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Aspiotis, S., S. Jung, F. Hauff, and R. L. Romer. "Petrogenesis of a late-stage calc-alkaline granite in a giant S-type batholith: geochronology and Sr–Nd–Pb isotopes from the Nomatsaus granite (Donkerhoek batholith), Namibia." International Journal of Earth Sciences 110, no. 4 (April 7, 2021): 1453–76. http://dx.doi.org/10.1007/s00531-021-02024-w.
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AbstractThe late-tectonic 511.4 ± 0.6 Ma-old Nomatsaus intrusion (Donkerhoek batholith, Damara orogen, Namibia) consists of moderately peraluminous, magnesian, calc-alkalic to calcic granites similar to I-type granites worldwide. Major and trace-element variations and LREE and HREE concentrations in evolved rocks imply that the fractionated mineral assemblage includes biotite, Fe–Ti oxides, zircon, plagioclase and monazite. Increasing K2O abundance with increasing SiO2 suggests accumulation of K-feldspar; compatible with a small positive Eu anomaly in the most evolved rocks. In comparison with experimental data, the Nomatsaus granite was likely generated from meta-igneous sources of possibly dacitic composition that melted under water-undersaturated conditions (X H2O: 0.25–0.50) and at temperatures between 800 and 850 °C, compatible with the zircon and monazite saturation temperatures of 812 and 852 °C, respectively. The Nomatsaus granite has moderately radiogenic initial 87Sr/86Sr ratios (0.7067–0.7082), relatively radiogenic initial εNd values (− 2.9 to − 4.8) and moderately evolved Pb isotope ratios. Although initial Sr and Nd isotopic compositions of the granite do not vary with SiO2 or MgO contents, fSm/Nd and initial εNd values are negatively correlated indicating limited assimilation of crustal components during monazite-dominated fractional crystallization. The preferred petrogenetic model for the generation of the Nomatsaus granite involves a continent–continent collisional setting with stacking of crustal slices that in combination with high radioactive heat production rates heated the thickened crust, leading to the medium-P/high-T environment characteristic of the southern Central Zone of the Damara orogen. Such a setting promoted partial melting of metasedimentary sources during the initial stages of crustal heating, followed by the partial melting of meta-igneous rocks at mid-crustal levels at higher P–T conditions and relatively late in the orogenic evolution.
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Milner, Simon C., Anton P. Le Roex, and Ronald T. Watkins. "Rb-Sr age determinations of rocks from the Okenyenya igneous complex, northwestern Namibia." Geological Magazine 130, no. 3 (May 1993): 335–43. http://dx.doi.org/10.1017/s001675680002001x.
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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.
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Reid, D. L., A. F. Cooper, D. C. Rex, and R. E. Harmer. "Timing of post–Karoo alkaline volcanism in southern Namibia." Geological Magazine 127, no. 5 (September 1990): 427–33. http://dx.doi.org/10.1017/s001675680001517x.
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AbstractNew radiometric age data are reported for alkaline centres in southern Namibia, and are discussed together with published age data in terms of models put forward to account for post-Karoo (Mesozoic–Recent) alkaline magmatism within the African plate. Agreement between K–Ar and Rb–Sr ages indicate emplacement of the Dicker Willem carbonatite in southern Namibia at 49 ± 1 Ma. Alkaline rocks associated with the Gross Brukkaros volcano show a discordant radiometric age pattern, but the best estimate for the age of this complex is 77 ± 2 Ma, similar to that obtained for the neighbouring Gibeon carbonatite-kimberlite province. The Dicker Willem carbonatite is therefore younger than the Luderitz alkaline province (133 ± 2 Ma), and the Gross Brukkaros volcano, but is older than the Klinghardt phonolite field (29–37 Ma). The new age data argue against a distinct periodicity in alkaline igneous activity in southern Africa, thereby ruling out possible controls by episodic marginal upwarping of the subcontinent. Although the available age data do not appear to be consistent with the passage of one or even two hotspots under southern Namibia, it is argued that the surface expression of hotspots under continents may be so large and overlapping that within-plate magmatism attributed to these thermal anomalies need not necessarily be confined to narrow linear belts or show an age progression. The role of hotspots in continental alkaline magmatism is most likely one of melt generation, while local crustal structure probably controls the distribution and timing of eruption. Major tectonic boundaries in the Precambrian basement underlying southern Namibia seem to have controlled the development of Tertiary alkaline centres in that region.
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Cuney, Michel. "Felsic magmatism and uranium deposits." Bulletin de la Société Géologique de France 185, no. 2 (February 1, 2014): 75–92. http://dx.doi.org/10.2113/gssgfbull.185.2.75.
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Abstract The strongly incompatible behaviour of uranium in silicate magmas results in its concentration in the most felsic melts and a prevalence of granites and rhyolites as primary U sources for the formation of U deposits. Despite its incompatible behavior, U deposits resulting directly from magmatic processes are quite rare. In most deposits, U is mobilized by hydrothermal fluids or ground water well after the emplacement of the igneous rocks. Of the broad range of granite types, only a few have U contents and physico-chemical properties that permit the crystallization of accessory minerals from which uranium can be leached for the formation of U deposits. The first granites on Earth, which crystallized uraninite, dated at 3.1 Ga, are the potassic granites from the Kaapval craton (South Africa) which were also the source of the detrital uraninite for the Dominion Reef and Witwatersrand quartz pebble conglomerate deposits. Four types of granites or rhyolites can be sufficiently enriched in U to represent a significant source for the genesis of U deposits: peralkaline, high-K metaluminous calc-alkaline, L-type peraluminous and anatectic pegmatoids. L-type peraluminous plutonic rocks in which U is dominantly hosted in uraninite or in the glass of their volcanic equivalents represent the best U source. Peralkaline granites or syenites are associated with the only magmatic U-deposits formed by extreme fractional crystallization. The refractory character of the U-bearing minerals does not permit their extraction under the present economic conditions and make them unfavorable U sources for other deposit types. By contrast, felsic peralkaline volcanic rocks, in which U is dominantly hosted in the glassy matrix, represent an excellent source for many deposit types. High-K calc-alkaline plutonic rocks only represent a significant U source when the U-bearing accessory minerals (U-thorite, allanite, Nb oxides) become metamict. The volcanic rocks of the same geochemistry may be also a favorable uranium source if a large part of the U is hosted in the glassy matrix. The largest U deposit in the world, Olympic Dam in South Australia is hosted by highly fractionated high-K plutonic and volcanic rocks, but the origin of the U mineralization is still unclear. Anatectic pegmatoids containing disseminated uraninite which results from the partial melting of uranium-rich metasediments and/or metavolcanic felsic rocks, host large low grade U deposits such as the Rössing and Husab deposits in Namibia. The evaluation of the potentiality for igneous rocks to represent an efficient U source represents a critical step to consider during the early stages of exploration for most U deposit types. In particular a wider use of the magmatic inclusions to determine the parent magma chemistry and its U content is of utmost interest to evaluate the U source potential of sedimentary basins that contain felsic volcanic acidic tuffs.
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Watson, Ken, Lawrence C. Rowan, Timothy L. Bowers, Carmen Anton‐Pacheco, Pablo Gumiel, and Susanne H. Miller. "Lithologic analysis from multispectral thermal infrared data of the alkalic rock complex at Iron Hill, Colorado." GEOPHYSICS 61, no. 3 (May 1996): 706–21. http://dx.doi.org/10.1190/1.1443998.
