Upper-intercept Age Research Articles

Geochemical and geochronological evidence of meteorite impact excavating the Archean basement at Lonar Crater, Central India

The effect of meteorite impact on basement rocks remains widely debated at Lonar Crater, Central India. We investigate an impact melt-bearing rock from the ejecta layer of Lonar to assess the potential interaction of the projectile with the underlying Archean basement. The investigated sample is a melanocratic, aphanitic melt rock comprising glassy inclusions, and plagioclase and quartz clasts. Plagioclase and quartz show shock-induced features including diaplectic glass and well-developed planar deformation features (PDFs), respectively. The decrease in shock pressure is thus established with the plagioclase of overlying basalt showing shock features like diaplectic glass whereas the quartz from lower Archean basement showing PDF development. Laser Raman spectroscopy reveals the presence of cristobalite. The impact melt-bearing rock shows major element geochemistry similar to that of Deccan basalt with a tholeiitic trend, but the Harker variation plots reveal an independent field for the melt rock between the two end members: Deccan basalt and Archean basement represented by Peninsular gneiss. The REE geochemistry also reveals more fractionation for the melt rock. Zircon U-Pb data indicate an upper intercept age around 3.0 to 3.1 Ga. Chondrite normalized zircon REE data display a positive Eu anomaly indicating a reheating during the impact event. This indicates that the melt rock sample was generated by the incorporation of Archean basement gneiss components, which is compatible with the estimated depth of excavation for an impact structure of this size. This specifies a depth to basement in the range between 522 and 570 m.

The Troia–Pedra Branca mafic–ultramafic complex, Borborema Province, Brazil: Record of 2.04 Ga post–collisional Alaskan–type magmatism and PGE mineralization

The Troia–Pedra Branca complex is the most extensive exposure of mafic–ultramafic rocks in the Borborema Province, northeastern Brazil. These rocks have been known for a long time, particularly because of their platinum group element (PGE) mineralization associated with chromitites. The Troia–Pedra Branca complex consists of a succession of serpentinites (after dunite), metachromitites, metaperidotites, hornblendites and metagabbros. PGE–bearing metachromitites are hosted by the serpentinite–peridotite unit, occurring as dispersed blocks, well preserved from weathering. However, host metadunites are poorly preserved, and are only accessed by drill core samples. Scanning Electron Microscopy (SEM) analysis in thin sections of selected metachromitite samples revealed that most of the platinum group minerals (PGM) occur in the chlorite–serpentine matrix, generally in contact with chromite grains. The main PGM are sperrylite (PtAs2), cooperite (PtS), irarsite ((Ir,Pt,Rh)AsS) and hollingworthite ((Rh,Pd,Pt,Ru)AsS). Within chromite grains, very few PGM were found, and sulfide inclusions are mainly chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8) and bornite (Cu5FeS4). Whole–rock geochemical data reveal that metagabbros are LILE–enriched and show subduction–related signature similar to that of Alaskan–type intrusions. Mineral chemistry of chromite and olivine is also compatible with arc–related Alaskan–type complexes. The U–Pb SHRIMP zircon age for a metachromitite sample yielded an upper intercept age of 2036 ± 27 Ma, which we interpret as the crystallization age. However, dispersion in the data implies that zircons lost variable amounts of radiogenic Pb at around 749 ± 54 Ma (lower intercept), which may be related to Neoproterozoic metamorphism. The age obtained for the Troia–Pedra Branca metachromitites (2036 Ma) is younger than the 2190–2130 Ma arc–related plutons of the area, and it is closely related in age to the 2.10–2.04 Ga syn– to late–collisional plutonism and high–temperature metamorphism. Therefore, this Alaskan–type mafic–ultramafic magmatism may be related to the post–collisional setting of the 2.2–2.0 Ga Eburnean/Transamazonian orogeny.

