Sensitive High-resolution Ion Microprobe Research Articles

Tectonic implications of a rare metamorphic event in eastern China during the Earth's ‘middle age’

The Mesoproterozoic Era has long been considered a relatively stable, silent and even ‘boring' period in Earth history, during which the lithosphere was tectonically inactive. However, an increasing amount of evidence suggests that metamorphism and magmatism were much more widespread and intense than previously thought, implying that this period was dynamic. Here, we report metamorphism at c. 1.4 Ga along the southeastern margin of the North China Craton in eastern China. We conducted analyses using a TESCAN Integrated Mineral Analyser (TIMA) combined with electron microprobe analysis (EMPA) and zircon sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled plasma mass spectrometry U–Pb dating on garnet amphibolites and their country rocks. The detailed mineral composition of a representative garnet amphibolite sample was identified and determined by TIMA. Phase equilibria modelling using pseudo-sections combined with the EMPA data indicated the peak metamorphism stage (M 2 ) of the garnet amphibolites at a P – T condition of 10.6–11.2 kbar and 820°C. The SHRIMP zircon U–Pb analysis yielded an age of 1366 ± 17 Ma for the garnet amphibolites, which is slightly younger than that of the country rock schist (1376 ± 22 Ma). Integrating the metamorphic event with geochemical signatures, magmatic records and sedimentation patterns in analogous geological settings on other continents, our findings collectively demonstrate a protracted and pervasive orogenic process along the periphery of the Columbia supercontinent.

Generation of Cu-Au skarn mineralization by mafic magma recharge: Insights from the Bozymchak deposit in the Chatkal-Kurama region, Kyrgyzstan, West Tianshan

Large numbers of igneous intrusions are emplaced in the magmatic arcs above subduction zones, but only a small fraction of them are mineralized with Cu ± Au ± Mo. Despite significant advances in recognizing the importance of magma source, volatile content, and redox condition in controlling mineralization, the magmatic processes required to form a mineral deposit are still poorly understood. Here, we demonstrate that mafic recharge to the magma chamber is critical for Cu-Au mineralization, as exemplified by the Bozymchak Cu-Au skarn deposit in the Chatkal-Kurama arc of Kyrgyzstan, West Tianshan. The Bozymchak intrusion associated with the mineralization comprises monzonite porphyry and granodiorite porphyry. A sensitive high-resolution ion microprobe zircon U-Pb age of 305 ± 2 Ma was obtained for the monzonite porphyry, which is consistent with the U-Pb age of zircon from the granodiorite porphyry (304 Ma) and an Re-Os age for the molybdenite (305 Ma). These ages indicate that both the monzonite porphyry and granodiorite porphyry were coeval with mineralization. The monzonite porphyry exhibits high-K calc-alkaline (K2O = 3.36−3.50 wt%), metaluminous and magnesian-rich characteristics, enrichment in large ion lithophile elements (LILEs; e.g., Cs, Th, Ba, K, and Pb), and negative anomalies in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). These geochemical signatures, combined with the Sr-Nd-Hf isotopic signatures [ISr(avg) = 0.7059, εNd(t) = −3.5 to −2.8, εHf(t) = −3.8 to +1.4], suggest that the magma was the product of partial melting of a metasomatized mantle wedge and was subjected to subsequent assimilation and fractional crystallization (AFC). The development of disequilibrium textures in both the monzonite porphyry and the granodiorite porphyry, such as acicular apatite, reverse zoning of plagioclase, and coexisting high-Al and low-Al amphiboles, indicates that the magma evolved in an open crustal magma chamber with mafic magma recharge. The magmas were moderately oxidized (∼ΔFMQ +1.0; FMQ is the fayalite-magnetite-quartz oxygen buffer) and water-rich, with 5.9 wt% and 6.5 wt% H2O in the granodiorite porphyry and monzonite porphyry, respectively. The repeated influx of hot mafic magma batches prolonged the existence of the magma chamber and promoted the maturation of the arc magma. In addition to introducing more Cu and Au, the replenishment of mafic magma in the chamber also introduced additional H2O and Cl, as shown by the presence of high-Al amphiboles and the elevated oxygen fugacity, which further promoted Cu-Au mineralization.

Multi-mineral geochronology reveals a protracted Mesoproterozoic tectonic and metallogenic link between southeastern Australia and Laurentia

