Yangtze Craton Research Articles

A ca.2.1 Ga Andean-type margin built on metasomatized lithosphere in the northern Yangtze craton, China: Evidence from high-Mg basalts and andesites

We report for the first time the presence of a suite of Paleoproterozoic (2.12Ga) metamorphosed high-Mg basalts and andesites in the Huangling dome, northern Yangtze craton, China, which provide new insights into crustal growth processes in the craton during the Paleoproterozoic era. The high-Mg basalts and andesites are mineralogically and texturally amphibolites and quartz-bearing amphibolites, respectively, and occur as deformed layers within the metasedimentary rocks of the Shuiyuesi Group in the northern Huangling dome. We present new field and petrographic observations, zircon U-Pb ages, in situ zircon Hf isotope and whole-rock major and trace element and Nd isotope data, to assess the petrogenetic origin and geodynamic setting of these high-Mg rocks. The igneous zircons from the andesites yield a weighted age of ca. 2.12Ga that is interpreted to be the formation age of the magmatic protolith. The basalts are characterized by moderate SiO2 (49wt.%), low TiO2 (0.63–0.65wt.%), high MgO (13.7–14.6wt.%) and high Mg-numbers (58–59). The andesites have 53–60wt.% SiO2, 0.45–0.50wt.% TiO2 and 6.6–9.5wt.% MgO contents, yielding high Mg-numbers (63–76). Both the basalts and andesites are enriched in LILE and LREE, but depleted in Nb, Ta, and HREE. Zircons in the andesites have εHf (t) values between −0.2 and +3.3 and corresponding Hf isotopic model age (TDM1) of 2.4Ga, reflecting various degrees of crustal input. They have negative εNd (t) values, ranging from −4.4 to −2.7. Geochemical characteristics of the basalts and andesites indicate that they are equivalent to high-Mg basalts and andesites, respectively, occurring in Phanerozoic suprasubduction zones. The Nd and Hf isotope compositions of the high-Mg rocks suggest that they were derived from partial melting of subarc lithospheric mantle above a subducting oceanic slab. The formation age and general geochemical characteristics of the high-Mg rocks reveal the presence of a Paleoproterozoic (2.1–2.2Ga) Andean-type continental margin developed on metasomatized lithospheric mantle in the northern Huangling dome. The geochemical and geochronological data presented in this study provide important insights into Paleoproterozoic evolution of the Yangtze craton.

A revisit to the Higo terrane, Kyushu, Japan: The eastern extension of the North China–South China collision zone

The Higo and Manotani high-T metamorphic rocks belong to the Higo terrane, central Kyushu, Japan. This terrane has been suggested as the eastern extension of the North China–South China collision zone based mainly on the similar petrological and geochronological data to the north Dabie high-grade rocks. In this study, detrital zircon U–Pb dating for the Higo metamorphic rocks yielded a maximum depositional age of 197Ma with a prominent component at 193–284Ma (62–95%) and a subsidiary peak at 1847–1875Ma (19–28%); whereas for the Manotani metamorphic rocks, the maximum depositional age is 263Ma with a prominent component at 263–304Ma (98%). These results suggest that the protoliths of the Higo and Manotani metamorphic rocks might be equivalent to those of the early–middle Jurassic Chizu and late Triassic Suo high-P metamorphic rocks, southwest Japan, respectively. The absence of middle Neoproterozoic detrital zircon (c. 700–825Ma) is incompatible with a proposition that the proto-Japan might have been located close to the South China craton, where Permian–Jurassic sedimentary rocks contain a significant amount of middle Neoproterozoic detrital zircon. The zircon provenance of the Higo terrane, as well as of the Chizu and Suo belts, could therefore be the North China craton, suggesting that the Suo metamorphic rocks, including the Manotani metamorphic rocks, have formed at the subduction plate boundary in the east, which changed into the contemporaneous North China–South China collision plate boundary in the west, likely passing through the Korean Peninsula. The transformation of the Chizu and Suo metamorphic rocks to the Higo and Manotani metamorphic rocks can be accounted for by the oceanward advance of the volcanic arc during trench retreating. The Cretaceous high-T metamorphism and related igneous intrusions of the Higo terrane would be regarded as the initial phase of the Ryoke event.

Geochemical constraints on the nature of magma sources for Triassic granitoids from South Qinling in central China

