Shangdan Suture Research Articles

Fabrics and geochronology of the Taibai ductile shear zone: Implications for tectonic evolution of the Qinling Orogenic Belt, central China

The Taibai ductile shear zone affecting the northern margin of the Taibai granitic pluton which emplaced in the North Qinling Orogenic Belt, is crucial to understand the amalgamation and post-collision history between the North China Block and the South China Block. The shear sense indicators and the kinematic vorticity numbers (0.42–0.59) suggest that the Taibai shear zone is an elongating general shear zone involving both sinistral shear and pure shear components. The quartz lattice preferred orientations mainly indicate the activation of prism 〈a〉 slip system combined with rhomb 〈a〉 slip to prism 〈a〉 slip system. The dynamic recrystallization of quartz is accommodated by sub-grain rotation and bulging recrystallization while plagioclase mainly displays bulging recrystallization. These characteristics suggest that the mylonites have experienced ductile shear deformation under greenschist facies conditions at temperatures of ∼400–550 °C. Zircons from granitic mylonite yield upper and lower intercept ages of 425 ± 4 Ma and 144 ± 1 Ma, respectively, suggesting the protolith of the mylonite crystallized at ca. 425 Ma and was involved into a hydrothermal activity at ca. 144 Ma. Muscovite 40Ar/39Ar dating of the mylonite sample yield a plateau age of 346 ± 5 Ma and an inverse isochron age of 344 ± 4 Ma, constraining the ductile shear deformation occurring during the Early Carboniferous time. We suggest that the Taibai shear zone was formed by NE-SW oblique compression with top-to-the-NW shear sense during the Early Carboniferous time, following the N-S shortening caused by collision between the North China Block and South China Block along the Shangdan suture at ca. 400 Ma.

Re-Os geochronology, O isotopes and mineral geochemistry of the Neoproterozoic Songshugou ultramafic massif in the Qinling Orogenic Belt, China

The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO2 (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ18O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ18O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ18O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, 187Re/188Os and 187Os/188Os ratios from harzburgite to dunite suggest an 187Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (TRD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction.

Minor and trace element variations in chromite from the Songshugou dunites, North Qinling Orogen: Evidence for amphibolite-facies metamorphism

The Songshugou dunites are part of the ultramafic sequence of the Songshugou ophiolite within the Qinling Group metamorphic rocks, which are distributed in the northern part of the Shang–Dan Suture Zone (SDSZ) in central China. The dunites include lenses of coarse- and medium-grained dunite set in a fine-grained dunite host. Disseminated chromite within these dunites can be classified into three microstructural types: (1) partly altered chromite with primary cores surrounded by porous chromite rims; (2) porous chromite with abundant pores that are filled with chlorite; and (3) homogeneous chromite. The primary magmatic cores of the partly altered chromite (Chr) is depleted in Cr and enriched in Al and Mg. Porous chromite includes Cr-rich Chr and ferritchromite (Fe-Chr) that is enriched in Fe2+ but depleted in Al. Homogeneous chromite is mainly Cr-magnetite (Cr-Mag) that is enriched in Fe3+ and extremely depleted in Al. The primary Cr-poor chromite was gradually transformed to porous Cr-rich Chr and/or by Fe-Chr during metamorphism. Reaction of Cr-poor Chr with olivine (Ol) produces chlorite (Chl) and secondary Chr enriched in Cr and Fe2+. Mg and Al diffuse away from primary Chr, leaving a residual Al- and Mg-depleted, Cr-rich Chr and Fe-Chr. Porous Chr is enriched in Zn, Co, Mn, but depleted in Ga, Ni, and Sc, due to the fluid-assisted formation of Chl within the pores of porous chromite. Zn, Co, and Mn from the metamorphic fluids diffuse toward the cores of chromite. The elements Sc, Ti, V, and Ni diffuse from Cr-poor Chr core to Cr-mag rim with increasing Fe3+/(Fe3+ + Fe2+). Late precipitation of Cr-mag occurred under oxidizing and hydrous conditions. The infiltration of fluid produced non-porous Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within chromite. These fluids further promoted the reordering of octahedral sites, enhancing higher mobilization and enrichment of Ga, Ni, Sc, V, and Ti. The infiltration of high oxygen fugacity fluids promoted diffusion of Ga, Ni, Sc, and Ti toward the rims, and diffusion of Zn, Co, and Mn toward the cores. All chromite grains from the Songshugou dunites have low Mg# [molar 100 × Mg/(Mg + Fe2+)] values, and high Cr# [molar 100 × Cr/(Cr + Al)] and Fe3+/(Cr + Al + Fe3+), similar to metamorphic chromite formed under amphibolite-facies conditions. The observed positive Zn–Co–Mn anomalies and negative Ga–Ni–Sc anomalies in chromite, and the crystallization of Chl, are fingerprints of amphibolite-facies metamorphism. The compositions of chromite, the inferred mineral assemblage in which it occurs, and mineral equilibria modeling of the assemblage all suggest amphibolite-facies metamorphic temperatures ranging from 600 °C to 700 °C.