Corundum (ruby) growth during the final assembly of the Archean North Atlantic Craton, southern West Greenland

We investigate the petrogenesis and tectonic significance of corundum (ruby)-bearing late Neoarchean rocks from the Maniitsoq region of southern West Greenland using petrology, thermobarometry, phase equilibrium modelling, and carbon isotopes as well as U–Pb zircon and rutile geochronology. Corundum was generated by amphibolite-facies metamorphism of an aluminosilicate schist. Thermobarometry and phase equilibrium modelling constrain the P–T conditions of corundum growth to ~ 600–700 °C at ~ 4–7 kbar. The presence of graphite suggests corundum growth in the presence of a mixed hydrous–carbonic fluid; this fluid was responsible for the transfer of SiO2 out of the precursor rock generating quartz-undersaturated conditions that enabled corundum growth. Carbon isotope values of graphite from corundum-bearing schist indicate a biogenic origin and are inconsistent with the fractionation of non-biogenic carbon. Zircon U–Pb geochronology documents regional metamorphism at c. 2.72–2.60 Ga. U–Pb rutile geochronology yields an upper intercept age of c. 2.5 Ga, which is interpreted as the best age estimate of corundum growth. The growth of corundum in southern West Greenland reflects heating associated with post-tectonic magmatism after the assembly of the North Atlantic Craton.

Ocean Plate Stratigraphy of a long-lived Precambrian subduction-accretion system: The Wutai Complex, North China Craton

Ocean Plate Stratigraphy (OPS) is the travelogue of the oceanic lithosphere from its birth at the Mid Ocean Ridge to its demise at the subduction zone in convergent plate margins. Here we present an OPS succession preserved onland in the Wutai Complex of the North China Craton (NCC), representing a prolonged subduction-accretion-collision system and continental growth during the Neoarchean-Paleoproterozoic. The OPS succession in Wutai includes metabasalts and banded iron formation representing the pelagic sequences together with intercalated metacarbonates. Phyllite, greenschist, and mica schist represent the hemipelagic sequence. Quartzite and conglomerate correspond to the turbidite sequence, with a progressive southward accretion towards a magmatic arc in the north. We present results from petrological, geochemical, zircon U-Pb, REE, and Lu-Hf studies from representative rocks in the OPS sequence of the Wutai Complex. The trace elements including the rare-earth element (REE) data of the metabasalt show transitional features between normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB). The REY (lanthanide elements and Y) data of the banded iron formation (BIF), greenschists, and phyllite suggest the interaction of high-temperature hydrothermal fluids and seawater. Magmatic zircon grains from the quartzite show 206Pb/207Pb upper intercept age of 2443 ± 21 Ma, whereas zircon grains in the phyllite show upper intercept age of 2494 ± 32 Ma, with minor xenocrysts with older ages of 2726 ± 46 Ma and 2672 ± 48 Ma. Zircon grains in the metaconglomerate show upper intercept age of 2569 ± 32 Ma. Those from the siliceous and intercalated bands in the BIF yield upper intercept ages of 2583 ± 34 Ma and 2524 ± 49 Ma. The zircon Lu-Hf data on zircon grains from the BIF, phyllite, quartzite, and metaconglomerate all show positive εHf(t) values (+1.3 to +7.4) in the range of depleted mantle to new crust, suggesting Neoarchean intra oceanic arc subduction. Combined with the zircon U-Pb and Hf isotopic data previously reported from the Wutai Complex, we infer the following four tectonic evolution stages: ca. 2800 Ma subduction initiation, ca. 2750–2300 Ma arc-accretionary complex, ca. 2300–2100 Ma back-arc rifting, and ca.1900–1750 Ma arc-continent collision. The metavolcanics and part of the metasedimentary rock assemblages in the Wutai Complex represent a deformed and dismembered OPS sequence generated through northward subduction and closure of the Wutai Ocean.

Insights into characterization and genesis of the Tieshanmiao banded iron formation deposit, China: Evidence from zircon U–Pb dating and geochemistry

