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1
Sun, Xilin, Klaudia F. Kuiper, Yuntao Tian, Chang’an Li, Zengjie Zhang, and Jan R. Wijbrans. "Comparison of Detrital Zircon U-Pb and Muscovite 40Ar/39Ar Ages in the Yangtze Sediment: Implications for Provenance Studies." Minerals 10, no. 7 (July 20, 2020): 643. http://dx.doi.org/10.3390/min10070643.
Анотація:
Detrital zircon U-Pb and muscovite 40Ar/39Ar dating are useful tools for investigating sediment provenance and regional tectonic histories. However, the two types of data from same sample do not necessarily give consistent results. Here, we compare published detrital muscovite 40Ar/39Ar and zircon U-Pb ages of modern sands from the Yangtze River to reveal potential factors controlling differences in their provenance age signals. Detrital muscovite 40Ar/39Ar ages of the major tributaries and main trunk suggest that the Dadu River is a dominant sediment contributor to the lower Yangtze. However, detrital zircon data suggest that the Yalong, Dadu, and Min rivers are the most important sediment suppliers. This difference could be caused by combined effects of lower reaches dilution, laser spot location on zircons and difference in closure temperature and durability between muscovite and zircon. The bias caused by sediment laser spot targeting a core or rim of zircon and zircon reworking should be considered in provenance studies.
2
Vetrov, Evgeny, Johan De Grave, Natalia Vetrova, Fedor Zhimulev, Simon Nachtergaele, Gerben Van Ranst, and Polina Mikhailova. "Tectonic History of the South Tannuol Fault Zone (Tuva Region of the Northern Central Asian Orogenic Belt, Russia): Constraints from Multi-Method Geochronology." Minerals 10, no. 1 (January 9, 2020): 56. http://dx.doi.org/10.3390/min10010056.
Анотація:
In this study, we present zircon U/Pb, plagioclase and K-feldspar 40Ar/39Ar and apatite fission track (AFT) data along the South Tannuol Fault Zone (STFZ). Integrating geochronology and multi-method thermochronology places constraints on the formation and subsequent reactivation of the STFZ. Cambrian (~510 Ma) zircon U/Pb ages obtained for felsic volcanic rocks date the final stage of STFZ basement formation. Ordovician (~460–450 Ma) zircon U/Pb ages were obtained for felsic rocks along the structure, dating their emplacement and marking post-formational local magmatic activity along the STFZ. 40Ar/39Ar stepwise heating plateau-ages (~410–400 Ma, ~365 and ~340 Ma) reveal Early Devonian and Late Devonian–Mississippian intrusion and/or post-magmatic cooling episodes of mafic rocks in the basement. Permian (~290 Ma) zircon U/Pb age of mafic rocks documents for the first time Permian magmatism in the study area creating prerequisites for revising the spread of Permian large igneous provinces of Central Asia. The AFT dating and Thermal history modeling based on the AFT data reveals two intracontinental tectonic reactivation episodes of the STFZ: (1) a period of Cretaceous–Eocene (~100–40 Ma) reactivation and (2) the late Neogene (from ~10 Ma onwards) impulse after a period of tectonic stability during the Eocene–Miocene (~40–10 Ma).
3
Ma, Svieda M., Dawn A. Kellett, Laurent Godin, and Michael J. Jercinovic. "Localisation of the brittle Bathurst fault on pre-existing fabrics: a case for structural inheritance in the northeastern Slave craton, western Nunavut, Canada." Canadian Journal of Earth Sciences 57, no. 6 (June 2020): 725–46. http://dx.doi.org/10.1139/cjes-2019-0100.
Анотація:
The north–northwest-striking Bathurst fault in the northeastern Slave craton displaced the 1.9 Ga Kilohigok basin and the ca. 2.02–1.96 Ga Thelon tectonic zone, and projects beneath the 1.7 Ga Thelon basin where unconformity-associated uranium deposits are spatially associated with basement faults. Here we investigate the deformation–temperature–time history of the Bathurst fault rocks using structural and microstructural observations paired with U–(Th–)Pb and 40Ar/39Ar geochronology. Highly strained hornblende-bearing granitoid rocks, the predominant rock type on the northeastern side of the Bathurst fault in the study area, show ambiguous sense of shear suggesting flattening by coaxial deformation. Quartz and feldspar microstructures suggest ductile deformation occurred at ≥500 °C. Along the main fault trace, brittle features and hydrothermal alteration overprint the pervasive ductile flattening fabric. In situ U–Th–Pb dating of synkinematic monazite suggests ductile fabric formation at ca. 1933 ± 4 Ma and ca. 1895 ± 11 Ma, and zircon from a cross-cutting dyke constrains the brittle deformation to ≤1839 ± 14 Ma. 40Ar/39Ar dating of fabric-defining minerals yield cooling ages of ca. 1920–1900 Ma and ca. 1900–1850 Ma for hornblende and muscovite, respectively, and a maximum cooling age of ca. 1840 Ma for biotite. We suggest the ca. 1933–1895 Ma ductile flattening fabric developed during orthogonal collision and indentation of the Slave craton into the Thelon tectonic zone and Rae craton. Brittle deformation on the Bathurst fault was localised parallel to the ductile flattening fabric after ca. 1840 Ma and preceded Thelon basin deposition. Brittle deformation features in Bathurst fault rocks preserve evidence for fluid–rock interaction and enhanced basement permeability, suggesting the fault is a possible conduit structure for mineralising fluids.
4
Allibon, James, Maria Ovtcharova, François Bussy, Michael Cosca, Urs Schaltegger, Denise Bussien, and Éric Lewin. "Lifetime of an ocean island volcano feeder zone: constraints from U–Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary IslandsThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh." Canadian Journal of Earth Sciences 48, no. 2 (February 2011): 567–92. http://dx.doi.org/10.1139/e10-032.
Анотація:
High-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U–Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U–Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699–988 °C) to amphibole (500–600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.
5
Szopa, Krzysztof, Roman Włodyka, and David Chew. "LA-ICP-MS U-Pb apatite dating of Lower Cretaceous rocks from teschenite-picrite association in the Silesian Unit (southern Poland)." Geologica Carpathica 65, no. 4 (August 1, 2014): 273–84. http://dx.doi.org/10.2478/geoca-2014-0018.
Анотація:
Abstract The main products of volcanic activity in the teschenite-picrite association (TPA) are shallow, sub-volcanic intrusions, which predominate over extrusive volcanic rocks. They comprise a wide range of intrusive rocks which fall into two main groups: alkaline (teschenite, picrite, syenite, lamprophyre) and subalkaline (dolerite). Previous 40Ar/39Ar and 40K/40Ar dating of these rocks in the Polish Outer Western Carpathians, performed on kaersutite, sub-silicic diopside, phlogopite/biotite as well as on whole rock samples has yielded Early Cretaceous ages. Fluorapatite crystals were dated by the U-Pb LA-ICP-MS method to obtain the age of selected magmatic rocks (teschenite, lamprophyre) from the Cieszyn igneous province. Apatite-bearing samples from Boguszowice, Puńców and Lipowa yield U-Pb ages of 103± 20 Ma, 119.6 ± 3.2 Ma and 126.5 ± 8.8 Ma, respectively. The weighted average age for all three samples is 117.8 ± 7.3 Ma (MSWD = 2.7). The considerably smaller dispersion in the apatite ages compared to the published amphibole and biotite ages is probably caused by the U-Pb system in apatite being less susceptible to the effects of hydrothermal alternation than the 40Ar/39Ar or 40K/40Ar system in amphibole and/or biotite. Available data suggest that volcanic activity in the Silesian Basin took place from 128 to 103 Ma with the the main magmatic phase constrained to 128-120 Ma.
6
Canada, Andrew S., Elizabeth J. Cassel, Allen J. McGrew, M. Elliot Smith, Daniel F. Stockli, Kenneth A. Foland, Brian R. Jicha, and Brad S. Singer. "Eocene exhumation and extensional basin formation in the Copper Mountains, Nevada, USA." Geosphere 15, no. 5 (July 16, 2019): 1577–97. http://dx.doi.org/10.1130/ges02101.1.
Анотація:
Abstract Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.
7
Van Niekerk, H. S., R. Armstrong, and P. Vasconcelos. "The Grenvillian assembly of Rodinia: Timing of accretion on the western margin of the Kalahari (Kaapvaal) Craton." South African Journal of Geology 123, no. 4 (October 26, 2020): 441–64. http://dx.doi.org/10.25131/sajg.123.0042.
