 34, n. 4 (1 aprile 1997): 426–43. http://dx.doi.org/10.1139/e17-036.
Testo completoAbstract (sommario):
The Karelian epoch of tectono-magmatic activity resulted in an intensive structural–tectonic rearrangement of Archean crustal blocks, origination, development, and orogenesis of the Pechenga – Varzuga belt. Being emplaced on the Archean crust of the continental type, the Pechenga – Varzuga belt is an intracontinental paleorift system formed during four stages: prerifting (2.55–2.30 Ga), early rifting (2.30–2.20 Ga), late rifting (2.20–1.95 Ga), and orogenic (1.95–1.70 Ga). During the stage of 2.55–2.30 Ga, as a result of formation of an extensive asthenolens whose projection to the surface covered most of the Kola – Lapland – Karelian province, there appeared paleoaulacogen depressions and mantle and crustal magma associations with normal alkalinity: gabbronorite dykes (2.55–2.40 Ga), low-Ti picrite–basalt (2.45–2.41 Ga), basalt, andesite–basalt (initial 87Sr/86Sr = 0.7042) and dacite volcanics, peridotite–pyroxenite–gabbronorite (2.50–2.43 Ga, εNd = −1, −2), lherzolite–gabbronorite (or drusite) (2.45 Ga), and gabbro–anorthosite (2.45 Ga) layered intrusions characterized by chromite, platinum, and titanomagnetite mineralization. As the rocks of Archean blocks were generally warmed up, intracrustal chambers of granitoid magmas were common. During the stage of 2.30–2.20 Ga, the asthenolens broke up and differentiation of its fragments significantly increased. Over the most heated fragments, the paleorift system (Pechenga–Varzuga belt) appeared, accompanied by generation of mantle melts with higher alkalinity (volcanic series of picrite – trachybasalt – trachyandesitic basalts, 87Sr/86Sr = 0.7035). During the third stage (2.20–1.95 Ga), rifting reached its maximum owing to intense sinistral fault-rifting, and mantle sources of deep ferropicritic (87Sr/86Sr = 0.7032; εNd = +1.6) and shallow tholeiitic basalt (87Sr/86Sr = 0.7021) melts formed at different depths; eruption of these magmas gave rise to thick volcanic sequences (1.98 Ga), Ni – Cu-bearing differentiated gabbro–wehrlite intrusions (1.98–1.90 Ga, 87Sr/86Sr = 0.7029; εNd = +1.5), and cogenetic, peridotite – olivine gabbro dyke swarms (1.96 Ga, εNd = +1.4), which are characterized by the elevated Fe, Ti, P, and light rare earth element contents. Intrusions of sulfide-bearing gabbronorite and websterite formed in the Lapland–Kolvitsa granulite belt, which experienced collision and high-grade (6–10 kbar (1 kbar = 100 MPa)) metamorphism. During the final stage (1.95–1.70 Ga), enclosure and orogenesis of the paleorift system took place; these events were accompanied by extensive development of mixed mantle–crustal and crustal sources, the formation of calc-alkaline volcanic and sedimentary orogenic associations, and the emplacement of P–Ti-bearing alkaline gabbro – nepheline syenite and U – Mo-bearing monzonite–granodiorite intrusions.
32
McGoldrick, Siobhan, Alex Zagorevski e Dante Canil. "Geochemistry of volcanic and plutonic rocks from the Nahlin ophiolite with implications for a Permo–Triassic arc in the Cache Creek terrane, northwestern British Columbia". Canadian Journal of Earth Sciences 54, n. 12 (dicembre 2017): 1214–27. http://dx.doi.org/10.1139/cjes-2017-0069.
Testo completoAbstract (sommario):
In northwestern British Columbia, the Permian Nahlin ophiolite in the northern Cache Creek terrane comprises spinel harzburgite tectonite with minor lherzolite, lower crustal mafic and ultramafic cumulates, gabbroic rocks including dikes intruding mantle harzburgite, and basaltic volcanic and volcaniclastic rocks. New lithogeochemical data from the Menatatuline Range area confirm that plutonic and volcanic rocks of the ophiolite are tholeiitic and arc related, while only a minor component of volcanic rocks are alkaline intraplate basalts. Tholeiitic basalts of the Nahlin ophiolite represent the products of 2%–20% fractional melting, and their complementary residue may be peridotite from the ophiolite mantle section. Correlative tholeiitic volcanic sections can be found elsewhere in the northern Cache Creek terrane, and they may be linked to a regionally extensive (∼200 km) intraoceanic arc. The arc tholeiite geochemistry of the plutonic and volcanic rocks, and the highly depleted nature of the mantle residues, imply that the Nahlin ophiolite formed in a supra-subduction zone environment. The Nahlin ophiolite therefore occupied the upper plate during intraoceanic collision prior to emplacement of the Cache Creek terrane. The volumetrically minor ocean island basalt type volcanic rocks in the northern Cache Creek terrane are associated with carbonate successions bearing Tethyan fauna. These sequences are likely fragments of oceanic plateaux and their carbonate atolls sliced off of the subducting plate and are unrelated to the Nahlin ophiolite-arc system.