Plate tectonic reconstructions place Laurentia and eastern Australia together during the Mesoproterozoic. Specifically, the Belt-Purcell Basin in the USA and Canada has been linked to the Rocky Cape Group in northwest Tasmania based on detrital zircon studies. The Belt-Purcell Basin is highly endowed in economic deposits of Cu, Pb, Zn, Ag and critical metals such as Co, formed during the Mesoproterozoic. The Rocky Cape Group of northwest Tasmania hosts several sediment-hosted Cu-Pb-Zn ± Co prospects, however they were previously thought to be Devonian in age. New laser ablation inductively-couple plasma mass spectroscopy (LA-ICP-MS) in situ U-Pb monazite, xenotime and apatite data, sensitive high resolution ion microprobe (SHRIMP) in-situ U-Pb monazite data, and LA-ICP-MS galena Pb isotope data were collected from these prospects. By using multiple minerals with different U–Pb closure temperatures, we have unravelled a complex and protracted history of thermal pulses in the Rocky Cape region associated with Cu-Pb-Zn ± Co mineralisation. These data indicate that primary sulphide mineralisation formed at c. 1350 Ma, with several resetting or remobilisation events recorded at c. 1250 Ma, c. 1100 Ma and c. 950 Ma. These ages are contemporaneous with previously published authigenic monazite and metamorphic monazite ages from the Rocky Cape Group. This suggests that mineralisation is broadly syn-sedimentary and related to the development of the basin into which the Rocky Cape Group was deposited. These new ages are similar to those of sediment-hosted ore deposits in the Belt-Purcell Basin in the USA and Canada, further supporting the proposed link between northwest Tasmania and North America during the Mesoproterozoic. The high metal endowment of age-equivalent rocks in the Belt-Purcell Basin, highlight the potential for new discoveries of base and critical metals in southeastern Australia.

Timing and geometry of the Chemehuevi Formation reveal a late Pleistocene sediment pulse into the Lower Colorado River

The Chemehuevi Formation is a distinctive 50−150-m-thick wedge-shaped Pleistocene sedimentary unit deposited by the Colorado River. It lines the perimeters of the river’s floodplains and bedrock canyons for more than 600 km between the mouth of the Grand Canyon and the delta region in the Gulf of California. The formation is composed of a basal tan to light-yellowish-brown and pale-orange mud-dominated facies overlain and interbedded by a light-yellow-brown sand-dominated facies. The unit is one of two extensively exposed aggradational packages in the Lower Colorado River corridor, in addition to a series of other smaller alluvial terrace deposits. The Chemehuevi Formation appears to represent the response of a fully integrated Colorado River system to a significant perturbation, in contrast to the Bullhead Alluvium, which is likely a unique result of Pliocene river integration. The aggradation of the Chemehuevi Formation in the Lower Colorado River corridor may be similarly due to a unique event in the Colorado River system, or it may instead be a well-preserved sedimentary sequence recording typical behavior of the Colorado River below the Grand Canyon in the late Pleistocene. As such, multiple causal mechanisms have been proposed, but no study to date has conclusively explained the Chemehuevi Formation. To help resolve its timing, duration, and origin, we applied post-infrared infrared stimulated luminescence, carbonate U-Th series, and zircon sensitive high-resolution ion microprobe U-Th series geochronology to determine the ages of key exposures of the unit over a wide spatial area. These new data demonstrate that the Chemehuevi Formation was deposited ca. 110−90 ka. The depositional ages collectively overlap, suggesting that deposition occurred rapidly relative to the resolution of the geochronometers. The new depositional timing coincides with a shift from glacial to interglacial conditions after the marine isotope stage 5-6 transition. This observation is consistent with a climate-induced sediment pulse as a causal mechanism, yet correlations with similar deposits in the Colorado River headwaters or in neighboring catchments appear elusive. Potentially, climate transitions between glacial and interglacial periods induced a sediment pulse from hillslopes of the Colorado River system that resulted in the Chemehuevi Formation. An alternative or additional explanation is that the Chemehuevi Formation represents release of lava dam−impounded sediment in the Grand Canyon. The surface geometry of the Chemehuevi Formation projects upstream to the approximate location of lava dams, and the largest possible lava dam impoundment (the Upper Prospect dam) is comparable in volume to the formation. The lava dam hypothesis appears to be a possible explanation for the Chemehuevi Formation. However, tying deposition to a specific lava dam or series of lava dams remains challenging due to discrepancies in timing and volume. The combined effects of a series of lava dams may have led to the Chemehuevi Formation, as the last Pleistocene lava dam eruption coincides with the onset of deposition. Alternatively, the formation may result from the combined effects of both regional climate transitions and the lava dams that created a transient reservoir to compound a climate transition−driven sediment pulse. The geochronologic data presented here do not allow us to distinguish between the lava dam or climate transition hypotheses but will need to be reconciled with any future proposed depositional model.

New discovery of 3.84–3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance

Abstract Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838–3754 Ma trondhjemitic gneisses (4 samples), 3786–3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786–3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4–3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4–56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86–1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2–11.3) and lower Eu/Eu* values (0.62–0.79). The potassic granites have (La/Yb)N ratios of 20.7–55.2 and Eu/Eu* values of 0.75–1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3–3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1–3.5 Ga (mainly 4.1–3.8 Ga) and δ18O of 4.6‰–7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.