Анотація:
Abstract During the Grenvillian assembly of Rodinia, the Namaqua-Natal Metamorphic Province (NNMP) was formed as a result of the convergence of the Laurentia and Kalahari cratons. A detailed model for this accretion along the south-eastern margin of the Kalahari Craton has been established, but the tectonic history of the NNMP along the western margin of the Kalahari Craton has remained highly controversial. U-Pb SHRIMP zircon age dating of gneiss in the Kakamas Domain of the NNMP, as well as U-Pb SHRIMP age dating of detrital zircons and 40Ar/39Ar dating of metamorphic muscovite from sediments overlying the gneiss, confirms the presence of at least two separate events during the Namaqua-Natal Orogeny at ~1 166 Ma and 1 116 Ma. These events occurred after the Areachap Terrane was accreted onto the western margin of the Proto-Kalahari Craton during the Kheis Orogeny. 40Ar/39Ar ages derived from metamorphic muscovite formed in the metasediments of the Kheis terrane does not provide evidence for the timing of the Kheis Orogeny but suggests that it most likely only occurred after ~1 300 Ma and not at 1 800 Ma as commonly accepted. A U-Pb concordia age of ~1 166 Ma was derived from granitic gneiss in the Kakamas Domain of the Bushmanland Subprovince, possibly reflecting subduction and the initiation of continent-continent collision between the Proto-Kalahari Craton and the Bushmanland Subprovince. This granitic gneiss is nonconformably overlain by the metasediments of the Korannaland Group that contains metamorphic muscovite with 40Ar/39Ar ages of ~1 116 Ma. This age suggest that complete closure of the ocean between the Proto-Kalahari Craton and Bushmanland Subprovince probably occurred about 50 Ma after the intrusion of the ~1 166 Ma granitic gneisses.
8
Parry, W. T., P. N. Wilson, D. Moser, and M. T. Heizler. "U-Pb Dating of Zircon and 40Ar/39Ar Dating of Biotite at Bingham, Utah." Economic Geology 96, no. 7 (November 1, 2001): 1671–83. http://dx.doi.org/10.2113/gsecongeo.96.7.1671.
9
MacIntyre, D. G., and M. E. Villeneuve. "Geochronology of mid-Cretaceous to Eocene magmatism, Babine porphyry copper district, central British Columbia." Canadian Journal of Earth Sciences 38, no. 4 (April 1, 2001): 639–55. http://dx.doi.org/10.1139/e00-107.
Анотація:
New U/Pb and 40Ar/39Ar isotopic dating in the Babine porphyry copper district of central British Columbia documents three distinct magmatic events at 107104, 8578, and 5450 Ma. The earliest event involved emplacement of rhyolite domes into submarine volcanic rocks of the Rocky Ridge Formation. The rhyolite domes and related dacitic to basaltic volcanic rocks gave a UPb age of 107.9 ± 0.2 Ma and an 40Ar/39Ar age of 104.8 ± 1.2 Ma. The rhyolites, which were previously mapped as Eocene, are reinterpreted to be part of a previously unrecognized mid-Cretaceous cauldron subsidence complex. The regionally extensive Late Cretaceous magmatic event is also recognized in the Babine district and is represented by 40Ar/39Ar ages of 85.2 ± 2.8 and 78.3 ± 0.8 Ma on two Bulkley intrusions, one of which has associated porphyry copper mineralization. The final magmatic event is the most widespread and involved emplacement of the Babine intrusions and formation of numerous porphyry copper deposits including the Bell and Granisle past producers. Twenty-one new 40Ar/39Ar isotopic ages for these intrusions and coeval andesites of the Newman Formation have a narrow range from 53.6 ± 0.9 to 49.9 ± 0.6 Ma, whereas previous KAr isotopic dating had a possible range of 15 Ma. The mid-Cretaceous, Late Cretaceous, and Eocene magmatic suites in the Babine district are interpreted to be part of a long-lived volcano-plutonic complex that was the site of periodic magmatism and porphyry copper mineralization over a 60 Ma time period. This complex may have evolved within a zone of extension (pull-apart basins) situated between dextral strike-slip faults that were active during periods of rapid oblique plate convergence.
10
Pálfy, J., P. L. Smith, and J. K. Mortensen. "A U-Pb and 40Ar/39Ar time scale for the Jurassic." Canadian Journal of Earth Sciences 37, no. 6 (June 1, 2000): 923–44. http://dx.doi.org/10.1139/e00-002.
Анотація:
Published time scales provide discrepant age estimates for Jurassic stage boundaries and carry large uncertainties. The U-Pb or 40Ar/39Ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration. A radiometric age database consisting of fifty U-Pb and 40Ar/39Ar ages was compiled to construct a revised Jurassic time scale. Accepted ages have a precision of ±5 Ma (2σ) or better and are confined to no more than two adjacent stages. The majority of these calibration points result from integrated bio- and geochronologic dating in the western North American Cordillera and have not been previously used in time scales. Direct dates are available only for the Triassic-Jurassic boundary and the initial boundary of the Crassicosta chron and the Callovian stage. The chronogram method was used to estimate all Early and early Middle Jurassic zone boundaries (attempted here for the first time), late Middle Jurassic substage boundaries, and Late Jurassic stage boundaries. Significant improvement is achieved for the Pliensbachian and Toarcian, where six consecutive zone boundaries are determined. The derived zonal durations are disparate, varying between 0.4 and 1.6 Ma. The latest Jurassic isotopic database remains too sparse, therefore chronogram estimates are improved using interpolation based on magnetochronology. The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous): 141.8+2.5– 1.8 Ma; Tithonian: 150.5+3.4– 2.8 Ma; Kimmeridgian: 154.7+3.8– 3.3 Ma; Oxfordian: 156.5+3.1– 5.1 Ma; Callovian: 160.4+1.1– 0.5 Ma; Bathonian: 166.0+3.8– 5.6 Ma; Bajocian: 174.0+1.2– 7.9 Ma; Aalenian: 178.0+1.0– 1.5 Ma; Toarcian: 183.6+1.7– 1.1 Ma; Pliensbachian: 191.5+1.9– 4.7 Ma; Sinemurian: 196.5+1.7– 5.7 Ma; Hettangian (Triassic-Jurassic): 199.6 ± 0.4 Ma.
11
Reiners, Peter W., Terry L. Spell, Stefan Nicolescu, and Kathleen A. Zanetti. "Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating." Geochimica et Cosmochimica Acta 68, no. 8 (April 2004): 1857–87. http://dx.doi.org/10.1016/j.gca.2003.10.021.
12
Melleton, Jérémie, Michel Faure, and Alain Cocherie. "Monazite U-Th/Pb chemical dating of the Early Carboniferous syn-kinematic MP/MT metamorphism in the Variscan French Massif Central." Bulletin de la Société Géologique de France 180, no. 3 (May 1, 2009): 283–92. http://dx.doi.org/10.2113/gssgfbull.180.3.283.
Анотація:
AbstractIn situ U-Th-Pb geochronology on monazite using Electron Probe Micro Analyser, constrained by structural and textural observations, has been performed on four samples from the Limousin area (northwest part of the French Massif Central) in order to date the syn-kinematic MP/MT metamorphism related to the top-to-the-NW shearing that deformed the stack of nappes in this zone of the Variscan belt. All the analyzed samples lead to a mean age at 360 ± 4 Ma. The close range of ages obtained during this study (360 Ma) and with the previous 40Ar-39Ar ones (360–350 Ma) suggests fast processes of cooling and exhumation during the Early Carboniferous in internal zones of the Variscan belt. The geodynamic significance of this Early Carboniferous event is discussed at the scale of the Ibero-Armorican orocline.
13
Rochelle-Bates, N., N. M. W. Roberts, I. Sharp, U. Freitag, K. Verwer, A. Halton, E. Fiordalisi, et al. "Geochronology of volcanically associated hydrocarbon charge in the pre-salt carbonates of the Namibe Basin, Angola." Geology 49, no. 3 (November 20, 2020): 335–40. http://dx.doi.org/10.1130/g48019.1.
Анотація:
Abstract In volcanic rifted margins, the timing of hydrocarbon charge is difficult to predict, but is important in understanding fluid genesis. We investigated whether igneous activity was linked to hydrocarbon charge in the prolific South Atlantic pre-salt petroleum system. To do this, we applied in situ carbonate U-Pb geochronology, a relatively novel tool for dating hydrocarbon migration, to bituminous veins in pre-salt travertines from the rifted onshore Namibe Basin (Angola). To test if fluid flow was synchronous with known volcanic pulses, we also obtained new 40Ar/39Ar geochronology from a nearby volcanic complex. Bitumen is associated with calcite in a first generation of veins and vugs, and with dolomite in younger veins. The dated calcite veins yielded a pooled U-Pb age of 86.2 ± 2.4 Ma, which overlaps the volcanism 40Ar/39Ar age of 89.9 ± 1.8 Ma. The overlapping dates and the localized bitumen occurrence around the dated volcanic center show a clear genetic relationship between Late Cretaceous igneous activity and hydrocarbon charge. The dolomite was dated at 56.8 ± 4.8 Ma, revealing a previously unknown Paleocene/Eocene fluid-flow phase in the basin.
14
Estrada, Solveig, and Friedhelm Henjes-Kunst. "40Ar-39Ar and U-Pb dating of Cretaceous continental rift-related magmatism on the northeast Canadian Arctic margin." Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 164, no. 1 (March 1, 2013): 107–30. http://dx.doi.org/10.1127/1860-1804/2013/0005.