Tectonic evolution of the North Qinling Orogenic Belt, Central China: Insights from metamafic rocks of the Songshugou Complex

High‐pressure (HP) and ultrahigh‐pressure (UHP) rocks have been widely recognized in the North Qinling Orogenic Belt (NQOB). The nature and age of their protoliths can provide insights into the formation and evolution of the NQOB. The Songshugou Complex, which crops out along the Shangdan Suture Zone, mainly consists of high‐grade metamorphosed mafic and ultramafic rocks, and thus, this area is an important study site for understanding the tectonic evolution of the NQOB. The metamafic rocks that enclose the ultramafic rocks are overwhelmingly composed of amphibolites that primarily exhibit amphibolite‐facies metamorphism, and minor garnet amphibolites (retrograde eclogite) and HP mafic granulites occur as discontinuous lenses or blocks within these amphibolites. Laser inductively coupled plasma mass spectrometry (LA‐ICPMS) U–Pb dating on zircons yielded ages of 499 ± 6, 507 ± 5, and 504 ± 10 Ma for the HP mafic granulite, garnet amphibolite (retrograde eclogite), and amphibolite, respectively. Those ages were interpreted to represent timing of the HP granulite or amphibolite metamorphism that occurred during later exhumation. Relict omphacites discovered in both garnet and zircon as well as in the matrix of the garnet amphibolite indicate that the rock underwent eclogite‐facies metamorphism and subsequent rapid exhumation. However, the timing of the different metamorphic stages cannot be distinguished from the zircon ages obtained using the LA‐ICPMS technique. The protolith age of the complex is constrained to be the early Neoproterozoic. Geochemical data indicate that the protoliths of the Songshugou metamafic rocks are of continental origin. These Neoproterozoic continental mafic rocks most likely formed during the breakup of the supercontinent Rodinia and were mainly derived from low‐degree partial melting of the asthenosphere with some lithospheric mantle contribution, within an extensional regime. All the available data reveal that the NQOB was one of the components of the Rodinia supercontinent and that it was likely involved in the Grenville orogenic belt and subsequently separated from the supercontinent at approximately 830–740 Ma.

Geology, Pb Isotope Geochemistry and Ore Genesis of the Liziyuan Gold Deposit, West Qinling Orogen, Central China

The Liziyuan gold deposit, situated on the south side of the Shangdan suture zone, West Qinling Orogen, occurs in metamorphic volcanic rocks (greenschist facies) of the early Paleozoic Liziyuan Group and in Indosinian Tianzishan monzogranite. Orebodies in the Liziyuan gold field are controlled by the ductile‐brittle shear zone, and by thrusting nappe faults related to the Indosinian orogeny. In detail, this paper analyzed the geological characteristics of the Liziyuan gold field, and the Pb isotopes of the Lziyuan host rocks, granitoids (Tianzishan monzogranite and Jiancaowan syenite porphyry), sulfides, and auriferous quartz veins by multiple‐collector inductively coupled plasma mass spectrometry (MC–ICP–MS). In addition, previous data on the sulfur, hydrogen, and oxygen isotopes were employed to discuss the possible sources of the ore‐forming fluids and materials, and to further understand the tectonic setting of the Liziyuan gold deposit. The sulfides and their host rocks (Lziyuan Group), Tianzishan monzogranite and Jiancaowan syenite porphyry, and auriferous quartz veins have similar Pb isotopic compositions. Zartman's plumbotectonic model diagram shows that most of the data for the deposit fall near the orogenic Pb evolutionary curve or within the area between the orogenic and mantle Pb evolutionary curves. In the Δβ‐Δγ diagram, which genetically classifies the lead isotopes, most of the data fall within the range of the subduction‐zone lead mixed with upper crust and mantle. This indicates that a complex source of the ore lead formed in the orogenic environment. The δ34S values of the sulfides range from 3.90 to 8.50‰ (average 6.80‰), with a pronounced mode at 5.00‰–8.00‰. These values are consistent with that of orogenic gold deposits worldwide, indicating that the sulfur sourced mainly from reduced metamorphic fluids. The isotopic hydrogen and oxygen compositions support a predominantly metamorphic origin of the ore‐forming fluids, with possible mixing of minor magmatic fluids, but the late stage was dominated by meteoric water. The characteristics of the Liziyuan gold deposit formed in the Indosinian orogenic environment of the Qinling Orogen are consistent with those of orogenic gold deposits found worldwide.