33
Brandon, Alan D. "Constraints on magma genesis behind the Neogene Cascade Arc: Evidence from major and trace element variation of high-alumina and tholeiitic volcanics of the Bear Creek Area". Journal of Geophysical Research 94, B6 (1989): 7775. http://dx.doi.org/10.1029/jb094ib06p07775.
Testo completo
34
de Wit, M. J., B. Linol, H. Furnes, T. Muedi e K. Valashiya. "Pillow Talk: Volcanic rocks of the Karoo that formed many leagues under the Gondwanan Sea". South African Journal of Geology 123, n. 3 (1 settembre 2020): 297–330. http://dx.doi.org/10.25131/sajg.123.0021.
Testo completoAbstract (sommario):
Abstract New field mapping has discovered numerous sections of pillow lavas with hyaloclastites at the lowest levels of Jurassic Karoo volcanic sequences across Lesotho and South Africa. Vesicularity (content and size) and geochemistry of the ca. 1 to 50 m thick pillow lavas sequences presently preserved at 1 670 to 2 150 m asl reveal they originated below 500 m of water. Most of the initial 87Sr/86Sr ratios of the pillow lavas scatter around ca. 0.708, which is also the value of Jurassic seawater. There is an increase in the δ18O values concomitantly with increasing alteration, a well-known feature at low-temperature when marine basalts alter to high-δ18O bearing minerals, such as clays and zeolites, stripping 18O by mass balance from seawater. This feature, combined with element behavior of the most altered samples, as well as the Sr-isotope signatures, suggest that the Karoo pillow lavas and hyaloclastites most likely altered under low-temperature conditions in seawater. Geochemistry of the major oxides and trace elements of the Karoo pillow lavas reveal they define tholeiitic basalt with minor basaltic andesite. The pillow lavas define Th/Yb-ratios well above the mantle array indicating derivation from subduction-related components, and their Ti-V relations are similar to mid-oceanic ridge basalt (MORB). The Nd-Sr isotope data for the pillow lavas indicate that the primary magma assimilated ca. 10 to 15% of continental crust. This is consistent with knowledge from mapping that sills and dykes cover close to 50% of the surface geology surrounding the sub-horizontal pillow lava sequences, and from drill-cores that these dolerites reached a minimum crustal depth of ca. 6 km below Jurassic sea level. The Karoo pillow lavas erupted between 186 and 184 Ma in relatively deep seawater of a possible rift system linked to fragmented continental lithosphere surrounded by ocean spreading domains, such as at the open-end of the East African rift across the Afar. By 183 Ma, Karoo volcanics extruded above sea level and after 182 Ma the pillow lavas were uplifted by at least 1 700 m. Thereafter, during farther up warping across the Karoo Large Igneous Province (KLIP), lavas across Lesotho reached heights over 3 500 masl by 140 Ma, and then eroded to their near present heights around 120 to 100 Ma as determined through a number of thermo-chronology and offshore sedimentation analyses. Thus, unlike the volcanics of the Ethiopian LIP, which erupted across continental crust at 2 000 masl ca. 30 to 40 million years prior to extensional tectonism across the Afar triple junction, KLIP reached its highest topography ca. 30 to 40 Myrs after early initiation across a triple junction system well-below sea level. This difference in timing between crustal extension and epeirogeny has implications for geodynamic and geochemical models that trace dispersal across southern Gondwana between Africa and Antarctica.
35
Abramov, B. N. "On petrogeochemical zoning of mesozoic volcanites of the ore fields of gold and polymetallic deposits of the Eastern Transbaikalia". Доклады Академии наук 487, n. 1 (19 luglio 2019): 65–68. http://dx.doi.org/10.31857/s0869-5652487165-68.
Testo completoAbstract (sommario):
Transverse petrogeochemical zoning in the location of volcanites of Shadaronsky (J2-3) and Mulinsky (J2-3) series, characteristic of volcanic arcs, has been revealed in the Eastern Transbaikalia. Volcanites of the tholeiitic and calc-alkaline series are developed in the frontal zone of the volcanic arc, in the suture zone of the Mongolo-Okhotsk suture (MOS), in the rear zone of the MOS - volcanites of the calc-alkaline series. The volcanites of the frontal and rear parts differ in their petrogeochemical composition, in the degree of oxidation of iron and in the distribution of rare earth elements (REE). These differences are characteristic of the volcanic arcs of the transition zones from the ocean to the continent. Vulcano-plutonic structures of the frontal zone of MOS produce gold mineralization, the rear zone of MOS - polymetallic mineralization.
36
Smellie, J. L. "Lithostratigraphy of Miocene–Recent, alkaline volcanic fields in the Antarctic Peninsula and eastern Ellsworth Land". Antarctic Science 11, n. 3 (settembre 1999): 362–78. http://dx.doi.org/10.1017/s0954102099000450.
