Central China Orogen Research Articles

Deep subduction and exhumation of micro-continents in the Proto-Tethys realm: Evidence from metamorphism of HP-UHT rocks in the North Qinling Orogen, central China

The North Qinling Orogen in central China contains a typical continental high- and ultrahigh-pressure (HP-UHP) metamorphic belt, but its metamorphic evolution remains controversial. We report a combined investigation on petrology, geochronology and phase modelling for felsic granulite (samples SN1504 & SN1517) and garnet clinopyroxenite (sample SN1505) from the Songshugou area. Four stages of metamorphic evolution are constrained: (i) A possible prograde from HP amphibolite facies to peak UHP eclogite facies stage (~3–8 GPa/700–1080 °C) recorded by the chemical zoning of garnet core and mantle in sample SN1504 and also the presence of supersilicic titanite in sample SN1505. (ii) A post-peak decompression evolution is recorded by the formation of coronary garnet and plagioclase around kyanite in sample SN1504 and high-Ti amphibole and plagioclase in sample SN1505, including the evolution from eclogite facies to HP–ultrahigh temperature (UHT) conditions of 1.3–2.2 GPa/900–980 °C for sample SN1504 and 1.0–2.1 GPa/960–1010 °C for SN1505, respectively. (iii) Further decompression is revealed from the growth of corundum + spinel in local domains in sample SN1504, suggesting a low-pressure (LP) condition of ~0.8 GPa/950 °C. (iv) A subsequent cooling evolution is recorded by thin films of K-feldspar + quartz + biotite in sample SN1504 and low-Ti amphibole in sample SN1505, indicative of melt crystallization towards the solidus with conditions of 830–770 °C/0.55–0.8 GPa and 880–820 °C/0.8–0.9 GPa, respectively. Zircon and monazite U-Pb dating yields three metamorphic ages of c. 521 Ma, c. 503 Ma and c. 500–480 Ma, which are interpreted to represent the prograde HP amphibolite facies stage, the peak eclogite facies stage and the late cooling stage, respectively. The metamorphic P–T–t path suggests a complete history of deep continental subduction to depths of >90–250 km, followed by rapid exhumation to crustal depths for the Qinling micro-continent during the early Proto-Tethys evolution.

HP–UHT granulites in the East Kunlun Orogen, NW China: Constraints on the transition from compression to extension in an arc setting of the Proto‐Tethys Ocean

AbstractThe East Kunlun Orogen (EKO) of the northwest part of the Central China Orogenic belt records long‐lived orogenic events from the Cambrian to the Triassic, involving the tectonic evolution from Proto‐Tethys to Palaeo‐Tethys. Although high‐ and ultra high pressure (HP‒UHP) metamorphism has been recognized in the EKO, other metamorphic events associated with the orogenic history are little known. The metamorphic history of high‐ and ultra high temperature granulites and their pressure–temperature–time (P‒T–t) path is keys to reconstructing tectonic events associated with a subduction‐to‐collision transition. In this contribution, we present P‒T–t results for the mafic and pelitic granulite facies metamorphic rocks in the Qingshuiquan region of the EKO, determined using phase equilibrium modelling and zircon geochronology. The Jingshuikou granulite terrane records a clockwise P–T path with high‐P and ultra high temperature (HP–UHT) conditions at Pmax ~900 to 950°C at 13 kbar, at Tmax of ~950°C at 9‒12 kbar, and a final stage of 800‒895°C at 6‒10 kbar. LA‐ICP‐MS U–Pb dating of zircons provides systemic constraints on a long‐lived metamorphic age span of 530‒465 Ma, with a peak of c. 503 Ma. The HP–UHT metamorphism of the Qingshuiquan granulite terrane reveals a complete process from arc compression at the early stage of subduction to extension on the active continental margin related to rollback of the subducted slab in the Early Palaeozoic, driven by heat transfer from upwelling of the sub‐arc hot asthenosphere. The timing and P‒T conditions of the high geothermal gradient metamorphism can be used to understand the subduction and collision process of the Proto‐Tethys Ocean, including the relationship between high‐P and high‐ to ultra‐temperature metamorphism in the East Kunlun and other orogenic belts.