Detrital zircon ages and proposed provenance of the Koegas Subgroup of the Ghaap Group, and overlying Makganyene Formation, of the Postmasburg Group, Transvaal Supergroup

Abstract The late Archaean to early Palaeoproterozoic strata of the Transvaal Supergroup of southern Africa is renowned for hosting geological units that preserve some the Earth’s most significant geological events. The glaciogenic Makganyene Formation is one such significant unit, given that it has been associated with the Snowball Earth Event. The maximum age of deposition of this formation, and subsequent timing of this event, has come into question, mostly because of concordant detrital zircon ages as young as ~2.2 Ga reported by Beukes et al. (2013). These ages are younger than the recently revised ca. 2.43 Ga baddeleyite age inferred for the overlying Ongeluk Formation and subsequently led to a significant revision of a long-held correlation between the upper Postmasburg- and Pretoria groups of the respective sub-basins of the Transvaal Supergroup. The primary objective of this study is to investigate the mode of occurrence of the ~2.2 Ga zircons in the Makganyene Formation as well as selected formations of the underlying Koegas Subgroup. Here, we report a total of 183 near concordant U-Pb zircon ages for the Koegas Subgroup (Pannetjie- and Naragas formations) and 967 such ages for the Makganyene Formation, extracted from seventeen samples from across the outcrop area for these units. Sensitive High Resolution Ion MicroProbe (SHRIMP) as well as three different Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) techniques were used for U-Pb age measurements. We assessed the quality of the respective data sets and possible shortcomings of the techniques to constrain the maximum age of deposition of the units and to infer possible source areas for the detritus. In contrast to zircon ages determined for the Koegas Subgroup, zircons younger than ~2.4 Ga are a prominent feature of the Makganyene Formation. Upon careful consideration of each data set, we concluded that these so-called young grains have likely suffered significant Pb-loss and that a possible overcorrection for common Pb during data reduction could not be ruled out. Although the age distribution generated using the respective techniques were comparable, none of the four techniques were successful in shedding light on the reliability of the ~2.2 Ga ages. The maximum age of deposition of the Makganyene Formation could not be constrained with confidence and therefore the revised correlation between the Postmasburg- and Pretoria groups is not contested. The detrital zircon age distribution of the Koegas-Makganyene succession was however found to be remarkably similar, with a major ~2.5 Ga age fraction and subordinate fractions at ~2.65 to ~2.9 Ga and older than ~3.0 Ga. The major ~2.5 Ga zircon age fraction points towards a prominent, likely orogenic source of detritus. We assign the origin of the major ~2.5 Ga and older zircon age fractions to a Rae-type craton, which we proposed to have been attached to the north of the Vaalbara Craton at time of deposition of these units and consider a tectono-magmatic event characteristically associated with this family of cratons as the possible cause of alteration and Pb-loss in zircon grains dated at ~2.2 Ga.

Geochronological constraints on the Hangenberg Event of the latest Devonian in South China

The Hangenberg Event was one of the most significant mass extinctions accompanied by large-scale climate changes at the end of the Devonian period. Lack of precise and accurate geochronological constraints in the eastern Paleotethys has hindered a high-resolution global correlation of the Hangenberg Event. Here, we report the U–Pb dates of zircons from the bentonite samples intercalated with strata recording the Hangenberg Event in the Muhua region, Guizhou, South China, using both sensitive high-resolution ion microprobe (SHRIMP) and chemical abrasion - isotope dilution - thermal ionization mass spectrometry (CA-ID-TIMS) methods. A bentonite within the Hangenberg Event strata in the Muhua area was dated at 360.47 ± 0.68 Ma by CA-ID-TIMS, which is more precise than the results obtained by SHRIMP methods (361.5 ± 2.9 Ma and 359.8 ± 1.5 Ma). Thus, we propose that the Hangenberg Event took place at 360.47 ± 0.68 Ma in South China, based on the first precise CA-ID-TIMS zircon age from the Daposhang section. The age obtained from South China for the Devonian–Carboniferous boundary overlaps with that in Geologic Time Scale 2020, within the total uncertainties.

Identical metamorphic record in distinct petrochemical systems: Case study of microscopically interlayered garnet amphibolite and metapelite from the Danba dome, SW China

It is known that different metamorphic rocks may have different abilities in recording metamorphic processes. Here, we report microscopically interlayered garnet amphibolite and metapelite layers in a rock from the Danba dome in eastern Tibetan Plateau, SW China. Both rock types exhibit similar garnet zoning patterns, biotite and plagioclase compositions, as well as peak P-T conditions (7.3–8.0 kbar / 700–725 °C for metapelite and 7.1–8.4 kbar / 690–710 °C for garnet amphibolite, respectively), indicating a global thermodynamic equilibrium state and the same metamorphic record of the two distinct layers. However, medium-sized garnet exhibits homogeneous core compositions and biotite preserves lower Ti and Fe# [= Fe2+ / (Fe2+ + Mg)] at the lithological interface, possibly indicating more intense chemical homogenization at the metamorphic peak and retrograde modification at the interface caused by the possible presence of vast fluid. Sensitive high-resolution ion microprobe (SHRIMP) UPb dating of metamorphic zircon show a wide dispersion of ages (c. 200–155 Ma), overlapping previous age data reported for this region. These results indicate that differences in petrochemical compositions may not lead to significant distortion in the metamorphic records, including garnet zoning patterns and peak P-T conditions. This further suggests that metamorphic rocks originating from diverse protoliths, which exhibit varying peak P-T conditions within the same locality, could either be a result of the tectonic juxtaposition of distinct tectonic slices or the influence of varying fluid conditions.