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Airborne thermal‐infrared multispectral scanner (TIMS) data of the Iron Hill carbonatite‐alkalic igneous rock complex in south‐central Colorado are analyzed using a new spectral emissivity ratio algorithm and confirmed by field examination using existing 1:24 000‐scale geologic maps and petrographic studies. Color composite images show that the alkalic rocks could be clearly identified and that differences existed among alkalic rocks in several parts of the complex. An unsupervised classification algorithm defines four alkalic rock classes within the complex: biotitic pyroxenite, uncompahgrite, augitic pyroxenite, and fenite + nepheline syenite. Felsic rock classes defined in the surrounding country rock are an extensive class consisting of tuff, granite, and felsite, a less extensive class of granite and felsite, and quartzite. The general composition of the classes can be determined from comparisons of the TIMS spectra with laboratory spectra. Carbonatite rocks are not classified, and we attribute that to the fact that dolomite, the predominant carbonate mineral in the complex, has a spectral feature that falls between TIMS channels 5 and 6. Mineralogical variability in the fenitized granite contributed to the nonuniform pattern of the fenite‐nepheline syenite class. The biotitic pyroxenite, which resulted from alteration of the pyroxenite, is spatially associated and appears to be related to narrow carbonatite dikes and sills. Results from a linear unmixing algorithm suggest that the detected spatial extent of the two mixed felsic rock classes was sensitive to the amount of vegetation cover. These results illustrate that spectral thermal infrared data can be processed to yield compositional information that can be a cost‐effective tool to target mineral exploration, particularly in igneous terranes.
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West, David P., Dwight Bradley, and Raymond Coish. "The Litchfield Pluton in South-Central Maine: Carboniferous Alkalic Magmatism in northern New England, USA." Atlantic Geology 52 (June 30, 2016): 169. http://dx.doi.org/10.4138/atlgeol.2016.008.
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The Litchfield pluton is a poorly exposed 7 km2 composite alkalic intrusive complex that cuts previously deformed and metamorphosed Silurian turbidites in south-central Maine. The pluton includes a variety of alkaline syenites, including the type locality of “litchfieldite”, a coarse-grained cancrinite, sodalite, and lepidomelane bearing nepheline syenite first recognized over 150 years ago and common in many petrologic collections. A new U-Pb zircon age of 321 ± 2 Ma from the nepheline syenite is interpreted to represent the crystallization age of the plutonic complex. A new biotite 40Ar/39Ar age of 239 ± 1 Ma from the syenite is similar to previously published mica ages from the surrounding country rocks and dates the time of regional cooling in the area below ~ 300°C. Whole rock geochemical analyses from rocks of the Litchfield pluton are compatible with strongly alkaline A-type granitoid rocks that formed in a within plate or continental rift tectonic setting. The age and geochemical characteristics of the alkalic igneous rocks near Litchfield are consistent with a model that invokes the generation of a small volume of alkalic magma beneath south-central Maine during a period of Carboniferous transcurrent tectonism in the northern Appalachian orogen.
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Barr, Sandra M., Daniel Brisebois, and Alan S. Macdonald. "Carboniferous volcanic rocks of the Magdalen Islands, Gulf of St. Lawrence." Canadian Journal of Earth Sciences 22, no. 11 (November 1, 1985): 1679–88. http://dx.doi.org/10.1139/e85-176.
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Volcanic rocks of Mississippian age occur on the Magdalen Islands as cap rocks and within collapse breccias above salt diapirs that have formed the islands. They consist of coarse volcaniclastic deposits and basaltic flows, intruded by minor mafic dykes and plugs. Petrologic studies of the basaltic rocks show that they are extensively altered. Original plagioclase, clinopyroxene, olivine, and interstitial glass are partially to entirely replaced by mixtures of chlorite, sericite, smectite, sphene, carbonate, epidote, albite, potassium feldspar, and iron oxides, and the samples display a relatively wide range in chemical compositions. Especially mobile were K, Na, and Ca, and most samples are classified as potash spilites (poenites). Using standard discriminant diagrams for mafic igneous rocks, it can be seen that the basalts appear to range from continental tholeiitic to continental alkalic. However, relict clinopyroxene compositions and the presence of kaersutitic amphibole and titaniferous biotite in some samples imply that the suite may originally have been more alkalic than tholeiitic.
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Buhlmann, Arndt L., Patricia Cavell, Ronald A. Burwash, Robert A. Creaser, and Robert W. Luth. "Minette bodies and cognate mica-clinopyroxenite xenoliths from the Milk River area, southern Alberta: records of a complex history of the northernmost part of the Archean Wyoming craton." Canadian Journal of Earth Sciences 37, no. 11 (November 1, 2000): 1629–50. http://dx.doi.org/10.1139/e00-058.
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Minettes exposed in southern Alberta near the Milk River are the northern outliers of the Eocene Sweet Grass Hills igneous complex of the Montana alkalic igneous province. These minettes often contain coarse-grained xenoliths of phlogopite + clinopyroxene ± apatite. The parent magmas of the minettes were generated at pressures [Formula: see text]17 kbar in equilibrium with clinopyroxene + phlogopite ± olivine. Fractional crystallization and mixing provided a spectrum of evolved minettes and cumulates, the latter of which were sampled by subsequent minette magmas as xenoliths. Two xenoliths were dated at 49.0 ± 0.8 Ma and 52 ± 1.7 Ma. The host dyke of the latter xenolith gave an age of 50 ± 0.3 Ma. The minettes and their xenoliths have overlapping values of 87Sr/86Sri, εNdT, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, similar to those of alkaline igneous rocks from farther south in the Montana alkalic igneous province. The Sweet Grass Hills lie north of the Great Falls Tectonic Zone, previously interpreted as a Proterozoic suture zone separating the Archean Medicine Hat block from the Archean Wyoming craton to the south. Geochemical data for the Milk River minettes provide evidence for a history of the mantle underneath the Medicine Hat block, similar to that found previously for mantle-derived rocks of the Wyoming craton, including a significant Proterozoic mantle enrichment event. Given this similarity, we suggest that the Wyoming craton extends into southern Alberta, and that the Great Falls Tectonic Zone does not represent a Proterozoic suture of two Archean blocks.
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Morris, Paul A., Tetsumaru Itaya, Shigeru Iizumi, Hiroo Kagami, R. John Watling, and Hisashi Murakami. "Age relations and petrology of alkalic igneous rocks from Oki Dozen, Southwest Japan." GEOCHEMICAL JOURNAL 31, no. 3 (1997): 135–54. http://dx.doi.org/10.2343/geochemj.31.135.
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Barr, Sandra M. "Geochemistry and tectonic setting of late Precambrian volcanic and plutonic rocks in southeastern Cape Breton Island, Nova Scotia." Canadian Journal of Earth Sciences 30, no. 6 (June 1, 1993): 1147–54. http://dx.doi.org/10.1139/e93-097.