Zircon U–Pb geochronology and geochemistry of the Lajimiao mafic complex in the Shangdan Suture Zone, Qinling orogen: Petrogenesis and tectonic implications

The Shangdan Ocean represents the eastern branch of the Proto-Tethys Ocean, which is important to decipher the evolution of the Central China Orogenic Belt. However, its evolution is still in debated. In this contribution, we carried out an integrated study of zircon UPb ages, whole-rock geochemistry, as well as zircon HfO isotope compositions for the Lajimiao mafic complex in the Shangdan Suture Zone in the North Qinling orogen. The Lajimiao mafic complex is composed of coarse-grained gabbros to norite-gabbros (Group 1) in the inner and fine-grained diorites (Group 2) in the outer part. The Group 1 samples are comprised predominantly of salite to augite (Mg# = 75–80), magnesi-hornblende (Mg# = 65–67) and Ca-rich plagioclase (An = 45–67); the Group 2 samples are comprised predominantly of magnesi-hornblende (Mg# = 60–66) and plagioclase (An = 28–53). New SIMS and LA-ICP-MS UPb dating of zircon grains from a gabbro and a diorite sample yielded similar weighted mean ages of 412 ± 4 Ma and 414 ± 2 Ma, indicating that they were formed simultaneously. The Group 1 samples have relatively low SiO2 (49.56–50.78 wt%), high MgO (6.67–7.70 wt%) and Fe2O3T (7.46–9.61 wt%) contents, while the Group 2 samples have relatively high SiO2 (57.74–59.53 wt%), low MgO (3.28–3.42 wt%) and Fe2O3T (5.76–7.01 wt%) contents. They both belong to calc-alkalic series and enriched in large ion lithophile elements (LILEs), light rare earth element (LREEs) and depleted in high field strength elements (HFSEs), which are typical of arc-related rocks. The Group 2 diorites exhibit relative enrichment in light rare earth element (LREE) ((La/Yb)N = 10.9–13.1) with slightly negative Eu anomalies (Eu/Eu⁎ = 0.82–0.93), compared to the Group 1 samples ((La/Yb)N = 5.2–9.6; Eu/Eu⁎ = 0.91–1.43). Both of them have enriched initial Sr (0.7049–0.7054), and chondritic to slightly depleted Nd (ƐNd(t) = −0.09 ~ +1.50), but depleted whole-rock and zircon Hf isotopic compositions (ƐHf(t) = 7.81–8.10 and 8.2–9.1). The HfNd isotopic systems are decoupled with positive △ƐHf values (5.22 to 5.92) (△ƐHf = ƐHf(t) – (1.59 × ƐNd(t) + 1.28)). The zircon grains display higher δ18O values of 6.3 to 6.5‰ than those of the normal mantle zircon. Whole-rock Pb isotopic compositions of the studied samples are of 15.610–15.624 for 207Pb/204Pb, 38.027–38.178 for 208Pb/204Pb, and 18.100–18.269 for 206Pb/204Pb. These geochemical features indicate that the Lajimiao mafic complex was derived from relatively depleted mantle that had been metasomatized by 2–5% subducted zircon-barren oceanic sediments. Combining with regional geological, geochronological, mineralogical, and geochemical data, the Lajimiao mafic complex was suggested to be generated by partial melting of a phlogopite-bearing mantle source at relatively low pressure in an arc setting. We suggested that the generation of the Lajimiao mafic complex was induced by the subduction of Shangdan Ocean during the early Devonian and thus the eastern branch of Proto-Tethys Ocean might have not been closed until that time.

Metamorphic history and Neoarchean–Paleoproterozoic crustal growth of the central Trans-North China Orogen: Evidence from granulite- to amphibolite-facies rocks of the Hengshan complex

The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional orogen exposed within the North China Craton (NCC). The orogen has undergone a complex tectonic evolution, but its role in the growth of continental crust is contentious. High-grade metamorphic rocks may carry key information required to clarify these issues. Here, using petrography, mineral chemistry, and zircon U–Pb–Hf isotopes, we investigate a suite of Neoarchean–Paleoproterozoic high-grade metamorphic rocks in the Hengshan Complex within the central TNCO. Garnet-bearing mafic granulites and amphibolites occur as lenses or dikes within tonalite–trondhjemite–granodiorite (TTG) gneisses. Three stages of metamorphic mineral assemblages are identified in a mafic granulite from the northern Hengshan Complex: (1) the prograde assemblage is preserved as small plagioclase and apatite inclusions within garnet porphyroblasts; (2) the peak mineral assemblage is recorded by a high-Ca garnet core coexisting with clinopyroxene, plagioclase, hornblende, biotite, and quartz; and (3) the retrograde assemblage is represented mainly by vermicular symplectites of hornblende + plagioclase and plagioclase coronae surrounding low-Ca garnet rims. Phase equilibria modeling and conventional geothermobarometry suggest that the mafic granulite records a clockwise P–T path, including decompression and cooling from the granulite-facies peak (12.5–14 kbar and >750 °C) to amphibolite-facies conditions during retrogression (7.3–8.3 kbar and 615–721 °C). In contrast, the amphibolite in the southern Hengshan Complex preserves only amphibolite-facies assemblages (hornblende + plagioclase + quartz), consistent with the metamorphic pressures of <7.1 kbar and temperatures below 720 °C.Zircon U--Pb dating of magmatic zircons shows that the protoliths of the TTG gneisses and amphibolite were emplaced at ca. 2.5 and 2.1 Ga, respectively. The metamorphic zircons reveal ages of 1.92–1.80 Ga for the mafic granulite (weighted mean of ca. 1.87 Ga) and 1.92–1.83 Ga for the amphibolites (weighted mean of ca. 1.88 Ga). These 1.88–1.87 Ga metamorphic ages are interpreted to represent the time when the rocks cooled to the solidus. By comparing Paleoproterozoic metamorphic ages across the TNCO, we infer that the subduction–collision–amalgamation between the Western and Eastern Blocks of the NCC lasted from 1.95 to 1.80 Ga at least, marking the final cratonization of the NCC. The ca. 2.5 Ga magmatic zircons within the TTG gneisses show positive εHf(t) values (+0.49 to +7.12) with model ages of 2.8–2.7 Ga, suggesting that the protoliths were derived from the partial melting of Neoarchean juvenile mafic crust. Zircon Hf model ages (TDM = 2.1–2.0 Ga) of the mafic granulite may represent the approximate time at which the mafic magma was generated in the mantle. Thus, the Hengshan Complex within the central TNCO records two episodes of crustal growth at 2.8–2.7 and 2.1–2.0 Ga, respectively. To further understand the processes involved in crustal evolution, we reconstructed a layered crust model for the Hengshan Complex, and the layered crust was strongly influenced by Paleoproterozoic thermal events.