Testo completoAbstract (sommario):
Miocene–Recent alkaline volcanic rocks form numerous outcrops scattered widely throughout the Antarctic Peninsula and eastern Ellsworth Land. They occur mainly as short-lived (typically 1–2 million years) monogenetic volcanic fields but include a large outcrop area in northern Antarctic Peninsula which includes several substantial polygenetic shield volcanoes that were erupted over a 10 million year period (the James Ross Island Volcanic Group (JRIVG)). As a whole, the outcrops are of considerable importance for our understanding of the kinematic, petrological and palaeoenvironmental evolution of the region during the late Cenozoic. Until now, there has been no formal stratigraphical framework for the volcanism. Knowledge of the polygenetic JRIVG is still relatively poor, whereas a unifying lithostratigraphy is now possible for the monogenetic volcanic fields. For the latter, two new volcanic groups and twelve formations are defined, together with descriptions of the type sections. The volcanic fields (both polygenetic and monogenetic) vary in area from c. 1 to 4500 km2, and aeromagnetic data suggest that one may exceed 7 000 km2. The rocks are divisible into two contrasting petrological ‘series’, comprising basanites–phonotephrites and alkali basalts–tholeiites. The JRIVG is dominated by alkali basalts–tholeiites but also contains rare basanites, and phonotephrite–tephriphonolite compositions occur in minor pegmatitic segregations in sills. By contrast, in the monogenetic volcanic fields, basanites–phonotephrites generally form the older outcrops (mainly 15–5.4 Ma) and alkali basalts–tholeiites the younger outcrops (4(?)–<1 Ma). Throughout the region, erupted volumes of alkali basalts–tholeiites were an order of magnitude greater, at least, than those of basanite–phonotephrite compositions. Interpretation of the lithofacies indicates varied Miocene–Recent palaeoenvironments, including eruption and deposition in a marine setting, and beneath Alpine valley glaciers and ice sheets. Former ice sheets several hundred metres thick, and fluctuating ice surface elevations, which were generally higher during the eruptive periods than at present, can also be demonstrated.
37
Panter, Kurt S., Jenna Reindel e John L. Smellie. "Chapter 5.3b Mount Early and Sheridan Bluff: petrology". Geological Society, London, Memoirs 55, n. 1 (2021): 499–514. http://dx.doi.org/10.1144/m55-2019-2.
Testo completoAbstract (sommario):
AbstractThis study discusses the petrological and geochemical features of two monogenetic Miocene volcanoes, Mount Early and Sheridan Bluff, which are the above-ice expressions of Earth's southernmost volcanic field located atc.87° S on the East Antarctic Craton. Their geochemistry is compared to basalts from the West Antarctic Rift System to test affiliation and resolve mantle sources and cause of melting beneath East Antarctica. Basaltic lavas and dykes are olivine-phyric and comprise alkaline (hawaiite and mugearite) and subalkaline (tholeiite) types. Trace element abundances and ratios (e.g. La/Yb, Nb/Y, Zr/Y) of alkaline compositions resemble basalts from the West Antarctic rift and ocean islands (OIB), while tholeiites are relatively depleted and approach the concentrations levels of enriched mid-ocean ridge basalt (E-MORB). The magmas evolved by fractional crystallization with contamination by crust; however, neither process can adequately explain the contemporaneous eruption of hawaiite and tholeiite at Sheridan Bluff. Our preferred scenario is that primary magmas of each type were produced by different degrees of partial melting from a compositionally similar mantle source. The nearly simultaneous generation of lower degrees of melting to produce alkaline types and higher degrees of melting forming tholeiite was most likely to have been facilitated by the detachment and dehydration of metasomatized mantle lithosphere.
38
KEPPIE, J. D., e J. DOSTAL. "Birth of the Avalon arc in Nova Scotia, Canada: geochemical evidence for ∼700–630 Ma back-arc rift volcanism off Gondwana". Geological Magazine 135, n. 2 (marzo 1998): 171–81. http://dx.doi.org/10.1017/s0016756898008322.
Testo completoAbstract (sommario):
Central Cape Breton Island in Nova Scotia, Canada, is host to ∼700–630 Ma felsic and associated mafic volcanic rocks that are relatively rare in other parts of the Avalon Composite Terrane, occurring elsewhere only in the Stirling Block of southern Cape Breton Island and in parts of eastern Newfoundland. The mafic rocks of central Cape Breton Island are typically intraplate tholeiitic basalts generated by melting of a garnet-bearing mantle source. They lack a continental trace element and εNd imprint although they were emplaced on continental crust; they resemble oceanic island basalts. Contemporaneous volcanism in the Stirling Block is calc-alkaline and formed in a volcanic arc setting. In the absence of evidence for an intervening trench complex or suture, it may be inferred that the central Cape Breton tholeiites formed in a back-arc setting relative to the Stirling Block. This rifting may represent the initial stages of separation of an Avalonian arc from western Gondwana. The arc rifted further between ∼630–610 Ma when the younger Antigonish-Cobequid back-arc basin formed. Subsequently, the extensional arc became convergent, telescoping the back-arc basin. Northwestward migration of calc-alkaline arc magmatism may be related to shallowing of the associated Benioff zone through time.