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Late Precambrian volcanic–sedimentary belts in the Mira (Avalon) terrane of southeastern Cape Breton Island display differences in rock types, petrochemistry, and age, showing that they did not form contemporaneously above a single northwest-dipping subduction zone, as proposed in earlier models. The oldest rocks are 680 Ma mafic and felsic flows and tuffs, and abundant, mainly tuffaceous, sedimentary rocks in the Stirling belt. They are interpreted to have formed in a trough within or peripheral to a volcanic-arc complex. Northwest of the Stirling belt, the East Bay Hills, Coxheath Hills, and Sporting Mountain belts consist of ca. 620 Ma mafic to felsic subaerial pyroclastic rocks and flows and contemporaneous dioritic to granitic plutons. Both volcanic and plutonic rocks are calc-alkalic to high-K calc-alkalic suites, formed in a continental margin volcanic arc. A correlative 620 Ma plutonic suite intruded the western margin of the Stirling belt, suggesting that subduction may have been toward the present southeast. The ca. 575 Ma Coastal belt, located southeast of the Stirling belt, is significantly younger than the other belts and appears to represent a less evolved calc-alkalic to low-K continental margin volcanic-arc and intra-arc basin formed above a northwest-dipping subduction zone. These various volcanic–sedimentary belts were juxtaposed by lateral movements along major faults in the late Precambrian to form this part of the Avalon composite terrane. Subduction-related, calc-alkalic magmatism at ca. 620 Ma was apparently widespread throughout the Avalon terrane of the northern Appalachian Orogen. However, ca. 680 Ma magmatism like that in the Stirling belt has been documented elsewhere only in the Connaigre Bay Group of Newfoundland. Circa 575 Ma and younger subduction-generated igneous activity like that in the Coastal belt has been recognized in southern New Brunswick, but alkaline magmas were forming in extensional regimes in other areas of the Avalon terrane at that time.
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MEDEIROS, SILVIA R., CRISTINA M. WIEDEMANN-LEONARDOS, and SIMON VRIEND. "Evidence of mingling between contrasting magmas in a deep plutonic environment: the example of Várzea Alegre, in the Ribeira Mobile Belt, Espírito Santo, Brazil." Anais da Academia Brasileira de Ciências 73, no. 1 (March 2001): 99–119. http://dx.doi.org/10.1590/s0001-37652001000100009.
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At the end of the geotectonic cycle that shaped the northern segment of the Ribeira Mobile Belt (Upper Proterozoic to Paleozoic age), a late to post-collisional set of plutonic complexes, consisting of a wide range of lithotypes, intruded all metamorphic units. The Várzea Alegre Intrusive Complex is a post-collisional complex. The younger intrusion consists of an inversely zoned multistage structure envolved by a large early emplaced ring of megaporphyritic charnoenderbitic rocks. The combination of field, petrographic and geochemical data reveals the presence of at least two different series of igneous rocks. The first originated from the partial melting of the mantle. This was previously enriched in incompatible elements, low and intermediate REE and some HFS-elements. A second enrichment in LREE and incompatible elements in this series was due to the mingling with a crustal granitic magma. This mingling process changed the composition of the original tholeiitic magma towards a medium-K calc-alkalic magma to produce a suite of basic to intermediate rock types. The granitic magma from the second high-K, calc-alkalic suite originated from the partial melting of the continental crust, but with strong influence of mantle-derived melts.
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Rowan, Lawrence C., Marguerite J. Kingston, and James K. Crowley. "Spectral reflectance of carbonatites and related alkalic igneous rocks; selected samples from four North American localities." Economic Geology 81, no. 4 (July 1, 1986): 857–71. http://dx.doi.org/10.2113/gsecongeo.81.4.857.
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Heyl, A. V., and M. R. Brock. "Mineral deposits related to Phanerozoic alkalic igneous rocks of the central part of the United States." Global Tectonics and Metallogeny 4, no. 1-2 (January 1, 1991): 61–64. http://dx.doi.org/10.1127/gtm/4/1991/61.
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Pe-Piper, Georgia, and David JW Piper. "Geochemical evolution of Devonian-Carboniferous igneous rocks of the Magdalen basin, Eastern Canada: Pb- and Nd-isotope evidence for mantle and lower crustal sources." Canadian Journal of Earth Sciences 35, no. 3 (March 1, 1998): 201–21. http://dx.doi.org/10.1139/e97-106.
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Magmatism associated with the extensional Magdalen basin includes voluminous tholeiitic gabbro and basalt and local granite and rhyolite. Pb- and (or) Nd-isotope determinations have been made on 70 igneous rocks from throughout the basin, and a further 15 samples of Avalonian basement from the southern margin of the basin, to characterize the contribution of lower crustal blocks and mantle sources to the magmatism and to constrain tectonic models for the basin. Five phases of magmatic evolution are distinguished in the Magdalen basin. (1) Middle to Late Devonian partial melting of lithospheric mantle, producing principally tholeiites and minor alkalic basalt. Tholeiites have Pb isotopic compositions similar to that of younger Triassic tholeiites generated from the same mantle, but experienced less crustal contamination. Regional variations in trace element composition of the mantle can be recognized. (2) The mafic magma triggered anhydrous base-of-crust melting, principally along the transpressive Cobequid and Rockland Brook faults, producing A-type granites in which radiogenic Pb increases northeastward. (3) In the latest Devonian, a large base-of-crust fractionating magma chamber evolved. It contained immiscible mafic and minor felsic magma, with uniform Nd isotopes, and high Ti in the mafic magma. (4) Although late Tournaisian dykes are not strongly fractionated, their evolution involved more crustal assimilation than earlier mafic rocks. (5) Local Viséan-Westphalian alkalic magmas, which ascended along crustal-scale faults, have Pb and Nd isotopic compositions resembling mantle plumes or their mixtures with lithospheric mantle sources. Only these youngest rocks show any isotopic evidence for input from an asthenospheric plume source, suggesting that regional extension was responsible for most of the magmatism.
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Dudás, Francis Ö. "Geochemistry of igneous rocks from the Crazy Mountains, Montana, and tectonic models for the Montana Alkalic Province." Journal of Geophysical Research: Solid Earth 96, B8 (July 30, 1991): 13261–77. http://dx.doi.org/10.1029/91jb00246.
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Schandl, E. S., M. P. Gorton, and D. W. Davis. "Albitization at 1700 ± 2 Ma in the Sudbury – Wanapitei Lake area, Ontario: implications for deep-seated alkalic magmatism in the Southern province." Canadian Journal of Earth Sciences 31, no. 3 (March 1, 1994): 597–607. http://dx.doi.org/10.1139/e94-052.
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U–Pb geochronology of hydrothermal monazite in albitized rocks from two gold deposits east of the Sudbury complex indicates that albitization in the Sudbury – Wanapitei Lake area occurred at 1700 ± 2 Ma and was coeval with a period of granitic plutonism in the Southern structural province between 1750 and 1700 Ma.A variety of rare earth element (REE) minerals, such as two generations of hydrothermal monazite, bastnäsite, synchysite, and gadolinite were identified in the albitized Huronian sediments in the Espanola – Sudbury – Wanapitei Lake areas. The presence of these REE minerals, the extraordinary light rare earth element enrichment in rocks from the Sheppard gold property east of the Sudbury igneous complex and the elevated REE concentrations in some albitized rocks suggests that sodium-rich fluids may have been generated by carbonatitic or alkalic intrusions at depth.Gold mineralization occurs in rocks that have been altered by at least two different types of fluids: (1) peralkaline; Na–REE bearing and (2) low pH, Co bearing. The high Co content of most mineralized samples and the relatively weak correlation between Au and Na2O suggests that gold was probably concentrated to economic grade by the low pH, Co-bearing fluids. The spatial association of albite and gold suggests that the albitized rocks may represent earlier fluid conduits that were subsequently refractured and invaded by the mineralizing solutions.