A deforming plate tectonic model of the South China Block since the Jurassic

The South China Block has experienced multiple phases of tectonic deformation since the Middle Jurassic. Understanding this deformation history helps reveal the driving mechanisms responsible and sheds light on the associated magmatism and mineralization. Here, we construct deformable plate tectonic reconstructions for the South China Block from 170 Ma to the present using GPlates. The deformation starts with a NW–SE-oriented shortening phase from 170 to 136 Ma. Subsequently, South China experienced two episodes of extension in the Cretaceous, during 136–118 Ma and 100–80 Ma, respectively. The Cenozoic extension occurred mostly along the southeast continental margin, including the East China Sea Shelf Basin, and rift basins in the northern South China Sea margin and the Taiwan margin segment. The rifting was initiated nearly simultaneously along the continental margin at ~65 Ma and ceased at ~34–30 Ma. The northern portion of the Okinawa Trough started rifting at 15 Ma, while the southern portion rifted in the last few million years. The Luzon Volcanic Arc’ thrusting onto the southeasternmost South China Block since 6.5 Ma caused the still-active Taiwan Orogeny. The predicted accumulated deformation at present-day is characterized by overall shortening onshore with contraction factors mostly between 0.8 and 1 and extension offshore with stretching factors generally between 1 and 2. The Central China Orogen and Taiwan Orogen experienced more intense shortening relative to the interior South China by factors down to 0.6. The Okinawa Trough experienced intense stretching by factors up to 2.5 in its central part. The intraplate deformation in the South China Block is closely related to the subduction of the Izanagi and Pacific plates. The Jurassic and earliest Cretaceous shortening was likely induced by trench advance, and the extension during the Cretaceous and the Cenozoic was mostly driven by subduction retreat.

Paleoproterozoic tectonic evolution of the northern Yangtze craton from oceanic subduction through continental collision to continental rifting: Geochronological and geochemical records of metabasites from the Tongbai orogen in central China

We report for the first time the existence of multistage Paleoproterozoic collision- and rift-related tectonothermal events in the northern margin of the Yangtze craton. This is achieved through an integrated study of geochronology and geochemistry for Phanerozoic metabasites from the Tongbai orogen in central China. Zircon U-Pb dating and trace element analysis reveal two episodes of magmatism and one episode of metamorphism from ca. 1.96 Ga to 1.63 Ga. Whole-rock geochemistry indicates that the 1.96 Ga metabasite shows typical arc-like trace elemental distribution patterns with enrichment in LREE but depletion in HFSE, whereas the 1.63 Ga metabasites are characterized by enriched mid-ocean ridge basalts (MORB)- or ocean island basalts (OIB)-like trace element distribution patterns with enrichment in LREE but no depletion in HFSE. In addition, a 1.84 Ga granulite facies metamorphic event is recorded by sector zoned domains of zircon from the 1.96 Ga metabasites, showing flat HREE patterns with markedly negative Eu anomalies and a high formation temperature of 825 °C. Integrating the available data from the northern Yangtze craton, we suggest that the 1.96 Ga metabasite was produced by decompressional melting of the metasomatic mantle domain subsequent to exhumation of the high-pressure metamorphic rocks, the 1.84 Ga granulite facies metamorphism was induced by asthenospheric upwelling in response to post-collisional extension, and the 1.63 Ga magmatism was associated with lithospheric extension due to continental rifting. Therefore, the Paleoproterozoic metabasites in the Tongbai orogen witness the tectonic evolution from oceanic subduction through continental collision to continental rifting, indicating that the northern margin of the Yangtze craton was involved in the assembly and breakup of supercontinent Columbia. In comparison to the coeval rift-related igneous rocks in other blocks, the geochemical features of 1.63 Ga metabasites in the northern Yangtze craton are similar to those in the northern Australia, but are significantly different from those in the southern Siberia and the northern Laurentia. This indicates a possible connection between the northern Yangtze and the northern Australia before the Columbia breakup.