15
Stockli, Daniel F., and Yani M. R. Najman. "Earth’s Dynamic Past Revealed by Detrital Thermochronometry." Elements 16, no. 5 (October 1, 2020): 311–17. http://dx.doi.org/10.2138/gselements.16.5.311.
Анотація:
A dvances in detrital noble gas thermochronometry by 40Ar/39Ar and (U–Th)/He dating are improving the resolution of sedimentary provenance reconstructions and are providing new insights into the evolution of Earth’s surface. Detrital thermochronometry has the ability to quantify tectonic unroofing or erosion, temporal and dynamic connections between sediment source and sink, sediment lag-times and transfer rates, the timing of deposition, and postdepositional burial heating. Hence, this technique has the unique ability to use the detrital record in sedimentary basins to reconstruct Earth’s dynamic long-term landscape evolution and how basins are coupled to their hinterlands.
16
Andersen, Nathan L., Brian R. Jicha, Brad S. Singer, and Wes Hildreth. "Incremental heating of Bishop Tuff sanidine reveals preeruptive radiogenic Ar and rapid remobilization from cold storage." Proceedings of the National Academy of Sciences 114, no. 47 (November 7, 2017): 12407–12. http://dx.doi.org/10.1073/pnas.1709581114.
Анотація:
Accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate, biologic evolution, and magmatic processes throughout Earth history. The conventional approach to dating these eruptive products using the 40Ar/39Ar method is to fuse dozens of individual feldspar crystals. However, dispersion of fusion dates is common and interpretation is complicated by increasingly precise data obtained via multicollector mass spectrometry. Incremental heating of 49 individual Bishop Tuff (BT) sanidine crystals produces 40Ar/39Ar dates with reduced dispersion, yet we find a 16-ky range of plateau dates that is not attributable to excess Ar. We interpret this dispersion to reflect cooling of the magma reservoir margins below ∼475 °C, accumulation of radiogenic Ar, and rapid preeruption remobilization. Accordingly, these data elucidate the recycling of subsolidus material into voluminous rhyolite magma reservoirs and the effect of preeruptive magmatic processes on the 40Ar/39Ar system. The youngest sanidine dates, likely the most representative of the BT eruption age, yield a weighted mean of 764.8 ± 0.3/0.6 ka (2σ analytical/full uncertainty) indicating eruption only ∼7 ky following the Matuyama−Brunhes magnetic polarity reversal. Single-crystal incremental heating provides leverage with which to interpret complex populations of 40Ar/39Ar sanidine and U-Pb zircon dates and a substantially improved capability to resolve the timing and causal relationship of events in the geologic record.
17
Bukovská, Zita, Petr Jeřábek, Ondrej Lexa, Jiří Konopásek, Marian Janák, and Jan Košler. "Kinematically unrelated C—S fabrics: an Lexample of extensional shear band cleavage from the Veporic Unit (Western Carpathians)." Geologica Carpathica 64, no. 2 (April 1, 2013): 103–16. http://dx.doi.org/10.2478/geoca-2013-0007.
Анотація:
Abstract Discontinuous and kinematically unrelated C-S fabrics have been recognized along the contact between the Gemeric and Veporic Units in the Western Carpathians. The formation of S and C fabrics within orthogneiss, quartzite and chloritoid-kyanite schist of the Veporic Unit is associated with Cretaceous syn-burial orogen-parallel flow and subsequent exhumational unroofing. The formation of the two fabrics characterized by distinct quartz deformation microstructure and metamorphic assemblage is separated by an inter-tectonic growth of transversal chloritoid-, kyanite-, ± monazite-bearing assemblage. The monazite U-Th-Pb concordia age of 97 ± 4 Ma was obtained by the laser ablation ICP-MS dating method. The age of this inter-tectonic metamorphic stage together with existing 40Ar/39Ar ages on exhumation of the Veporic Unit indicate that despite the similar appearance to shear bands or C-S mylonites there is a time span of at least 10 Myr between the formation of homogeneous S fabrics and superposed discrete C fabrics in the studied rocks
18
Queen, M., J. A. Hanes, D. A. Archibald, E. Farrar, and L. M. Heaman. "40Ar/39Ar phlogopite and U – Pb perovskite dating of lamprophyre dykes from the eastern Lake Superior region: evidence for a 1.14 Ga magmatic precursor to Midcontinent Rift volcanism." Canadian Journal of Earth Sciences 33, no. 6 (June 1, 1996): 958–65. http://dx.doi.org/10.1139/e96-072.
Анотація:
Thin lamprophyre dykes are abundant in the vicinity of the ca. 1.1 Ga Midcontinent Rift System and the late Archean – Early Proterozoic Kapuskasing Uplift in the Canadian Shield northeast of Lake Superior. However, the relationship between these dykes and spatially associated alkalic-carbonatite complexes has been unclear. To ascertain the temporal relationship between the two, we have determined, by 40Ar/39Ar and U–Pb dating, precise emplacement ages for six lamprophyre dykes from the region. The [Formula: see text] Ma U–Pb date for perovskite from the western Marathon and Wawa areas is indistinguishable from the 1144 ± 7 Ma 40Ar/39Ar date for phlogopite from the eastern Kapuskasing Uplift, and it is concluded that these dykes are part of a regional 1.14 Ga swarm that extends over 300 km. These dykes were therefore emplaced some 35 Ma prior to the 1109 Ma commencement of Midcontinent Rift volcanism and coeval alkaline-complex activity, but essentially simultaneously with emplacement of the 1141 ± 2 Ma Abitibi diabase dyke swarm. It is suggested that these widespread 1144 Ma lamprophyre dykes and the Abitibi diabase dykes represent the first magmatic response of initiation of the Midcontinent rifting event, and were precursors to the main rift magmatism at 1109 Ma.
19
Volkova, N. I., E. I. Mikheev, A. V. Travin, A. G. Vladimirov, A. S. Mekhonoshin, and V. V. Khlestov. "P–T CONDITIONS, U/Pb AND 40Ar/39Ar ISOTOPIC AGES OF UHT GRANULITES FROM CAPE KALTYGEI, WESTERN BAIKAL REGION." Geodynamics & Tectonophysics 12, no. 2 (June 23, 2021): 310–31. http://dx.doi.org/10.5800/gt-2021-12-2-0526.
Анотація:
The study is focused on metapelitic granulites of Cape Kaltygei (Western Baikal region) that contain a diagnostic mineral assemblage of ultrahigh temperature (UHT) metamorphic rocks (orthopyroxene+sillimanite+quartz). The pseudosection-based thermobarometry yields peak metamorphic temperature and pressure values (T=950 °C, P=~9 kbar) and suggests near-isobaric cooling (IBC) conditions during the retrograde evolution of the granulites. The U/Pb zircon age estimates for metamorphism (~1.87 Ga) support the data published by other researchers. The SHRIMP-II U-Pb dating of zircon cores yields a minimum protolith age of 1.94–1.91 Ga. Biotites and amphiboles from granulites of Cape Kaltygei show the 40Ar/39Ar isotopic ages that are close to the Early Paleozoic accretion-collision system of the Western Baikal region.
20
Lanphere, M. A., D. E. Champion, M. A. Clynne, J. B. Lowenstern, A. M. Sarna-Wojcicki, and J. L. Wooden. "Age of the Rockland tephra, western USA." Quaternary Research 62, no. 1 (July 2004): 94–104. http://dx.doi.org/10.1016/j.yqres.2004.03.001.
Анотація:
The age of the Rockland tephra, which includes an ash-flow tuff south and west of Lassen Peak in northern California and a widespread ash-fall deposit that produced a distinct stratigraphic marker in western North America, is constrained to 565,000 to 610,000 yr by 40Ar/39Ar and U–Pb dating. 40Ar/39Ar ages on plagioclase from pumice in the Rockland have a weighted mean age of 609,000 ± 7000 yr. Isotopic ages of spots on individual zircon crystals, analyzed by the SHRIMP-RG ion microprobe, range from ∼500,000 to ∼800,000 yr; a subpopulation representing crystal rims yielded a weighted-mean age of 573,000 ± 19,000 yr. Overall stratigraphic constraints on the age are provided by two volcanic units, including the underlying tephra of the Lava Creek Tuff erupted within Yellowstone National Park that has an age of 639,000 ± 2000 yr. The basaltic andesite of Hootman Ranch stratigraphically overlies the Rockland in the Lassen Peak area and has 40Ar/39Ar ages of 565,000 ± 29,000 and 565,000 ± 12,000 yr for plagioclase and groundmass, respectively. Identification of Rockland tephra in ODP core 1018 offshore of central California is an important stratigraphic age that also constrains the eruption age to between 580,000 and 600,000 yr.
21
Zhang, Yong, Jing-Gui Sun, Shu-Wen Xing, and Zeng-Jie Zhang. "Geochronology and geochemistry of the Cuihongshan Fe-polymetallic deposit, northeastern China: implications for ore genesis and tectonic setting." Canadian Journal of Earth Sciences 55, no. 5 (May 2018): 475–89. http://dx.doi.org/10.1139/cjes-2017-0178.