The Tieshanmiao hosted by the metamorphosed Paleoproterozoic Upper Taihua Group, is a large-scale BIF–hosted Fe deposit in the southern margin of the North China Craton. The formation age, BIF type, depositional condition as well as the genesis of pyroxene–rich BIF is under heated debate. In this study, we present petrology, whole–rock geochemistry, zircon U–Pb geochronology and Lu–Hf isotope analyses on the Tieshanmiao Fe deposit. The Fe ore of the deposit can be divided into two sub-types, including banded pyroxene–magnetite quartzite (BMQ) and disseminated magnetite pyroxenite (DMP). The hanging wall rock pyroxene biotite gneiss yields an upper intercept age of 2406 ± 24 Ma, interpreted as the lower limited age for BIF deposition. The foot wall rock garnet–bearing biotite plagioclase gneiss yields a weighted mean 207Pb/206Pb age of 2739 ± 12 Ma, interpreted as its maximal depositional age. Lu-Hf isotope systematics indicates that pyroxene biotite gneiss is derived from the Paleoproterozoic mantle contaminated by the crust, and the provenance of garnet–bearing biotite plagioclase gneiss is derived from a Mesoarchean crust.The primary mineral assemblages of the pyroxene–rich Tieshanmiao Fe deposit are Fe–silicate, Ca/Mg–carbonate, ferrihydrite, and minor quartz. The BMQs have high and variable Y/Ho ratios (40–80), recording periodic alkaline hydrothermal injection that is progressively diluted by seawater, while the DMPs have low and constrained Y/Ho ratios, archiving well–supplied alkaline hydrothermal injection. We propose a REY (lanthanide elements and Y) modelling of multi–Fe sources for the Tieshanmiao BIF deposit. The BMQs are sourced from seawater and high–T hydrothermal fluid, while the DMPs are sourced from seawater, high–T hydrothermal fluids and one or several additional Fe fluxes (e.g. high-Fe dissolved riverine fluxes, low–T hydrothermal fluids or benthic Fe shuttles driven by dissimilatory iron reduction).The basic gneisses as metabasalts associated with the Tieshanmiao Fe deposit are characterized by high–Fe assay, variable LREE enrichment, relatively depletion in Nb, Ta, Zr, Ti, Y, and Th. The geochemistry of the basic gneisses shows affinities to both MORBs and volcanic–arc basalts, indicating a back–arc basin setting of the BIF deposition. It is most likely a Superior–type BIF–hosted Fe deposit, which is formed in a back–arc basin along the craton–side continental shelf.

Petrogenesis and tectonic implications of TTG granitoids from the Daqingshan Complex of the Khondalite Belt, North China Craton

Located in the Western Block of the North China Craton, the Khondalite Belt is one of the three Paleoproterozoic tectonic belts that were linked to the final assembly of the craton. At present, a popular model is that the Khondalite Belt was formed by the collision between the Yinshan and Ordos blocks at ∼1.95 Ga. However, the initiation of oceanic subduction and its related arc magmatism and accretionary process before the collisional event were poorly constrained. The Daqingshan Complex is located in the middle East part of the Khondalite Belt, and contains highly deformed and metamorphosed rock assemblages, and thus represents a key area to decipher the above issue. In this study, we carried out petrological, geochemical and geochronological analysis on the TTG granitoids of the Daqingshan Complex. Zircon U-Pb results from three typical TTG samples yielded upper intercept ages of 2545 ± 50 Ma, 2484 ± 68 Ma and 2452 ± 32 Ma, indicating that the TTG granitoids were emplaced in the late Neoarchean. Metamorphic zircons from two samples gave ²⁰⁷Pb/²⁰⁶Pb weighted mean ages of 1892 ± 53 Ma and 1906 ± 27 Ma, respectively, recording the timing of a continent-to-continent collisional event. Thirteen TTG granitoid samples are geochemically low-, medium- and high-K calc-alkaline, with metaluminous to peraluminous trends and are enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, La, Ce, Nd, and depleted in high field strength elements (HFSEs) such as Nb and Ta. Chondrite-normalized rare earth element (REE) patterns show fractionation with (La/Yb) _N ratios ranging from 8.20 to 27.47, with weak Eu negative anomalies (δEu = 0.50 – 0.98). In addition, TTG granitoids of the Daqingshan Complex belong to I-type granitoids, and their igneous protoliths were intimately related to a subduction-related magmatic arc environment. New results of this study reveal that the initial oceanic lithosphere subduction operated since ∼2.55 Ga along the southern margin of the Yinshan Block, and generated the coeval arc-related TTG granitoids. Closure of the ocean led to the final collision between the Yinshan and Ordos blocks and the amalgamation of the Western Block at 1.95 to 1.85 Ga.

Geology of the Mesoproterozoic Pillar Lake Volcanics and Inspiration Sill, Armstrong, Ontario: evidence of early Midcontinent Rift magmatism in the northwestern Nipigon Embayment