39
Fyffe, L. R., e S. M. Barr. "Petrochemistry and tectonic significance of Carboniferous volcanic rocks in New Brunswick". Canadian Journal of Earth Sciences 23, n. 9 (1 settembre 1986): 1243–56. http://dx.doi.org/10.1139/e86-121.
Testo completoAbstract (sommario):
Carboniferous volcanic rocks from the New Brunswick Platform in the Maritimes Basin are divided into three age groups. Late Tournaisian to early Visean volcanic rocks are tholeiitic basalts and andesites that, in southern New Brunswick, are inter-bedded with abundant calc-alkalic rhyolite. Late Visean to Namurian volcanic rocks consist of an interbedded sequence of alkalic basalts and trachyandesites. Late Westphalian volcanic rocks change in composition up section from trachyte to peralkalic rhyolite. All three age groups display petrochemical features indicative of an intraplate tectonic setting. The volcanic geochemistry is consistent with the development of the Maritimes Basin either as a failed rift formed along the margin of a late Paleozoic ocean or as a rhomb graben formed within a transcurrent zone; the former model is preferred. The change in basaltic composition from tholeiitic to alkalic apparently coincided with a decrease in rate of extension between the Tournaisian and Namurian. Local peralkalic volcanism occurred during regional sagging of the basin as extension ceased and basement rocks cooled in the Late Carboniferous.
40
Leat, P. T., e R. S. Thorpe. "Ordovician volcanism in the Welsh Borderland". Geological Magazine 123, n. 6 (novembre 1986): 629–40. http://dx.doi.org/10.1017/s0016756800024146.
Testo completoAbstract (sommario):
AbstractBasaltic, andesitic and rhyodacitic/rhyolitic volcanism was widespread during Ordovician time in the Welsh Basin. New chemical data are presented for Llanvirn to Caradoc lavas and tuffs from the Welsh Borderland which, during Ordovician time, formed the southeastern margin of the Welsh Basin. In view of the observed chemical alteration, immobile elements are used in the interpretation of the original geochemical character. The data indicate that the Llanvirn Stapeley volcanic group of the Shelve inlier was a bimodal basalt/basaltic andesite – rhyodacite/rhyolite association. The basalts have trace element contents of tholeiitic associations with a subduction-related character. The Caradoc Whittery and Hagley volcanic groups of the Shelve inlier comprise lavas and tuffs of calc-alkaline andesite. Blocks sampled from the Breidden Hills show that these were also derived from a calc-alkaline volcano. Associated Caradoc pumice- and ash-flow deposits from the Breidden Hills are probably of altered calc-alkaline rhyodacite/rhyolite composition. The Sibdon Carwood basalt flow, the only known example of Ordovician volcanism east of the Pontesford–Linley and Church Stretton lineaments, has transitional tholeiitic to alkaline character, with trace element contents influenced by subduction-related processes. The overall tholeiitic to calc-alkaline nature of the magmatism is consistent with the view that, during Llanvirn to Caradoc time, the Welsh Basin was an ensialic marginal basin.
41
Maurice, Charles, Don Francis e Louis Madore. "Constraints on early Archean crustal extraction and tholeiitic-komatiitic volcanism in greenstone belts of the Northern Superior Province". Canadian Journal of Earth Sciences 40, n. 3 (1 marzo 2003): 431–45. http://dx.doi.org/10.1139/e03-006.
Testo completoAbstract (sommario):
Numerous small remnants of Archean greenstone belts in the Northern Superior Province (ca. 28752710 Ma) have chemical characteristics similar to those of the larger greenstone belts of the Southern Superior Province, and preserve direct evidence of crustal conditions prior to the major volcanic events of the late Archean (WawaAbitibi subprovinces; ca. 27602700 Ma). Three of the best preserved belts are engulfed in tonalite intrusions of the Faribault-Thury Complex (FTC) and exhibit common chemical characteristics, which may imply a similar origin. The dominant tholeiitic basalts typically have MgO contents > 7 wt.%, TiO2 < 1 wt.% and nearly flat rare-earth element (REE) patterns (La/Smn = 0.771.22; Gd/Ybn = 0.861.20). Associated komatiites have flat to depleted REE patterns (La/Smn = 0.450.95), high Al2O3/TiO2 (>15), low CaO/Al2O3 (<1.2), and chondritic Gd/Yb ratios similar to 2.7 Ga Al-undepleted komatiites. The trace-element ratios of komatiitic rocks are indistinguishable from those of the associated tholeiites, suggesting either a derivation from similar mantle sources or a comagmatic relationship (Nb/Thpm = 0.81.1; La/Cepm = 0.91.3; Nb/Ce = 0.70.9; Y/Hopm ~1; and Th/Lapm = 0.71.1). Numerical modelling of trace and major elements during low-pressure crystal fractionation reproduces the spectrum of both inferred liquid and cumulate compositions and is consistent with a comagmatic origin between the komatiites and tholeiites. The relatively low Nb/Th ratios of these mid-Archean volcanic rocks relative to both modern day basalts and late Archean basalts may indicate that they were derived from a mantle source that had not lost its crustal components, nor seen significant recycled oceanic crust (high Nb/Th). The extraction of continental crust from this Archean mantle source might then postdate the FTC volcanism, and may be associated with the generation of the voluminous tonalites that engulf the belts.
