Central China Orogen Research Articles

Compositional diversity of late Mesozoic granites rooting from the subducted crust in the Qinling–Dabie orogenic belt

Continental subduction has been identified in many orogens, such as the Alps, the Himalayas, and the Dabie; however, better understanding of the mechanism and outcome of different subduction processes remains desirable. The Qinling–Dabie orogen in Central China shows an asymmetric distribution of Triassic ultrahigh-pressure metamorphic rocks and syn- and post-tectonic granitoid plutons, thereby providing an excellent means by which to probe the diverse processes of continental subduction. Many late Mesozoic granitoid plutons that formed in post-orogenic or even anorogenic settings are widely distributed in both the Qinling and Dabie tectonic units and can be useful for studying the composition of materials beneath the crust because of their conspicuous compositional diversity. In the present study, we report zircon ages and NdHf isotopic compositions for the Shangcheng and Heyu granite plutons, which represent late Mesozoic plutons in Dabie and Qinling. When compared with those of the Shangcheng pluton, the wide range of Nd isotopic values and the slightly depleted Hf isotopic compositions of the Heyu pluton indicate that there may be juvenile crust beneath the Qinling tectonic unit that is not present beneath the southern margin of North China. The currently undetectable juvenile material in this region may be an important candidate source of the late Mesozoic magmatism and metal mineralization in the southern margin of North China. The compositional diversity of these granites, as indicated by the NdHf isotopic compositions, is attributed to the nature of the deep crust within the orogenic belt, and the magmatic origins of this region are discussed in the present study.

Unraveling the genesis of wolframite mineralization in the West Qinling Belt, China: Evidence from geochronology, geochemistry, and fluid inclusion study

The West Qinling Belt is a pivotal part of the Central China Orogenic Belt renowned for producing abundant Au-polymetallic deposits. Nonetheless, reports of wolframite mineralization within this region were absent. The Xuehuashan deposit is the premier documented quartz-vein type wolframite deposit in West Qinling, which has significant implications of W mineralizing potential in the West Qinling Belt. The ore bodies are predominantly composed of wolframite-quartz veins and/or veinlets which are proximal or within the Late Triassic Baijiazhuang granitoid. To elucidate the age and genesis of the Xuehuashan wolframite mineralization, we presented analyses of U-Pb dating and chemical composition on wolframite, fluid inclusion study, and hydrogen–oxygen isotopes on wolframite and quartz. The Xuehuashan wolframite U-Pb dating yielded an age of 213.0 ± 6.7 Ma, which is consistent with the age of the Baijiazhuang granitoid. The correlation among trace elements and the Y/Ho vs. Zr/Hf diagram suggests trace elements in wolframite were influenced by crystalline chemical factors and the geochemistry of parental ore-forming fluid. Homogeneous texture and stable chemical composition of wolframite, as well δD vs. δ18O isotopes, suggest a single magmatic fluid from the Baijiazhuang granitoid. Fluid inclusions in wolframite and quartz have high homogenization temperatures and varying salinities of 0.4 – 16.4 wt%, suggesting that intense decompression and fluid boiling during fracture open caused the wolframite precipitation. Compared with the Late Triassic granitoids in the Qinling Belt, the Baijiazhuang granitoid shows higher ratios of Rb/Sr and lower ratios of Zr/Hf, K/Rb, Nb/Ta, LREE/HREE, as well as negative Eu anomalies, which are consistent with the W-related highly evolved granitoids but distinct to W-barren granitoids. Hence, this study highlights W mineralization potential in the Qinling belt associated with the highly evolved Late Triassic granitoids.

New P-T path of Huwan high-pressure unit from Hong'an orogen, Central China Orogenic Belt: Implications for oceanic to continental subduction and exhumation in the subduction zones