A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock major-trace elements and Sr-Nd isotopes as well as mineral chemistry and O isotopes was carried out for Triassic granitoids from the South Qinling orogen in central China. Model calculations were also performed to examine the trace element fractionation during partial melting of crustal rocks. The results provide insights into the nature of magma sources for these granitoids. LA-ICPMS zircon U-Pb dating yields concordant ages of 208±2 to 216±3Ma for these granitoids from the Shahewan (SHW), Caoping (CP) and Zhashui (ZS) plutons, and no relict zircon cores are identified by the CL imaging and U-Pb dating. The SHW and CP granitoids contain hornblende and are metaluminous with A/CNK ratios of 0.84 to 0.93. They exhibit relatively low SiO2 contents (62.88–69.04wt.%) but high contents of FeOT, MgO and TiO2, and slightly to negligibly negative Eu anomalies (δEu=0.79–0.89). Zircons from them show mantle-like δ18O values of 4.71 to 5.72‰. In contrast, the ZS granites contain no hornblende and are metaluminous to weakly peraluminous with A/CNK ratios of 0.99 to 1.03. They show relatively high SiO2 contents (69.32–75.94wt.%) but low FeOT, MgO and TiO2 contents, and moderate negative Eu anomalies (δEu=0.63–0.81). They have slightly low zircon δ18O values of 4.60 to 4.83‰. All of these granitoids show arc-like trace element distribution patterns with enrichment in LREE and LILE (e.g., Rb, K and Pb) but depletion in HFSE (e.g., Nb, Ta and Ti). Geochemical comparison and modeling indicate that these granitoids are different from adakitic rocks originating from the thickened lower continental crust. Compared with the composition of felsic melts produced by petrological experiments of various lithologies, it appears that these granitoids are derived from dehydration melting of metabasaltic sources at normal lower crustal depths, and experienced varying degrees of fractional crystallization. The SHW and CP granitoids were crystallized under the conditions of variably low pressures of 0.7–2.3kbar, temperatures of 628–766°C and higher fO2 than the ZS granites. On the other hand, these granitoids show moderate whole-rock initial 87Sr/86Sr ratios of 0.7045 to 0.7055 and slightly negative εNd(t) values of −4.0 to −1.5 as well as slightly negative to positive zircon εHf(t) values of −1.3 to 3.2. They have two-stage Nd model ages of 1.05–1.38Ga and Hf model ages of 1.04–1.39Ga. These whole-rock εNd(t) and zircon εHf(t) values are comparable with those for Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in South Qinling but inconsistent with those for Neoproterozoic Bikou metavolcanics and mafic-ultramafic intrusions in the northern margin of the Yangtze craton. Together with their non-adakitic features, it is suggested that they were derived from partial melting of the mafic lower crust in South Qinling rather than the subducted Yangtze continental crust beneath South Qinling. The continental collision between the North China Block and South China Block in the Late Triassic would have brought about reworking of the mafic lower crust for the granitoid magmatism in the Qinling orogen.

Recycling of metal-fertilized lower continental crust: Origin of non-arc Au-rich porphyry deposits at cratonic edges

Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383–445 ppm) and Au (7–12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6–16 ppb) with higher Au/Cu ratios (2 × 10 −4 to 8 × 10 −4 ) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au–fertilized low-crustal cumulates at 40–30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits.

Toxic elements in the stream sediments of an urbanized basin, Eastern China: urbanization greatly elevates their adverse biological effects.

The concentration of toxic elements (Hg, Cd, Cr, Cu, Pb, Zn, Ni and As) was measured in the sediments of the Qinhuai River in Eastern China along a rural to urban gradient. Multiple approaches were undertaken to evaluate the degree of enrichment and adverse biological effects of these elements. The results showed that the concentration of Hg, Cd, Zn, Cu and Pb increased exponentially from the agricultural headwater to the urbanized downstream, which reflects a severe anthropogenic influence. In addition, area-specific references, such as the local soil background (LSB) and upper continental crust (UCC) derived from the Yangtze craton, were more applicable for evaluating the enrichment of toxic elements in the Qinhuai River than was global UCC. In addition, Cd and Hg had the highest enrichment factor values (EF, with averages of 9.18 and 7.14, respectively); Zn, Pb and Cu had moderate EFs (averages from 1.52 to 2.40), while the average EFs of Ni, Cr and As were approximately equal to 1. Based on consensus-based sediment quality guidelines (SQGs), the contamination characteristics of all of the samples studied were associated with low to moderate priority of adverse biological effects (ABEs) in the rural upstream area, while it was associated with a high to moderate priority of ABEs in the urban sections of the Qinhuai River. Our results suggest that the adverse biological effects of elevated levels of toxic elements were strongly related to the degree of anthropogenic pollution.

A relic slice of archean–early Paleoproterozoic basement of Jiaobei Terrane identified within the Sulu UHP belt: Evidence from protolith and metamorphic ages from meta-mafic rocks, TTG–granitic gneisses, and metasedimentary rocks in the Haiyangsuo region