Metamorphic evolution of high-pressure and ultrahigh-temperature granulites from the Alxa Block, North China Craton: Implications for the collision and exhumation of Paleoproterozoic orogenic belts

High-pressure (HP) and ultrahigh-temperature (UHT) granulites with a high geothermal gradient (greater than 500 °C/GPa) are prominent features of Paleoproterozoic orogenic belts and may represent paired metamorphic belts present during the early stages of plate tectonics. Understanding their pressure−temperature−time (P-T-t) paths and metamorphic evolutionary relationships could provide valuable constraints on the tectonic processes of Paleoproterozoic orogenic belts. Here, we describe garnet mafic and clinopyroxene-orthopyroxene (Cpx-Opx) granulites from the Diebusige area of the Alxa Block in the western part of the Khondalite Belt, North China Craton. Through detailed petrographic, phase equilibrium modeling, and Ti-in-amphibole thermometric studies, we obtained the preserved peak mineral assemblages of two types of mafic granulites: garnet + clinopyroxene + amphibole + plagioclase + quartz + ilmenite, and clinopyroxene + orthopyroxene + plagioclase + amphibole + garnet (rare) + ilmenite. The preserved peak P-T conditions were determined to be 850−890 °C/11.4−13.2 kbar (HP granulite-facies) and 950−970 °C/8.2−9.2 kbar (UHT conditions), with thermal gradients of ∼70 °C/kbar (moderate differential temperature/differential pressure, dT/dP) and ∼110 °C/kbar (high dT/dP), respectively. Using sensitive high-resolution ion microprobe U-Pb dating and rare earth element analysis of zircons, we found that the garnet mafic granulite recorded an HP granulite-facies metamorphic age of ca. 1.95 Ga and a retrograde cooling age of ca. 1.8 Ga, while the Cpx-Opx granulite recorded a consistent retrograde cooling age of ca. 1.8 Ga. By combining these results with the metamorphic evolution and timing (ca. 1.92−1.91 Ga) of UHT rocks from the Khondalite Belt, we suggest that the garnet (HP) mafic and Cpx-Opx (UHT) granulites may represent different stages of the same metamorphic event, shedding light on the processes involved in the collision and subsequent exhumation of Paleoproterozoic orogenic belts.

UHP metamorphism, decompression anatexis and retrogression of garnet-bearing metagranite and granitic gneiss from the Dabie orogen during continental collision

Metamorphosed granitic rocks are commonly volumetrically dominant in continental collisional terranes, which are crucial targets for resolving the metamorphic evolution of deeply subducted continental crust. However, their metamorphic processes in ultrahigh pressure (UHP) belts are often poorly understood, because they are commonly intensely retrogressed and presently dominated by amphibolite-facies minerals with very rare petrological evidences of UHP metamorphism. It is known that, in these lithologies, zircon can potentially preserve evidence of the UHP history; therefore, the P-T-t path of metafelsic rocks may be estimated using a multidisciplinary approach which combines zircon microanalysis with phase equilibria modelling. In the central Dabie UHP metamorphic zone (CDZ) (China), two types of metamorphosed garnet-bearing granitic rocks, i.e. metagranite and granitic gneiss, whose massive vs. gneissic structure reflects a different degree of deformation, are closely associated with eclogites. Opposite to the eclogites, their detailed metamorphic evolution is poorly understood. In this study, a three-stage metamorphic evolution is reconstructed for these lithologies, based on field observation, bulk-rock elemental geochemistry, sensitive high-resolution ion microprobe zircon UPb dating, Raman micro-spectroscopy analysis of zircon inclusion, mineral chemistry, conventional thermobarometry and phase equilibria modelling. The first UHP stage is represented by the assemblage coesite-rutile-garnet-phengite preserved in the metamorphic domains of zircon and by garnet core in the matrix, revealing minimum peak pressures of 44 kbar and 30 kbar for granitic gneiss and metagranite, respectively, at 231 ± 4 Ma. The second stage is represented by anatexis at 18–20 kbar, 725–870 °C and 222 ± 3 Ma, evidenced by leucosomes at the outcrop scale and by quartzofeldspathic films with low dihedral angles, melt inclusions preserved in peritectic garnet and quartz/K-feldspar/plagioclase- garnet intergrowth at the micro-scale. The third stage is represented by amphibolite-facies overprint at 590–700 °C, 9–14 kbar, at 214–219 Ma, recorded by biotite-plagioclase-quartz symplectites developed at the expenses of phengite, re-equilibration of the peritectic garnet rim, and amphibole-quartz (biotite-titanite) inclusions in zircon overgrowth rims. The resulting P-T-t path overlaps with previous results estimated from other CDZ UHP rocks. Besides, the protoliths of both metagranite and granitic gneiss were emplaced during the mid-Neoproterozoic (683–786 Ma) period, as already documented throughout the Dabie orogen. Compared with previously published data for meta-granitic rocks in the CDZ, the weakly deformed metagranite does not appear to represent low-grade metamorphic rocks. Instead, this study (i) suggests that the two groups of lithologies experienced a coeval metamorphic evolution; (ii) reconstructs a multistage P-T-t history based on zircon microanalysis and pseudosections; (iii) reveals that anatexis, retrogression and deformation facilitate the elimination of (U)HP mineralogical records of metagranitoids. Overall, these results provide new constraints on the metamorphic evolution and exhumation processes of deeply subducted granitic rocks during continental collision.