Geology, fluid inclusions, and H–O–S–Pb isotopes of the Chigou porphyry Cu deposit in Southern Qinling, central China: Implication for ore genesis

The Chigou porphyry Cu deposit is located in eastern segment of the Southern Qinling metallogenic belt, on the southern margin of the Central China Orogenic Belt. Copper mineralization is associated with quartz diorite, quartz diorite porphyry and granite porphyry, and occurs as veins, veinlets, disseminations, or veinlet-disseminations within host porphyries. Ore-forming process could be divided into three stages: stage I quartz + K-feldspar + pyrite ± chalcopyrite, stage II quartz + chlorite + chalcopyrite ± molybdenite and stage III quartz + calcite + pyrite. Four types of fluid inclusions (FIs) were distinguished in quartz-bearing veins, including vapor-rich two-phase (V-type), liquid-rich two-phase (L-type), hypersaline (H-type), and CO2-rich (C-type) inclusions. The FIs were homogenized at temperatures of 330–473 °C for stage I, 174–345 °C for stage II, and 128–272 °C for stage III, with corresponding salinities of 3.8–53.6 wt% NaCl equiv., 1.4–15.1 wt% NaCl equiv., and 0.4–7.7 wt% NaCl equiv., respectively. Hydrogen and oxygen isotope data indicate that the ore-forming fluids have a dominant magma signature and were diluted by meteoric water during the late stage. Chalcopyrite and pyrite from different stages record a narrow range of δ34S values from −0.2 to −2‰, consistent with the sulfur isotopic values of typical magmatic-hydrothermal system. Chalcopyrite has similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios to those of porphyry intrusions. All stable and radiogenic isotope results reveal that the ore-forming components (i.e. metal and sulfur) were sourced from a magmatic reservoir with some crustal materials. Laser Raman micro spectroscopy data and fluid inclusion analytical results suggest that the Chigou deposit was formed by an initially high- to intermediate-temperature, high-oxygen fugacity (fO2) and CO2-rich fluid system at an estimated depth of 0.7–2.1 km. Decreases in temperature and fO2 as well as fluid boiling were critical factors controlling sulfide precipitation at Chigou.

Tectonic Setting of the Eastern Margin of the Sino-Korean Block in the Pennsylvanian: Constraints from Detrital Zircon Ages

To test the previous hypothesis that upper Paleozoic sediments in the eastern Sino-Korean Block were mostly derived from the paleo-orogen located to the east, we compared published and new U–Pb age data of detrital zircons from Pennsylvanian strata distributed in the Sino-Korean Block (SKB). The age distributions of detrital zircons from different localities of Pennsylvanian strata in North China reflect varying contributions from the Inner Mongolia Paleo-uplift in the north and the Central China Orogenic Belt in the south. The supply of detritus from the northern source to distant areas, however, appears to have been limited during the Pennsylvanian times. The age distributions of detrital zircons from Korean Pennsylvanian strata located in the east of the SKB are characterized by a dense cluster of 1.84–1.90 Ga and differ from those of North China. The Korean age characteristic is best explained by strong influences of the detritus derived from the Paleoproterozoic Yeongnam Massif in southeastern Korea. Along with the significant number of zircons that record syn- to near-depositional magmatic activities, this observation supports the hypothesis of the existence of an active continental margin setting in the east of the SKB.

Petrogenesis of Triassic granitoids in the Fengxian–Taibai ore cluster, Western Qinling Orogen, central China: Implications for tectonic evolution and polymetallic mineralization