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Pivarunas, Anthony F., and Joseph G. Meert. "Protracted magmatism and magnetization around the McClure Mountain alkaline igneous complex." Lithosphere 11, no. 5 (June 27, 2019): 590–602. http://dx.doi.org/10.1130/l1062.1.
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Abstract The McClure Mountain–Iron Mountain igneous complex is an alkalic intrusive center in the northern Wet Mountains of southern Colorado. It was emplaced in early Cambrian time into gneissic/granitic 1.75–1.45 Ga Proterozoic host rocks. Numerous dikes are associated with the complex, primarily along the western side. Although the main intrusive nepheline-syenite body is well dated, the ages of the surrounding dikes are poorly known. Crosscutting relationships and poorly defined K-Ar dates suggest that the dikes are younger than the main intrusion. Paleomagnetic samples were collected from dikes associated with the McClure Mountain igneous complex. Geochronologic samples were also collected from two dikes sampled for their paleomagnetism. We obtained U-Pb zircon ages of 526 ± 8 Ma for a lamprophyric extracomplex dike and 483 ± 2 Ma for a trachytic extracomplex dike. These ages suggest either multistage or protracted dike intrusion around the ca. 524 Ma McClure Mountain complex. Our paleomagnetic data are consistent with previously published results. Dikes of the complex primarily exhibit southeast and shallow paleomagnetic directions, with variable declinations. Results from several baked contact tests indicate that the magnetizations are secondary. A steeply inclined magnetization is pervasive and was acquired over a protracted interval from late Laramide time to the present day.
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Muir, R. J., W. R. Fitches, and A. J. Maltman. "The Rhinns Complex: Proterozoic basement on Islay and Colonsay, Inner Hebrides, Scotland, and on Inishtrahull, NW Ireland." Transactions of the Royal Society of Edinburgh: Earth Sciences 85, no. 1 (1994): 77–90. http://dx.doi.org/10.1017/s0263593300006313.
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ABSTRACTThe Precambrian basement on the islands of Islay, Colonsay and Inishtrahull comprises a deformed igneous association of mainly syenite and gabbro, with minor mafic and felsic intrusions. This association is collectively referred to as the Rhinns Complex. Isotopic data indicate that the complex represents new addition of material to the crust at c. 1·8 Ga. The igneous protolith was juvenile mantle-derived material, not reworked Archaean crust. Overall, the complex has an alkalic composition, with major and trace element patterns similar to igneous rocks generated in a subduction-related setting: high LILE/HFSE and LREE/HREE ratios, together with negative Nb, P and Ti anomalies.The formation of the Rhinns Complex was contemporaneous with the Laxfordian tectonothermal cycle in the Lewisian Complex. These Proterozoic events are most likely associated with an extensive 1·9–1·7 Ga mobile belt around the southern margin of Laurentia-Baltica. As part of this belt, the Rhinns Complex forms a link between the Ketilidian province of South Greenland and the Svecofennian of Scandinavia.Inherited isotopic signatures in the Caledonian granites on the north side of the Highland Boundary Fault may reflect the presence of a large area of Proterozoic basement (?Rhinns Complex) beneath Scotland and NW Ireland. Alternatively, the Proterozoic signature could be derived from the incorporation of Moine or Dalradian sediment into the granitic magmas.
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Corfu, F., S. L. Jackson, and R. H. Sutcliffe. "U–Pb ages and tectonic significance of late Archean alkalic magmatism and nonmarine sedimentation: Timiskaming Group, southern Abitibi belt, Ontario." Canadian Journal of Earth Sciences 28, no. 4 (April 1, 1991): 489–503. http://dx.doi.org/10.1139/e91-043.
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The paper presents U–Pb ages for zircons of the calc-alkalic to alkalic igneous suite and associated alluvial–fluvial sedimentary rocks of the Timiskaming Group in the late Archean Abitibi greenstone belt, Superior Province. The Timiskaming Group rests unconformably on pre-2700 Ma komatiitic to calc-alkalic volcanic sequences and is the expression of the latest stages of magmatism and tectonism that shaped the greenstone belt. An age of 2685 ± 3 Ma for the Bidgood quartz porphyry, an age of about 2685–2682 Ma for a quartz–feldspar porphyry clast in a conglomerate, and ages ranging from 2686 to 2680 Ma for detrital zircons in sandstones appear to reflect an early stage in the development of the Timiskaming Group. The youngest detrital zircons in each of three sandstones at Timmins, Kirkland Lake, and south of Larder Lake define maximum ages of sedimentation at about 2679 Ma; the latter sandstone is cut by a porphyry dyke dated by titanite at [Formula: see text], identical to the 2677 ± 2 Ma age for a volcanic agglomerate of the Bear Lake Formation north of Larder Lake. Similar ages have previously been reported for syenitic to granitic plutons of the region. The dominant period of Timiskaming sedimentation and magmatism was thus 2680–2677 Ma. Xenocrystic zircons found in a porphyry and a lamprophyre dyke have ages of 2750–2720 Ma, which correspond to the ages of the oldest units in the belt, predating the volumetrically dominant ca. 2700 Ma greenstone sequences. The presence of these xenocrysts and the onlapping of the Timiskaming Group on all earlier lithotectonic units of the southern Abitibi belt support the concept that the 2700 Ma ensimatic sequences were thrust onto older assemblages during a phase of compression that culminated with the generation of tonalite and granodiorite at about 2695–2688 Ma. Published geochemical data for the Timiskaming igneous suite, notably the enrichments in large-ion lithophile elements and light rare-earth elements and the relative depletion of Nb, Ta, and Ti compare with the characteristics of suites at modern convergent settings such as the Eolian and the Banda arcs and are consistent with generation of the melts from deep metasomatized mantle in the final stages of, or after cessation of, subduction. Late- and post-Timiskaming compression caused north-directed thrusting and folding. Turbiditic sedimentary units of the Larder Lake area which locally structurally overly the alluvial–fluvial sequence and were earlier thought to be part of the Timiskaming Group, appear to be older "flyschoid" sequences, possibly correlative with sedimentary rocks deposited in the Porcupine syncline at Timmins between 2700 and 2690 Ma.
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Hébert, Claude, Anne-Marie Cadieux, and Otto van Breemen. "Temporal evolution and nature of TiFeP mineralization in the anorthositemangeritecharnockitegranite (AMCG) suites of the south-central Grenville Province, Saguenay Lac St. Jean area, Quebec, Canada." Canadian Journal of Earth Sciences 42, no. 10 (October 1, 2005): 1865–80. http://dx.doi.org/10.1139/e05-050.