The Midcontinent Rift System of North America is a ∼1.1 Ga large igneous province comprising mainly flood basalts and intrusive rocks. We present new data for the Pillar Lake Volcanics and Inspiration Sill from the northern edge of the Midcontinent Rift in the northwestern Nipigon Embayment. The Pillar Lake Volcanics comprise a ∼20–40 m thick, flat-lying sequence of mafic pillowed and massive flows, pillowed flow breccia, and hyaloclastite breccia. They are characterized by SiO2 of 52–54 wt%, TiO2 of 1.2–1.3 wt%, and K2O of 0.9–1.1 wt%. They are light rare earth element (LREE) enriched, with La/Smn of 3.0–4.4 with fractionated heavy rare earth elements (HREE) (Gd/Ybn = 1.4–1.7). The Inspiration diabase sill is ≤ 50 m thick and is in direct contact with the underlying Pillar Lake Volcanics. Baddeleyite and zircon data from the Inspiration Sill yield a combined U–Pb upper intercept age of 1105.6 ± 1.6 Ma. The Inspiration Sill is characterized by uniform SiO2 of 52–53 wt%, TiO2 of 1.1–1.2 wt%, and K2O of 0.9–1.2 wt%. Inspiration Sill samples are LREE enriched with La/Smn of 3.2–3.3 and fractionated HREE (Gd/Ybn = 1.6). The Pillar Lake Volcanics are at least 1120 Ma, and perhaps as old as 1130 Ma, and represent an early, thin, and restricted mafic volcanic sequence, largely preserved below the younger Inspiration Sill. The Pillar Lake Volcanics and Inspiration Sill display a marked geochemical similarity, suggesting that they may represent magmatism associated with the earliest stages of Midcontinent rifting.

Extent and age of Mesoarchean components in the Nagssugtoqidian orogen, West Greenland: Implications for tectonic environments and crust building in cratonic orogenic belts

The Nagssugtoqidian orogen (NO), which forms the northern margin of the North Atlantic craton in West Greenland, is largely comprised of ~2870 to 2720 Ma juvenile continental crust reworked during Paleoproterozoic orogenic events. Sparse evidence of components older than 3100 Ma in the area (U–Pb zircon ages and bulk-rock Sm–Nd isotopes) have hinted at a unit called the Qorlortoq gneiss but were not substantiated by regional studies. Here we report the “rediscovery” of the Qorlortoq gneiss, documented through a new geochemical and geochronological dataset of this orthogneiss and related components, which include ultramafic cumulate enclaves and late basaltic dykes. Laser Ablation Split Stream (LASS) U-Pb/Lu-Hf isotopic analyses of zircon from the gneiss give a U–Pb concordia upper-intercept age of 3177 ± 12 Ma and weighted mean εHf(t) of 1.7 ± 0.5, indicating a juvenile protolith extracted from a depleted mantle source.Ultramafic enclaves of cumulate origin make up a key component of the Qorlortoq gneiss. Their trace element systematics indicate a parental melt with tholeiitic affinities, sourced from depleted mantle beneath relatively thin lithosphere (<80 km), that was subsequently contaminated by crustal assimilation. Re–Os isotope systematics of the cumulates and their relationship with the younger Qorlortoq dykes place the age of these cumulates close to that of the Qorlortoq gneiss.The age of the Qorlortoq gneiss makes it the oldest known component in the NO by ~300 Ma. The extended timescale between episodes of crustal production in this area, could indicate a geodynamic environment characterized by stagnant-lid or episodic mobile-lid tectonics, at least on a regional scale. These constraints provide important information about the diversity and nature of Archean geodynamic environments.

Chicheng high-pressure granulites record the paleoproterozoic tectonic evolution in the northern North China Craton

High-pressure (HP) mafic granulites are widely distributed along the Trans-North China Orogen (TNCO), and they provide important insights into the timing and tectonic processes involved in the formation of the TNCO. We discovered three types of HP granulite, including mafic, intermediate, and felsic granulites, exposed in Chicheng area, in the northern segment of the TNCO. Their mineral assemblages of prograde, peak, and post-peak metamorphic stages were identified based on petrographic investigations. SIMS U-Pb zircon analyses yielded an upper intercept age of 2547 ± 38 Ma and a lower intercept age of 1916 ± 100 Ma for one HP mafic granulite and a discordant 207Pb/206Pb age of 2479 Ma with a weighted mean 207Pb/206Pb age of 1905 ± 9 Ma for the other HP mafic granulite. For the HP intermediate granulite, an upper intercept age of 2544 ± 20 Ma was obtained. The HP felsic granulite yielded an upper intercept age of 2524 ± 23 Ma and a concordant 207Pb/206Pb age of 1906–1907 Ma. These results reveal that the protoliths of HP granulites were formed at ~ 2.5–2.55 Ga and underwent high pressure granulite facies metamorphism at ~ 1.9 Ga. The elevated Hf isotopic compositions of the metamorphic zircons from two granulite samples indicate that metamorphic fluids that have high 176Hf/ 177Hf ratios but low 176Lu/177Hf ratios were involved in their metamorphism. The low 18O isotopic features of the zircons from the intermediate granulite suggest alteration by δ18O-depleted seawater or meteoric fluids at high temperatures. By integrating geochronological data from previous studies, we infer that the formation of the TNCO may have begun as early as ~ 1.9 Ga and lasted until ~ 1.8 Ga, supporting the hypothesis that it was formed by a long-lived tectono-metamorphic event