Late Neoarchean charnockite in Daqingshan terrane, North China Craton: petrogenesis and implications for continental crustal evolution

ABSTRACT Modern analogues of Archaean charnockite are important because they facilitate investigations into processes of early crustal formation and tectonic transition. Herein, we present the results of the petrography, geochemistry, and isotope geochronology of Late Neoarchean charnockites in the Daqingshan terrane at the northern margin of the North China Craton. The sources, petrogenesis, and tectonic implications of the samples obtained are determined based on their geochemical characteristics. Charnockites mainly exhibit magnesian and calc–alkaline characteristics; furthermore, they contain high amounts of light rare-earth elements, with slightly positive Eu anomalies. Analysis of zircon U–Pb dating using a sensitive high-resolution ion microprobe reveals that Hademengou–Xiwulanbulang (HDMG–XWLBL) charnockites were emplaced during the (i) Late Neoarchean (ca. 2500 Ma), with zircon εHf(t) values ranging from 1.57 to 7.81; (ii) single-stage model ages (TDM1) from 2529 to 2637 Ma; and (iii) two-stage model ages (TDM2) from 2710 to 2820 Ma. A comparative study has performed on the petrography, geochemistry, and geochronology of charnockite and granulite shown that the parent magma of the charnockites is the product of the partial melting of granulite. Combining our results with geological data pertaining to the Archaean structural evolution of the Daqingshan terrane, we speculate that the Late Neoarchean HDMG–XWLBL charnockite was formed during a subduction-related thermo-tectonic event in the northern margin of the North China Craton.

Fluid-fluxed partial melting of the Buncheon granitic gneiss in the Yeongnam Massif, Korea: Protracted (ca. 1.86–1.84 Ga) reworking of the Paleoproterozoic Korean arc

The fluid-fluxed melting of igneous protoliths provides an opportunity to assess the role of fluid for the formation and modification of continental crust during high-temperature metamorphism. In order to investigate such an anatexis in the Buncheon granitic gneiss, Yeongnam Massif, we combined field, microstructural, and geochemical studies with the U-Pb and oxygen isotopic analyses of zircon and monazite. The granitic gneisses comprise metatexitic migmatites containing leucosomes, although neither melanosomes nor peritectic phases are associated with neosomes. These migmatites underwent syn-kinematic wet melting in the presence of ca. 0.9–1.4 wt% H2O (estimated from the pseudosection modeling) via the reaction, plagioclase + K-feldspar + quartz + H2O = melt, that took place at ∼650–750 °C and 4–6 kbar. The leucosomes differ in the degree of Eu anomalies and have variable amounts of rare earth elements, Zr, Y, and Th. Such variations are attributed to the segregation and migration of melts as well as the entrainment of residual phases. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon and monazite were determined from two granitic gneisses and three leucosomes together with an adjacent, undeformed Hongjesa granite. Zircon grains are oscillatory-zoned, except for homogeneous overgrowth rims rarely present in two leucosomes. Both zircon domains were dated at ca. 1.98–1.97 Ga and ca. 1.86 Ga, respectively, suggesting that igneous protoliths were partly recrystallized during the late Orosirian. Monazites from two gneiss samples contain submicroscopic polycrystalline inclusions of K-feldspar + quartz + apatite, reflecting the presence of former melts, and yielded a pooled 207Pb/206Pb age of 1859 ± 6 Ma (n = 30). In contrast, monazites from a leucosome are inclusion-free, and yielded the youngest age of 1840 ± 16 Ma (n = 6). This result is interpreted to represent the melt crystallization at ca. 1.84 Ga following the ca. 1.86 Ga anatexis. On the other hand, monazite from the Hongjesa granite, yielded a weighted mean 207Pb/206Pb age of 1878 ± 5 Ma (n = 11), most likely reflecting a prograde metamorphic growth. The δ18O values of ca. 1.98–1.97 Ga igneous zircon in granitic gneisses are 7.1–7.2 ‰, but those from the leucosome are reduced at 5.9–6.7 ‰. The latter is largely identical to 5.7–6.7 ‰ estimated from ca. 1.86 Ga domains of zircon in the leucosome and monazite in the granitic gneiss; in contrast, a slightly lower δ18O value of 5.46 ± 0.61 ‰ is recorded in the ca. 1.84 Ga monazite of leucosome. The relatively low δ18O values are most likely accounted for by an influx of fluid that has apparently triggered partial melting in the granitic gneiss, and the fluid itself is possibly linked to the crystallization of mafic magmas in an arc complex. Taken together, in contrast to the majority of zircons preserving the crystallization age of igneous protoliths, monazites are more susceptible to recrystallization during the fluid-present melting and permit us to decode a protracted (∼20 m.yr.) episode from partial melting to melt crystallization in the Yeongnam Massif. Such a longevity of melt-present system typifies the prolonged crustal reworking of Paleoproterozoic Korean arc situated at the eastern margin of the North China Craton.