Full textAbstract:
In the south-central Grenville Province, Quebec, Canada, anorthositemangeritecharnockitegranite (AMCG) magmatism took place during four distinct episodes between 1327 and 1008 Ma. AMCG rocks crosscut several gneiss complexes composed of ~1506 Ma supracrustal rocks and massive to gneissic igneous rocks that were emplaced during two distinct episodes: ~1434 and 13931383 Ma. The four episodes of AMCG magmatism are (i) the 1327 ± 16 Ma labradorite-type De La Blache Mafic Plutonic Suite, (ii) the 11601135 Ma labradorite- and andesine-type Lac St. Jean Anorthositic Suite, (iii) a 10821045 Ma unnamed plutonic suite, and (iv) the 10201008 Ma andesine-type Valin Anorthositic Suite. The Valin Anorthositic Suite includes the 1016 ± 2 Ma andesine-type Mattawa Anorthosite, the 10101008 Ma andesine-type Labrieville Alkalic Anorthositic Massif, the 1020 ± 4 Ma St. Ambroise Pluton, the 1018+73 Ma Farmer Monzonite; the 1010 ± 2 Ma Gouin Charnockite, and the 1010 ± 3 Ma La Hache Monzonite. Study of the TiTeP mineral occurrences in these four AMCG units in the south-central Grenville Province has shown that (i) apatite-bearing rocks are related only to andesine-type anorthosites, (ii) titaniferous magnetite is restricted to labradorite-type anorthosites, and (iii) hemo-ilmenite occurs only in andesine-type anorthosite and associated oxideapatite-rich gabbronorites (OAGN) and nelsonites.
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MacDonald, Lisa A., Sandra M. Barr, Chris E. White, and John WF Ketchum. "Petrology, age, and tectonic setting of the White Rock Formation, Meguma terrane, Nova Scotia: evidence for Silurian continental rifting." Canadian Journal of Earth Sciences 39, no. 2 (February 1, 2002): 259–77. http://dx.doi.org/10.1139/e01-074.
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The White Rock Formation in the Yarmouth area of the Meguma terrane of southern Nova Scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows, epiclastic and clastic sedimentary rocks, and minor intermediate and felsic crystal tuff. It is divided into seven map units that appear to young from west to east, inconsistent with a previously assumed synclinal structure. The White Rock Formation is flanked on both northwest and southeast by mainly the Cambrian to Lower Ordovician Halifax Formation; the western contact is interpreted to be a sheared disconformity, whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels. The granitic Brenton Pluton forms a faulted lens within the eastern shear zone. A felsic tuff from the upper part of the White Rock Formation yielded a UPb zircon age of 438+32 Ma, identical within error to published ages for the Brenton Pluton and felsic volcanic rocks near the base of the White Rock Formation in the Torbrook area of western Nova Scotia. The chemical characteristics of the mafic volcanic rocks and associated mafic intrusions consistently indicate alkalic affinity and a continental within-plate setting. The felsic volcanic rocks and Brenton Pluton have chemical characteristics of within-plate anorogenic granitic rocks, and the pluton is interpreted to be comagmatic with the felsic volcanic rocks. The igneous activity may have occurred in response to extension as the Meguma terrane rifted away from Gondwana in the latest Ordovician to Early Silurian. Epsilon Nd values are similar to those in voluminous Devonian plutonic rocks of the Meguma terrane, and the magmas appear to have been derived from similar sources.
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Macey, P. H., R. J. Thomas, H. P. Smith, D. Frei, and P. J. le Roux. "Lithostratigraphy of the Naros Granite (Komsberg Suite), South Africa and Namibia." South African Journal of Geology 124, no. 3 (September 1, 2021): 795–804. http://dx.doi.org/10.25131/sajg.124.0040.
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Abstract The Naros Granite occurs as a large, northwest-trending ovoid batholith roughly 30 km long and 15 km wide straddling the Orange River border between South Africa and Namibia, 25 km northeast of Onseepkans. It consists mainly of a leucocratic to mesocratic grey, coarse-grained equigranular hornblende-biotite granite-granodiorite that is locally mildly feldspar porphyritic. Small, ovoid mafic autoliths are common and characteristic of the Naros Granite. The composition of the unit varies from granite to granodiorite with a minor leucogranitic phase observed along the southern margin of the batholith. Hornblende and biotite are ubiquitous mafic minerals but small amounts of orthopyroxene occur locally. The Naros Granite has yielded tightly-constrained U-Pb zircon ages between 1 114 Ma and 1 101 Ma. The Naros Granite is generally unfoliated to weakly deformed with only localised shearing along contacts with the surrounding country rocks giving rise to orthogneissic fabrics. It has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic autoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant chemical weathering. The Naros Granite is the youngest and most evolved member of the ~1.11 Ga Komsberg Suite, a collection of late- to post-tectonic I-type metaluminous, intermediate to felsic, biotite ± hornblende granitoids and their charnockitic equivalents that have intruded the older pre-tectonic gneisses of the Kakamas Domain of the Namaqua Metamorphic Sector.
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Corfu, F., and H. Wallace. "U–Pb zircon ages for magmatism in the Red Lake greenstone belt, northwestern Ontario." Canadian Journal of Earth Sciences 23, no. 1 (January 1, 1986): 27–42. http://dx.doi.org/10.1139/e86-004.
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U–Pb dating was carried out on nine volcanic rocks and two felsic intrusions from the Red Lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization.The data indicate a protracted period of igneous activity spanning at least 270 Ma. Felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages of [Formula: see text] and [Formula: see text]. A third unit, dated at [Formula: see text], contains inherited zircons older than 2982 Ma, which casts some uncertainty on the validity of the inferred intercept age. Rocks in the western part of the belt, previously believed to form a relatively young calc-alkalic sequence but now known to be dominantly tholeiitic, are shown to be relatively old, with ages of [Formula: see text] and [Formula: see text]. These two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units.Another volcanic horizon in the centre of the belt is dated at 2830 ± 15 Ma, and calc-alkaline volcanic sequences on the southern and northern flanks of the belt yield ages of 2739.0 ± 3.0 and [Formula: see text], respectively. An age of [Formula: see text] was determined for tholeiitic pyroclastic rocks near the base of the predominantly calc-alkaline Heyson sequence.The major gold deposits of the Red Lake belt appear to be present dominantly within older supracrustal sequences. On the other hand, they are also associated with late deformation zones that postdate the intrusion of the Dome Stock dated at 2718.2 ± 1.1 Ma ago. The time of an earlier folding event is bracketed by this age and by the age of [Formula: see text] for an isoclinally folded felsic dike.
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Barr, S. M., C. E. White, N. G. Culshaw, and J. WF Ketchum. "Geology and tectonic setting of Paleoproterozoic granitoid suites in the Island Harbour Bay area, Makkovik Province, Labrador." Canadian Journal of Earth Sciences 38, no. 3 (March 1, 2001): 441–63. http://dx.doi.org/10.1139/e00-086.