Polymetamorphic events in the Jiao-Liao-Ji Belt, North China Craton：Evidence from integrated zircon, xenotime, and monazite LA–ICP–MS U–Pb dating

ABSTRACT Poly-metamorphism has been observed in many Precambrian metamorphic terranes. The metamorphic overprinting in the poly-metamorphic rocks makes it difficult to determine the timing and P − T path of each metamorphic event. In this study, in situ U–Pb dating of zircon, xenotime, and monazite, with simultaneously determination of trace element compositions, is utilized to provide new constraints on the timing of discrete metamorphic events involving reactivation of the Jiao-Liao-Ji Belt, the eastern edge of the North China Craton. Detrital zircons from metasandstones in the Gaixian Formation record a mixed provenance that includes late Neoarchean (ca. 2532 Ma) and middle Paleoproterozoic (ca. 2181 Ma). Xenotimes from the Gaixian pelitic schists yield an upper intercept age of 1906 ± 25 Ma, and this age is interpreted as the timing of greenschist-facies regional metamorphism. However, monazites from the same pelitic schists yield a distinctly younger age of 242 Ma, with late Paleoproterozoic inherited ages of 1894 and 1809 Ma, which are broadly consistent with the xenotime ages. In addition, magmatic zircons from a granitic mylonite vein yield a similar age of 245 Ma, making it coeval with the monazite. The interpretation of these ages is also reinforced by the mineral assemblage of inclusions in the dated accessory mineral grains. These new geochronological data, combined with published data from the Jiao-Liao-Ji Belt and the related regions, suggest that the Gaixian Formation in the Yanghe area was likely some time deposited at 2181–1900 Ma, and experienced both late Paleoproterozoic and Triassic low-grade metamorphic events related to two distinct orogenic phases in the eastern North China Craton.

Paleoproterozoic arc magmatism in the southern Amazonian Craton, Brazil: Constraints from geology, geochemistry, and geochronology of granitic rocks

This work presents new geological, geochemical, and geochronological data of Paleoproterozoic granites located in the south-central sector of the Amazonian Craton. This region falls within the Peixoto de Azevedo Domain in the southern Tapajós-Parima Tectonic Province. The results here presented are of the utmost importance in advancing geological knowledge of the Alta Floresta Gold Province. The study area encompasses granitic rocks of the Matupá Intrusive Suite, and of the newly individualized Teles Pires Intrusive Suite. The latter includes two granitic bodies that have been previously ascribed to the Matupá Intrusive Suite. The Matupá Intrusive Suite was divided into two facies: Facies 1, composed of syenogranites and monzogranites, and Facies 2, composed of granodiorites. Granodiorites of Facies 2 have chemical characteristics similar to that of magnesian and metaluminous granites, while syenogranites and monzogranites of Facies 1 are ferrous and slightly peraluminous. Petrographic and geochemical features demonstrate the calc-alkaline nature of the Matupá Intrusive Suite which is typical for I-type granites from of volcanic arc settings. Magmatic zircon crystals yielded U–Pb LA-ICP-MS ages at 1864 ± 19 Ma and 1859 ± 8 Ma for syenogranite and monzogranite of Facies 1, respectively, and an age of 1881 ± 5 Ma for granodiorite of Facies 2. These are the crystallization ages for intrusive bodies of the Matupá Intrusive Suite. The Teles Pires Intrusive Suite varies from syenogranite to monzogranite whose calc-alkaline signature can be linked to the setting of a mature magmatic arc. U–Pb zircon in situ dating with LA-ICP-MS yielded upper intercept ages at 1790 ± 6 Ma and 1793 ± 7 Ma which are interpreted as its magmatic crystallization. Besides these crystallization ages, inherited zircon crystals also provided an older intercept age of 1870 ± 14 Ma, which is correlatable with the Matupá Intrusive Suite. Crustal assimilation and/or contamination are suggested for both the Matupá and Teles Pires intrusive suites due to the presence of inherited zircon populations of ca. 1960 Ma and 1900 Ma, and ca 1896 Ma and 1880 Ma, respectively. These ages are coherent with the Creporizão (Creporizão orogenic) Intrusive Suite, Tropas Intrusive Suite (Tropas orogenic), and Parauari Intrusive Suite (Parauari orogenic), which are all part of the Tapajós Domain. The two intrusive suites outcropping in the study area are therefore attributed to the Matupá Intrusive Suite, Peixoto de Azevedo Domain, and to the Teles Pires Intrusive Suite, Juruena Domain, which in turn is part of the Rondônia-Juruena Tectonic Province.