Oceanic to continental subduction is a pivotal process in plate tectonics, yet characterizing this tectonic transition and subsequent exhumation remains a challenging task. This study integrates our novel pressure-temperature (P-T) estimates on a metabasite lens and its hosting metasedimentary rock in the Xiongdian area of the Huwan high-pressure unit of the Hong'an orogen, Central China Orogenic Belt, with existing geochronological data from the same and neighboring outcrops. Our preliminary but comprehensive findings reveal the evolution of this subduction transition process. After the opening at ca. 430–400 Ma the Paleotethyan ocean subducted northward and formed a warm oceanic subduction zone at ca. 400–385 Ma, marked by relatively low P and high T conditions (∼610–740 °C and ∼1.5–1.8 GPa; M1 stage). Subsequently, the Paleotethyan oceanic plate continued its subduction and cooled the overlying mantle until its final closure during ca. 385–300 Ma. This phase led to the subduction of South China Block (SCB) and the formation of a cold continental subduction zone at ca. 300–270 Ma, featured by relatively high P and low T conditions (∼570–610 °C and ∼2.1–2.7 GPa; M2 stage). Following the delamination from the subducted SCB continental lithospheric mantle, the detached high-pressure rocks experienced a two-stage accelerated exhumation process, i.e., early slow exhumation (∼0.7 ± 0.4 mm/a) to shallower mantle depth (∼610–730 °C and ∼1.5–2.1 GPa; M3 stage) at ca. 270–243 Ma and later rapid exhumation (>2.5 ± 0.9 mm/a) to approximately the Moho depth (∼550–600 °C and ∼0.8–1.3 GPa; M4 stage) at ca. 243–233? Ma. To facilitate the buoyancy-driven exhumation of eclogite blocks from their maximum depth (M2 stage), a minimum of ∼16–25 vol% of low-density metasedimentary rock is required. The increasing buoyancy caused by decreasing densities of the high-pressure rocks with exhumation (from M2 to M4 stage), slab break-off induced rebound forces, and other tectonic factors may jointly explain the accelerated exhumation pattern. Our density results also emphasize that, partly owing to a stronger contribution from a buoyancy-driven mechanism, the high-pressure rocks would be preferentially exhumed in warm to extremely warm oceanic subduction zones compared to their intermediate to cold counterparts.

Petrogenesis and geodynamic settings of the Devonian adakites and A-type granites in the North Qinling orogen

The Devonian adakites and A-type granites developed in the eastern section of the North Qinling (NQ) orogen in Central China records the evolution of the East Qinling orogen. In this study, petrological, zircon UPb and LuHf isotopic, whole rock element and SrNd isotopic analyses were carried out to investigate the origin and geodynamic setting of the Sikeshu and Shenxianya plutons in NQ. The Sikeshu two-mica granites formed at 415.7 ± 2.3 Ma possess geochemical characteristics similar to those of adakites, while the Shenxianya monzogranites formed at 386.4 ± 1.8 Ma exhibit A-type granite characteristics. Both plutons show enriched SrNd isotopic characteristics and display enrichment and depletion of the zircon Hf isotope. Results show that the Sikeshu two-mica granites were derived by partial melting of a combined source from Erlangping mafic rocks and the Qinling Group, while the Shenxianya monzogranites were derived by partial melting of lower crust granulite. In the late Late Silurian, the mantle-derived magma produced by the slab failure underplated the thickened lower crust, forming the Sikeshu two-mica granites with adakitic features. The NQ underwent a transition from compression to extension during the late Late Silurian-Early Devonian following the closure of the Shandfan Ocean. The mantle-derived magma formed by delamination underplated the lower crustal granulites, generating the Shenxianya A-type granite.

Geophysical constraints on continental rejuvenation in central China: Implications for outward growth of the Tibetan Plateau

Continental rejuvenation results from the tectonic reactivation of crustal structures and lithospheric reworking by mantle flow. Geochemical observations and field mapping have traditionally provided the primary evidence for the secular evolution of crustal composition and tectonic processes during continental rejuvenation. Nonetheless, the impact of continental rejuvenation on the observed present-day strain rate and orogenic-scale lithospheric structure has not been well constrained. The pre-existing E-W−trending Central China Orogenic Belt has been overprinted by the N-S−trending Central Longitudinal Seismic Belt and constitutes the intracontinental West Qinling Syntaxis in central China, where the tectonic setting changes eastward from contraction to extension. Combining updated global positioning system data and high-resolution crustal seismic tomography, we reveal a modern continental rejuvenation process within the West Qinling Syntaxis in central China. The northward extrusion of the Tibetan Plateau’s weak lithospheric layer (middle-lower crust and lithospheric mantle) of southwestern China relative to the rigid Sichuan Basin/Ordos Block of the eastern West Qinling Syntaxis results in regional dextral shearing that shapes the Central Longitudinal Seismic Belt and defines the eastern Tibetan Plateau margin. The pre-existing E-W−trending Central China Orogenic Belt has been preserved above the brittle-ductile transition zone, and the northward movement of the deep lithospheric layer drives the deformation of the upper crust in the West Qinling Syntaxis. Our results, along with previous studies, suggest the presence of an intracontinental lithospheric interchange structure in central China. The continental rejuvenation of the West Qinling Syntaxis results from a combination of fault reactivation in the upper crust (Stage I, Eocene−Oligocene) and reworking of the deep lithosphere (Stage II, middle−late Miocene) related to the plateau-wide shift in stress accommodation ultimately driven by the redistribution of mass outward from the central Tibetan Plateau. At present, the transition zone between the high- and low-velocity anomalies along the Central Longitudinal Seismic Belt not only shapes the landscape boundary but controls the size and recurrence interval of earthquakes within the West Qinling Syntaxis in central China.