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Paleoproterozoic granitoid rocks in the Island Harbour Bay area (Kaipokok domain, Makkovik Province, Labrador) are divided into four separate suites on the basis of field relations, petrology, and age. The redefined Island Harbour Bay plutonic suite consists of ca. 18951870 Ma dioritic to granitic (mainly granodioritic and granitic) units. The rocks are variably foliated as a result of emplacement under amphibolite-facies conditions in a dextral transpressive regime during Andean-type subduction. The dominant mafic mineral is biotite, and accessory epidote, allanite, and titanite are abundant. The suite is calc-alkalic, but with rare-earth element patterns similar to those of Archean tonalitictrondhjemiticgranodioritic suites. It is interpreted to have formed deep in an Andean-type magmatic arc at the margin of the Nain continent. In contrast, the younger Hares Islands and Drunken Harbour granites (emplaced at ca. 1805 and 1790 Ma, respectively) were part of widespread late-orogenic magmatic activity in the Makkovik Province. In contrast to the Island Harbour Bay plutonic suite, these units retain igneous textures and are either unfoliated or display magmatic foliation, locally modified by emplacement in active shear zones. The ca. 1716 Ma Blacklers Bight granite varies from porphyritic to equigranular, is fluorite-bearing, and has chemical features approaching those of continental A-type granites. Similar granite occurs farther south in the Makkovik Province, reflecting widespread anorogenic magmatic activity at that time, perhaps related to mafic magma underplating. Variable interaction with Archean (Nain Province) crust by granitic magmas of all three ages is evidenced by εNd values ranging from 7.2 to 2.5.
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Abrahams, Y., and P. H. Macey. "Lithostratigraphy of the Mesoproterozoic Donkieboud Granodiorite (Komsberg Suite), South Africa and Namibia." South African Journal of Geology 123, no. 3 (September 1, 2020): 421–30. http://dx.doi.org/10.25131/sajg.123.0028.
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Abstract The Donkieboud Granodiorite pluton forms an extensive intrusion across the border region between South Africa and southeast Namibia. The mesocratic grey, weakly to moderately K-feldspar porphyritic biotite ± hornblende ± orthopyroxene granodiorite represents the most extensive member of the late- to post-tectonic Komsberg Suite (~1 125 to 1 105 Ma) which intruded as sheet-like bodies into the older high grade paragneisses and orthogneisses (~1 230 to 1 140 Ma) of the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Province. The Donkieboud Granodiorite comprises three main textural variations namely:a porphyritic to weakly porphyritic, relatively undeformed rock with randomly orientated ovoid and twinned feldspar phenocrysts;a weakly- to well-foliated gneiss with between 3 to 10% feldspar phenocrysts set in a medium-grained matrix anda patchy metamorphic charnockite variety. Large inclusions of the strongly foliated Twakputs (~1 210 Ma) and the Witwater (~1 140 Ma) garnetiferous granite gneisses occur within the Donkieboud Granodiorite and mafic xenoliths are common. The Donkieboud Granodiorite is variably deformed ranging from unfoliated to being gneissic. The foliation developed during its intrusion into an existing but waning regional stress field with the strain increasing towards the contacts with the surrounding country rocks. Subsequent km-scale open folding resulted in the reorientation of the gneissic foliation and locally, intense reworking of the fabrics along the margins of the folds. In places, the Donkieboud unit is crosscut by discrete mylonitic shears with a west to northwest trend. U-Pb zircon dating of the Donkieboud Granodiorite samples yielded concordia ages of between 1 118 and 1 107 Ma. Overall the Donkieboud Granodiorite has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic xenoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant amount of chemical weathering. The εNd values of -1.15 and -0.11 and TDM values of 1 615 and 1 505 Ma are typical of the Komsberg Suite and indicate a significant contribution of older crustal material to the magma of the Donkieboud pluton.
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Dallmeyer, R. David, and R. Damian Nance. "Tectonic implications of 40Ar/39Ar mineral ages from late Precambrian – Cambrian plutons, Avalon composite terrane, southern New Brunswick, Canada." Canadian Journal of Earth Sciences 29, no. 11 (November 1, 1992): 2445–62. http://dx.doi.org/10.1139/e92-192.
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Within the Avalon composite terrane exposed in southern New Brunswick, late Precambrian, low-grade volcanic–sedimentary sequences are juxtaposed against late Precambrian gneisses (Brookville Gneiss) and older platformal metasedimentary rocks (Green Head Group) along the Caledonia Fault. Both assemblages host petrographically similar suites of calc-alkalic dioritic and granodioritic plutons. Those intruding volcanic–sedimentary sequences (Caledonia terrane) record ca. 615–625 Ma crystallization ages typical of arc-related magmatism throughout the Avalon composite terrane. However, 40Ar/39Ar age data from stocks intruding gneisses and platformal metasedimentary rocks (Brookville terrane) suggest significantly younger crystallization ages.36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende are interpreted to closely date postmagmatic cooling within six plutons: Fairville Granite (547 ± 1 Ma); French Village Quartz Diorite (539 ± 2 and 537 ± 1 Ma); Rockwood Park Granodiorite (529 ± 2 and 523 ± 3.5 Ma); Musquash Granite (526 ± 2 Ma); Milkish Head Granite (Red Bridge pluton, 520 ± 1.5 Ma); Lepreau Diorite (Talbot Road pluton, 519 ± 2 Ma and Hansen Stream pluton, 518 ± 1.5 Ma. A hornblende isotope correlation age of 530 ± 2 Ma from penetratively foliated amphibolite within the French Village Quartz Diorite suggests that the magmatic activity was locally accompanied by ductile shear. Muscovite within granitic pegmatite in the Brookville Gneiss records a 40Ar/39Ar plateau age of 510 ± 1 Ma interpreted to date final phases of associated magmatic activity.Arc-related magmatism extending into the Cambrian contrasts with the characteristic tectono-stratigraphic record in the Avalon composite terrane where late Precambrian igneous rocks are overstepped by Cambrian–Ordovician shallow-marine strata with only a local and minor record of rift-related volcanic activity. Although the Brookville terrane shows affinities with the Avalon composite terrane during the late Precambrian, the 40Ar/39Ar age data suggest that it was isolated as a distinct tectono-stratigraphic element by the Early Cambrian.
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Emeleus, C. H. "The Tertiary lavas and sediments of northwest Rhum, Inner Hebrides." Geological Magazine 122, no. 5 (September 1985): 419–37. http://dx.doi.org/10.1017/s0016756800035342.
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AbstractSeveral small outliers of Tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from Torridonian sediments and Tertiary granophyre. Erosion continued during the period of sediment and lava accumulation. Four formations are recognized; these are, in order of increasing age, the Orval Formation (hawaiite and basaltic hawaiite lavas), the Guirdil Formation (icelandite lavas, interbedded conglomerates), the Upper Fionchra Formation (tholeiitic basaltic andesite lavas, hyaloclastite deposits, basal conglomerate) and the Lower Fionchra Formation (alkali and transitional basalt, basaltic hawaiite and hawaiite lava flows, basal conglomerate); each is separated by an erosional interval. Clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses, Torridonian sediments, igneous rocks derived from the Rhum Tertiary Central Complex (including allivalites), and Tertiary lavas of local origin but also including, in the oldest conglomerates, tholeiitic basalts not now preserved on or near Rhum. Prior to and during lava and sediment accumulation, erosion on Rhum had cut down to a level similar to that of the present day, although not to the extent that high-grade thermally altered rocks, which are a marked feature of the Central Complex, were being eroded in any quantity. A sequence of east–west trending valleys, possibly initiated on the line of the earlier Main Ring Fault, drained the area of the Central Complex which then, as now, must have been high ground. Small lakes occasionally formed in the valleys allowing the accumulation of fine-grained sediment with plant remains, and promoting the formation of hyaloclastite deposits when buried by later flows. No source for any of the lava formations is preserved on Rhum; they are thought to have come from feeders north of Rhum, possibly near Canna, and to have ponded against the hills and valleys near and in the Central Complex.The oldest tholeiitic lavas, not now found in situ, were followed by alkali and transitional flows compositionally similar to the Skye Main Lava Series but characteristically feldsparphyric; the most mafic also contain phenocrysts of magnesian olivine (with included Cr-Al-rich spinels) and aluminous spinel. Both the early alkalic/transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures < 9 kb; the tholeiitic basaltic andesites and icelandites equilibrated at relatively shallows depths.Apart from a few N–S to NW–SE-trending basalt dykes, the lava formations represent the youngest Tertiary igneous event on Rhum.