Zircon U–Pb age of Paleoproterozoic granite gneiss from the Ogcheon Belt in the Geumsan area, South Korea

A Paleoproterozoic granite gneiss body was discovered in the Jurassic Daebo granite of the Ogcheon Belt, Geumsan area, South Korea. The granite gneiss is a two-mica variety with a small amount of garnet, and has a peraluminous bulk-rock composition. U–Pb isotope analysis of zircon grain cores by LA-ICP-MS yields a 207Pb/206Pb magma crystallization age of ca. 2.5 Ga. The granite gneiss body is therefore one of the second oldest rocks in South Korea. Zircon grain mantle and rim data yield an upper intercept age of ca. 1.89 Ga and a lower intercept age of 174±74 Ma along the discordia line, which are interpreted to be metamorphic ages related to Paleoproterozoic and Jurassic magmatism, respectively.

Ca 2.85 Ga Nb‐enriched mafic dikes from the southern Huangling dome of the Yangtze Block, southern China: Implications for Mesoarchean subduction zone processes

In this study, we report the first occurrence of Archean Niobium‐enriched mafic dikes in the southern Huangling dome within the Yangtze Block, southern China. Zircon U–Pb dating of three mafic dike samples yielded weighted mean 207Pb/206Pb ages of 2,858 ± 24 Ma (MSWD = 1.14), 2,889 ± 11 Ma (MSWD = 0.98), and 2,840 ± 11 Ma (MSWD = 0.17), respectively, which are interpreted as the intrusion ages of the dikes. Zircons from the host granodioritic gneiss show different trace element patterns and variable Eu anomalies, and yield an upper intercept age of 2,891 ± 32 Ma. On the basis of previous studies, the 2,891 Ma age is interpreted as the metamorphic recrystallization time caused by late tectono‐thermal events. The mafic dikes possess high Nb contents ranging from 4.7 to 18.4 ppm, which are similar to those reported from Archean, Proterozoic, and Phanerozoic Nb‐enriched basaltic rocks occurring worldwide. The Nb‐enriched mafic dikes show HFSE‐depleted patterns with significant negative Zr, Hf, and Ti anomalies, and higher Nb/U (8.29–48.42), Nb/Thpm (0.59–1.71), and Nb/Lapm (0.76–1.34) ratios than “normal” island arc volcanic rocks that are consistent with a subduction zone geochemical affinity. Positive εNd(t) values (+5.0 to +11.8) of the Nb‐enriched mafic dikes point to an extended depleted mantle characterized by elevated Sm/Nd ratio, whereas εNd(t) values (+0.3 to −2.3) and tectonic environment of the host granodioritic rocks, as well as the most widely accepted petrogenetic models of the TTG series indicate enriched mafic crustal sources. Therefore, we propose that the Nb‐enriched mafic dikes were generated from a depleted mantle with an extended history of elevated Sm/Nd ratio that was metasomatized by slab‐derived melts at ca. 2.90–2.85 Ga.

Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based onP-T-tpaths of the eclogite-bearing Baqing metamorphic complex

AbstractSubduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex.The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines in the massive chromitite, and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former is characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter shows a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain variable PGE abundances (ΣPGE = 8-244 ppb) with enrichments of Os, Ir and Ru, relative to Pt and Pd . These features suggest that the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting, , which underwent crustal contamination. The ReOs isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with UPb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of an early stage of metamorphism and hydrothermal alteration. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.