A Late Triassic gold metallogenic event within the extended tectonic evolution of the West Qinling Orogen, central China: Constraints from U-Pb and Ar-Ar geochronology in the Yangshan gold district

The West Qinling Orogen in central China resulted from Late Devonian to Triassic collisional orogenesis involving accreted terranes along the southwestern margin of the North China Block with subsequent ocean closure and collision of the South China Block with those terranes. The Yangshan gold district, with a pre-mining resource of more than 308 t gold, is situated along the Mianlue Suture Zone on the southern margin of the orogen. The gold orebodies are mainly controlled by the ENE-trending Anchanghe-Guanyinba Fault and its subparallel second-order faults. The relatively low pressure–temperature mineralization is dominated by disseminated pyrite, arsenopyrite, and stibnite hosted in phyllite and granite dikes, accompanied by intense muscovite-silica-carbonate alteration.Four major phases of deformation related to collisional tectonics are recorded in the Yangshan gold district. Early N-S-oriented compressional deformation (D1) is characterized by an alignment of muscovite, overgrowths on framboidal pyrite, and quartz veinlets. D2 is characterized by a series of continuous ENE-trending reverse faults and folds at multiple scales related to SSE-NNW-oriented compression. D3 is related to gold mineralization and is characterized by a series of ENE-trending, largely normal faults, formed during transtensional tectonics. D4 post-dates gold mineralization and is associated with the development of nearly N-S-trending, low-displacement normal faults.New high-precision SHRIMP zircon U-Pb dating of three types of granite dikes in the Yangshan gold district yields broadly coincident ages of 216 ± 3.0 Ma to 210 ± 9.7 Ma, which is a range identical to that of existing dates of 217–211 Ma for more regional granite magmatism in this district. New Ar-Ar dating of gold-related muscovite gives an age of 210.9 ± 1.6 Ma for the Yangshan gold deposit, indicating that hydrothermal activity overlapped or post-dated the final stages of granite intrusion. Pre-226 Ma peak metamorphism of the ore-hosting rocks occurred prior to extensional ductile shear deformation that is constrained by a published 40Ar/39Ar muscovite cooling age of 223 ± 2 Ma. In good agreement, U-Pb constrained ages of retrograde metamorphism and associated late magmatism of around 215 Ma are reported for rocks along the Mianlue Suture Zone.These geological and geochronological data suggest that relatively shallow-level gold mineralization was broadly coeval with Late Triassic transtensional, orogen-parallel deformation along the regional-scale fault system, late in the tectonic evolution of the West Qinling Orogen. This is consistent with its classification as epizonal orogenic gold mineralization.

A detrital zircon dataset for the eastern Central China Orogenic Belt (East Qinling, Dabie and Sulu orogens)

AbstractThe Qinling‐Dabie‐Sulu orogen is the eastern part of the Central China Orogenic Belt. Looking back on previous studies of the sedimentary record we systematically compiled the dataset of detrital zircon U–Pb ages and Hf isotopes for this orogenic belt. The dataset incorporates a comprehensive compilation of 32,449 detrital zircon U–Pb ages and 979 Hf isotopes. The findings demonstrate that distinct tectonic units within the orogenic belt exhibit unique tectonic evolution histories during various geological epochs. This dataset can be used to constrain the tectonic framework and orogenic evolution including ocean subduction, continental collision and orogen exhumation. It can be integrated with other geological datasets to improve our understanding of the Central Orogenic Belt in China, and possibly plate tectonics globally.