42
Dostal, J., C. Dupuy e J. L. Poidevin. "Geochemistry of Precambrian basaltic rocks from the Central African Republic (Equatorial Africa)". Canadian Journal of Earth Sciences 22, n. 5 (1 maggio 1985): 653–62. http://dx.doi.org/10.1139/e85-072.
Testo completoAbstract (sommario):
The two Archaean greenstone belts (Bandas and Bogoin) in the Central African Republic (Equatorial Africa) are 250 and 150 km long. The metavolcanic rocks in the belts are predominantly komatiitic and tholeiitic basalts. Komatiites include both Al-depleted and Al-undepleted types. The komatiites and light-REE-depleted tholeiites were probably derived from a similar upper mantle source. However, the tholeiitic basalts enriched in light REE from the upper volcanic strata of the Bandas belt were generated from a different source. The dolerites from Proterozoic dyke swarms and sills differ from the basalts mainly in their abundances and ratios of several incompatible elements such as K, Rb, Th, and light REE. They were derived from a distinct, incompatible-element-enriched upper mantle source.The average background gold levels in the Bandas belt and dolerite dyke swarms are comparable to those in equivalent rocks from North America. The exception is the Bogoin greenstone belt, which has anomalously high gold abundances.
43
Dostal, J., R. Laurent e J. D. Keppie. "Late Silurian – Early Devonian rifting during dextral transpression in the southern Gaspé Peninsula (Quebec): petrogenesis of volcanic rocks". Canadian Journal of Earth Sciences 30, n. 12 (1 dicembre 1993): 2283–94. http://dx.doi.org/10.1139/e93-198.
Testo completoAbstract (sommario):
The Upper Silurian – Lower Devonian volcanic rocks in the southern Gaspé Peninsula of the Quebec Appalachians crop out at the northeast end of the Connecticut Valley – Gaspé Synclinorium. These shallow marine and subaerial sequences reach a thickness of up to at least 2000 m and comprise two groups: (1) the Late Silurian volcanic rocks, which are mainly transitional alkalic–tholeiitic basalts with steeply sloping REE patterns; (2) the Early Devonian volcanic rocks, which include a significant proportion of intermediate rocks in addition to tholeiitic basalts. Compared with the Silurian rocks, the Devonian basalts have lower abundances of strongly incompatible trace elements such as Ba, Th, Ta, Nb, and light REE and relatively flat heavy REE patterns. Basalts of both groups display negative Nb and Ta anomalies (relative to Th and La).Although the basalts of both sequences were derived from lithospheric mantle, the Silurian basalts were generated from garnet peridotite at ~ 80 km depth while the Devonian basalts appear to have resulted from a larger degree of melting of spinel peridotite at a shallower depth (~ 60 km). Devonian intermediate rocks are probably the result of mixing of the basaltic magma with upper crustal material through assimilation – fractional crystallization processes. The basalts are interpreted to have formed in a northwest-trending rift zone located in the Quebec Reentrant during dextral transpression along the Appalachian Orogen. Rotation during and after the volcanism reoriented the rift zone to a northeast trend. The high density layer at the base of the crust under the Magdalen Basin may be the former magma chamber for the Silurian–Devonian volcanism. The change from transitional to tholeiitic volcanism at the Silurian–Devonian boundary suggests that the stretching value (ratio of final to initial surface area) increased from < 2 to > 2 at that time. This boundary is also coincident with the Salinic disturbance that is inferred to have been produced by erosion of the thermally uplifted block associated with rifting.
44
LAPIERRE, H., D. BOSCH, A. NARROS, G. H. MASCLE, M. TARDY e A. DEMANT. "The Mamonia Complex (SW Cyprus) revisited: remnant of Late Triassic intra-oceanic volcanism along the Tethyan southwestern passive margin". Geological Magazine 144, n. 1 (21 dicembre 2006): 1–19. http://dx.doi.org/10.1017/s0016756806002937.