The petrogenesis and tectonic setting of the Triassic granitoids, and their relationship with the coeval Au and Pb–Zn polymetallic mineralization in the Western Qinling Orogen (WQO) in central China remain debated. The Fengxian–Taibai Ore Cluster (FTOC) in the eastern part of the WQO hosts large-scale Triassic granitoids and numerous polymetallic deposits, providing an excellent opportunity to constrain the controversies mentioned above. In this work, we choose the Huahongshuping granodiorite pluton, Guji biotite monzogranite pluton, Taoerling biotite monzogranite dike, Dongtangzi granite porphyry dike and Dagou dioritic dikes in the FTOC for a combined study on the zircon U–Pb geochronology, whole-rock major, trace elements and Sr–Nd isotopes. LA–ICP–MS zircon U–Pb dating yields ages of ~225 Ma, ~224 Ma and 227–217 Ma for the Huahongshuping pluton, Guji pluton and granitoid dikes, respectively. All these granitoids have metaluminous and high-K calc-alkaline series affinities, and are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE) and depleted in high field-strength elements (HFSE) with a marked LREE/HREE fractionation ((La/Yb)N = 10.66–32.42), fractionation of LREE ((La/Sm)N = 2.72–6.60), and negative Eu anomalies (δEu = 0.45–0.99). They have initial Sr isotopic ratios of 0.7062–0.7082, and negative εNd(t) values of –9.42 to –8.09. The above features indicate that the FTOC granitoids were mainly derived from the partial melting of the Neoproterozoic meta-volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite, with direct contribution of mantle materials. Combined with previous data on sedimentation, deformation and metamorphism, and tectonics, we suggest that the WQO underwent the Early Mesozoic tectonic evolution from the subduction at 248–235 Ma, through syn-collision at 235–225 Ma, to post-collision at 225–195 Ma. Integrating with the ore geology, geochronology, mineralogy, and quartz and carbonate H–O, and in-situ S isotope data of sulfides from different mineralization stages at representative Au and Pb–Zn deposits in literatures, we propose a conceptive model that the post-collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the FTOC.

Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

AbstractA combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U–Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490–450 Ma and involves high‐P (HP) metamorphism (M1) at 785–815°C and 10–14 kbar followed by decompressional heating to 840–880°C and 8–9 kbar for medium‐pressure granulite facies metamorphism (M2), defining a clockwise P–T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K‐feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P–T path with continuous heating to ultrahigh‐temperature (UHT) metamorphism (M3) at 890–980°C and 9–11 kbar, followed by decompressional cooling to 740–880°C and 7–9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh‐pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision‐thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.

Qinling gneiss domes and implications for tectonic evolution of the Early Paleozoic Orogen in Central China

The Qinling arc terrane (QAT) at the southern margin of the North Qinling Early Paleozoic orogen, Central China, is composed of gneiss domes overridden by cold allochthon nappes. The gneiss domes are characterized by migmatitic and plutonic cores of granite or gabbro and overlying upper amphibolite to granulite facies metamorphic sequences with pervasive injection of leucogranitic veins. The gneiss domes display a high temperature domal foliation dipping radially away the core and a down-dipping stretching lineation. Cascading folds occur in the mantling metamorphic rocks. The new data of zircon/monazite U-Pb dating suggest that the gabbroic and granitic plutons formed at 514–476 Ma and 495–441 Ma, respectively, whereas the high-grade metamorphism of the mantling rocks took place at 469–412 Ma, and the leucogranitic veins formed at two main episodes (472–440 Ma and 416–413 Ma). Mica 40Ar/39Ar dating yields 402–328 Ma for the cooling ages of the leucogranitic veins and gneissic rocks. The basaltic magma underplated into the QAT lower crust resulted in partial melting, producing granitic magma at the Late Cambrian to Middle Ordovician during northward subduction of the Shangdan Proto-Tethyan oceanic slab. Buoyant upwelling of the migmatitic and plutonic core was promoted by constriction in the lower crust and decompression-extension in the upper crust during the Silurian. Since the closure of the Shangdan Ocean and the collision between the North China Block and South China Block (～400 Ma), the Qinling gneiss domes experienced rapid uplift, and were finally overthrust by the upper allochthon transported from the North Qinling.

Crustal density structures and isostasy beneath the Western North China craton, Trans-North China Orogen, and surrounding regions

To determine the lateral and vertical variations in crustal structure and their influence on the seismicity of the Western North China Craton, the Trans-North China Orogen, and the surrounding regions, the wavelet multi-scale structures, Moho depth, crustal density structures, and isostatic state are modelled using Bouguer gravity anomaly data, topography, and earthquake focal mechanisms. We obtained homogeneous crustal densities and deviations of <1 ​km between the crustal thicknesses estimated from the isostatic model and those inverted from the Bouguer gravity anomalies in the Ordos Block, the Inner Mongolia Suture Zone, the Sichuan Basin, and the Jizhong Depression. These results provide new evidence for relatively simple and stable continental crustal structures, and indicate that these regions will remain stable in both the vertical and lateral directions. The Hetao Graben, Yinchuan Graben, Weihe Basin, and Shanxi graben system have heterogeneous crustal densities and are isostatically over-compensated. In contrast, the crust beneath the Yinshan Uplift, Lvliang Uplift, and northern and central Taihang Uplift is thin and under-compensated. The heterogeneous crustal densities and non-isostatic state beneath the Tibetan Plateau and Qinling Central China Orogen indicate that these two blocks are unstable in the vertical and lateral directions. Although Cenozoic deformation of the North China Craton is thought to be driven by lithospheric stresses related to the India-Eurasia collision and Pacific slab retreat in South East Asia, we suggest that gravitational potential energy created by the heterogeneous crustal structure modulates these first-order forces. The results of this study could constrain the causes of seismicity in systems surrounding the Ordos Block.