The gold mineral systems of the West Qinling orogen, central China: New insight from the Baguamiao gold deposit

The Baguamiao gold deposit is the first large deposit discovered in the West Qinling orogen in central China in the 1990 s. However, the occurrence of invisible gold in the deposit has not been reported. There is also no consensus on whether magmatic activity is involved in the gold mineralization, the style of mineralization, and the mechanism of gold precipitation. In this study, the in-situ trace elements, and chemical mapping of pyrite and pyrrhotite in the Baguamiao gold deposit were carried out for the first time. These data combined with the C-H-O isotope data of quartz, ankerite and calcite were used to determine mineralization process. In the Baguamiao gold deposit, five types of pyrite (Py0 to Py4) and pyrrhotite (Po0 to Po4) were identified, corresponding to the sedimentary-diagenetic period and four stages of mineralization (Stage I to Stage IV), respectively. From stage I to stage IV, gold mineralization in pyrite and pyrrhotite occurs as either visible or invisible crystals. The latter occurs dominantly as lattice-bound solid solutions (Au+) and is related to the content of Cu and Te. Co and Ni contents and Co/Ni ratios indicate that pyrite and pyrrhotite are magmatic-hydrothermal origin and affected by fluid-rock interaction. The change of Se and Te concentrations in pyrite and pyrrhotite indicates that oxygen fugacity (ƒO2) was increased from stage I to stage II, and then decreased during stage III and stage IV. In addition, the δ18O values of quartz, calculated δ18OH2O values of the ore-forming fluids and δ13C values range from 16.9 to 20.3 ‰, 7.7 to 12.9 ‰, and − 19.3 to + 0.2 ‰, respectively. Such isotopic signature records typical characteristics of orogenic gold deposit. The ore-forming fluid is a mixed hydrothermal solution involving metamorphic water, magmatic water, and meteoric water, and mixed with marine carbonate and sedimentary organic matter. Gold was precipitated by phase separation, fluid-rock interaction, and fluid mixing. Combined with the tectono-magmatic evolution, metamorphic deformation, and gold mineralization age of the West Qinling orogen, it can be concluded that with the closure of the Mianlue portions of the East Paleo-Tethys Ocean, the Baguamiao gold deposit was formed in the post-collisional extensional environment between the South China Block and the North China Block in the Late Triassic and is an orogenic gold deposit reference to the crustal continuum model.

Microcontinental block amalgamation in the northern Tibetan Plateau and its significance for understanding the closure of the Proto-Tethyan Ocean

The Qilian-Qaidam Orogenic Belt is a component of the western Central China Orogenic Belt, and records a nearly complete evolutionary history of the South Qilian and North Qaidam oceans (branches of the Proto-Tethyan Ocean). However, the timing and process of amalgamation of the microblocks of the western Central China Orogenic Belt and the closure time of the South Qilian and North Qaidam oceans remain uncertain. The Olongbuluke Terrane (OLT), located in the central of the Qilian-Qaidam Orogenic Belt, contains well-developed Precambrian to Paleozoic sedimentary successions that shed light on the tectonic history of these ocean basins. In this paper, detrital zircon UPb ages combined with ophiolitic, magmatic and high-pressure to ultrahigh-pressure metamorphic records and field investigations, indicate that the OLT was part of a passive continental margin during the early Paleozoic and there was a microblock, named the North Qaidam Terrane (NQT), lying between the Qaidam Block and the OLT. A marked provenance change occurs in the OLT after ca. 517 Ma, which is inferred to record the collision between the OLT and NQT. Later, the North Qaidam oceanic plate subducted under the NQT, and the North Qaidam Ocean closed at ca. 450 Ma. The similar detrital zircon UPb age spectra of contemporaneous deposit flysch sequences in the Shihuigou Formation (OLT) and the Balonggonggaer Formation (the South Qilian Accretionary Belt) indicates the onset of collision between the OLT and Central Qilian Terrane occurred at ca. 458 Ma. The detrital zircon UPb age spectra of microblocks from the early Paleozoic strata of the northern Tibetan Plateau share a North Indian affinity, indicating that the closure of the Proto-Tethyan Ocean may relate to the formation of the Gondwanan landmass.

Evidence of Vertical Slab Tearing in the Late Triassic Qinling Orogen (Central China) From Multiproxy Geochemical and Isotopic Imaging