Анотація:
The Lesser Xing’an Range is located in the eastern segment of the Central Asian Orogenic Belt. It hosts an important polymetallic metallogenic belt that contains more than 20 large- to small-scale porphyry Mo, epithermal Au, and skarn Fe-polymetallic deposits. The Cuihongshan Fe-polymetallic deposit is one of the largest polymetallic deposits in northeastern China. To better understand the formation of the Cuihongshan Fe-polymetallic deposit, we investigated the geological characteristics of the Cuihongshan deposit and applied geochemistry and geochronology to constrain the timing of the mineralization, and characteristics of the magmas. Zircon U–Pb dating of the alkali-feldspar granite and monzogranite yielded weighted mean 206Pb/238U ages of 495 ± 1.6 and 203 ± 1 Ma, respectively. Re–Os dating on molybdenite yielded an isochron age of 203.2 ± 1.4 Ma, and 40Ar/39Ar dating on phlogopite yielded an age of 203.4 ± 1.3 Ma. These data suggest that mineralization occurred during the Late Triassic, and is closely related with the monzogranite emplacement. These rocks belong to the high-K calc-alkaline and subalkaline series, are enriched in Rb, U, and Th, are depleted in Nb, Ta, and Ti, and show strong Eu anomalies, implying that they are A-type post-orogenic rocks. The Cuihongshan Fe-polymetallic formation is possibly related to an extensional environment resulting from the final closure of the Paleo-Asian Ocean.
22
Creaser, Robert A., Larry M. Heaman, and Philippe Erdmer. "Timing of high-pressure metamorphism in the Yukon – Tanana terrane, Canadian Cordillera: constraints from U – Pb zircon dating of eclogite from the Teslin tectonic zone." Canadian Journal of Earth Sciences 34, no. 5 (May 1, 1997): 709–15. http://dx.doi.org/10.1139/e17-057.
Анотація:
Zircon from eclogite near Last Peak in the Teslin tectonic zone yielded a U–Pb isotopic age of 269 + 2 Ma (2σ), the first precise age for such a rock in the Yukon –Tanana terrane of the Canadian Cordillera. Both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin, and the U – Pb age therefore constrains the timing of peak high-pressure metamorphism in this rock. The U – Pb age demonstrates for the first time that an Early Permian high-pressure metamorphic event occurred in rocks now making up the Teslin tectonic zone, and possibly elsewhere in the Yukon – Tanana terrane. This U – Pb age provides a new geochronologic "pin" in the evolution of the Yukon – Tanana terrane prior to its Mesozoic accretion to the North American continental margin and, combined with recent 40Ar/39Ar muscovite data, indicates that high-pressure metamorphism at this time was a relatively short-lived event.
23
Krautz, Jana, Mandy Hofmann, Andreas Gärtner, Ulf Linnemann, and Arno Kleber. "Capability of U–Pb dating of zircons from Quaternary tephra: Jemez Mountains, NM, and La Sal Mountains, UT, USA." E&G Quaternary Science Journal 67, no. 1 (January 31, 2018): 7–16. http://dx.doi.org/10.5194/egqsj-67-7-2018.
Анотація:
Abstract. Two Quaternary tephras derived from the Jemez Mountains, New Mexico – the Guaje and Tsankawi tephras – are difficult to distinguish due to their similar glass-shard chemical composition. Differences in bulk chemical composition are small as well. Here we examine the feasibility to assign an age to a distal tephra layer in the La Sal Mountains, Utah, by U–Pb dating of zircons and to correlate it with one of the two Jemez eruptions. We also dated original Jemez tephras for comparison. Even though the tephras are very young, we obtained reasonable age determinations using the youngest cluster of zircon grains overlapping in age at 2σ. Thereafter, the Guaje tephra is 1.513 ± 0.021 Myr old. The La Sal Mountains tephra is correlated with the Tsankawi tephra. Three samples yielded a common age range of 1.31–1.40 Myr. All ages are in slight disagreement with published age determinations obtained by 40Ar ∕ 39Ar dating. These findings indicate that distal Jemez tephras can be distinguished by U–Pb dating. Furthermore, we encourage giving this method a try for age assignments even of Quaternary volcanic material.
24
Yang, Tianjian, Xiaoming Sun, Guiyong Shi, and Ying Liu. "LA-ICP-MS U–Pb Dating of Cenozoic Rutile Inclusions in the Yuanjiang Marble-Hosted Ruby Deposit, Ailao Shan Complex, Southwest China." Minerals 11, no. 4 (April 19, 2021): 433. http://dx.doi.org/10.3390/min11040433.
Анотація:
Among the marble-hosted ruby deposits in the Himalayan tectonic belt, which yields the highest-quality rubies in the world, the Yuanjiang deposit is the only economically viable one located in China. More attempts are necessary to put constraints on the ore-forming age of these marble-hosted ruby deposits. Here, we dated rutile inclusions in the Yuanjiang rubies using the LA-ICP-MS U–Pb method, which yielded a lower intercept 206Pb/238U age of 20.2 ± 1.2 Ma on the Tera-Wasserburg plot, close to the 22.5–22.2 Ma 40Ar/39Ar ages of phlogopite from the ruby host matrix assemblage. Our U–Pb rutile age put a constraint on the cooling history of the Yuanjiang rubies deposit. The new rutile age is consistent with our previous model that shows the ca. 28–22 Ma left lateral shearing plays an important role in transporting the ruby deposit toward the surface. This study provides the first example of in-situ U–Pb dating of rutile in the Himalayan tectonic belt, demonstrating the great potential of U–Pb rutile geochronology for Cenozoic mineral deposits.
25
Dunlop, David J. "Grenvillia and Laurentia — a Precambrian Wilson cycle?" Canadian Journal of Earth Sciences 51, no. 3 (March 2014): 187–96. http://dx.doi.org/10.1139/cjes-2013-0101.
Анотація:
John Tuzo Wilson coined the term “plate” in plate tectonics. He is famous for inventing transform boundaries, hot spot tracks, and the Wilson cycle of ocean birth, growth, and decline. Less well remembered is his work in the 1950s on tectonic and radiometric age provinces of the Canadian Shield, as part of which he fathered U/Pb geochronology in Canada. This work gave strong support to the notion of continental growth through accretion of successively younger terranes onto an ancient cratonic core. The present paper reviews how paleomagnetism can trace the motions of continents to test Wilson’s ideas. Continental accretion often involves deep burial of one of the colliding elements through subduction or crustal underplating; such was the case with the Grenville orogen and its subprovinces in their Proterozoic accretion onto the Laurentian craton. The resulting heating and metamorphism erases most pre-collisional magnetic information but adds something new: the possibility of following the post-metamorphic uplift and cooling history, in time and space. The time element is provided by a new form of isotopic geochronology, thermochronometry, which provides dates for specific minerals together with the temperatures at which they became closed to isotopic migration. U/Pb dating of sphene is one method used; another is the 40Ar/39Ar variant of K/Ar dating applied to hornblende, micas, and feldspars, which have a wide range of Ar closure temperatures. The two specific Grenville studies described deal with parallel uplift histories determined by 40Ar/39Ar dating and by magnetics for the accreted terranes of the Central Metasedimentary Belt in Ontario and with the paleomagnetic detection of the post-1240 Ma closing of a small ocean between the Elsevir terrane and Laurentia during the Grenvillian orogeny.
26
Wu, Meng, Liang Li, Jing-gui Sun, and Rui Yang. "Geology, geochemistry, and geochronology of the Laozuoshan gold deposit, Heilongjiang Province, Northeast China: implications for multiple gold mineralization events and geodynamic setting." Canadian Journal of Earth Sciences 55, no. 6 (June 2018): 604–19. http://dx.doi.org/10.1139/cjes-2018-0038.
Анотація:
The Laozuoshan gold deposit, located in the central part of the Jiamusi Massif, is hosted by the contact zone between granitic complex and Proterzoic strata. In this study, we present the results of geochronology and geochemistry of ore-related granodiorite and diorite porphyry, and hydrothermal sericite 40Ar/39Ar dating. The granodiorite and diorite porphyry in the Laozuoshan gold deposit are calc-alkaline and high-K (calc-alkaline) series, which are enriched in LREE and LILE and depleted in HFSE, with no depletion of Eu. The geochronology data show that zircon U–Pb ages of the granodiorite and diorite porphyry are ∼262 Ma and ∼105 Ma, respectively. The sericite 40Ar/39Ar ages are ∼194 Ma and ∼108 Ma. On the basis of previous researches, ore geology and geochronology studies show that the Laozuoshan gold deposit underwent at least two gold mineralization events. We suggest that the first one, which was related to skarnization, resulted from the collision between the Jiamusi and Songnen Massifs in Late Permian. The subsequent gold mineralization resulted from the subduction of the paleo-Pacific Plate in Early Cretaceous.