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Hollings, P., R. Wolfe, D. R. Cooke, and P. J. Waters. "Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits and a Case for Slab Roll-Back?" Economic Geology 106, no. 8 (November 17, 2011): 1257–77. http://dx.doi.org/10.2113/econgeo.106.8.1257.
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Lipman, Peter W. "Evolution of silicic magma in the upper crust: the mid-Tertiary Latir volcanic field and its cogenetic granitic batholith, northern New Mexico, U.S.A." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 79, no. 2-3 (1988): 265–88. http://dx.doi.org/10.1017/s0263593300014279.
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ABSTRACTStructural and topographic relief along the eastern margin of the Rio Grande rift, northern New Mexico, provides a remarkable cross-section through the 26-Ma Questa caldera and cogenetic volcanic and plutonic rocks of the Latir field. Exposed levels increase in depth from mid-Tertiary depositional surfaces in northern parts of the igneous complex to plutonic rocks originally at 3–5 km depths in the S. Erosional remnants of an ash-flow sheet of weakly peralkaline rhyolite (Amalia Tuff) and andesitic to dacitic precursor lavas, disrupted by rift-related faults, are preserved as far as 45 km beyond their sources at the Questa caldera. Broadly comagmatic 26 Ma batholithic granitic rocks, exposed over an area of 20 by 35 km, range from mesozonal granodiorite to epizonal porphyritic granite and aplite; shallower and more silicic phases are mostly within the caldera. Compositionally and texturally distinct granites define resurgent intrusions within the caldera and discontinuous ring dikes along its margins; a batholithic mass of granodiorite extends 20 km S of the caldera and locally grades vertically to granite below its flat-lying roof. A negative Bouguer gravity anomaly (15–20 mgal), which encloses exposed granitic rocks and coincides with boundaries of the Questa caldera, defines boundaries of the shallow batholith, emplaced low in the volcanic sequence and in underlying Precambrian rocks. Palaeomagnetic pole positions indicate that successively crystallised granitic plutons cooled through Curie temperatures during the time of caldera formation, initial regional extension, and rotational tilting of the volcanic rocks. Isotopic ages for most intrusions are indistinguishable from the volcanic rocks. These relations indicate that the batholithic complex broadly represents the source magma for the volcanic rocks, into which the Questa caldera collapsed, and that the magma was largely liquid during regional tectonic disruption.Volcanic and plutonic magmas (1) changed from early high-K calc-alkaline to alkalic prior to caldera eruptions; (2) differentiated to a weakly peralkaline rhyolite and equivalent acmiteartvedsonite granite cap (underlain by calc-alkaline granite) when the caldera formed at 26·5 Ma; then (3) reverted to calc-alkaline compositions. Concentrations of alkalis and minor elements such as Rb, Th, U, Nb, Zr, and Y reached maxima at the caldera stage. The volcanic rocks constitute intermittently quenched samples of upper parts of Questa magma bodies at early stages of crystallisation; in contrast, the comagmatic granitic rocks preserve an integrated record of protracted crystallisation of the magmatic residue as eruptions diminished. Multiple differentiation processes were active during evolution of the Questa magmatic system: crystal fractionation, replenishment by mantle and lower crustal melts of varying chemical and isotopic character, mixing of evolved with more primitive magmas, upper crustal assimilation, and perhaps volatile-transfer processes. As a result, an evolving batholithic cluster of coalesced magma chambers generated diverse assemblages of broadly cogenetic rocks within a few million years. Evolution of the Questa magmatic system and similar high-level Tertiary granitic batholiths nearby in the southern Rocky Mountains provides broad insights into magmatic processes in continental regions such as the overall shapes of batholiths, time and compositional relations between cogenetic volcanic and plutonic rocks, density equilibration of magmas with country rocks, and thermal evolution of continental crust.
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Borisova, A. Y., R. Thomas, S. Salvi, F. Candaudap, A. Lanzanova, and J. Chmeleff. "Tin and associated metal and metalloid geochemistry by femtosecond LA-ICP-QMS microanalysis of pegmatite–leucogranite melt and fluid inclusions: new evidence for melt–melt–fluid immiscibility." Mineralogical Magazine 76, no. 1 (February 2012): 91–113. http://dx.doi.org/10.1180/minmag.2012.076.1.91.
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AbstractGranitic pegmatites are exceptional igneous rocks and the possible role of an immiscibility process in their origin is strongly debated. To investigate metal and metalloid behaviour in hydrous peraluminous systems (aluminium saturation index, ASI >1), we analysed 15 quartz-hosted primary melt and fluid inclusions from pegmatites in the Ehrenfriedersdorf Complex (Erzgebirge, Germany) and 26 primary melt inclusions from leucogranites of the Ehrenfriedersdorf district (Germany), Kymi (Finland) and Erongo (Namibia) by femtosecond laser ablation inductively coupled plasma quadrupole mass spectrometry. The results presented here for 32 elements provide evidence for metal and metalloid fractionation between two types of immiscible melts (A and B) and NaCl – HCl-rich brine in the pegmatite system. No evidence for the boundary layer effect was observed in the 40 – 500 μm size melt inclusions that were investigated. The data on the Ehrenfriedersdorf pegmatites allow quantification of the metal and metalloid partitioning between natural NaCl-rich brine and the two types of melt (e.g. KAsbrine/type-A,B melts = 0.01 – 1.7; KSbbrine/type-A,B melts = 10 – 285; KZnbrine/type-A,B melts ≥ 50; KPbbrine/type-A melt ≥ 50; KAgbrine/type-A melt = 46). These data are in accord with existing natural and experimental data on equilibrium fluid – melt partitioning as well as spectroscopic data on the metal and metalloid complexation in hydrous aluminosilicate melts and NaCl – HCl-rich fluids.
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McNeil, A. M., and R. Kerrich. "Archean lamprophyre dykes and gold mineralization, Matheson, Ontario: the conjunction of LILE-enriched mafic magmas, deep crustal structures, and Au concentration." Canadian Journal of Earth Sciences 23, no. 3 (March 1, 1986): 324–43. http://dx.doi.org/10.1139/e86-035.