AbstractThe process of slab tearing profoundly affects interchanges of material and energy between the lithosphere and asthenosphere and has been invoked as a trigger of magmatism in modern and ancient subduction‐collision systems. Horizontal slab tears may occur after continental collision and subsequent slab break‐off, whereas vertical slab tears may occur in response to along‐strike variations in rollback velocity, thereby ripping the subducting lithosphere. Previous studies have documented tearing of subducting slabs in modern convergent settings using geophysical methods, but in ancient collisional belts, where these methods cannot be applied, detection of slab tearing is more difficult, particularly if the oceanic plate was fully subducted and the geological evidence for subduction is limited. Here, we employ multiproxy element‐isotope imaging, as well as zircon petrochronology and whole‐rock geochemistry, to assess evidence of vertical tearing of the Mianlue slab in the Triassic Qinling Orogen in central China. Our results show evidence of anomalous, inferred tear‐related magmatism east of 107°E, at 225–210 Ma. Vertical tearing of the northward subducting Mianlue slab is recorded by geochemically anomalous igneous rocks, and the recognition of N–S to NE–SW trending boundaries defined by Sr‐Nd‐Hf isotopes, elemental ratios (Nb/Ta, and Ca/Al), Mg#, and reduced crustal thickness between 107° and 108°E. The inferred vertical tear may have been driven by slab rollback of the Mianlue slab at 225–210 Ma. The results show that multiproxy element‐isotope imaging (e.g., of εNd(t), εHf(t), (87Sr/86Sr)i, Nb/Ta, Mg#, and Ca/Al) can be used to reconstruct the evolution of fossil subduction systems.

Geochronology and geochemistry of the Cryogenian sedimentary rocks and Early Paleozoic mafic dykes of the South Qinling belt: Implications for the tectonic evolution of the northwestern South China craton

The late Paleozoic–Mesozoic tectonic evolution of the Central China orogen between the North China and South China cratons was key in the growth of the Asian continent. The preceding Proterozoic–Early Paleozoic geologic history of the Central China orogen is comparatively less well understood yet critical to resolving preconditions for the assembly of Asia. The South Qinling belt located within the Central China orogen exposes a spectacular record of Cryogenian supracrustal sedimentary rocks and Early Paleozoic intermediate-mafic rocks that are related to the development of the northwestern South China craton. In this study, we investigated the Cryogenian sedimentary rocks and Early Paleozoic magmatic dykes/sills in the Daba Shan of the South Qinling belt to provide new constraints on the tectonic evolution of the orogen in the context of the Paleo-Tethyan domain. We report that Cryogenian sedimentary rocks predominantly contain ca. 850–695 Ma detrital zircon grains, which suggests that these short-transported materials were sourced from the northwestern South China craton. Gabbroic dykes that intrude Neoproterozoic rocks have UPb zircon ages ca. 458–446 Ma, which we interpret reflects the timing of regional crustal extension. Geochemical and SrNd isotopic data indicate that Early Paleozoic gabbroic dykes were sourced from the enriched mantle, possibly associated with an upwelling plume that interacted with the lithosphere along the northern margin of the South China craton.

Petrogenesis and tectonic affinity of Early Cretaceous potassic diorites in the northern Taihang Mountain, Trans-North China Orogen

• Early Cretaceous potassic diorites are identified within the central Trans-North China Orogen. • Lithospheric delamination and thermo-mechanical erosion explain lithospheric structures during the Early Cretaceous in the North China Craton. • Early Cretaceous magmatism in the study area was linked to slab sinking and rollback. • The subducted Izanagi plate may be a candidate for carbonated peridotite at ∼140 Ma. Mesozoic mantle-derived potassic magmatic rocks in the Trans-North China Orogen (TNCO) are important for constraining tectonic evolution. However, potassic intermediate plutons in this orogen are rarely reported in literature, despite the significance of such a potassic magmatic event being well understood. Here, we describe the petrography of the Longmen potassic diorites and present zircon U–Pb ages and elemental and Sr–Nd–Mg isotopic compositions for this pluton. The Longmen potassic diorites were emplaced during the Early Cretaceous (ca. 140 Ma), showing shoshonite features with marked depletion of high-field strength elements relative to large ion lithophile elements with a slightly negative Eu anomaly. It possesses restricted ranges of initial 87 Sr/ 86 Sr ratios (0.70592 to 0.70667), ε Nd (t) values (−15.86 to −13.26), and low δ 26 Mg (−0.37 to −0.26‰). Low-degree partial melting of the ancient and enriched mantle, characterised by amphibole-bearing lherzolite in the spinel–garnet transition zone, was the likely mantle source of the evaluated Longmen potassic diorites, resulting from fluid-related metasomatism and upwelling mantle (carbonated) peridotite. Minor crustal materials were present in the lithospheric mantle because of Eastern Block and Western Block subduction/collision during the Paleoproterozoic. Based on our data and previous studies, slab sinking and rollback of the Izanagi plate (Paleo-Pacific plate) may have created an ancient, enriched mantle beneath the East Asian continent, inducing lithospheric thinning and heterogeneity of the lithospheric mantle in the North China Craton and lithospheric extension via thermo-mechanical erosion at 145–110 Ma in the TNCO.