Petrogenesis of the Palaeoproterozoic (1.87 Ga) Damyang amphibolites in the north‐western margin of the Yeongnam Massif and tectonic implications of the Korean Peninsula

The Yeongnam Massif is located in the southern part of the Korean Peninsula and holds key elements in understanding the Palaeoproterozoic tectonic evolution of north‐east Asia during the amalgamation of the Columbia supercontinent. In this study, we have conducted geochronological and geochemical studies on the mafic intrusive along the northern margin of the Yeongnam Massif. Sensitive high‐resolution ion microprobe (SHRIMP) U–Pb zircon dating on two amphibolite and one gabbro samples in the Damyang area show that the amphibolites crystallized at ~1.87 Ga and the gabbro crystallized at ~87.6 Ma. The 1.87 Ga Damyang amphibolites are simultaneous with the Muju and Okbang amphibolites along the northern margin of the Yeongnam Massif. They also show similar geochemical characteristics, including tholeiitic basalt composition, enriched Light Rare Earth Elements (LREE) and large ion lithosphile elements (LILEs), depleted high field strength elements (HFSEs), and slightly enriched but various Hf isotopes, indicating that they originated from an enriched but heterogeneous mantle source with fluid addition. Thus, the 1.87 Ga amphibolites along the northern margin of the Yeongnam Massif were probably emplaced in an arc‐related setting. Combined with the 2.02–1.96 Ga magmatism, it is proposed that the subduction might have lasted during the 2.02–1.85 Ga interval and mantle‐derived magma rose up due to the roll‐back process between 1.88–1.85 Ga.

Sr-Nd-Ca isotopic variations of Cenozoic calc-alkaline and alkaline volcanic rocks above a slab tear in Western Anatolia, Turkey

The tectonic mechanisms that may trigger a transition from calc-alkaline to alkaline volcanic activity above a subduction zone are enigmatic. We report major/trace elemental and Sr-Nd-Ca isotopic compositions of a suite of magmatic samples from the Miocene Gördes calc-alkaline dacites and the Quaternary Kula alkaline basalts from Western Anatolia, Turkey. Zircon sensitive high-resolution ion microprobe dating shows that the Gördes dacites formed at ca. 18 Ma. They are characterized by high-K calc-alkaline affinities, enrichment of light rare earth elements and large-ion lithophile elements (Rb, Th, U), positive Pb anomalies and negative Eu and high field strength element (HFSE) (Nb, Ta, and Ti) anomalies with high 87Sr/86Sri (0.70977−0.71010) and low εNd(t) (−7.3 to −7.0). They have small δ44/40Ca (0.59‰−0.69‰) values lower than that of mid-ocean ridge basalt (0.83% ± 0.11%). The Kula basalts exhibit alkaline affinities and are more enriched in incompatible elements (e.g., Rb, Ba, Th, Nb, Ta, Pb, and Sr) than those of typical oceanic island basalts. Their low 87Sr/86Sri (0.70307−0.70343) and high εNd(t) (+4.2 to +6.5) are consistent with an asthenospheric mantle source. Their δ44/40Ca values, ranging from 0.67‰ to 0.81‰, are higher than those of the dacites and lower than that of the bulk silicate earth (0.94‰ ± 0.10‰). Geochemical modeling suggests that variable degrees of partial melting of an asthenospheric mantle involving recycled oceanic crust may have resulted in low δ44/40Ca values of the Kula basalts. Combined with reported mantle tomographic images, we interpret that sub-slab asthenospheric mantle upwelling through a slab tear during subduction roll-back may have played a key role in the Sr-Nd-Ca isotopic variability from Miocene calc-alkaline dacites to Quaternary alkaline basalts in Western Anatolia.

A Late Triassic depositional age for the Xujiahe formation, Sichuan basin: Implications for the closure of the Paleo-Tethys Ocean

The Late Triassic transition from marine carbonate rock to clastic in the Sichuan Basin, east of the Tibetan Plateau, not only records the evolutional process of peripheral foreland basin, but also preserves the evidence of closure of the Paleo-Tethys Ocean. The depositional age of the Xujiahe Formation, Late Triassic clastic, is debated. A new radiometric date based on sensitive high-resolution ion microprobe U–Pb ages for zircons grains from a tuff sample is presented here, which is used to reinterpret existing magnetostratigraphic data from the Xujiahe Formation. From these data, we infer a 224-210 Ma depositional age range for the formation. In addition, a comprehensive data set of seismic-reflection profiles and wire-line logs indicates there are four basinal scale unconformities (including U/C-1, U/C-2, U/C-3 and U/C-4) in the Late Triassic in the Sichuan basin. Together, the age constraints and hiatus relationships not only imply that the initiation of the Sichuan foreland basin is represented by the unconformity in the Late Carnian. This unconformity (U/C-2) is interpreted here to be the result of the closure of the Paleo-Tethys Ocean, and also indicate that the Sichuan foreland tectonics probably ceased by 211 Ma following the extensive lithosphere delamination from the Qiangtang to Songpan-Ganzi terranes. The new understanding of the tectonic and depositional evolution of the Sichuan foreland basin provides insight into the quasi-simultaneous collisional geodynamic processes of the Paleo-Tethys Ocean.