A great quantity of metamorphic rocks (Haiyangsuo Complex), including high-pressure (HP) mafic granulites with ‘red-eye-circle’ textures, retrogressive products of Grt-, Qtz-, Ep-, and Cpx-bearing amphibolites, TTG–granitic gneisses, and metasedimentary rocks, is distributed extensively within the Haiyangsuo region, northeastern Sulu ultrahigh-pressure (UHP) belt. The Haiyangsuo Complex crops out over an area of at least 70km2, rather than the 15-km2 area mentioned in previous studies. Based on integrated studies of petrography, mineral phase transformations, mineral inclusions, cathodoluminescence (CL) images of zircons, and zircon U–Pb LA–ICP–MS and amphibole Ar–Ar age dates, together with previous P–T condition estimates, abundant Neoarchean to Early Paleoproterozoic igneous protoliths, including ca. 2.6–2.5Ga meta-mafic rocks and ca. 2.7Ga, ca. 2.55–2.5Ga, and ca. 2.4Ga TTG–granitic gneisses, were identified in the Haiyangsuo region, whereas the Late Paleoproterozoic (∼1.7Ga) and Neoproterozoic (ca. 800–650 Ma) protolith ages common in the Sulu HP–UHP rocks have not been observed. The metasedimentary rocks in the region studied have a deposition age of ca. 2.1–1.9Ga, showing protolith ages similar to those of the voluminous Khondalite series of the Jingshan and Fenzishan groups from the Jiaobei Terrane, southwestern Paleoproterozoic Jiao–Liao–Ji orogenic belt.Two discrete episodic HP granulite–facies metamorphic events were identified for the first time in the meta-mafic rocks of the region and their country rocks. The first episodic HP granulite-facies metamorphism (M1) is characterized by the relict coarse-grained mineral assemblage of garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)±quartz (Qtz), which formed under P–T conditions of 740–850°C and 1.0–1.26GPa. In contrast, the second HP granulite-facies metamorphism (M21) is characterized by ‘red-eye-circle’ textures, with a typical fine-grained mineral assemblage of Grt+Cpx+Pl±Qtz surrounding the relict coarse-grained HP granulite-facies minerals of M1. It formed under P–T conditions of 765–845°C and 1.14–1.75GPa. Both of the HP granulite-facies metamorphic events were overprinted and replaced by a late amphibolite-facies retrogression (M22), which is typified by a fine-grained mineral assemblage of Amp+Pl+Grt and formed under P–T conditions of 580–660°C and 0.6–0.8GPa. Abundant metamorphic zircon U–Pb dating revealed that the two discrete episodic HP granulite-facies metamorphic events in the Haiyangsuo Complex occurred and recorded in metamorphic zircon domains with HP granulite-facies mineral inclusions at Late Paleoproterozoic (ca. 1.87–1.86Ga) and Middle Triassic (ca. 245–225Ma), respectively, and that each was overprinted by a subsequent amphibolite-facies retrogression at∼1.84Ga and 220–214Ma, respectively. Various metamorphic rocks (Haiyangsuo Complex) in the Haiyangsuo region have rock association, petrographical, geochronological, and P–T evolution affinities to the Paleoproterozoic Jiaobei Terrane, SW Jiao–Liao–Ji Belt, but are distinguished from the Sulu UHP rocks. Hence, the Haiyangsuo Complex is exotic to the Sulu UHP belt and can be regarded as an inseparable part of the Jiaobei Terrane during the Paleoproterozoic. The unique Middle Triassic HP granulite-facies metamorphic event first identified within the Haiyangsuo Complex coincides with that (245–225Ma) of the adjacent Jiaodong Terrane, NE Sulu UHP belt, indicating that the Haiyangsuo Complex was also involved in Triassic collision–subduction and experienced HP granulite-facies metamorphism at a lower crustal level. The ultimate formation of the Haiyangsuo Complex along the southeastern margin of the Jiaobei Terrane, SW Jiao−Liao−Ji orogenic belt, was genetically related to the collision between the North China and South China cratons. Thus, the juxtaposition of the Haiyangsuo Complex to the Sulu UHP metamorphic belt took place in the Middle–Late Triassic rather than in the Jurassic.

A Paleoproterozoic ophiolitic mélange, Yangtze craton, South China: Evidence for Paleoproterozoic suturing and microcontinent amalgamation

We document for the first time the presence of a Paleoproterozoic ophiolitic mélange in the Archean–Paleoproterozoic Kongling Complex in the northern Huangling Dome, Yangtze craton, South China. Detailed field mapping, petrographic, geochronological and geochemical studies reveal that the mélange consists of a suite of amphibolite facies mafic and ultramafic tectonic blocks including serpentinized harzburgite, olivine pyroxenite, pyroxenite, diabase, gabbro and basalt dispersed in a strongly sheared metasedimentary matrix. The mélange displays polyphase deformation and northwest-verging nappe structures, and underwent amphibolite to granulite facies metamorphism in the Paleoproterozoic (2.0–1.95Ga). The metasedimentary rocks are composed mainly of garnet-biotite-plagioclase gneiss, mica schist, mica-graphite schist, marble, quartzite, and banded iron formation (BIF). The mafic–ultramafic rock association shares the lithological and geochemical characteristics of Phanerozoic suprasubduction zone ophiolites.LA-ICP-MS U–Pb dating of zircons from the diabase yield ages between 2142 and 2148Ma for the magmatic cores and between 2042–2048Ma for the metamorphic rims. Zircon cores have εHf (t) values ranging from +5.4 to +10.3 (average=+7.2) with corresponding TDM1 ages of 2.24Ga, indicating that these rocks are remnants of juvenile crust derived from the depleted mantle at ca. 2.2Ga. All rock types in the mélange underwent Paleoproterozoic (2.0–1.95Ga) amphibolite–granulite facies metamorphism and were intruded by syn-metamorphic granite (∼2.0Ga), post-collisional (ca.1.85Ga) granite and mafic dykes. Intrusion age of one quartz monzonitic dyke crosscutting the serpentinized harzburgite is 1999Ma, constraining the minimum emplacement age of the mafic–ultramafic complex. The mafic–ultramafic rocks within the metasedimentary matrix in the northern Huangling Dome are fragments of a ca. 2.15Ga suprasubduction zone ophiolite that was incorporated into the suture zone during the accretionary-collisional process. The mélange recognized in the northern Yangtze craton provides important evidence for the Paleoproterozoic subduction and accretion processes possibly associated with the amalgamation of the proposed Columbia supercontinent.