Petrogenesis of the Sanshilipu gabbro complex in the Shangdan Suture: New constraints on evolution of the Qinling Orogen

The Sanshilipu gabbro complex is located along the Shangdan Suture, which is important to understanding the evolution of the Qinling Orogen, Central China. The complex is mainly composed of norite, gabbro, and gabbro–diorite. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon dating yields a weighted mean 206Pb/238U age of 414 ± 2.8 Ma, interpreted to represent the crystallization age of the gabbro complex. The complex has SiO2 contents ranging from 48.6 to 51.3 wt% with enrichment of Na relative to K, showing calc‐alkaline feature. The studied samples are characterized by an enrichment of large‐ion lithophile elements (Ba, Sr, and Pb) and depletion of HFSE (Nb, Ta, Zr, Hf, and Ti) and a large range of Nb/Ta ratios (3.7 to 11.8). They also have early flattened‐LREE distribution patterns. These geochemical characteristics indicate that the Sanshilipu gabbro complex was formed in an island‐arc environment. They have slightly enriched Sr–Nd isotope compositions with (87Sr/86Sr)i = 0.7047–0.7063 and ɛNd(t) from −0.44 to −4.63, indicating that the formation of the complex was involved in some subducted sediments and oceanic crust and fluid released from the subducted slab. The identification of island‐arc‐originated mafic rocks suggests that the onset time of the collision between the Yangtze Craton and the North China Craton should be younger than 415 Ma.

Mylonitized peridotites of Songshugou in the Qinling orogen, central China: A fragment of fossil oceanic lithosphere mantle

The Songshugou mylonitized peridotites within the Qinling Group metamorphic rocks in Central China are distributed in the northern part of the Shang-Dan Suture Zone (SDSZ) and contain abundant dunites and harzburgites. The dunites were intensely deformed and mylonitized converting the coarse-grained type to medium- and fine-grained types which contain prominent lenticular structure and relict olivine (Ol) porphyroclasts. Mineralogical and geochemical compositions suggest that the protoliths of the mylonitized peridotites were coarse-grained peridotites of lithospheric mantle origin. The harzburgites occur as enclaves within mylonitic peridotites in the form of lenses or veins. The orthopyroxenes in harzburgites were formed at the expense of Ol and have similar compositions to those of metasomatized harzburgites, characterized by low Al2O3, CaO and Cr2O3 contents. The harzburgites exhibit the gently U-type REE patterns with enriched incompatible elements (Rb, Ba, Sr, Zr and Hf), suggesting the metasomatic origin. The obvious ductile deformation of the large porphyroclastic orthopyroxene (Opx) suggests that the metasomatism occurred before the deformation. Ductile shearing deformation is indicated by the small fold structures and net-style ductile shearing zones within the Songshugou peridotite massif. The process is also result in the alignment of elongated Ol grains from initially coarse-granular via porphyroclastic to fine-granular texture. The relatively low Fo olivine, together with high Al2O3, and CaO contents and the abnormally low total PGE abundance in the fine-grained dunites suggest the ingress of melt/fluid during the mylonitization. The presences of significant amount of amphibole in the peridotites indicate the ingress of hydrous fluids. In general, the Songshugou peridotites have similar compositional characteristics with peridotites of Oman and Troodos ophiolites which are fragments of oceanic lithosphere mantle. One coarse-grained dunite has a TRD age of 875Ma. Additionally two stages Paleozoic TRD ages are obtained from medium-grained and fine-grained dunites (491Ma and 550Ma; 446Ma and 476Ma). The broadly coeval nature of mylonitization with progressive metamorphism of surrounding amphibolites suggested that the Songshugou peridotites were generated before the early Paleozoic deformation. Our data, combined with the previous work on the surrounding HP/UHP metamorphic rocks, demonstrate that the Songshugou mylonitized peridotites represent fragments of the Neoproterozoic fossil oceanic lithospheric mantle that experienced extensive deformation during the Early Paleozoic subduction processes.