27
Schoene, Blair, Michael P. Eddy, C. Brenhin Keller, and Kyle M. Samperton. "An evaluation of Deccan Traps eruption rates using geochronologic data." Geochronology 3, no. 1 (April 16, 2021): 181–98. http://dx.doi.org/10.5194/gchron-3-181-2021.
Анотація:
Abstract. Recent attempts to establish the eruptive history of the Deccan Traps large igneous province have used both U−Pb (Schoene et al., 2019) and 40Ar/39Ar (Sprain et al., 2019) geochronology. Both of these studies report dates with high precision and unprecedented coverage for a large igneous province and agree that the main phase of eruptions began near the C30n–C29r magnetic reversal and waned shortly after the C29r–C29n reversal, totaling ∼ 700–800 kyr duration. These datasets can be analyzed in finer detail to determine eruption rates, which are critical for connecting volcanism, associated volatile emissions, and any potential effects on the Earth's climate before and after the Cretaceous–Paleogene boundary (KPB). It is our observation that the community has frequently misinterpreted how the eruption rates derived from these two datasets vary across the KPB. The U−Pb dataset of Schoene et al. (2019) was interpreted by those authors to indicate four major eruptive pulses before and after the KPB. The 40Ar/39Ar dataset did not identify such pulses and has been largely interpreted by the community to indicate an increase in eruption rates coincident with the Chicxulub impact (Renne et al., 2015; Richards et al., 2015). Although the overall agreement in eruption duration is an achievement for geochronology, it is important to clarify the limitations in comparing the two datasets and to highlight paths toward achieving higher-resolution eruption models for the Deccan Traps and for other large igneous provinces. Here, we generate chronostratigraphic models for both datasets using the same statistical techniques and show that the two datasets agree very well. More specifically, we infer that (1) age modeling of the 40Ar/39Ar dataset results in constant eruption rates with relatively large uncertainties through the duration of the Deccan Traps eruptions and provides no support for (or evidence against) the pulses identified by the U−Pb data, (2) the stratigraphic positions of the Chicxulub impact using the 40Ar/39Ar and U−Pb datasets do not agree within their uncertainties, and (3) neither dataset supports the notion of an increase in eruption rate as a result of the Chicxulub impact. We then discuss the importance of systematic uncertainties between the dating methods that challenge direct comparisons between them, and we highlight the geologic uncertainties, such as regional stratigraphic correlations, that need to be tested to ensure the accuracy of eruption models. While the production of precise and accurate geochronologic data is of course essential to studies of Earth history, our analysis underscores that the accuracy of a final result is also critically dependent on how such data are interpreted and presented to the broader community of geoscientists.
28
Baadsgaard, H., J. F. Lerbekmo, J. R. Wijbrans, C. C. Swisher III, and M. Fanning. "Multimethod radiometric age for a bentonite near the top of the Baculites reesidei Zone of southwestern Saskatchewan (Campanian–Maastrichtian stage boundary?)." Canadian Journal of Earth Sciences 30, no. 4 (April 1, 1993): 769–75. http://dx.doi.org/10.1139/e93-063.
Анотація:
A 15 cm thick bentonite at the top of the Baculites reesidei Zone in Cruikshank Coulee north of Herbert in southwestern Saskatchewan yielded biotite, sanidine, plagioclase, and zircon. Rb–Sr analyses give a mineral isochron with an age of 72.5 ± 0.2 Ma (± 2σ). U–Pb analyses of zircon produced concordant 206Pb/238U and 207Pb/235U ages of 72.4 ± 0.4 Ma (± 2σ) and 72.6 ± 0.4 Ma (± 2σ), respectively. Laser 40Ar/39Ar dating of sanidine and biotite revealed ages of 72.5 ± 0.2 Ma (± SE) and 72.6 ± 0.2 Ma (± SE), respectively. If the Campanian–Maastrichtian stage boundary is near the top of the Baculites reesidei Zone, then the overall average age of 72.5 ± 0.4 Ma dates the stage boundary.
29
Dong, Hanwen, Kyle P. Larson, Dawn A. Kellett, Zhiqin Xu, Guangwei Li, Hui Cao, Zhiyu Yi, and Lingsen Zeng. "Timing of slip across the South Tibetan detachment system and Yadong–Gulu graben, Eastern Himalaya." Journal of the Geological Society 178, no. 1 (September 29, 2020): jgs2019–197. http://dx.doi.org/10.1144/jgs2019-197.
Анотація:
The Yadong–Gulu graben preserves the kinematic and temporal relationships between east–west-directed extension in southern Tibet and north–south extensional strain in the Himalaya. In the Yadong region, distinct outer and inner top-down-to-the-north segments of the South Tibetan detachment system (STDS) are recognized. Herein, we combine high- to medium-T (U–Pb, 40Ar/39Ar) and low-T (apatite fission-track, apatite (U–Th)/He and zircon (U–Th)/He) thermochronometry to investigate the timing of slip across the STDS and Yadong–Gulu structures. These data demonstrate that the cessation of the Yadong shear zone, the structurally outer ductile segment of the STDS, occurred c. 20 Ma and that motion along the inner brittle–ductile Zherger La detachment continued after c. 16.6 Ma, ending by 11 Ma. The cooling history in the immediate STDS footwall is characterized by two main episodes of relatively rapid cooling and exhumation. The first occurred in the middle Miocene (c. 15–11 Ma), and is common along-strike of the innermost STDS footwall, related to cooling of the STDS. The second occurred in the late Miocene–Pliocene (c. 7–3 Ma), and is local to the Yadong–Gulu graben footwall in NW Bhutan, indicating that late Miocene–Pliocene slip along the graben system contributed to exhumation of the STDS east of the graben rift.Supplementary material: Tables of analytical data, dating results, and input data and model parameters of HeFTy are available at https://doi.org/10.6084/m9.figshare.c.5132941
30
Han, Jinsheng, Pete Hollings, Fred Jourdan, Yunchuan Zeng, and Huayong Chen. "Inherited Eocene magmatic tourmaline captured by the Miocene Himalayan leucogranites." American Mineralogist 105, no. 9 (September 1, 2020): 1436–40. http://dx.doi.org/10.2138/am-2020-7608.
Анотація:
Abstract The Miocene Cuonadong leucogranites in the easternmost section of the Tethyan Himalaya, Southern Tibet, are characterized by two types of tourmaline. Tourmaline occurs as needle-like crystals in the two-mica ± tourmaline granites (Tur G) and large patches in the pegmatites (Tur P). Both the granite and the pegmatites yield Miocene ages (ca. 20 Ma) based on monazite U(-Th)-Pb dating, whereas 40Ar/39Ar geochronology of the coarse-grained tourmalines (Tur P) crosscut by pegmatite veins yielded an Eocene mini-plateau age of 43 ± 6 Ma. Major element concentrations of tourmaline indicate that both Tur P and Tur G belong to the schorl group with a magmatic origin, but trace elements such as V indicate that they are not cogenetic. Boron isotopes suggest that Tur P (average –9.76‰) was derived from typical crustal sources, whereas Tur G (average –7.65‰) contains relatively more mafic input. The capture of Eocene tourmaline by the Miocene leucogranites at Cuonadong suggests that the crustally derived Eocene magmatism may have occurred in the southern Tethyan Himalaya. Identification of the inherited magmatic tourmaline (Tur P), although not common, challenges the current application of tourmaline chemistry to the investigation of magmatic-hydrothermal systems.
31
McDowell, Fred W., William C. McIntosh, and Kenneth A. Farley. "A precise 40Ar–39Ar reference age for the Durango apatite (U–Th)/He and fission-track dating standard." Chemical Geology 214, no. 3-4 (January 2005): 249–63. http://dx.doi.org/10.1016/j.chemgeo.2004.10.002.
32
McLeish, Duncan F., Stephen Johnston, Richard Friedman, and James Mortensen. "Stratigraphy and U–Pb Zircon–Titanite Geochronology of the Aley Carbonatite Complex, Northeastern British Columbia: Evidence for Antler-Aged Orogenesis in the Foreland Belt of the Canadian Cordillera." Geoscience Canada 47, no. 4 (December 18, 2020): 171–86. http://dx.doi.org/10.12789/geocanj.2020.47.165.