Geochemical and Nd isotopic compositions of the Palaeoproterozoic metasedimentary rocks in the Kongling complex, nucleus of Yangtze craton, South China block: implications for provenance and tectonic evolution

AbstractPalaeoproterozic metasedimentary rocks, also referred to as khondalites, characterized by Al-rich minerals, are extensively exposed in the nucleus of the Yangtze craton, South China block. Samples of garnet–sillimanite gneiss in the khondalite suite were collected from the Kongling complex for Nd isotopic and elemental geochemical study. These rocks are characterized by variable SiO2 contents ranging from 35.71 to 58.07 wt%, and have low CaO (0.45–0.84 wt%) but high Al2O3 (18.56–29.04 wt%), Cr (174–334 ppm) and Ni (42.5–153 ppm) contents. They have high CIW (Chemical Index of Weathering) values (90.4–94.7), indicating intense chemical weathering of the source material. The samples display light rare earth elements (LREE) enrichment with negative Eu anomalies (Eu/Eu*=0.40–0.68), and have flat heavy rare earth elements (HREE) patterns. The high contents of transition elements (e.g. Cr, Ni, Sc, V) and moderately radiogenic Nd isotopic compositions suggest that the paragneisses might be those of first-cycle erosion products of predominantly mafic rocks mixing with small amounts of felsic moderately evolved Archaean crustal source. Geochemical and Nd isotopic compositions reveal that at least some of the protoliths of Kongling khondalite were sourced from local pre-existing mafic igneous rocks in a continental arc tectonic setting. Combined with documented zircon U–Pb geochronological data, we propose that the Palaeoproterozoic high-pressure granulite-facies metamorphism, rapid weathering, erosion and deposition of the khondalites in the interior of the Yangtze craton might be related to a Palaeoproterozoic collisional orogenic event during 2.1–1.9 Ga, consistent with the worldwide contemporary orogeny, implying that the Yangtze craton may have been an important component of the Palaeoprotorozoic Columbia supercontinent.

Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze, eastern Tibet

In this paper we address the formation and exhumation of the Danba Metamorphic Complex (DMC) that represents the deepest structural level of the Songpan Ganze terrane situated along the eastern margin of the Tibetan plateau. The DMC comprises a variety of gneiss domes and offers a unique opportunity to decipher their development during orogenic evolution. For that purpose, PTtD paths of metamorphic rocks sampled at different structural levels have been reconstructed. The DMC is composed of Triassic metaturbidites of the Xikang group, Paleozoic metasedimentary cover and basement of the Yangtze craton. The DMC is structurally marked by transposition of the upright S1 foliation of the Triassic metaturbidites into a NW-SE trending S2 composite foliation dipping to the NE. Transposition is associated with a localized top-to-the-northeast shear zone along the northeastern edge of the DMC and with pervasive top-to-the-southwest shearing from the core to the border of the complex. These structures are consistent with extrusion of the core of the DMC relative to the lower grade Triassic metaturbidites. The position of the biotite isograd overlapping the structural boundary of the DMC suggests that the Triassic metaturbidites have been affected by an increase in temperature as a result of extrusion. Within the DMC, the position of the metamorphic index minerals relative to the composite S2 foliation reveals that biotite, garnet, staurolite and kyanite grew before the transposition into S2, in contrast with sillimanite which crystallizes in the hinge of F2 folds and along the axial planar S2 schistosity. The sillimanite isograd delineates regional-scale overturned F2 folds and cross-cuts the staurolite and kyanite isograds consistent with an increase in temperature during D2. The melt-in isograd characterizes the deepest structural level of the DMC. PT conditions for these metamorphic rocks, determined using pseudosections and conventional thermometry, indicate a temperature increase from 400°C to more than 600°C from the edge to the core of the DMC for a relatively homogeneous pressure ranging from 5 to 6.5kbar suggesting that isograds and isotherms represent the syn-D2 thermal structure of the orogenic crust. Migmatites exposed in the deepest structural level of the DMC yield a pressure significantly lower than the surrounding metamorphic rock suggesting that they crystallized after D2 and after some exhumation of their hosts. Three different types of gneiss domes are distinguished on the basis of their position relative to the isograds, their structural characteristics, and their position relative to the margin of the Yangtze craton. Close to the craton and at the highest structural level, the Gezong dome represents a basement-rooted tectonic slice, in an intermediate position, the Gongcai dome corresponds to a basement-cored nappe, and further away and at the deepest structural level, the Bawang, Cunuchan and Qingaling domes are migmatite-cored domes. The presence at the current-day surface of this variety of gneiss domes reflects the difference in burial of the margin during the Mesozoic Indosinian orogeny.

Genesis of the Xinqiao Cu–S–Fe–Au deposit in the Middle-Lower Yangtze River Valley metallogenic belt, Eastern China: Constraints from U–Pb–Hf, Rb–Sr, S, and Pb isotopes

The Xinqiao Cu–S–Fe–Au deposit in the Tongling ore district, one of important part of Middle-Lower Yangtze River Valley metallogenic belt in Eastern China, is located along the northern margin of the Yangtze craton. It consists of two major types of mineralization: a more economically significant stratiform orebody with pyrite-bearing quartz veins in its footwall, and the less economically important skarn-type orebodies.An integrated study of zircon U–Pb–Hf and trace elements analysis for the quartz diorite in Xinqiao Jitou stock, Rb–Sr isotope analysis for quartz fluid inclusions from the footwall stockwork mineralization, as well as a systematic S–Pb isotope analysis of sulfides was carried out, all based on detailed field investigations. U–Pb zircon dating of Jitou stock yielded an age of 139.6±1.5Ma (MSWD=0.80), close to the Rb–Sr isotope isochron age (138.0±2.3Ma, MSWD=5.40, an initial 87Sr/86Sr value of 0.71138±0.00014) of quartz fluid inclusions from the footwall stockwork mineralization. The εHf(t) values of zircon grains vary from −15.3 to −8.5 with TDM2 ranging from 2151 to 1729Ma, suggesting that the magma for Jitou stock was probably dominant in crustal-derived magmas with minor mantle material inputs, based on the previous studies on Sr isotope compositions, Nb/Ta ratios and Mg#. The ΔFMQ values (−4.44 to 7.77, average 4.36) suggest that the Jitou stock was derived from the highly oxidized magma and favorable for the Cu mineralization. The systematic S–Pb isotope data and the initial 87Sr/86Sr values demonstrate that the stratiform orebody and the footwall stockwork mineralization were likely to be genetically linked to magmatic-hydrothermal events of Yanshanian (Jurassic – Cretaceous) age, as confirmed by closeness of ages of mineralization and magmatism. Therefore, it is interpreted that the Xinqiao stratiform mineralization may have resulted from Yanshanian magmatic hydrothermal fluids associated with the Jitou stock, as may be the case also for the skarn-type mineralization hosted in the contact between the Yanshanian Jitou stock and the Lower Permian Qixia Formation limestone, based on the basis of ore deposit geology. Generally, the Xinqiao Cu–S–Fe–Au mineralization may have been generated by the Yanshanian tectono-thermal event in Eastern China.