Special issue on tectonic, magmatic and metallogenetic evolution of the central China orogenic belt

ABSTRACT Special Issue on Tectonic, Magmatic and Metallogenetic Evolution of the Central China Orogenic Belt.

Response of trace elements to partial melting of felsic crust at high to ultrahigh temperatures: Implications for granite geochemistry

It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected the composition of granites due to a limited knowledge of trace element behaviors during crustal anatexis. In order to quantify this issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted to be not stable at UHT conditions in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into anatectic melts. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominant factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with DBaK-feldspar/biotite of 2.5–3.0 and DSrK-feldspar/plagioclase values of 1.4–1.9, and ilmenite can preserve more Nb and Ta than biotite with DNbbiotite/ilmenite values of 0.07–0.1 and DTabiotite/ilmenite values of 0.04–0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and orthopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.

Differentiation between carbonate and silicate metasomatism based on lithium isotopic compositions of alkali basalts

Abstract Carbonate and silicate metasomatism occurring in subduction zones is an important mechanism underlying mantle heterogeneity and compositional diversity of mantle-derived rocks. However, distinguishing between the two kinds of metasomatism is often difficult. Lithium (Li) and its isotopes have great potential in this regard because of the different Li isotopic compositions of recycled marine carbonate and silicate components. We report Li isotopic data from Cenozoic and Mesozoic alkali basalts of the West Qinling orogen in central China. Relative to those for normal basalts, very high δ7Li values (up to +11.2‰) were observed for the Cenozoic alkali basalts, but significantly and systematically lower values (as low as −3.3‰) were estimated for the Mesozoic alkali basalts. Their abnormal Li isotopic compositions, combined with major- and trace-element contents and Sr-Mg isotope ratios, indicate that the Cenozoic and Mesozoic alkali basalts originated from carbonated and silicated mantle sources, respectively, reflecting metasomatism of the mantle by slab-derived carbonate and silicate melts during Paleotethyan oceanic subduction. Interactions of such melts with the mantle peridotite in subduction channels can account for the elemental and isotopic differences of the studied alkali basalts. The present study demonstrates an effective way to distinguish between carbonate and silicate metasomatism in subduction zones by studying Li isotopic compositions of alkali basalts and highlights the prospect of Li isotopes in tracing the deep carbon cycle.

Decoupling between Mg and Ca isotopes in alkali basalts: Implications for geochemical differentiation of subduction zone fluids

Chemical metasomatism of the mantle wedge by slab-derived fluids is an important mechanism for the compositional features of mafic magmas above oceanic subduction zones. The possible physicochemical differentiation would occur during the transport of metasomatic agents from the slab to the wedge. This process is indicated by decoupling between Mg and Ca isotopes in alkali basalts from the South Qinling Orogen in central China. These basalts exhibit ocean island basalts (OIB)-like trace element distribution patterns, variable (87Sr/86Sr)i ratios from 0.7040 to 0.7059, positive εNd(t) values of 3.3 to 3.5 and εHf(t) values of 6.2 to 6.6. They have extremely low SiO2 contents, high CaO contents and CaO/Al2O3 ratios. They also show light Ca isotopes with δ44Ca values of 0.64 to 0.96‰, similar to the sedimentary carbonate. In addition, igneous carbonate minerals are abundant in the basalts. They exhibit shifts in δ13C (−5.9 to −1.8‰) and δ18O (16.2 to 18.0‰) towards the sedimentary carbonate, but fall within the ranges of global carbonatite. These geochemical features indicate the mantle source of basaltic magmas was generated through metasomatism of the mantle wedge by slab-derived carbonated melts. However, these basalts have unusual heavy Mg isotope compositions with δ26Mg values of −0.36 to 0.03‰ relative to the mantle values of −0.25 ± 0.07‰, in contrast to typical sedimentary carbonated melts with light Mg isotope compositions. This points to the decoupling between Mg and Ca isotopes, which may be caused by the incongruent MgCa isotope fractionation during the metasomatism. The slab-derived fluid may undergo significant differentiation during their metasomatism of the overlying mantle wedge, and this process may have completely removed the carbonate effect on Mg isotopes but retained Ca isotopes as the “fingerprint” of the recycled carbonate. Our model calculation results further reveal that when the carbonated melts would be differentiated at considerable degrees (F > 0.1), the decoupling of MgCa isotopes begins to appear. Therefore, the MgCa isotopes decoupling in the basalts provide new insights into the differentiation behaviors of subduction zone fluids. This differentiation process may influence the composition of metasomatic agents and thus the resultant mafic igneous rocks.