K-Ar fault-gouge dating in the Lower Buller gorge constrains the formation of the Paparoa Trough, West Coast, New Zealand

ABSTRACT K-Ar dating of fault gouge from the intersection of the WNW-striking Ohika Detachment of the Paparoa Metamorphic Core Complex and a NNE-striking high-angle normal fault (Ohikaiti Fault) yielded an age of 103 ± 3.5 Ma (1σ uncertainty) for metamorphic white mica and an age of 35.2 ± 2.3 Ma for fault-gouge formation. The K-Ar four-point isochron is well defined and the upper-intercept age of 103 ± 3.5 Ma is inferred to reflect growth of metamorphic white mica in the footwall of the Paparoa Metamorphic Core Complex. We relate the fault gouge at the Ohikaiti Fault to the formation of the Paparoa Trough in the latest Eocene and the deposition of the Rapahoe Group. The NNE-striking Ohikaiti Fault either reactivated a favourably oriented segment of the Ohika Detachment or cut the detachment and displaced it when the Paparoa Trough formed. We discuss a model of relatively modest deformation in the Paparoa Trough as part of the Challenger Rift System because the northern West Coast region was close to the pole of plate rotation in the late Eocene and Oligocene.

Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U–Pb thermal ionization mass spectrometry (FIB-TIMS)

Abstract. Baddeleyite is a powerful chronometer of mafic magmatic and meteorite impact processes. Precise and accurate U–Pb ages can be determined from single grains by isotope dilution thermal ionization mass spectrometry (ID-TIMS), but this requires disaggregation of the host rock for grain isolation and dissolution. As a result, the technique is rarely applied to precious samples with limited availability (such as lunar, Martian, and asteroidal meteorites and returned samples) or samples containing small baddeleyite grains that cannot readily be isolated by conventional mineral separation techniques. Here, we use focused ion beam (FIB) techniques, utilizing both Xe+ plasma and Ga+ ion sources, to liberate baddeleyite subdomains directly, allowing their extraction for ID-TIMS dating. We have analysed the U–Pb isotope systematics of domains ranging between 200 and 10 µm in length and from 5 to ≤0.1 µg in mass. In total, six domains of Phalaborwa baddeleyite extracted using a Xe+ plasma FIB (pFIB) yield a weighted mean 207Pb∕206Pb age of 2060.1±2.5 Ma (0.12 %; all uncertainties 2σ), within uncertainty of reference values. The smallest extracted domain (ca. 10×15×10 µm) yields an internal 207Pb∕206Pb age uncertainty of ±0.37 %. Comparable control on cutting is achieved using a Ga+-source FIB instrument, though the slower speed of cutting limits potential application to larger grains. While the U–Pb data are between 0.5 % and 13.6 % discordant, the extent of discordance does not correlate with the ratio of material to ion-milled surface area, and results generate an accurate upper-intercept age in U–Pb concordia space of 2060.20±0.91 Ma (0.044 %). Thus, we confirm the natural U–Pb variation and discordance within the Phalaborwa baddeleyite population observed with other geochronological techniques. Our results demonstrate the FIB-TIMS technique to be a powerful tool for highly accurate in situ 207Pb∕206Pb (and potentially U–Pb in concordant materials) age analysis, allowing dating of a wide variety of targets and processes newly accessible to geochronology.

Geochemistry of two types of Palaeoproterozoic granites, and zircon U–Pb dating, and Lu–Hf isotopic characteristics in the Kuandian area within the Jiao‐Liao‐Ji Belt: Implications for regional tectonic setting

Palaeoproterozoic granites accompanied with granulite‐facies gneisses can be recognized in the Kuandian area, northern Jiao‐Liao‐Ji Belt (JLJB). This study highlights two episodes of Palaeoproterozoic magmatism, identified by granites and biotite monzogranitic gneisses herein. Zircon grains from biotite monzogranitic gneiss with homogeneous oscillatory zoning yielded a U–Pb age of 2,153 ± 16 Ma, in accordance with the age of zircon cores from the monzogranitic gneiss. Zircon grains from the two granites yielded upper intercept ages of 1,892 ± 38 Ma and 1,859 ± 36 Ma respectively. Monzogranitic gneiss samples are enriched in Th, Rb, Pb, U, and REE, but depleted in Sr and P. They have high 1,000 × Ga/Al ratios versus Zr, Ce, Nb, Y contents, which are usually interpreted as the features of classical A‐type granite that originated in a lithospheric extension setting. The ~1.89–1.86 Ga granite samples have relatively high Al, Rb, Th, U and low Sr, Zr, and Hf contents. These features, combined with the observation of minor apatite grains under the microscope are similar with the I‐type granites. In situ zircon Lu–Hf isotopic analyses revealed that ~1.89–1.86 Ga granite had a similar range and TDM2 ages with the monzogranitic gneiss, but with much lower εHf(t) values. The monzogranitic gneiss may have originated from an Archean crust, while the later granite may be derived from melting of the partial melting of the monzogranitic gneiss or the Archean crust that generated the monzogranitic gneiss. The ~1.89–1.86 Ga granites, together with the coeval S‐type granites in the JLJB, represent an orogeny process that may involve a Palaeoproterozoic subduction.