UHP metamorphism recorded by coesite-bearing metapelite in the East Kunlun Orogen (NW China)

AbstractThe East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U–Pb age systematics andP–Tconditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U–Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane.P–Tcalculations suggest that metapelite may have experienced a clockwiseP–Tpath with peakP/Tconditions of 685 ± 41 °C and &gt;28 kbar, and equilibrated at 482–566 °C and 5.6–8.9 kbar during subsequent exhumation. The high-pressure – ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints

Granitoids are important components of major orogenic belts, and provide important information about the regional geodynamic evolution. The emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed, although it still remains debated. The Qinling Orogen within the Central China Orogen was marked by the emplacement of numerous Late Triassic granitic plutons. Although the petrology, geochemistry and geochronology of these intrusions have been addressed in various studies, their tectonic setting remains controversial, particularly since the structural aspects not been evaluated in detail. In this study, we attempt to reconstruct the emplacement process of the Late Triassic Dongjiangkou pluton in the South Qinling Belt. Field observations show extensive syn-plutonic deformations both in the pluton and its contact zones. Microstructural observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high-T solid-state deformation. Zircon U–Pb ages reveal that the pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages (~210 Ma and ~200 Ma). Al-in-hornblende thermobarometer indicates that the pluton was formed at depths ranging from 4.7 km to 8.8 km, with an increasing depth trend from the inner unit to the outer unit. Distribution of the internal fabrics shows two concentric patterns which are concordant with pluton margins at the pluton scale and were probably induced by the regional sinistral transpression. Integrating these analyses, an incremental emplacement model is proposed for the syn-tectonic pluton. This model not only solves the ‘room problem’ but also accounts for the zoned petrological features of the pluton. Combined with previous studies, we suggest that the Late Triassic granite plutons in the Qinling Orogen were emplaced under a syn-collisional convergence setting, and that the granite magmatism was probably controlled by regional tectonics. Additionally, the incremental emplacement model may be a common mechanism for the Late Triassic granite plutons.

Late Mesozoic magmatism in the East Qinling Orogen, China and its tectonic implications

The Qinling Orogen in Central China records the history of a complex geological evolution and tectonic transition from compression to extension during the Late Mesozoic, with concomitant voluminous granitoids formation. In this study, we present results from petrological, geochemical, zircon U-Pb-Lu-Hf isotopic studies on the Lengshui felsic dykes from Luanchuan region in the East Qinling Orogen. We also compile published geochronological, geochemical, and Hf isotopic data from Luanchuan region and present zircon Hf isotopic contour maps. The newly obtained age data yield two group of ages at ∼145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes, with the ∼145 Ma interpreted as response to the peak of magmatism in the region, and the ∼140 Ma as the timing of formation of the felsic dykes. The corresponding Hf isotopic data of the granite porphyries display negative εHf(t) values of –16.67 to –4.61, and Hf crustal model ages (TDMC) of 2255–1490 Ma, indicating magma sourced from the melting of Paleo- to Mesoproterozoic crustal materials. The compiled age data display two major magmatic pulses at 160–130 Ma and 111–108 Ma with magmatic quiescence in between, and the zircon Hf isotopic data display εHf(t) values ranging from –41.9 to 2.1 and TDMC values of 3387–1033 Ma, suggesting mixed crustal and mantle-derived components in the magma source, and correspond to multiple tectonic events during the Late Mesozoic. The Luanchuan granitoids are identified as I-type granites and most of these are highly fractionated granites, involving magma mixing and mingling and crystal fractionation. The tectonic setting in the region transformed from the Late Jurassic syn-collision setting to Early Cretaceous within-plate setting, with E–W extension in the Early Cretaceous. This extension is correlated with the N–S trending post-collisional extension between the North China Craton and Yangtze Craton as well as the E–W trending back-arc extension triggered by the westward Paleo-Pacific Plate subduction, eventually leading to lithospheric thinning, asthenospheric upwelling, mafic magma underplating, and crustal melting in the East Qinling Orogen.