Testo completoAbstract (sommario):
Upper Triassic volcanic and sedimentary rocks of the Mamonia Complex in southwestern Cyprus are exposed in erosional windows through the post-Cretaceous cover, where the Mamonia Complex is tectonically imbricated with the Troodos and Akamas ophiolitic suites. Most of these Upper Triassic volcanic rocks have been considered to represent remnants of Triassic oceanic crust and its associated seamounts. New Nd and Pb isotopic data show that the whole Mamonia volcanic suite exhibits features of oceanic island basalts (OIB). Four rock types have been distinguished on the basis of the petrology and chemistry of the rocks. Volcanism began with the eruption of depleted olivine tholeiites (Type 1) and oceanic island tholeiites (Type 2) associated with deep basin siliceous and/or calcareous sediments. The tholeiites were followed by highly phyric alkali basalts (Type 3) interbedded with pelagic Halobia-bearing limestones or white reefal limestones. Strongly LREE-enriched trachytes (Type 4) were emplaced during the final stage of volcanic activity. Nd and Pb isotopic ratios suggest that tholeiites and mildly alkali basalts derived from partial melting of heterogeneous enriched mantle sources. Fractional crystallization alone cannot account for the derivation of trachytes from alkaline basalts. The trachytes could have been derived from the partial melting at depth of mafic material which shares with the alkali basalts similar trace element and isotopic compositions. This is corroborated by the rather similar isotopic compositions of the alkali basalts and trachytes. The correlations observed between incompatible elements (Nb, Th) and εNd and Pb isotopic initial ratios suggest that the Mamonia suite was derived from the mixing of a depleted mantle (DMM) and an enriched component of High μ (μ = 238U/204Pb, HIMU) type. Models using both Nd and Pb isotopic initial ratios suggest that the depleted tholeiites (Type 1) derived from a DMM source contaminated by an Enriched Mantle Type 2 component (EM2), and that the oceanic tholeiites (Type 2), alkali basalts (Type 3) and trachytes (Type 4) were derived from the mixing of the enriched mantle source of the depleted tholeiites with a HIMU component. None of the Mamonia volcanic rocks show evidence of crustal contamination. The Upper Triassic within-plate volcanism likely erupted in a small southerly Neotethyan basin, located north of the Eratosthenes seamount and likely floored by oceanic crust.
45
Phillips, E. R., R. P. Barnes, R. J. Merriman e J. D. Floyd. "The tectonic significance of Ordovician basic igneous rocks in the Southern Uplands, southwest Scotland". Geological Magazine 132, n. 5 (settembre 1995): 549–56. http://dx.doi.org/10.1017/s001675680002121x.
Testo completoAbstract (sommario):
AbstractIn the northern part of the Southern Uplands, restricted volumes of basic igneous rocks occur at or near the base of the Ordovician sedimentary strata. These rocks have previously been interpreted as ocean-floor tholeiites representative of the subducted Iapetus oceanic plate, preserved as tectonic slivers in a fore-arc accretionary prism. The alternative, back-arc basin model proposed for the Southern Uplands on sedimentological evidence raises questions over the origin of these rocks. New geochemical data and previously published data clearly indicate that the volcanic material does not have a simple single source. The oldest (Arenig) volcanic rocks from the Moffat Shale Group associated with the Leadhills Fault include alkaline within-plate basalts and tholeiitic lavas which possibly display geochemical characteristics of midocean ridge basalts. In the northernmost occurrence, alkaline and tholeiitic basalts contained within the Caradoc Marchburn Formation are both of within-plate ocean island affinity. To the south, in the Gabsnout Burn area, the Moffat Shale Group contains lenticular bodies of dolerite and basalt which have characteristics of island-arc to transitional basalts. This complex association of basaltic volcanic rocks is, at the present time, difficult to reconcile with either a simple fore-arc or back-arc setting for the Southern Uplands. However, the increasing arc-related chemical influence on basic rock geochemistry towards the southeast may tentatively be used in support of a southern arc-terrane, and as a result, a back-arc situation for the Southern Uplands basin. An alternative is that these volcanic rocks may represent the local basement to the basin and include remnants of an arc precursor to the Southern Uplands basin.
46
Bahlburg, Heinrich, Udo Zimmermann, Ramiro Matos, Jasper Berndt, Nestor Jimenez e Axel Gerdes. "The missing link of Rodinia breakup in western South America: A petrographical, geochemical, and zircon Pb-Hf isotope study of the volcanosedimentary Chilla beds (Altiplano, Bolivia)". Geosphere 16, n. 2 (10 gennaio 2020): 619–45. http://dx.doi.org/10.1130/ges02151.1.