Анотація:
The tectonic significance and age of carbonatite intrusions in the central Foreland Belt of the Canadian Cordillera are poorly constrained. Recent work has demonstrated that one of these carbonatite intrusions, the Aley carbonatite, was emplaced as a syn-kinematic sill, coeval with a major nappe-forming tectonic event. Determining the age of the Aley carbonatite thus provides a means of directly dating syn-tectonic magmatism. Attempts at dating carbonatite units failed due to low U–Pb content in sampled zircon; however, a U–Pb titanite age of 365.9 ± 2.1 Ma was obtained from the Ospika pipe, an ultramafic diatreme spatially and genetically related to the carbonatite. This U–Pb titanite age is further supported by respective 40Ar/39Ar phlogopite ages of 359.4 ± 3.4 Ma and 353.3 ± 3.6 Ma for the pipe and a spatially associated lamprophyre dyke. We interpret the Late Devonian U–Pb titanite age of the Ospikapipe to be the minimum possible age of the carbonatite and syn-magmatic nappe-forming tectonic event. The maximum possible age of the carbonatite is constrained by the Early Devonian age of the Road River Group, the youngest strata intruded by carbonatite dykes and involved in the nappe-forming event. Our dating results for the Aley carbonatite closely correlate with U–Pb zircon and perovskite ages obtained for the Ice River carbonatite complex in the central Foreland Belt of the southern Canadian Cordillera, and support the interpretation of carbonatite intrusions of the western Foreland Belt as genetically linked components of an alkaline-carbonatitic magmatic province. Structural, stratigraphic, and geochronological data from the Aley area indicate that deformation was similar in style to, and coeval with, structures attributable to the Antler orogeny, and are consistent with the Antler orogen having extended the length of the Cordilleran margin from the southern United States to Alaska.
33
Kenny, Gavin G., Irmeli Mänttäri, Martin Schmieder, Martin J. Whitehouse, Alexander A. Nemchin, Jeremy J. Bellucci, and Renaud E. Merle. "Age of the Sääksjärvi impact structure, Finland: reconciling the timing of small impacts in crystalline basement with regional basin development." Journal of the Geological Society 177, no. 6 (July 1, 2020): 1231–43. http://dx.doi.org/10.1144/jgs2020-034.
Анотація:
We report a new age for the Sääksjärvi impact structure, Finland, a 6 km diameter feature that formed in crystalline rocks of the Precambrian Baltic Shield. Two previous studies reported 40Ar/39Ar data for Sääksjärvi and suggested conflicting formation ages of ≤330 Ma or c. 560 Ma. The former age represents a possible complication for models which indicate that the region was covered by sediments of the Caledonian foreland basin throughout much of the Phanerozoic. We conducted a study combining imaging, microstructural analysis and U–Pb dating of shocked zircon from Sääksjärvi. The U–Pb dataset indicates a c. 600 Ma impact into predominantly c. 1850 Ma target rocks. A concordia age of 608 ± 8 Ma (2σ) confirms Sääksjärvi as the first known Ediacaran impact structure in the Baltic Shield and only the second worldwide. Our data indicate that the Sääksjärvi impact structure formed in exposed crystalline basement rocks of the Baltic Shield prior to the development of the Caledonian foreland basin. Given that most impact structures on Earth are relatively small features, radiometric dating of small impact structures in crystalline basement may place boundaries on the timing and spatial extent of palaeobasins that might otherwise be difficult to constrain.Supplementary material: Additional imaging and microstructural analysis of zircon, additional concordia plots, and U–Pb data tables are available at https://doi.org/10.6084/m9.figshare.c.5043462
34
Mercer, Cameron M., Kelsey E. Young, John R. Weirich, Kip V. Hodges, Bradley L. Jolliff, Jo-Anne Wartho, and Matthijs C. van Soest. "Refining lunar impact chronology through high spatial resolution 40Ar/39Ar dating of impact melts." Science Advances 1, no. 1 (February 2015): e1400050. http://dx.doi.org/10.1126/sciadv.1400050.
Анотація:
Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe 40Ar/39Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt–forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through 40Ar/39Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.
35
Picazo, Suzanne M., Tanya A. Ewing, and Othmar Müntener. "Paleocene metamorphism along the Pennine–Austroalpine suture constrained by U–Pb dating of titanite and rutile (Malenco, Alps)." Swiss Journal of Geosciences 112, no. 2-3 (September 13, 2019): 517–42. http://dx.doi.org/10.1007/s00015-019-00346-1.
Анотація:
Abstract We present in situ rutile and titanite U–Pb geochronology for three samples from the Ur breccia, which forms the boundary between the Malenco unit and the Margna nappe (Eastern Central Alps) near Pass d’Ur in southeast Switzerland. These sampled both oceanic brecciated material and a blackwall reaction zone in contact with a micaschist and serpentinized peridotite. Peak temperatures during Alpine metamorphism in these units were ~ 460 ± 30 °C. Textural observations combined with new geochronological data indicate that rutile and titanite both grew below their closure temperatures during Alpine metamorphism. We present a technique to calculate the most precise and accurate ages possible using a two-dimensional U–Pb isochron on a Wetherill concordia. Rutile from two samples gave a U–Pb isochron age of 63.0 ± 3.0 Ma. This age conflicts with previous 39Ar–40Ar data on heterogeneous amphiboles from which an age of 90–80 Ma was inferred for the high pressure part of the Alpine evolution, but is consistent with K–Ar ages and Ar–Ar ages on phengitic white mica. Titanite from three samples gave a U–Pb isochron age of 54.7 ± 4.1 Ma. This age is consistent with Rb–Sr isochron ages on mylonites along and in the footwall of the Lunghin–Mortirolo movement zone, a major boundary that separates ductile deformation in the footwall from mostly localized and brittle deformation in the hangingwall. Our ages indicate a Paleocene rather than upper Cretaceous metamorphism of the Pennine–Austroalpine boundary and permit at most ~ 15 Myr, and possibly much less, between the growth of rutile and titanite.
36
Suárez, M., A. Demant, R. De La Cruz, and C. M. Fanning. "40Ar/39Ar and U–Pb SHRIMP dating of Aptian tuff cones in the Aisén Basin, Central Patagonian Cordillera." Journal of South American Earth Sciences 29, no. 3 (April 2010): 731–37. http://dx.doi.org/10.1016/j.jsames.2009.11.003.
37
Pálfy, József, Roland Mundil, Paul R. Renne, Raymond L. Bernor, László Kordos, and Mihály Gasparik. "U–Pb and 40Ar/39Ar dating of the Miocene fossil track site at Ipolytarnóc (Hungary) and its implications." Earth and Planetary Science Letters 258, no. 1-2 (June 2007): 160–74. http://dx.doi.org/10.1016/j.epsl.2007.03.029.
38
Annikova, I. Yu, A. G. Vladimirov, S. A. Vystavnoi, D. Z. Zhuravlev, N. N. Kruk, E. N. Lepekhina, D. I. Matukov, et al. "U-Pb and 39Ar/40Ar dating and Sm-Nd and Pb-Pb isotopic study of the Kalguty molybdenum-tungsten ore-magmatic system, Southern Altai." Petrology 14, no. 1 (January 2006): 81–97. http://dx.doi.org/10.1134/s0869591106010073.
39
Kurz, W., A. Wölfler, and R. Handler. "CENOZOIC TECTONIC EVOLUTION OF THE EASTERN ALPS – A RECONSTRUCTION BASED ON 40AR/39AR WHITE MICA, ZIRCON AND APATITE FISSION TRACK, AND APATITE (U/Th)-He THERMOCHRONOLOGY." Bulletin of the Geological Society of Greece 43, no. 1 (January 19, 2017): 299. http://dx.doi.org/10.12681/bgsg.11182.
Анотація:
The Cenozoic tectonic evolution of the Eastern Alps is defined by nappe assembly within the Penninic and Subpenninic units and their subsequent exhumation. The units above, however, are affected by extension and related faulting. By applying distinct thermochronological methods with closure temperatures ranging from ~450° to ~40°C we reveal the thermochronological evolution of the eastern part of the Eastern Alps. 40Ar/39Ar dating on white mica, zircon and apatite fission track, and apatite U/Th-He thermochronology were carried out within distinct tectonic units (Penninic vs. Austroalpine) and on host rocks and fault- related rocks (cataclasites and fault gouges) along major fault zones. We use particularly the ability of fission tracks to record the thermal history as a measure of heat transfer in fault zones, causing measurable changes of fission track ages and track lengths. Additionally, these studies will provide a general cooling and exhumation history of fault zones and adjacentcrustal blocks.
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Chugaev, A. V., I. V. Chernyshev, E. Y. Rytsk, E. B. Salnikova, A. A. Nosova, A. V. Travin, A. B. Kotov, A. M. Fedoseenko, and I. V. Anisimova. "Relationship between magmatic, metamorphic and hydrothermal processes within the Baikal-Muya terrane (Eastern Siberia): constraints from a high-precision geochronological study of the Kedrovsky granitoid massif." Доклады Академии наук 489, no. 3 (November 29, 2019): 292–97. http://dx.doi.org/10.31857/s0869-56524893292-297.
Анотація:
High-precision dating of granitoids is significant for identification the ages of the main stages of crust formation in various blocks of the continental crust. In this work, we dating the Kedrovskiy diorite-granodiorite massif localized within the South Muya block of the Baikal-Muya accretion terrane (BMT) (Eastern Siberia) among the Neoproterozoic gabbroids of the Kedrovskiy complex and metasedimentary rocks of the Kedrovskaya Formation. The results of U-Pb (ID TIMS) and 39Ar-40Ar geochronological studies of the Kedrovskiy massif are discussed. The formation of the massif (781 3 Ma) occurred at an early stage formation of the Proterozoic continental crust of BMT during accretion of Baikalids with the Siberian craton. A later thermal event (626 11 Ma) of the Ediacaran stage evolution BMT is reflected in the formation of of biotite-quartz-feldspar veins that cut through the granitoids of the Kedrovka massif. The obtained geochronological data show that the gold mineralization of the Kedrovskiy deposit (273 4 Ma) was not generated by the Kedrovskiy massif granitoids.