A Mediterranean-style model for early Neoproterozoic amalgamation of South China

A Mediterranean-style model is proposed for early Neoproterozoic amalgamation of South China, based on recognition of the Guangxi promontory on the southeastern margin of the Yangtze craton through a synthesis of geologic, geophysical and chronological data. The model suggests that, while the Guangxi promontory collided with the overriding Cathaysia terrane along the Yunkai orogeny during the earliest Neoproterozoic, the convergence between the Yangtze craton and the Cathaysia terrane paused or slowed down, which triggered the upper Rhine Graben-style Kangdian impact rift along the western Yangtze margin and resulted in roll-back of the oceanic slab trapped in the embayments and sequential formation of backarc rifts in the Cathaysia margin during the early Neoproterozoic. This model predicts a complex diachronous Yangtze–Cathaysia collision history, first at the Guangxi promontory at ∼1000Ma and finally at the Guizhou reentrant at ∼830Ma.

Geochronology, geochemistry and Hf–Sr–Nd isotopes of the ore-bearing syenite from the Shapinggou porphyry Mo deposit, East Qinling-Dabie orogenic belt

The Shapinggou Mo deposit is located in the western Dabie mountains, the eastern part of the Qinling-Dabie molybdenum orogenic belt. Shapinggou Mo deposit is a concealed deposit with the ore body mainly hosted by explosive breccia of Gaijing and the granite porphyry as well as the syenite of Shapinggou. Geochemistry study show that the SiO2 contents of Shapinggou syenite range from 61.74 to 69.93%, and the A/CNK from 0.95 to 1.06, classified as metaluminous to weak peraluminous, belonging to alkalic to shoshonitic series. The Mo deposits in Qinling Mo belt formed in two main periods, i.e., the first period occurred in to the Early Cretaceous (145–130 Ma), the second period in the late Early Cretaceous (130–110 Ma). Most of the Mo deposits in Dabie region formed in the second period. The results of zircon U–Pb show that the age of the Shapinggou syenite is 111.3 ± 1.2 Ma, which belongs to the second period. Proterozoic-Archean inherited zircons suggest that it may include some more ancient crustal material like Kongling group. The ɛHf(t) values of Shapinggou syenite range from −15.6 to −8.0, TDM2(Hf) from 1.7 to 2.16 Ga, respectively. The ɛNd(t) values of the Shapinggou syenite range from −12.29 to −11.76, TDM2(Nd) from 1.85 to 1.89 Ga, the 87Sr/86Sr from 0.709 to 0.710, respectively. Results of zircon Hf isotope and whole rock Sr–Nd isotope of Shapinggou syenite indicate that the Mo ore-forming materials were mainly generated from old Yangtze craton, e.g., gneiss from Dabie orogeny, mixed with some juvenal mantle materials. The geodynamics of the Shapinggou Mo deposit corresponded to an extension period in Eastern China, which caused by large scale lithospheric thinning. The delamination caused asthenosphere upwelling and crust-mantle interaction, which provided the ore-forming material and heat.

Late Permian basalts in the Yanghe area, eastern Sichuan Province, SW China: Implications for the geodynamics of the Emeishan flood basalt province and Permian global mass extinction

We report the finding of a ∼20m thick sequence of massive pyroxene–plagioclase–phyric basalt lava flows in the Yanghe area of the northeastern Sichuan Basin, within the Yangtze craton of SW China, which were previously considered to be located outside the Emeishan flood basalt province. This basaltic sequence above the middle Permian Maokou Formation (Fm.) is overlain by the late Permian Longtan Fm. Thus, the Yanghe basalts should be stratigraphically correlated with the Emeishan flood basalts. The Yanghe basalts show typical oceanic island basalt (OIB) affinity, and geochemically resemble Emeishan basalts, especially in the case of high-Ti (HT) basalts from the eastern domain of the Emeishan flood basalt province. The rocks have low age-corrected (87Sr/86Sr)t (t=260Ma) ratios (0.704158–0.704929) and Pb isotopic ratios [206Pb/204Pb(t) (18.264–18.524), 207Pb/204Pb(t) (15.543–15.58), and 208Pb/204Pb(t) (38.147–38.519)], and positive εNd(t) values (+3.15 to +3.61), suggesting that the lavas have not undergone any significant crustal contamination. The crystallization temperature of clinopyroxene is estimated to be 1368–1420°C, suggesting anomalously thermal inputs from a mantle source and a possible plume-head origin. The fractionation of middle rare earth elements (MREE) to heavy REE (HREE) suggests that these rocks were produced by small degrees of partial melting of mantle peridotite within the garnet–spinel transition region. The stratigraphic relationships and similar geochemical signatures with the Emeishan flood basalts suggest that the Yanghe basalts are part of the Emeishan flood basalt province and can be considered as the northeastern limit of the Emeishan flood basalt province. Our finding extends the diameter of the Emeishan flood basalt province to ∼1200–1400km, covering an area of up to ∼7×105km2, two times more than previously estimated. The larger areal extent and giant eruption volume, incorporating the Sichuan Basin, lend support to the correlation between the end-Guadalupian mass extinction and the Emeishan flood basalt province.