Testo completoAbstract (sommario):
Abstract The assembly of Rodinia involved the collision of eastern Laurentia with southwestern Amazonia at ca. 1 Ga. The tectonostratigraphic record of the central Andes records a gap of ∼300 m.y. between 1000 Ma and 700 Ma, i.e., from the beginning of the Neoproterozoic Era to the youngest part of the Cryogenian Period. This gap encompasses the time of final assembly and breakup of the Rodinia supercontinent in this region. We present new petrographic and whole-rock geochemical data and U-Pb ages combined with Hf isotope data of detrital zircons from the volcanosedimentary Chilla beds exposed on the Altiplano southwest of La Paz, Bolivia. The presence of basalt to andesite lavas and tuffs of continental tholeiitic affinity provides evidence of a rift setting for the volcanics and, by implication, the associated sedimentary rocks. U-Pb ages of detrital zircons (n = 124) from immature, quartz-intermediate sandstones have a limited range between 1737 and 925 Ma. A youngest age cluster (n = 3) defines the maximum depositional age of 925 ± 12 Ma. This is considered to coincide with the age of deposition because Cryogenian and younger ages so typical of Phanerozoic units of this region are absent from the data. The zircon age distribution shows maxima between 1300 and 1200 Ma (37% of all ages), the time of the Rondônia–San Ignacio and early Sunsás (Grenville) orogenies in southwestern Amazonia. A provenance mixing model considering the Chilla beds, Paleozoic Andean units, and data from eastern Laurentia Grenville sources shows that >90% of the clastic input was likely derived from Amazonia. This is also borne out by multidimensional scaling (MDS) analysis of the data. We also applied MDS analysis to combinations of U-Pb age and Hf isotope data, namely εHf(t) and 176Hf/177Hf values, and demonstrate again a very close affinity of the Chilla beds detritus to Amazonian sources. We conclude that the Chilla beds represent the first and hitherto only evidence of Rodinia breakup in Tonian time in Andean South America.
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Hollings, Pete, Greg Stott e Derek Wyman. "Trace element geochemistry of the Meen-Dempster greenstone belt, Uchi subprovince, Superior Province, Canada: back-arc development on the margins of an Archean protocontinent". Canadian Journal of Earth Sciences 37, n. 7 (1 luglio 2000): 1021–38. http://dx.doi.org/10.1139/e00-007.
Testo completoAbstract (sommario):
Comprehensive trace element analyses of mafic and felsic volcanic rocks from the 2.85-2.74 Ga Meen-Dempster greenstone belt reveal a wide compositional diversity. The ~2.85 Ga Kaminiskag assemblage is dominated by mafic tholeiite characterized by predominantly unfractionated REE (La/Smn = 0.8-1.1). Rare intermediate and felsic pyroclastic flows (SiO2 = 62-74) with moderate to pronounced LREE enrichment are intercalated with the tholeiite. The Kaminiskag assemblage is overlain by the ~2825 Ma Meen assemblage, comprising dominantly dacitic tuffs and pyroclastic breccia, displaying enriched LREE (La/Smn = 3.7-7.2) and moderately fractionated HREE, in conjunction with pronounced negative Nb anomalies. Five distinct suites have been recognized in the ~2740 Ma Confederation assemblage: (1) tholeiitic basalt with flat to smoothly depleted REE, (2) tholeiite with flat to weakly depleted LREE in conjunction with pronounced negative Nb anomalies, (3) Fe-rich basalt with elevated Ti and P contents, LREE enrichment, and fractionated HREE, (4) LREE enriched basalt and andesite with negative Nb anomalies, and (5) dacite and rhyolite with enriched LREE, moderately fractionated HREE, and variable high field strength element anomalies. The geochemistry and geochronology of the Kaminiskag and Meen assemblages are consistent with the formation of an oceanic back arc (Kaminiskag assemblage), which formed the basement for a subduction-related arc complex (Meen assemblage) after a 15 Ma hiatus. The Confederation assemblage is interpreted to represent an Archean back arc, where the complex interplay of mantle sources allows for the eruption of tholeiite, subduction-modified tholeiite, ocean island basalt-like basalt, and subduction-related arc-type volcanic rocks. The recognition of back-arc basins within the Meen-Dempster greenstone belt emphasizes a continuity of crustal growth processes from the Archean to the present day.
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Skulski, Thomas, Robert P. Wares e Alan D. Smith. "Early Proterozoic (1.88–1.87 Ga) tholeiitic magmatism in the New Québec orogen". Canadian Journal of Earth Sciences 30, n. 7 (1 luglio 1993): 1505–20. http://dx.doi.org/10.1139/e93-129.
Testo completoAbstract (sommario):
The New Québec orogen contains two volcano-sedimentary sequences bounded by unconformities. Each sequence records a change from continental sedimentation and alkaline volcanism to marine sedimentation and tholeiitic volcanism. The first sequence records 2.17 Ga rifting and the development, by 2.14 Ga, of a passive margin along the eastern part of the Superior craton. The second sequence developed between 1.88 and 1.87 Ga in pull-apart basins that reflect precollisional dextral transtension along the continental margin. Second-sequence magmatism comprises (i) carbonatitic and lamprophyric intrusions and mildly alkaline mafic to felsic volcanic rocks; (ii) widespread intrusion of tholeiitic gabbro sills, and submarine extrusion of plagioclase glomeroporphyritic basalts and younger aphyric basalts and picrites; and (iii) late-stage, mafic to felsic volcanism and intrusion of carbonatites. Crustal thinning allowed primitive tholeiitic magmas to equilibrate at progressively lower pressures before more buoyant derivative liquids could erupt. Early primitive melts were trapped at the base of the crust and crystallized olivine and orthopyroxene with minor crustal contamination. Derivative melts, similar to transitional mid-ocean-ridge basalts, migrated upward into mid-crustal magma chambers where they became saturated in calcic plagioclase. Subsequent tapping of these magma chambers allowed plagioclase ultraphyric magmas to intrude the sedimentary pile and erupt on the sea floor. Prolonged lithospheric extension resulted in more voluminous mantle melting and eruption of picrites and basalts in the south. Primitive magmas in the north were trapped beneath thicker crust and crystallized wehrlite cumulates. Resulting basaltic melts intruded the volcano-sedimentary pile, or erupted as aphyric basalts.