Archean block rotation in Western Karelia: Resolving dyke swarm patterns in metacraton Karelia-Kola for a refined paleogeographic reconstruction of supercraton Superia

Rifting, breakup, and subsequent collision related to the ca. 1.92–1.79 Ga Svecofennian orogeny fragmented and deformed the western margin of the Archean Karelia-Kola craton into four crustal blocks: Pudasjärvi, Iisalmi, Kuhmo, and Taivalkoski. Detailed quantification of Svecofennian deformation is limited due to poorly exposed basement geology and an as yet incomplete dyke swarm record.New U-Pb ID-TIMS geochronological results on baddeleyite and zircon are presented for three key mafic dykes from the Pudasjärvi block, namely the Uolevinlehto, Myllykangas, and Sipojuntti dykes. The age of the 325°-trending Uolevinlehto dyke is estimated at ca. 2400 ± 12 Ma from discordant multigrain baddeleyite fractions, showing it to be younger than ca. 2450 Ma dykes across Karelia. The 350°-trending Myllykangas dyke has a minimum age of 2135.2 + 3.6/−3.7 Ma based on chemically abraded zircon. Results from single baddeleyite grains provide a precise upper intercept age of 2128.9 ± 1.2 Ma for the 320°-trending Sipojuntti dyke.Our new U-Pb ages are integrated with those from the literature to define six major dyke swarms in the Pudasjärvi block: the NNE-trending ca. 2.45 Ga Pääjärvi, NW-trending ca. 2.40 Ga Uolevinlehto, NW-trending ca. 2.13–2.10 Ga Tohmajärvi, WNW-trending ca. 2.07 Ga Palomaa, NNW-trending ca. 1.98 Ga Paukkajanvaara, and undated “East-West” dykes. Trends of contemporaneous dyke swarms in the Taivalkoski and Kuhmo blocks, however, are systematically offset by 35°. With subvertical dips, offset dyke swarms record 35° clockwise vertical-axis rotation of the Pudasjärvi block relative to the interior of Karelia, consistent with dextral transpression during the Svecofennian orogeny. Structural restoration of the Pudasjärvi block improves the constraints on regional dyke swarm patterns, and these are used to revise the position of the Karelia-Kola craton within the context of the paleogeographic reconstruction of supercraton Superia.

Mesoarchean gabbro-anorthosite complex from Singhbhum Craton, India

The Singhbhum Craton in Peninsular India preserves some of the oldest continental crust on the globe with Hadean and Paleoarchean zircon record. Here we investigate a gabbro-anorthosite complex from the eastern margin of the craton exposed at Besoi, Kuliana, Sarat and Jashipura areas. These rocks display moderate SiO2 (47.94–53.77 wt%), TiO2 (0.42–2.17 wt%), Al2O3 (8.97–14.96 wt%) MgO (6.14–10.30 wt%) and CaO (10.73–15.07 wt.) at low to moderate Cr (20–862 ppm), Ni (32–636 ppm) and V (81–666 ppm) and are classified as noritic gabbros and gabbros. Their rare earth and trace element patterns exhibit mild to moderate fractionation with coherent patterns (La/Yb = 1.71–14.59), negative to positive Eu anomalies (Eu/Eu* = 0.62–1.44) negative Nb, Ti and sparingly negative to positive ZrHf anomalies. The UPb analysis of zircon grains from the Jashipura gabbros yield upper intercept ages of 3047 to 3099 Ma and the 3047 Ma age is considered to mark the timing of emplacement. Our geochemical data suggest that these rocks were generated through fractional crystallization of magma in a magma chamber at a transitional depth of the spinel-garnet regime with subsequent crustal contamination at an active continental margin supporting a continental arc setting. The gabbro-anorthosite suites in this region are synchronous with the adjacent granites and we propose that the mafic magma provided the heat for crustal melting that generated the granitoids. This magmatic event coincides with the ca. 3.1 Ga widespread crustal growth event in the Singhbhum Craton.