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Dallmeyer, R. D. "40Ar/39Ar mineral age record of variably superimposed Proterozoic tectonothermal events in the Grenville Orogen, central Labrador." Canadian Journal of Earth Sciences 24, no. 2 (February 1, 1987): 314–33. http://dx.doi.org/10.1139/e87-033.
Анотація:
Penetratively cleaved slate–phyllite within supracrustal sequences (Seal Lake and Letitia Lake groups) in northernmost portions of the Grenville parauthochthon in central Labrador record 1018 ± 15 and 985 ± 14 Ma whole-rock 40Ar/39Ar plateau ages, which are interpreted as closely dating deformation and a concomitant low-grade metamorphism. The 40Ar/39Ar signature of Grenvillian tectonothermal effects is variably recorded in more southerly portions of the parautochthon. Locally within the extensive North Pole Brook igneous suite, there is no significant metamorphic retrogression. In these areas, igneous hornblendes record ca. 1650 Ma plateau ages similar to those suggested for emplacement of most of the suite by previously reported Rb–Sr whole-rock isochron and U–Pb zircon analyses. Low-temperature retrogression of igneous assemblages is observed throughout most of the North Pole Brook igneous terrane. Hornblende concentrates from five relatively undeformed samples are characterized by internally discordant 40Ar/39Ar spectra reflecting variable extraneous argon contamination and minor development of retrogressive blue–green amphibole. Biotite spectra are generally more concordant but display a wide range in total-gas and plateau ages (1534–2419 Ma) as a result of variable extraneous argon contamination. The North Pole Brook suite is progressively more deformed southwestward in the Grenville parautochthon, where it is largely represented by blastomylonitic orthogneisses that record hornblende plateau ages of 991–1013 Ma. Where not contaminated with extraneous argon, biotite records ages identical to those of coexisting hornblende, suggesting rapid post-Grenvillian metamorphic cooling. Paragneiss exposed in northeastern portions of the parautochthon displays a heterogeneous Grenvillian tectonothermal record. Muscovite within mylonitic rocks, developed along internal imbrication zones, yields plateau ages of 1046 and 1088 Ma. Hornblendes within amphibolite horizons in the paragneisses show little rejuvenation and record pre-Grenvillian thermal histories (1873 and 1315 Ma plateau dates). Hornblende within statically metamorphosed amphibolite near the structural base of the Wilson Lake allochthon records an early Grenville (1112 Ma) plateau age. Biotite within the allochthon has been previously shown to be regionally contaminated with extraneous argon components. The 40Ar/39Ar results support previous hypotheses that suggest that northwestward emplacement (at ca. 1100 Ma?) of the Wilson Lake and other allochthons was partially responsible for the burial and associated polydeformation and metamorphism of the Grenville parautochthon. The 985–1018 Ma low-grade metamorphism and penetrative deformation recorded in supracrustal sequences in the northernmost parautochthon are markedly younger and must reflect late Grenvillian orogenic activity with different tectonic implications.
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Zhang, Hongyuan, Junlai Liu, and Wenbin Wu. "Geochronology and Tectonic Evolution of the Lincang Batholith in Southwestern Yunnan, China." Journal of Geological Research 2012 (August 8, 2012): 1–11. http://dx.doi.org/10.1155/2012/287962.
Анотація:
Geochronological research of the Lincang Batholith is one key scientific problem to discuss the tectonic evolution of the Tethys. Two granitic specimens were selected from the Mengku-Douge area in the Lincang Batholith to perform the LA-ICPMS Zircon U-Pb dating based on thorough review of petrological, geochemical, and geochronological data by the former scientists. Rock-forming age data of biotite granite specimen from Kunsai is about 220 Ma, the Norian age. However, the west sample from Mengku shows 230 Ma, the Carnian age. The later intrusion in Kunsai area located east to the Mengku area shows directly their uneven phenomena in both space and time and may indirectly reflect the space difference of the contraction-extension transformation period of the deep seated faults. Considering the former 40Ar/39Ar data and the outcrop in Mengku, the Lincang Batholith should have experienced one tectonic exhumation and regional subsidence cycle. The regional subsidence cycle has close relations to the expansion of the Meso Tethys.
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MARK, Darren F., Clive M. RICE, Malcolm HOLE, and Dan CONDON. "Multi-chronometer dating of the Souter Head complex: rapid exhumation terminates the Grampian Event of the Caledonian Orogeny." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 111, no. 2 (June 2020): 95–108. http://dx.doi.org/10.1017/s1755691020000043.
Анотація:
ABSTRACTThe Souter Head sub-volcanic complex (Aberdeenshire, Scotland) intruded the high-grade metamorphic core of the Grampian Orogen at 469.1 ± 0.6 Ma (uranium-238–lead-206 (238U–206Pb) zircon). It follows closely peak metamorphism and deformation in the Grampian Terrane and tightly constrains the end of the Grampian Event of the Caledonian Orogeny. Temporally coincident U–Pb and argon/argon (40Ar/39Ar) data show the complex cooled quickly with temperatures decreasing from ca.800 °C to less than 200 °C within 1 Ma. Younger rhenium–osmium (Re–Os) ages are due to post-emplacement alteration of molybdenite to powellite. The U–Pb and Ar/Ar data combined with existing geochronological data show that D2/D3 deformation, peak metamorphism (Barrovian and Buchan style) and basic magmatism in NE Scotland were synchronous at ca.470 Ma and are associated with rapid uplift (5–10 km Ma−1) of the orogen, which, by ca.469 Ma, had removed the cover to the metamorphic pile. Rapid uplift resulted in decompressional melting and the generation of mafic and felsic magmatism. Shallow slab break-off (50–100 km) is invoked to explain the synchroneity of these events. This interpretation implies that peak metamorphism and D2/D3 ductile deformation were associated with extension. Similarities in the nature and timing of orogenic events in Connemara, western Ireland, with NE Scotland suggest that shallow slab break-off occurred in both localities.
44
Miller, Brent V. "Introduction to Radiometric Dating." Paleontological Society Papers 12 (October 2006): 1–23. http://dx.doi.org/10.1017/s1089332600001327.
Анотація:
Radiometric dating of rocks and minerals to constrain the age of the Earth, timing of geological events and paleobiological histories has its roots in the works of nuclear physicists of the early Nineteenth Century during the period of discovery of radioactivity and investigations into the nature of the atom. The intervening years since have seen great progress in using the long-lived radioactive elements to constrain the origin and evolution of the Earth and to place the rock and fossil record into a consistent, numerically quantifiable temporal framework.U-Th-Pb and40Ar/39Ar dating methods have emerged as the primary tools for calibrating most of Earth history. It is important for all geoscientists to appreciate the physical basis underlying these methods and to have the ability to evaluate dates by means of currently accepted practices of data presentation. This introduction, along with the accompanying chapters, is intended to help the consumers of radiometric dates to understand better the uses and limitations of radiometric dating methods in an effort to tailor methods and techniques to address specific geochronologic needs, including calibration of the geologic time scale.The ultimate goal of a fully calibrated rock record remains an on-going endeavor. The 2004 ICS geologic time scale is the latest compilation of those efforts. The numerical age calibration is constrained by only 213 radiometric dates, the vast majority of which are U-Pb and40Ar/39Ar dates. Radiometric age control is not evenly distributed through geologic time. There are virtually no radiometric dates in the late Cenozoic where magnetostratigraphy and cyclostratigraphic methods are more precise and applicable. Radiometric dating efforts are concentrated on biostratigraphically important segments of the rock record such as the Permian-Triassic and Cretaceous-Paleocene boundary events, and this is reflected in high-precision calibration of these boundaries. Large segments of geologic time, however, are constrained by either a few radiometric dates per chronostratigraphic unit (most of the Paleozoic) or none at all (Upper Triassic). The current status of radiometric age control on the rock record largely reflects real, underlying scientific issues in biostratigraphy and geochronology, and thus can help point the way to fruitful lines of collaboration between paleontologists, stratigraphers, and geochronologists.
45
Guo, Jian, Youyue Lu, Jianming Fu, Zhengwei Qin, Yongyun Ning, and Zunzun Zhang. "Geology and Geochronology of the Maozaishan Sn Deposit, Hunan Province: Constraints from Zircon U–Pb and Muscovite Ar–Ar Dating." Minerals 9, no. 12 (December 11, 2019): 773. http://dx.doi.org/10.3390/min9120773.