Triassic provenance shifts and tectonic evolution of southeast Ordos Basin, Central China

Tectonic evolution of the North China Craton underwent a complicated process during the early Mesozoic owing to almost simultaneous closures of the Mianlue Ocean to the south and the Palaeo-Asian Ocean to the north. The tectonic setting of the Triassic sedimentary basin in the southern North China Craton remains a subject of debate. The conventional view is that early Mesozoic basins resulted from plate collision and crustal shortening, whereas new petrological evidence of crustal extension has been identified in the southern North China Craton. In this study, we used petrology, sedimentology, subsidence and geochronology to trace basin provenance and reconstruct source-to-sink around the southeast Ordos Basin. The petrographic modal compositions showed that the provenance changed from recycled orogenic to a mixture of recycled orogenic and arc magmatic materials. Tectonic subsidence of the southeast Ordos Basin revealed two episodes of rapid subsidence, the second of which lasted ~10 Ma and caused the depocenter of the basin to move southward. Early Triassic detrital zircons with distinguishing ages of 1100–700 Ma were derived from the Central China Orogenic Belt and North China Craton; then, they were sourced from the Inner Mongolia Palaeo-uplift, based on zircon ages clustered at ~400–250 Ma. The petrographic modal composition, tectonic subsidence and zircon age distribution imply a provenance shift from short and rapid uplift (North Qinling Orogenic Belt) to remote continuous uplift (Inner Mongolia Palaeo-uplift); this is also supported by the palaeo-current direction. This shift defined a change in the Qinling Orogenic Belt from collision and uplift to extension, which caused the depositional centre of the Ordos Basin to move southward. This integrated tectono-sedimentary environment study provides new insight into the closure process of the Mianlue Ocean during the Triassic and its impact on the tectonic environment of the southern North China Craton.

Early Paleozoic arc-back-arc system evolution in the junction of the Qinling and Qilian Orogens: Geochemical constraints from ca. 445–430 Ma magmatic rocks in the Tianshui area

ABSTRACT The Tianshui area is located at the junction of the Qinling and Qilian Orogens, the evolution of which is crucial in understanding the Early Palaeozoic tectonic evolution of the Central China Orogenic System. We present here geochronological, geochemical, and Sr-Nd isotopic data for four representative Silurian magmatic rocks, termed the Dangchuan, Huamiao, Yanjiadian, and Hongtupu, along a cross section across the main rock units of the Tianhui area. The 432 Ma Dangchuan syenogranites are peraluminous with negative εNd(t) values, high initial 87Sr/86Sr ratios, and low Rb/Sr and high CaO/Na2O ratios. We interpret the syenogranites to be generated by partial melting of metagreywackes during a Silurian orogenic event. The 436 Ma Huamiao diorites are high-Mg diorites with high MgO contents and Mg# (63–65) and enriched Sr-Nd isotopic compositions. The 445 Ma Yanjiadian diorites are high-Mg adakitic rocks with high Sr and Mg# and low Y and Yb contents. They were both generated by the interaction between sediment/slab-derived fluids/melts and mantle peridotite in the subduction-related setting. The 445 Ma Hongtupu basalts include E-MORB and IAB with depleted Sr-Nd isotopic compositions. They were derived from enriched mantle and sediment-modified mantle, respectively, and formed in a back-arc basin setting. Synthesizing our new and published data, we propose a tectonic model involving generation of Baihua-Huluhe arc-back-arc system, subduction of the Huluhe back-arc oceanic basin, and collision between the North Qinling arc terrane and North China Block, which is closely associated with the closure of the Proto-Tethys Ocean.

Late Mesozoic Huangbeiling S-type granite in the East Qinling Orogen, China: Geochronology, petrogenesis and implications for tectonic evolution