Architecture of the Sulu crustal suture between the North China Craton and Yangtze Craton: Constraints from Mesozoic granitoids

The Yangtze Craton (YC) and the North China Craton (NCC) collided in the Triassic, producing the prominent NNE-trending Sulu high-ultrahigh pressure metamorphic belt and associated crustal thickening. Late Jurassic–Early Cretaceous granitic plutons in the Sulu orogenic belt and the Jiaobei terrane to the west were used to investigate the crustal architecture across the suture. Our new data show that the granitoids from these two regions have similar chemical and isotope compositions. They are all characterized by very high Sr and low Y–Yb contents, high Sr/Y and (La/Yb)N ratios, similar ƐNd(t) values from −18.2 to −21.4, and similar initial 87Sr/86Sr ratios from 0.7076 to 0.7119. The calculated Nd two-stage model ages (TDM2) based on whole rock data vary from 2415 to 2662Ma. Co-magmatic zircon crystals from the granitoids have variably negative ƐHf(t) values from −26.8 to −12.8, with the calculated Hf TDM2 from 2008 to 2892Ma. The inherited zircon crystals from these rocks are dominated by Neoproterozoic (800–600Ma) and Triassic–Early Jurassic (~220Ma and ~180Ma) ages. The ƐHf(t) values of the inherited zircon crystals with U–Pb ages between 180Ma and 800Ma from Sulu and Jiaobei range from −21.6 to 4.2 and from −23 to −1.9, respectively. They all plot within the field of crustal evolution between 1385 and 2583Ma. The similar whole rock geochemical signatures and similar inherited zircon data indicate a similar source for the granitoids in these two regions. We propose that the source regions across the suture all belong to the YC. The occurrence of the YC crust beneath the NCC at this location is thought to have resulted from the westward subduction of the YC beneath the NCC and subsequent continental collision in the Triassic. In this model, the abundant 800 to 230Ma inherited zircon crystals in the granitoids are interpreted to have been derived from the source region whereas the rare older inherited zircon crystals are thought to have been derived from the overlying NCC during magma ascent.

Remains of trilobites and other species discovered in a volcanic ash bed of the end-Permian, Yangtze craton, South China

ABSTRACTThe remains of trilobites and other species were unexpectedly discovered in a volcanic ash layer beneath the Permian–Triassic Boundary (PTB). Based on a biostratigraphic investigation of the Zhongliangshan section in Chongqing, South China, the quantity of the species gradually decreased with subsequent volcanism. This finding provides an opportunity to further understand the disappearance of trilobites and the evolution of the mass extinction event. The temporal coincidence between the volcanic eruption event and the loss of trilobites and other species supports the idea of a cause-and-effect relationship. The species remains in the ash bed appeared before the disappearance of Clarkina yini and the climax of the negative carbon isotope excursion, which implies that the onset of the mass extinction occurred at the end-Permian. The explosive volcanic events caused massive releases of CO2, toxic gases and volcanic ash and resulted in loss of habitat for certain species in the Tethys domain. This phenomenon may have led to the abrupt death of trilobites and the catastrophic collapse of biodiversity.

Widespread Neoarchean (~ 2.7–2.6 Ga) magmatism of the Yangtze craton, South China, as revealed by modern river detrital zircons

The Kongling Terrane, which is the Archean nucleus of the Yangtze craton, preserves Paleoarchean-Proterozoic rocks as old as 3.45Ga. However, the dominant stage of formation of this Archean terrane remains unclear. In this paper, U-Pb and Lu-Hf isotopes of detrital zircons from two rivers and one stream in the northern part of the Kongling Terrane were studied by LA-ICP-MS and LA-MC-ICP-MS, respectively. These zircons show complicated internal structures in cathodoluminescence images, but the majority of them have linear or oscillatory zoning patterns, indicating magmatic origins. In general, the detrital zircons from these three local rivers show similar U-Pb age distributions. Together, they yield age peaks at 3.3–3.1Ga (5%), 3.0–2.8Ga (18%), 2.7–2.6Ga (30%), 2.6–2.2Ga (15%), 2.0–1.9Ga (27%), 1.9–1.7Ga (1%), 1.7–1.5Ga (2%), and 1.0–0.8Ga (1%). This age distribution implies that the North Kongling Terrane formed primarily during the Meso- to Neoarchean (3.0–2.6Ga). Lu-Hf isotopic data reveal predominant subchondritic to chondritic εHf(t) values (−11.3 to 0) for the Mesoarchean zircons, indicating the dominant role of crustal reworking during this period. In comparison, most Neoarchean zircons exhibit near chondritic to suprachondritic εHf(t) values (up to +7.4), suggesting juvenile crustal additions. The 2.0–1.9Ga zircons have similar initial Hf isotopic compositions as those of the 2.7–2.6Ga zircons, suggesting a metamorphic recrystallization origin. This observation further implies that the proportion of Neoarchean ages might be underestimated. Previous studies have reported several 2.7–2.6Ga Archean outcrops, minor detrital zircons in Precambrian sedimentary rocks and xenocrystal zircons in volcanic rocks from the Yangtze craton. Combined with our data, we propose that the 2.7–2.6Ga magmatism may have played an important role in forming the Kongling Terrane and have widely affected other parts of the Yangtze craton as well, similar to many other Archean cratons worldwide. The ~2.7–2.6Ga may correspond to the initial stabilization of the Archean nucleus of the Yangtze craton. Thereafter, this craton shows comparable 2.5–2.2Ga age records to the recently proposed Nunavutia supercraton, which could provide further clues to the early tectonic process of the Yangtze craton.