Анотація:
The Maozaishan Sn deposit, located south of the Dayishan ore field in the Nanling Range, is a newly explored greisen-type Sn deposit. Two muscovite samples from tin-bearing ores yielded 40Ar/39Ar plateau ages of 154.7 ± 1.1 Ma (Mean standard weighted deviation (MSWD) = 0.48) and 152.6 ± 0.7 Ma (MSWD = 0.25), respectively. Zircon U–Pb dating result of fine-grained biotite monzogranite in the Maozaishan mining area shows that these zircon grains can be subdivided into two populations, with ages of 154.2 ± 2.0 Ma (MSWD = 0.51) and 159.6 ± 1.9 Ma (MSWD = 0.09), respectively, indicating that the monzogranite is formed by a multi-stage magmatic event. It is indicated that formation of the Maozaishan Sn deposit is closely related to the Middle Jurassic granitic magmatism. Based on the trace element compositions of zircon grains, the calculated magma temperatures and oxygen fugacity (log(fO2)) values range from 638 °C to 754 °C (mean = 704 °C) and from −18.9 to −15.8 (mean = −17.1), respectively. In addition, these intrusive rocks in the Dayishan ore field belong to highly fractionated granites and are characterized by low oxygen fugacity and crust–mantle origin, which are consistent to these tin-bearing granites in the Nanling Range and in favor of the Sn mineralization.
46
Michel, Véronique, Guanjun Shen, Chuan-Chou Shen, Chung-Che Wu, Chrystèle Vérati, Sylvain Gallet, Marie-Hélène Moncel, Jean Combier, Samir Khatib, and Michel Manetti. "Application of U/Th and 40Ar/39Ar Dating to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic Site in Ardèche, France." PLoS ONE 8, no. 12 (December 5, 2013): e82394. http://dx.doi.org/10.1371/journal.pone.0082394.
47
Zhang, Qian W. L., Zhen M. G. Li, Meng-Yan Shi, Yi-Chao Chen, Jia-Hui Liu, and Chun-Ming Wu. "40Ar/39Ar dating of hornblende and U-Pb dating of zircon in the Aketashitage orogen, NW China: Constraints on exhumation and cooling in the Paleoproterozoic." Precambrian Research 352 (January 2021): 106018. http://dx.doi.org/10.1016/j.precamres.2020.106018.
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Le Carlier De Veslud, Christian, Paul Alexandre, Michel Cuney, Gilles Ruffet, Alain Cheilletz, and Denis Virlogeux. "40 Ar/39Ar thermochronology and thermal evolution of Meso-Variscan granitoids of the Charroux-Civray plutonic complex (Seuil du Poitou)." Bulletin de la Société Géologique de France 175, no. 2 (March 1, 2004): 147–56. http://dx.doi.org/10.2113/175.2.147.
Анотація:
Abstract The eastern part of the “Seuil du Poitou” area had been selected by the French National radioactive waste management agency (ANDRA) as a potential site for building an underground laboratory in granitic rocks. 17 cored bore holes, completed by petrographical, geochemical [Cuney et al., 2001], geophysical [Virlogeux et al., 1999] and structural [Gros and Genter, 1999] studies, have provided a detailed knowledge of the Charroux-Civray complex, hidden under a Jurassic sedimentary cover. Three main types of magmas were distinguished : medium-K calk-alkaline, high-K calk-alkaline and peraluminous. The first two types are largely predominant and belong to the “Limousin Tonalitic line” (LTL) [Peiffer, 1985 ; 1986]. They were emplaced between 356 ± 5 Ma to 349 ± 5 Ma from U-Pb dating of zircon [Bertrand et al., 2001], at a depth of 14 ± 2 km [Freiberger et al., 2001]. This work aims to reconstruct the thermochronological evolution of the Charroux-Civray complex from 40Ar/39Ar dating of biotite and amphibole, combined with available U/Pb ages [Bertrand et al., 2001] and thermo-barometric data derived from mineral paragenesis and fluid inclusions [Freiberger et al., 2001]. This reconstruction may provide interesting constraints on a stage not well understood in the evolution of the French Massif Central : the emplacement of the LTL granitoids. The datings were performed on alteration-free, single grain of biotite and amphibole from the main petrologic types, according to the procedure described by Ruffet et al. [1991] and [1995]. The closure temperatures of the isotopic systems have been assumed to be 300 ± 30 oC for biotites, 500 ± 50 oC for amphiboles, and 850 ± 50 oC for zircons [Villa, 1998 ; McDougall and Harrison, 1999]. Six samples were dated : two tonalites (samples 112 and 212t), a monzogranite (sample 106), a monzogabbro-diorite (sample 115), a monzodiorite (sample 104), and a granodiorite (sample 105). Some of the analyses have been performed twice to test the reproducibility of the 40Ar/39Ar measurements. The 14 age spectra obtained may be divided into four groups : plateau ages, which provide robust ages for the amphiboles of samples 104, 106, 112, and biotites from samples 106, 115 and 212t ; pseudo-plateaux ages : three biotites (samples 104, 105 and 112) display spectrum shapes that could be interpreted as resulting from 39Ar recoil, related to an incipient chloritisation [Ruffet et al., 1991 ; McDougall and Harrison, 1999]. The most reliable ages are therefore close to the apparent ages given by the high temperature steps ; 40Ar* excess, as suggested by the spectrum shape of the amphibole from sample 212t [McDougall and Harrison, 1999]. The preferred age is defined with 83 % of released gas, and has been confirmed by a duplicate analysis ; a meaningless spectrum has been obtained on the amphiboles from sample 115. A duplicate analysis provided an approximate age of 347 ± 1 Ma, calculated on a relatively flat segment of the age spectrum. These results show that : (1) the closure of the isotopic system of the amphiboles occurred at approximately the same time over the entire complex (about 348 Ma) ; (2) the closure of the isotopic system of the biotites occurred slightly after the closure of the amphiboles, but spread over a larger time interval (350–343 Ma), (3) all the samples display high temperature gradients between 500 and 300 oC (> 40 oC.my-1). These results are in good agreement with mineralogical and fluid inclusion thermo-barometric data [Freiberger et al., 2001]. Two scénarios may be invoked to explain such high temperature gradients : a fast exhumation episode (several mm/y) during the 350–340 Ma period. This model is not acceptable because it is incompatible with pre- and post-intrusion conditions constrained by thermo-barometric data ; a fast thermal equilibration of the complex with surrounding rocks at the end of a succession of nearly-synchronous emplacement of calk-alkaline intrusions. First-order numerical models were used to simulate the thermal equilibration of the intrusive bodies with surrounding rocks, assuming a purely conductive heat regime [Carslaw and Jaeger, 1959]. These models show that according to the size of intrusions, the thermal equilibrium with surrounding rocks is reached in less than 5 to 10 m.y. The calculated temperature gradients derived from these models are compatible with those deduced from 40Ar/39Ar ages. These data confirm the existence of a major calk-alkaline magmatic event on the Seuil du Poitou, at about 355–350 Ma, which would be synchronous with the emplacement of the large peraluminous Guéret-type granodiorites in the northern Limousin. The 40Ar/39Ar biotite ages indicate that a regional temperature of 250–300 oC was reached at ca. 340 Ma at a depth of about 9 km.
49
Pochon, Anthony, Yannick Branquet, Eric Gloaguen, Gilles Ruffet, Marc Poujol, Philippe Boulvais, Charles Gumiaux, et al. "A Sb ± Au mineralizing peak at 360 Ma in the Variscan belt." BSGF - Earth Sciences Bulletin 190 (2019): 4. http://dx.doi.org/10.1051/bsgf/2019004.
Анотація:
40Ar/39Ar absolute dating on tobelite (an ammonium-rich white mica) has been performed in order to provide geochronological constraints on the Sb ± Au mineralization and hydrothermalism at the Saint-Aubin-des-Châteaux base metal-Sb ± Au occurrence (Variscan Central Armorican Domain, France). The results show that the Sb ± Au deposition occurred atca.360 Ma. Coupled with recent results obtained in neighboring areas, this occurrence seems to belong to a large-scale Early Carboniferous economic Sb ± Au mineralizing peak in the southeastern part of the Central Armorican Domain. The emplacement of a coeval widespread mafic magmatism in the region appears to represent a major trigger for this mineralizing system at shallow depths (less than 3 km). In the light of these new data, this Early Carboniferous mafic magmatic event must be considered for the overall understanding of the genesis of mineralizing systems at the scale of the whole Variscan belt. Finally, at Saint-Aubin-des-Châteaux, evidence of an Early Permian hydrothermal event is also reported through LA-ICP-MS U-Pb dating of fluorapatite, arguing for the re-use and re-opening of Early Carboniferous mineralizing plumbing system by late (i.e.Permian) fluid flow pulses although no metal mobilization was associated with this event.
50
Gutiérrez-Navarro, Rodrigo, Elisa Fitz-Díaz, José Rafael Barboza-Gudiño, and Daniel F. Stockli. "Shortening and exhumation of Sierra de Catorce in northeastern Mexico, in light of 40Ar/39Ar illite dating and (U-Th)/He zircon thermochronology." Journal of South American Earth Sciences 111 (November 2021): 103334. http://dx.doi.org/10.1016/j.jsames.2021.103334.