Oceanic subduction to continental collision in the NE Proto-Tethys revealed by early Paleozoic eclogites with high-temperature granulite-facies overprinting in the East Kunlun orogenic belt, northern Tibet

The East Kunlun orogenic belt in the northern Tibetan Plateau records a long-term accretionary and collisional history in the northeastern Proto-Tethys Ocean, which is important for reconstructing the paleogeography of early Paleozoic East Asia. Here, we present an integrated study combining petrology, geochemistry, geochronology, and metamorphic pressure−temperature (P−T) data of newly found eclogites in the middle Nuomuhong segment of the East Kunlun orogenic belt. The eclogites are composed mainly of garnet, omphacite and low-sodium clinopyroxene, amphibole and plagioclase with minor orthopyroxene, biotite, quartz, accessory rutile, ilmenite, titanite, and zircon. Detailed petrographic observations, conventional geothermobarometry, and phase equilibrium modeling point to the presence of five metamorphic mineral assemblages with corresponding P−T conditions related to: (1) prograde M1 stage P−T estimates at >14.0 kbar/∼470−506 °C; (2) Pmax M2 eclogite-facies stage P−T conditions of ∼26 kbar/∼570 °C; (3) early retrograde M3 high-pressure granulite-facies stage; (4) subsequent M4 retrograde medium-pressure granulite facies at Tmax of ∼860−900 °C; and (5) later M5 retrograde amphibolite-facies stage P−T conditions of <6.2 kbar/∼710−730 °C. These P−T estimates define a clockwise P−T path characterized by heating during the Pmax formation of the eclogite facies, to the Tmax exhumation stage of granulite-facies lithologies, the latter of which is identified for the first time in retrograde eclogites from the East Kunlun orogenic belt. Whole-rock geochemical compositions indicate a mid-oceanic-ridge basalt (MORB) affinity for the eclogite protoliths and a fragmented oceanic crust origin. Sensitive high-resolution ion microprobe (SHRIMP) zircon U−Pb isotopic analyses of the eclogite yielded two groups of weighted mean 206U/238Pb ages of 464 ± 8 Ma and 419 ± 4 Ma, which are interpreted as the age of the eclogite protoliths and the lower threshold for peak eclogite-facies metamorphism, respectively. Our new data, together with regional eclogite-facies metamorphism, suggest a ca. 520−460 Ma age for the subduction of the eastern Kunlun oceanic crust, within the northern Proto-Tethys Ocean, to a depth of ∼83 km, with early subduction−accretionary orogenesis at ca. 419 Ma. Overprinting by high-temperature granulite-facies points to exhumation of oceanic crust to the middle to shallow crustal level at this time. Collectively, the preserved eclogite and high-temperature granulite mineral assemblage provide new constraints on the tectonic evolution and the detailed accretionary-to-collisional orogenesis of the Proto-Tethys Ocean. They suggest that the ca. 428−411 Ma subduction-collisional event marked the termination of the Proto-Tethys Ocean and the eventual formation of an ∼500-km-long, high- to ultra-high−pressure metamorphic belt in the East Kunlun orogenic belt.

Radiogenic Pb in xenotime trapped in nanoscale inclusions of apatite during fluid alteration

This study focuses on the low-temperature mineralogical response of xenotime, a phosphate mineral routinely used as a geochronometer, to fluid-assisted alteration. The studied xenotime grain (z6413) comes from a ∼ 1000 Ma pegmatite from the Grenville Province, Canada, and is commonly used as reference material for UPb analyses. At the microscale, the grain has a mottled texture, sub-micrometer porosity, and small domains dark in backscattered electron (BSE) images that are characterised by curviplanar, sharp boundaries. The small dark BSE domains are associated with Th-U-rich inclusions and larger porosity (2–3 μm) and are interpreted to result from localised fluid-assisted coupled dissolution-reprecipitation. Sensitive high-resolution ion microprobe (SHRIMP) UPb analyses of unaltered and fluid-affected domains yield concordant crystallisation dates, irrespective of the textural domains. The apparently unaltered xenotime domain was characterised at the nanoscale to determine if the grain was affected by fluids beyond the altered domains defined by BSE imaging. Transmission electron microscopy (TEM) imaging results indicate the presence of randomly distributed Ca + Pb nanoscale precipitates. Atom probe tomography (APT) reveals the presence of spherical clusters (4 to 18 nm in size) enriched in radiogenic Pb, Ca, and Si atoms, which, combined with TEM observations, are interpreted as nanoscale inclusions of apatite. In addition to the inclusions, a dislocation enriched in Ca and fluid mobile elements such as Cl, Li, Na, and Mn was imaged from APT data indicating percolating of fluids further than the reaction front. APT 206Pb/238U nanogeochronology indicates that the nanoscale inclusions of apatite formed at 863 ± 28 Ma, 100–150 Ma after crystallisation of the host xenotime, with its formation attributed to fluid metasomatism. This study shows that fluid-xenotime reaction caused Pb* to be redistributed at the nanoscale, recording the timing of metasomatism. However, at the scale of SHRIMP analytical spot (10 μm), xenotime is concordant, indicating that Pb was not mobile at the microscale and fluid-altered xenotime can preserve its crystallisation age. Although the studied grain shows a limited amount of altered domains in BSE imaging, nanoscale analyses reveal a more pervasive re-equilibration of the minerals through the percolation of fluids along dislocations.