The Dewulu reduced Au-Cu skarn deposit in the Xiahe-Hezuo district, West Qinling orogen, China: Implications for an intrusion-related gold system

The Triassic West Qinling orogen, which formed by the continental collision between the South China block and North China craton following the subduction and closure of the paleo-Tethyan ocean, hosts numerous gold deposits that have commonly considered as products of metamorphic dehydration during the oceanic subduction and subsequent continental collision. However, whether or not there are gold deposits that are genetically related to regional magmatism remains poorly understood. Here we present mineralogical, compositional, and geochronological data of the Dewulu Au-Cu skarn deposit in the Xiahe-Hezuo district to understand the ore genesis and its genetic link to many coeval sediment-hosted disseminated and magmatic-hosted vein gold deposits in the same district, which collectively reveal a possible intrusion-related gold system. The Dewulu Au-Cu skarn deposit in the eastern zone of the Xiahe-Hezuo district is associated with the early Triassic I-type, ilmenite-series Dewulu quartz diorite pluton that intrudes Permian marine clastic and carbonate rocks. Ore-related hydrothermal biotite separates yield a well-defined 40Ar/39Ar plateau age of 239.9±1.4Ma (2σ), which agrees with a previous zircon U-Pb age (238.6±1.5Ma at 2σ) of the Dewulu quartz diorite and thus demonstrates a temporal and likely genetic link between the two. Both prograde and retrograde skarn assemblages are well developed in the Dewulu Au-Cu deposit. Prograde garnet and pyroxene are compositionally dominated by grossularite and hedenbergite, respectively, indicating a reduced skarn system. Au-Cu mineralization is largely represented by the arsenopyrite-loellingite-chalcopyrite-pyrrhotite-bornite assemblage and is associated with retrograde skarns. Gold is mostly hosted in arsenopyrite, loellingite and chalcopyrite, and has close textural relations with native bismuth or bismuthides. Mineral stability relationships and fluid inclusion microthermometric data confirm that the prograde skarns formed at a low oxygen fugacity, high temperature (>630°C), and a depth range of 2.9 to 6.5km. Arsenopyrite and chlorite geothermometers indicate that the retrograde skarns and late quartz-sulfide-calcite assemblages formed at 550 to 345°C and <350°C, respectively. The oxygen and sulfur fugacities of ore fluids were below the pyrite-pyrrhotite buffer during the retrograde stage, confirming a reduced fluid system. The reduced nature of ore-forming fluids is inherited from the reduced Dewulu quartz diorite, which was likely caused by significant assimilation of Permian to early Paleozoic carbonaceous and pyritic sedimentary sequences into the parental magmas that originally had a high oxidation state. The Xiahe-Hezuo district also hosts several major sediment-hosted disseminated and magmatic-hosted vein gold deposits, which are broadly coeval with the Dewulu Au-Cu skarn deposit. All those disseminated and vein gold deposits have also been recognized to be genetically related to reduced granitoid intrusions in the Xiahe-Hezuo district. These deposits combined constitute the first intrusion-related gold system in the West Qinling orogen.

Petrogenesis of Late Triassic ultramafic rocks from the Andong Ultramafic Complex, South Korea

To constrain the source and tectonomagmatic processes that gave rise to the Andong Ultramafic Complex (AUC) in South Korea, we determined the clinopyroxene Sr–Nd–Hf–Pb isotope and trace element compositions as well as the whole-rock and mineral compositions for the Late Triassic (ca. 222Ma) ultramafic rocks from the complex. They are composed of dunites, wehrlites, pyroxene/hornblende peridotites, and pyroxenites. The constituent minerals are olivines, diopsides/augites, bronzites, calcic-amphiboles, and spinels. Clinopyroxenes exhibit a convex-upward rare earth element (REE) pattern, with an apex at Sm. The whole-rock compositions plot away from the residual mantle peridotite trends, with variable but lower Al2O3 and SiO2 contents, and higher CaO, FeO*, and TiO2 contents at a given value of MgO. Estimated equilibrium temperatures for the AUC rocks range from 420 to 780°C. These observations, together with the absence of reaction or melt impregnation textures, indicate that the AUC ultramafic rocks are magmatic cumulates emplaced within the crust rather than residual mantle or mantle-melt reaction products. The AUC clinopyroxenes have compositions intermediate between the oceanic island basalt- and arc basalt-related cumulate clinopyroxenes. The AUC spinels have lower Cr#s than the arc-related magmatic cumulate spinels. They plot within the field for spinels from mid-ocean ridge basalts (MORB) on a TiO2 vs. Cr# diagram. However, the AUC clinopyroxenes have much more radiogenic Sr ([87Sr/86Sr]i=0.70554 to 0.70596), unradiogenic Nd ([εNd]i=−1.0 to −0.3), and Hf ([εHf]i=+4.4 to +6.6) isotopic compositions than those of the MORB or fore-arc basalts (FAB). In the Sr–Nd isotopic correlation diagram, the AUC clinopyroxenes plot in the enriched extension of the “mantle array”. They also have more elevated 207Pb/204Pb ratios at a given 206Pb/204Pb than those of the MORB or FAB. In the Nd-Hf isotope space, the AUC clinopyroxenes have somewhat elevated 176Hf/177Hf ratios at a given 143Nd/144Nd compared to the “mantle–crust” array. These observations indicate that the sub-continental lithospheric mantle (SCLM) overprinted by secondary volatile-rich silicate melts might be the principal source of the AUC magmatism. Heat from the upwelling asthenosphere, through the slab window produced by detachment of the oceanic slab from the buoyant continental lithosphere during continental collision between the North and South China Cratons, might lead to partial melting of the overlying metasomatized SCLM, resulting in the post-collisional Triassic magmatism in South Korea.