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At the Canadian Arrow deposit, Matheson, separate base- and precious-metal-bearing fracture arrays are associated with a trondhjemitic stock transected by lamprophyre dykes. Peripheral to the stock, high-Fe tholeiitic pillow basalts (Fe2O3 17.8 wt.%, TiO2 2.4 wt.%, Al2O3/TiO2 = 13.7), with interbedded felsic units, experienced amphibolite-facies hydrothermal or regional metamorphism and intense ductile deformation focussed within mylonite zones at the interflow horizons. The trondhjemite stock experienced pervasive reaction with marine water at temperatures that diminished to ≤ 150 °C: this was accompanied by conversion of plagioclase of igneous origin to albite, concomitant enrichment of sodium (Na2O = 8.32 ± 1.16 wt.%, 1σ), and shifts of albite up to δ18O = 14, with attendent, variably negative, quartz–albite fractionations.Galena-dominated veins were precipitated at 220–255 °C from hypersaline CaCl2–NaCl hydrothermal fluids with 22–34 equivalent wt.% NaCl, δ18O = +0.6 to +2.5; these were of evolved marine origin. Later gold-bearing quartz veins formed during hydraulic fracturing under conditions where Pfluid ≥ σ3 + tensile strength. Alteration in vein selvages involved the reaction of albite (δ18O = 14) to K-feldspar (δ18O = 11.3), with gains of Si, K, Rb, Ba, CO2, and S from the hydrothermal fluids and concomitant losses of Fe, Mg, and Na. Volume dilatations were up to +56%. Hydrothermal fluids implicated in this vein array were at 320 ± 20 °C, possessed low salinity, δ18OH2O = 8 ± 0.5, and underwent transient effervescence of CO2; they were of metamorphic or magmatic origin. Coexisting K-feldspar and hematite signify higher aqueous K+/H+ and more oxidizing conditions of deposition than that of most Archean lode gold deposits.Lamprophyre dykes containing trondhjemitic xenoliths were injected along two major subparallel fracture systems showing the same geometry and orientation as the gold-bearing veins. The dykes possess the low SiO2 (39.6 wt.%) and elevated incompatible (P, Th, Zr, Hf, LREE) and mafic (Cr, Ni, Co) trace elements characteristic of calc-alkaline lamprophyres; they are thought to be mantle derivatives. The dykes feature chondrite-normalized troughs at Ta–Nb and Ti. Fe–Mg carbonates in the least-altered lamprophyric dykes (δ13C = −2.8 ± 0.6) may have been derived from a magmatic carbonate reservoir, whereas hydrothermal calcites associated with gold veins (δ13C = −3.2 to −4.6) are interpreted to represent donation of carbon from a uniform reservoir containing carbonate, magmatic C, and carbonaceous components.The conjunction of trondhjemitic and lamprophyric rocks with major structures and gold deposits is interpreted in terms of transcrustal fractures utilized as a conduit for high-Na magmas from the base of the crust, for alkalic magmas from the mantle, and for discharge of hydrothermal fluids from a metamorphic or magmatic reservoir.
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Cheng, Zhiguo, Tong Hou, Jakob K. Keiding, Ilya V. Veksler, Vadim S. Kamenetsky, Marko Hornschu, and Robert B. Trumbull. "Comparative Geothermometry in High-Mg Magmas from the Etendeka Province and Constraints on their Mantle Source." Journal of Petrology 60, no. 12 (December 1, 2019): 2509–28. http://dx.doi.org/10.1093/petrology/egaa016.
Full textAbstract:
Abstract There is still debate whether Large Igneous Provinces (LIPs) are caused by high mantle temperatures induced by thermal plumes or by other factors that enhance melt production from the mantle. A prerequisite for assessing the thermal plume model is a reliable estimate of liquidus temperatures of the magmas produced, preferably based on more than one method of geothermometry. The study reported here compares multiple geothermometers for the Etendeka LIP, which is among the largest Phanerozoic examples and one that shows several features suggestive of a plume origin (continental flood basalt province linked via an age-progressive volcanic ridge to an active hotspot). Magnesium (Mg)-rich magmas emplaced as dikes in NW Namibia are the most primitive rocks known from this province and are thus best suited to determine the composition and melting conditions of their mantle source. Earlier studies of the Etendeka Mg-rich dikes reported high liquidus temperatures based on olivine-melt Mg–Fe equilibria. We extend that work to a larger set of samples and compare the results of olivine-melt Mg–Fe thermometry with other methods based on spinel-melt and spinel–olivine equilibria (Al-in-olivine thermometry), as well as olivine-melt trace-element exchange (Sc/Y thermometry and V oxybarometry). All methods used the same starting assumptions of nominally anhydrous melts and a crystallization pressure of 0·5 GPa. Only mineral-melt or mineral-mineral pairs consistent with compositional equilibrium were used for calculating temperatures. The trace-element compositions of olivine are also used to discuss the relative proportion of peridotite and pyroxenite in the mantle source for these magmas. Twelve dike samples were studied, with whole-rock MgO concentrations ranging from 8·4 to 19·4 wt %. Diagnostic element ratios of transition metals in olivine (e.g., Mn/Fe, Mn/Zn, Zn/Fe) indicate a peridotite-dominated mantle source for the magmas, which is consistent with the other indicators based on whole-rock data e.g., 10 000×Zn/Fe, CaO–MgO trend, FeO/MnO and FC3MS (FeO/CaO–3×MgO/SiO2). The temperature variations show a positive correlation with the Fo-content of host olivines, and values from high-Fo olivine agree well with olivine and spinel liquidus temperatures calculated from thermodynamic models of bulk-rock composition. All methods and most samples yielded a temperature range between 1300 °C and 1400 °C. An exceptional few samples returned temperatures below 1300 °C, the minimum being 1193 °C, whereas several samples yielded temperatures above 1400 °C, the upper range being 1420–1440°C, which we consider to be a robust estimate of the maximum liquidus temperatures for the high-Mg magmas studied. The conversion to mantle potential temperatures is complicated by uncertain depth and degree of melting, but the functional relationship between Tp and primary melt MgO contents, using melt inclusions from olivine phenocrysts with of Fo > 90, indicate a Tp range from 1414 to 1525 °C ( 42 °C), which is 100–150°C higher than estimates of ambient upper mantle Tp in the South Atlantic today.
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Goodenough, Kathryn M., Eimear A. Deady, Charles D. Beard, Sam Broom-Fendley, Holly A. L. Elliott, Frederick van den Berg, and Hüseyin Öztürk. "Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings: Storehouses of Rare Earth Elements." Journal of Earth Science, August 23, 2021. http://dx.doi.org/10.1007/s12583-021-1500-5.
Full textAbstract:
AbstractThe rare earth elements (REE) are critical raw materials for much of modern technology, particularly renewable energy infrastructure and electric vehicles that are vital for the energy transition. Many of the world’s largest REE deposits occur in alkaline rocks and carbonatites, which are found in intracontinental, rift-related settings, and also in syn- to post-collisional settings. Post-collisional settings host significant REE deposits, such as those of the Mianning-Dechang belt in China. This paper reviews REE mineralisation in syn- to post-collisional alkaline-carbonatite complexes worldwide, in order to demonstrate some of the key physical and chemical features of these deposits. We use three examples, in Scotland, Namibia, and Turkey, to illustrate the structure of these systems. We review published geochemical data and use these to build up a broad model for the REE mineral system in post-collisional alkaline-carbonatite complexes. It is evident that immiscibility of carbonate-rich magmas and fluids plays an important part in generating mineralisation in these settings, with REE, Ba and F partitioning into the carbonate-rich phase. The most significant REE mineralisation in post-collisional alkaline-carbonatite complexes occurs in shallow-level, carbothermal or carbonatite intrusions, but deeper carbonatite bodies and associated alteration zones may also have REE enrichment.