S-type granites are typical features of collisional orogenic belts and could provide insights into the tectonic process associated with the final phase of orogeny. The East Qinling Orogen, one of significant segments in the Central China Orogen, witnessed complex tectonic evolution during the Late Mesozoic. The rare S-type granites in this orogen can be used as important proxies to understand the Late Mesozoic tectonic processes. Although a few previous studies suggested that the Huangbeiling pluton in the East Qinling Orogen might be S-type granite, detailed studies are lacking. Thus, we report the results from a systematic petrological, whole-rock geochemical and zircon U-Pb-Lu-Hf isotopic studies on the Huangbeiling pluton, with a view to constrain the timing of magmatism, petrogenetic evolution and genetic type, and to evaluate the implications for Late Mesozoic tectonic evolution of the East Qinling Orogen. Zircon U-Pb analysis yield 206Pb/238U spot ages in the range of 156.7–132.2 Ma, with weighted 206Pb/238U mean ages varying from 146.8 to 141.9 Ma, suggesting the Huangbeiling pluton formed during the Late Jurassic to Early Cretaceous. Zircon Lu-Hf isotopic data show negative εHf(t) values of −21.5 to −14.9 and two-stage Hf model ages of 2546–2131 Ma, which are correlated with the Neoarchean to Paleoproterozoic (3.0–2.1 Ga) meta-sedimentary rocks from nearby Taihua Group, indicating that the magma was sourced from reworked ancient crustal components involving meta-sedimentary rocks. Whole-rock geochemical data display enrichment of LREEs, Pb, Hf and Y as well as depletion in HREEs, Ba and HFSEs (e.g., Ta, P, Ti), with weakly negative Eu anomalies. The Huangbeiling granitoids are identified as S-type granites, which generated through partial melting of lower-middle crust and upper crustal fractional crystallization in syn-collisional settings. In conjunction with published information related to the tectonic evolution of the East Qinling Orogen, we propose that the Late Mesozoic Huangbeiling S-type granites might response to the complex tectonic evolution related to extensional tectonics induced by multi-directional (intra-continental) subductions from the Yangtze and North China Cratons as well as the Paleo-Pacific Plate during the Late Mesozoic.

Sericite 40Ar/39Ar and zircon U-Pb dating of the Liziyuan gold deposit, West Qinling orogen, central China: Implications for ore genesis and tectonic setting

The West Qinling orogen in central China contains several dozens of gold deposits. However, absolute ages of gold mineralization and its relations to regional tectonism, magmatism, and metamorphism remain not tightly constrained for most of these gold deposits. Here we integrate 40Ar/39Ar dating of sericite from various hydrothermal alteration assemblages and U-Pb dating of zircon grains from magmatic rocks within and surrounding the Liziyuan gold deposit in the northern belt of the West Qinling orogen to provide new insights into the age and tectonic setting of gold mineralization. The Liziyuan gold deposit consists of six ore camps (Yuzigou, Liushagou, Jiancaowan, Kuangou, Suishizi, and Yingfang) that are hosted in Ordovician greenschist facies metamorphosed sedimentary-volcanic rocks and, less significantly, the Middle Triassic Tianzishan monzogranite pluton intruding the metamorphic rocks. Gold mineralization consists of auriferous quartz-pyrite veins and subordinate sulfide disseminations in hydrothermally altered wall rocks, with minor calcite-polymetallic sulfide veins in the monzogranite-hosted orebodies. Ore-related alteration assemblages are dominated by quartz, sericite, pyrite, chlorite, and calcite, with gold occurring mostly as native gold grains enclosed in or filling microfractures of pyrite and quartz. Barren alteration assemblages, dominated by K-feldspar, quartz, sericite, and pyrite, are locally developed along eastern margin of the Tianzishan monzogranite. Two sericite aggregates extracted from gold ores in the Liushagou camp yield well-defined 40Ar/39Ar plateau ages of 200.1 ± 1.2 and 198.8 ± 1.1 Ma (2σ), whereas those from barren alteration assemblages in the Tianzishan monzogranite, which hosts the Suishizi ore camp, give rise to older 40Ar/39Ar plateau ages of 211.9 ± 1.2 and 211.3 ± 1.3 Ma (2σ). The Tianzishan monzogranite pluton and several dioritic to granitic stocks and dykes in and surrounding the gold mines have zircon U-Pb ages ranging from 241.3 ± 1.2 to 212.4 ± 0.9 Ma (2σ). These new isotopic ages suggest that the Liziyuan gold deposit formed at the Triassic-Jurassic boundary, arguably representing the youngest orogenic-type gold mineralization event over the northern belt of the West Qinling orogen. Gold mineralization postdates magmatism in the Liziyuan gold deposit at least by about 10 million years, precluding a possible genetic relation between the two. Rather, the ore genesis was likely related to metamorphic fluids released from Paleozoic sedimentary and volcanic rocks in relation to the Qinling orogenesis. In contrast, the barren hydrothermal alteration assemblages were likely caused by fluids exsolved from a magma presumably represented by the ca. 212 Ma diorite dyke. Results presented here, when combined with independent geological and geochemical data, suggest that the Liziyuan gold deposit formed most likely under a post-collisional extensional setting related to the West Qinling orogeny.