Early Indosinian high-Mg# and high-Sr/Y ratio granodiorites in the Xiahe area, West Qinling, Central China: Petrogenesis and geodynamic implications

Indosinian granitic rocks are widespread in the West Qinling Orogen (WQO) in central China. Some of the early Indosinian granitoids have high Sr and low Y contents, and therefore high Sr/Y ratios, and they have been interpreted as adakitic rocks that formed by partial melting of thickened continental lower crust. However, several studies have revealed that granitoids with high Sr/Y ratios can be formed by various geological processes. Here, we report new zircon U-Pb ages, whole–rock geochemical data, and Sr–Nd–Hf isotope data for the Ganjiagongma pluton in the Xiahe area of the WQO, and we discuss whether the WQO underwent crustal thickening during the early Indosinian. The zircon U-Pb dating results indicate that the Ganjiagongma granodiorite and its mafic microgranular enclaves (MMEs) crystallized at ca 242 Ma. The MMEs are characterized by low SiO2 contents and Sr/Y ratios, and high MgO, Cr, and Ni contents, as well as high Mg# values, and they have evolved zircon Hf and whole–rock Sr-Nd isotopes, indicating they were produced by the partial melting of metasomatized continental lithospheric mantle. The granodiorite samples have some adakitic characteristics with SiO2 = 65.22 to 68.01 wt%, MgO = 1.07 to 1.71 wt%, Mg# = 59 to 62, Sr = 357 to 620 ppm, Y = 9.7 to 17.4 ppm, Sr/Y = 30.26 to 47.46, and no significant Eu anomalies. However, the granodiorites have relatively flat HREE patterns, indicating that amphibole, rather than garnet was the main residue in the source area, and this proposal is consistent with the relatively low but constant Nb/Ta values and varied Zr/Sm ratios. Simple two-end member modeling results indicate that the granodiorite could have been formed in normal or slightly thickened crust by mixing of 20%–30% mantle-derived melt with 70%–80% melt derived from the partial melting of basement rocks in the continental lower crust of the WQO. Taking into account other geological evidence, we propose that the formation of the Ganjiagongma pluton was related to the subduction of the A'nimaque-Mianlue oceanic crust, and that no significant thickening of the continental crust occurred in the WQO during the early Indosinian.

Source and possible tectonic driver for Jurassic–Cretaceous gold deposits in the West Qinling Orogen, China

The West Qinling Orogen (WQO) in Central China Orogenic Belt contains numerous metasedimentary rock-hosted gold deposits (>2000 t Au), which mainly formed during two pulses: one previously recognized in the Late Triassic to Early Jurassic (T3–J1) and one only recently identified in the Late Jurassic to Early Cretaceous (J3–K1). Few studies have focused on the origin and geotectonic setting of the J3–K1 gold deposits.Textural relationships, LA-ICP-MS trace element and sulfur isotope compositions of pyrites in hydrothermally altered T3 dykes within the J3–K1 Daqiao deposit were used to constrain relative timing relationships between mineralization and pyrite growth in the dykes, and to characterize the source of ore fluid. These results are integrated with an overview of the regional geodynamic setting, to advance understanding of the tectonic driver for J3–K1 hydrothermal gold systems. Pyrite in breccia- and dyke-hosted gold ores at Daqiao have similar chemical and isotopic compositions and are considered to be representative of J3–K1 gold deposits in WQO. Co/Ni and sulfur isotope ratios suggest that ore fluids were derived from underlying Paleozoic Ni- and Se-rich carbonaceous sedimentary rocks. The geochemical data do not support the involvement of magmatic fluids. However, in the EQO (East Qinling Orogen), J3–K1 deposits are genetically related to magmatism. Gold mineralization in WQO is contemporaneous with magmatic deposits in the EQO and both are mainly controlled by NE- and EW-trending structures produced by changes in plate motion of the Paleo-Pacific plate as it was subducted beneath the Eurasian continent. We therefore infer that the J3–K1 structural regime facilitated the ascent of magma in the EQO and metamorphic fluids in the WQO with consequent differences in the character of contemporaneous ore deposits. If this is correct, then the far-field effects of subduction along the eastern margin of NE Asia extended 1000's of km into the continental interior.

Tectonic units of the Early Precambrian basement within the North China Craton: Constraints from gravitational and magnetic anomalies

The tectonic divisions of the North China Craton (NCC) basement has always been a hot and yet unresolved topic. We collected the latest gravity and magnetic data, and then used a two-dimensional discrete wavelet transform method to decompose the Bouguer gravity anomaly of the NCC into the 1st to 8th Approximations and Details, respectively. We then compared the dynamic tectonic division schemes with the gravity and magnetic division schemes for the Precambrian basement at the end of the Neoarchean and Archean. Our results show that the 2nd-order Detail field of the Bouguer gravity anomalies is well consistent withthe Paleoproterozoic tectonic divisions of the NCC by Zhao et al. (2005), for it clearly outlines the 3-unit layout of the Eastern and Western blocks and the central Trans-North China Orogen, with some anomalous zones identically trending with the dike swarms. Faults and volcanic rocks generally produce magnetic anomalies, but the main anomalous zones in the magnetic field are merely distributed around the plate boundaries of the NCC. It may indicate that the amalgamation of the NCC was mostly completed in the Paleoproterozoic. The 5th-order Detail field of Bouguer gravity anomalies of the NCC corresponds well with its tectonic division at the end of the Neoarchean, for it well reflects the general boundaries and axial trends of the Archean units. These Archean units are mainly separated into three Paleoproterozoic tectonic units by Paleoproterozoic tectonic boundaries, respectively. The magnetic anomalies within the Western and Eastern blocks have separately similarities with the Trans-North China Orogen. The 6th-order Detail fields, however, match well the boundaries of the 10 continental nucleii of the Archean NCC, but the anomalous distributions in the 6th-order Detail are also divided by the Paleoproterozoic tectonic borders. In view of the gravity and magnetic anomalies, it further reveals that the NCC basement may have experienced a dynamic evolutionary process from continental nucleation to micro-continental amalgamation, and to a final unified crystalline basement.