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Abdel-Karim, Abdel-Aal M., El-Nuri M. Ramadan e Mohamed R. Embashi. "Multiphase Alkaline Basalts of Central Al-Haruj Al-Abyad of Libya: Petrological and Geochemical Aspects". Journal of Geological Research 2013 (18 giugno 2013): 1–12. http://dx.doi.org/10.1155/2013/805451.
Testo completoAbstract (sommario):
Al-Haruj basalts that represent the largest volcanic province in Libya consist of four lava flow phases of varying thicknesses, extensions, and dating. Their eruption is generally controlled by the larger Afro-Arabian rift system. The flow phases range from olivine rich and/or olivine dolerites to olivine and/or normal basalts that consist mainly of variable olivine, clinopyroxene, plagioclase, and glass. Olivine, plagioclase, and clinopyroxene form abundant porphyritic crystals. In olivine-rich basalt and olivine basalt, these minerals occur as glomerophyric or seriate clusters of an individual mineral or group of minerals. Groundmass textures are variably intergranular, intersertal, vitrophyric, and flow. The pyroclastic, clastogenic flows and/or ejecta of the volcanic cones show porphyritic, vitrophric, pilotaxitic, and vesicular textures. They are classified into tholeiite, alkaline, and olivine basalts. Three main groups are recorded. Basalts of phase 1 are generated from tholeiitic to alkaline magma, while those of phases 3 and 4 are derived from alkaline magma. It is proposed that the tholeiitic basalts represent prerift stage magma generated by higher degree of partial melting (2.0–3.5%) of garnet-peridotite asthenospheric mantle source, at shallow depth, whereas the dominant alkaline basalts may represent the rift stage magma formed by low degree of partial melting (0.7–1.5%) and high fractionation of the same source, at greater depth in an intra-continental plate with OIB affinity. The melt generation could be also attributed to lithosphere extension associated with passive rise of variable enriched mantle.
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Imamverdiev, Nazim A., Araz I. Orudzhov, Anar A. Valiyev e Samir S. Mursalov. "Petro-geochemical features of the Bajocian island-arc volcanism in the Lesser Caucasus (Azerbaijan)". Journal of Geology, Geography and Geoecology 31, n. 2 (3 agosto 2022): 280–92. http://dx.doi.org/10.15421/112226.
Testo completoAbstract (sommario):
This article discusses the petro-geochemical features of the Bajocian volcanism in the Azerbaijan Lesser Caucasus. Jurassic volcanism, manifested with varying intensity in the Lok-Karabakh zone, took place in various geodynamic settings, depending on the alternation of extension and compression processes in the island arc. Under these conditions, in the Lesser Caucasus during Middle Jurassic Epoch, two volcanic associations were formed: the Bajocian basalt-rhyolite and the basalt-andesite-dacite-rhyolite of the Bathonian age. It was found that the volcanic rocks of the Lower Bajocian complex belong to the tholeiitic series, and the Upper Bajocian rocks belong to the calc-alkaline series. In the rocks of the association, light REE slightly prevail over heavy ones and form almost flat spectra, the normalized plots are characterized by the chondritic nature of the distribution of rare earth elements, and the lines are parallel to the spectrum of the distribution of rare earth elements in MORB. In such rocks particularly the europium ratio (Eu / Eu * = 0.81–1.21) approaches 1 and low La / Yb ratios are observed. In some samples of more basic rocks, the content of heavy REE increases. Such a distribution of REE in the studied volcanic rocks is common for basic rocks of the tholeiitic series in typical island arcs. In the analyzed single rhyolite sample, a negative Eu anomaly is observed (Eu / Eu * = 0.56). The volcanic rocks on the primitive mantle normalized multi-element plots are characterized by depletion in Ta, Nb and enrichment in LILE (Rb, Ba, Pb, U, Th), which is characteristic of island arc-related volcanic rocks of supra-subduction zones (SSZ). The rocks are also depleted in titanium, potassium, and phosphorus. It was concluded that, in terms of geochemical features, the Middle Jurassic igneous rocks were formed at the ensimatic island arc, which was the initial stage of the development of the island arc tectonic setting, replaced in the Upper Jurassic by ensialic subduction.