Evolutionary, paleoecological, and biostratigraphic implications of the Ediacaran-Cambrian interval in West Gondwana

The Ediacaran-Cambrian transition interval is described for the west part of the Gondwana Supercontinent. This key interval in Earth’s history is recorded in the upper and lower part of the Tagatiya Guazú and Cerro Curuzu formations, Itapucumi Group, Paraguay, encompassing a sedimentary succession deposited in a tidally influenced mixed carbonate-siliciclastic ramp. The remarkable presence of cosmopolitan Ediacaran shelly fossils and treptichnids, which are recorded in carbonate and siliciclastic deposits, respectively, suggests their differential preservation according to lithology. Their distribution is conditioned by substrate changes that are related to cyclic sedimentation. The associated positive steady trend of the δ13C values in the carbonate facies indicates that the Tagatiya Guazú succession is correlated to the late Ediacaran positive carbon isotope plateau. Sensitive high-resolution ion microprobe U-Pb ages of volcanic zircons from an ash bed ∼30 m above the fossil-bearing interval in the Cerro Curuzu Formation indicate an Early Cambrian (Fortunian) depositional age of 535.7 ± 5.2 Ma. As in other coeval sedimentary successions worldwide, the co-occurrence of typical Ediacaran skeletal taxa and relatively complex trace fossils in the studied strata highlights the global nature of key evolutionary innovations.

Emplacement ages and petrogenesis of the Sunchang and Namwon granitoids, South Korea

We present new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages for two adjacent plutons (Sunchang and Namwon) along the Middle Jurassic Honam Shear Zone, South Korea, and report their major- and trace-element and Sr-Nd-Pb isotopic compositions to constrain the emplacement ages and petrogenesis of the plutons. The Sunchang foliated granites yield U-Pb ages of 186–178 Ma, suggesting long-lasting magmatic activity. The emplacement age of the Namwon pluton (192–180 Ma) indicates that it was broadly coeval with or slightly older than the Sunchang pluton. Some zircon grains contain ancient (Neoarchean to Paleo-Proterozoic) inherited cores. The REE patterns of the granitoids are roughly U-shaped with no strong negative Eu anomalies and show arc-like enrichments in LILEs and depletion in Nb or Ta, similar to I-type granites. The Sunchang granites yield initial 87Sr/86Sr ratios of 0.71543–0.71675, εNd(t) values of −29.0 to −26.0, and 206Pb/204Pb ratios of 18.44–18.90, and the Namwon granitoids yield initial 87Sr/86Sr ratios of 0.71229–0.71652, εNd(t) values of −27.5 to −20.9, and 206Pb/204Pb ratios of 17.93–18.23. The granitoids have adakite-like elevated Sr/Y (25–78) and La/Yb (15–66) ratios but are potassic, which is inconsistent with slab-derived melts. Furthermore, they have relatively low MgO at a given SiO2 content compared with high-SiO2 adakites and low Mg# (30–48) and Ni (<5 ppm), Co (<8 ppm) and Cr (<52 ppm) contents. They resemble C-type adakites, which form by the partial melting of thickened lower continental crust with residual garnet and amphibole. Underplating mafic magma from the mantle wedge or shear heating may have been the source of heat for the crustal melting. The close spatiotemporal association of these C-type adakitic rocks suggests a genetic link between them. The theory that the massive to weakly foliated Namwon granitoids were emplaced post-tectonically needs to be revised, as the Namwon and Sunchang granitoids formed during the same magmatic episode in the regional-scale shear zone.

SHRIMP U–Pb geochronology of Mesoproterozoic basement and overlying Ocoee Supergroup, NC–TN: dating diagenetic xenotime and monazite overgrowths on detrital minerals to determine the age of sedimentary deposition

Sedimentary rocks of the Ocoee Supergroup crop out in the Appalachian Blue Ridge of western North Carolina and eastern Tennessee. This ∼12 km thick sequence of strata was mapped as nonconformably overlying Mesoproterozoic crystalline basement (about 1.20–1.02 Ga) and beneath the lower Cambrian Chilhowee Group, leading to the traditional interpretation that the Ocoee Supergroup was deposited between about 1.02 and 0.54 Ga. This interpretation was challenged by Unrug and Unrug (1990, Geology, 18: 1041–1045) and Unrug et al. (2000, Geological Society of America Bulletin. Vol. 112, pp. 982–996), who claimed that Paleozoic fossils were recovered from rocks of the Walden Creek Group (upper Ocoee), and thereby created a new tectonic model for the origin of Laurentia. To resolve the age controversy concerning the time of deposition of the Ocoee Supergroup, diagenetic xenotime overgrowths on detrital zircon and diagenetic monazite on detrital monazite were dated by the U–Pb method. Sensitive High Resolution Ion Microprobe ages of diagenetic xenotime from three samples of sandstone (Thunderhead Sandstone, Cades Sandstone, and Shields Formation) indicate that the Ocoee Supergroup was deposited in the interval of about 580–550 Ma. A sample of Sandsuck Formation (uppermost Ocoee) yielded xenotime overgrowths of about 410 Ma, probably related to post-depositional low-grade metamorphism. Various origins of xenotime overgrowths may be distinguished by trace element distributions. Ages and trace element concentrations of monazite overgrowths support the xenotime age results, although concordia systematics are complicated by high concentrations of common Pb and inheritance of ages (∼1.2 and 0.6 Ga). These age data support the observed field relations for a late Neoproterozoic depositional age of the Ocoee Supergroup.