A westward propagating slab tear model for Late Triassic Qinling Orogenic Belt geodynamic evolution: Insights from the petrogenesis of the Caoping and Shahewan intrusions, central China

Late Triassic granitoid intrusions are widespread in the South Qinling Belt (SQB), providing excellent subjects to understand the geodynamic evolution of the Qinling Orogenic Belt and the collision between the North China Craton (NCC) and Yangtze Craton (YZC). This study shows newly obtained geological, geochemical and zircon U–Pb–Hf isotopic data of the Caoping and Shahewan intrusions, revealing that the Caoping intrusion consists of ~215Ma fined-grained granites, and ~221–215Ma porphyritic and coarse to medium-grained tonalites, granodiorites and monzogranites, which assemble with coeval mafic magmatic enclaves (MMEs). The Shahewan intrusion is composed of ~215–210Ma porphyritic granodiorites and monzogranites, which also assemble with coeval MMEs. The fine-grained granites from the Caoping intrusion are characterized by high SiO2, Rb and (La/Yb)N ratio, but low MgO, CaO and Sc contents, with εHf(t) values of −8.6 to +4.3 and TDM2(Hf) ages of 883–1596Ma, suggesting that they are mainly derived from partial melting of the Meso- to Neoproterozoic metagreywackes. The porphyritic and coarse to medium-grained granitoid rocks from both Caoping and Shahewan intrusions are characterized by higher MgO, CaO, Sc, Mg# values, but low SiO2, Rb contents and (La/Yb)N ratio, with εHf(t) values of −0.7 to +2.8 and TDM2(Hf) values of 961–1158Ma, suggesting that they are mainly formed by magma mixing between melts that were derived from Meso- to Neoproterozoic basement rocks of the SQB and metasomatized lithospheric mantle. The MMEs from Caoping and Shahewan intrusions are characterized by low SiO2, Sr/Y ratio, high MgO, K2O, Rb, Sc, total REE contents, with εHf(t) values of +0.5 to +6.1 and TDM(Hf) values of 661–846Ma, suggesting that they are produced by partial melting of metasomatized lithospheric mantle. The rapakivi-like textures of the rocks from Shahewan intrusion may be caused by continued underplating and injection of mafic magma resulting in higher temperature (TZr=770–817°C, comparing with Caoping intrusion of TZr=727–773°C), together with the magma mixing and isothermal decompression processes. Integrated with previous regional data, the SQB shows that mantle-derived magmatic activations initiated at ~234Ma near Wudang in the east, and westward systematically decreased to ~222–219Ma near Caoping and to 212–208Ma near Yangba. We attribute this temporal–spatial distribution of mantle-derived magmatism within the SQB to the progressive tear of the subducted oceanic slab. Combined with sedimentary studies on the foreland basins, paleomagnetic studies as well as numerical and seismic tomography models of slab break-off, we propose that the ~225–205Ma magmatism in the SQB resulted from a westward slab tear following the collision between the NCC and YZC.

Refertilization of serpentinites in the Sulu UHP terrane, Eastern China: constraints from geochronology, mineral composition, geochemistry, and Re–Os isotopes

ABSTRACTSerpentinites from Junan (JN), Rizhao (RZ), and Rongcheng (RC) in the Sulu ultra-high-pressure (UHP) terrane, China, were analysed for U–Pb zircon geochronology, mineral chemistry, whole-rock major and trace element chemistry (including rare-earth elements (REEs) and platinum-group elements (PGEs)), and Re–Os isotopes, in order to better constrain their petrogenesis and geodynamic process. The serpentinite zircons yield two age groups: 731 ± 10 to 780 ± 10 Ma for relic magmatic zircon cores, which may indicate early crystallization and emplacement of the peridotite in the Yangtze crust, and 209 ± 2 to 218 ± 3 Ma for metamorphic zircon, which coincides with Triassic UHP metamorphism. The spinels in the serpentinites exhibit significant Cr# variation (0.6–0.91) and have undergone multi-stage metamorphism. The serpentinites are characterized by enrichment in incompatible trace elements, low Ni and IPGE concentrations, and high Pd/Ir ratios, and the bulk-rock major elements plot in the ultramafic cumulate region. Their Re and Os concentrations are similar to those of typical orogenic peridotite, but they have high 187Os/188Os ratios (0.12433–0.14423). We believe that the serpentinite’s protolith consisted of cumulates from an asthenosphere-derived melt that intruded into the continental crust of the Yangtze craton in the Neoproterozoic. These cumulates were later subducted and metamorphosed during the subduction of the Yangtze craton in the Triassic. The serpentinites underwent melt–rock interactions and fluid enrichment, both prior to and during serpentinization.