Geological reconstructions of the East Asian blocks: From the breakup of Rodinia to the assembly of Pangea

Pangea is the youngest supercontinent in Earth's history and its main body formed by assembly of Gondwana and Laurasia about 300–250 Ma ago. As supported by voluminous evidence from reliable geological, paleomagnetic and paleontological data, configurations of major continental blocks in Pangea have been widely accepted. However, controversy has long surrounded the reconstructions of East Asian blocks in Pangea. To determine whether or not the East Asian blocks were assembled to join Pangea before its breakup, we carried out geological and paleomagnetic investigations on East Asian blocks and associated orogenic belts, supported by a NSFC Major Program entitle “Reconstructions of East Asian blocks in Pangea”. Our results indicate that the breakup of Rodinia around 750 Ma ago led to the opening of the Proto-Tethys and Paleo-Asian oceans in East Asia, with the former separating the South China, North China, Alex Qaidam and Tarim blocks from other East Asian blocks at the margins of Australia and India, whereas the Paleo-Asian Ocean existed between the East Asian blocks and Siberia-Eastern Europe. The Proto-Tethys Ocean closed in the early Paleozoic (500–420 Ma), leading to the collision of South China, North China, Alex, Qaidam and Tarim with other East Asian blocks at the northern margin of Gondwana. The subduction of the Paleo-Asian Ocean formed the Central Asian Orogenic Belt, the largest accretionary orogen in Earth's history, and its closure was diachronous, with its western, central and eastern segments closing at 310–280 Ma, 280–265 Ma and 260–245 Ma, respectively, leading the Tarim, Alex and North China blocks to join Eastern Europe-Siberia as part of Pangea. During the early Devonian (420–380 ma), the East Paleo-Tethys Ocean opened with two branches, of which the north branch is called the Mianlue Ocean that separated the Tarim-Qaidam-Central Qilian-Alex and North China blocks in the north from North Qiangtang-Indochina-South China in the south, and the south branch is the stricto sensu East Paleo-Tethys Ocean that separated North Qiangtang-Indochina-South China from the Sibumasu and South Qiangtang-Lhasa blocks at the northern margin of Gondwana. In the Triassic, the East Paleo-Tethys Ocean (stricto sensu) closed along the Longmu Co – Shuanghu – Changning – Menglian – Inthanon belt, leading to the collision of North Qiangtang-Indochina-South China with Sibumasu and South Qiangtang-Lhasa, forming a single southern continent, which then collided with the Tarim-Qaidam-Central Qilian-Alex and North China blocks to form a coherent East Asian continent that had become part of Pangea by 220 Ma, when the Mianlue Ocean closed, leading to the formation of the E-W-trending Central China Orogenic System.

HP–UHP Metamorphic Belt in the East Kunlun Orogen: Final Closure of the Proto-Tethys Ocean and Formation of the Pan-North-China Continent

The East Kunlun Orogen, the northwestern part of the Central China Orogenic Belt, is a long-lived accretionary orogenic belt that records the evolution and eventual destruction of branches of the Tethys Ocean, from the Cambrian to the Triassic. Here we report an Early Paleozoic eclogite belt that extends for ∼ 500 km within the East Kunlun Orogen. This belt consists of eclogite blocks, meta-sedimentary rocks and minor serpentinite blocks, accompanied by ophiolites (530-460 Ma) and concurrent arc volcanic sequences and granitic plutons. Geochemical data show that the eclogites have N-MORB- to OIB-like compositions. U-Pb dating of metamorphic zircons from eclogites and their surrounding rocks gave peak and retrograde metamorphic ages of 430-410 Ma. Coesite pseudomorphs in garnet, quartz exsolution rods in omphacite, as well as P-T calculations, suggests that some eclogites experienced UHP metamorphic conditions at 29-30 kbar and 610-675 °C; these could represent oceanic crust subducted to and exhumed from coesite-forming depths (100–120 km). The UHP metamorphic eclogite belt in the East Kunlun Orogen may represent the final closure of the Proto-Tethys Ocean (opening at ∼580 Ma, subduction initiating at ∼520 Ma) at ∼ 430-410 Ma in the East Kunlun, with the formation of Pan-North-China Continent in the Early Paleozoic and expansion of Paleo-Tethys Ocean in the south.