Fabrics and geochronology of the Wushan ductile shear zone: Tectonic implications for the Shangdan suture zone in the Qinling orogen, Central China

The ductile shearing along the Shangdan suture zone during the Paleozoic time is a key to understand the collisional deformation and tectonic regime of amalgamation between the North China Block and the South China Blocks. The Wushan ductile shear zone, a branch of the Shangdan suture, records mylonitic deformation that affected granitic and felsic rocks outcropping in an over 1km wide belt in the western Qinling Orogenic belt. Shear sense indicators and kinematic vorticity number (0.79–0.99) of the mylonites reveal a dextral shear deformation. The quartz c-axis fabrics indicate activation of combined basal 〈a〉 and rhomb 〈a〉 slip, prism 〈a〉 slip and prism 〈c〉 slip. The dynamic recrystallization of quartz is accommodated by combined subgrain rotation and grain boundary migration. These characteristics suggest that the mylonites experienced ductile shear deformation under amphibolite facies conditions at temperatures of ∼500–650°C. Zircons from granitic mylonite yield a U-Pb age of 910±4.8Ma, which represents the formation age of the protolith of the mylonite. The ductile shear zone was intruded by a granitic dyke, which yields a zircon U-Pb age of 403±3.5Ma constraining the minimum age of the ductile shear deformation. Together with regional geology and available geochronological data, these structural characteristics and ages indicate that the Wushan ductile shear zone was formed by dextral shearing following the N-S shortening as a result of collision between the North China and South China blocks along the Shangdan suture.

Tectonic evolution of a complex orogenic system: Evidence from the northern Qinling belt, Central China

The Qinling orogenic belt preserves the records of a long evolution history of convergence between the North China Craton and the South China Craton. In this study, we present results from new geological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic investigations on a suite of orthogneisses from the Taihua Group (THG) and Tietonggou Group (TGG). We also present geochronological data on schist and migmatite from the North Qinling belt (NQB) and metasedimentary rocks from the Wuguan unit (WGU) which is exposed along the southern side of the Shangdan suture zone. Two orthogneisses from the THG define several stages of arc magmatism at ∼2.51Ga, ∼2.34Ga, ∼2.28Ga and ∼2.16Ga, followed by metamorphism at 1930±31Ma. Zircons from the TGG trace a tectonothermal event at 1897±15Ma. Geochemical data on biotite gneiss from the THG and the TTG (tonalite–trondhjemite–granodiorite) gneiss from the TGG classify these rocks as dacite and display volcanic arc affinity. Zircon Lu–Hf isotopic results suggest that the parental magma for the protolith of felsic gneisses were derived from Mesoarchean crustal components (TDMC=2766–3067Ma). The amphibole gneiss from the THG classifies as metabasalt and the zircons from this rock show dominantly negative εHf(t) values varying from −10.3 to −4.6 and TDMC range of 3088–3437Ma, suggesting magma derivation by melting of Paleoarchean–Mesoarchean subducted oceanic crust.Zircons in the schist from the NQB show a wide age population in the range of 937–1131Ma (peak at 1035Ma). Zircons from the melanosomes of the migmatite in the NQB define ages between 405 and 379Ma, correlating with the melting event in the NQB during 450–380Ma. The WGU shows age populations of 854–803Ma (with a peak at 829–824Ma), ∼2460Ma, ∼1802Ma and 1180–1000Ma, which are markedly different from that of the NQB and SQB. The ages obtained in our study correlate with the widely reported Grenvillian-aged magmatism in the NQB, and suggest that the NQB might have been a discrete micro-continent during Paleo- and Mesoproterozoic which has been overprinted by Paleozoic tectonic event.

Geological characteristics and ore-forming process of the gold deposits in the western Qinling region, China

The western Qinling, belonging to the western part of the Qinling–Dabie–Sulu orogen between the North China Block and South China Block, is one of the most important gold regions in China. Isotopic dates suggest that the Mesozoic granitoids in the western Qinling region emplaced during the Middle-Late Triassic, and the deposits formed during the Late Triassic. Almost all gold deposits in the western Qinling region are classified as orogenic, Carlin-type, and Carlin-like gold deposits, and they are the products of Qinling Orogenesis caused by the final collision between the North China Block and the South China Block. The early subduction of the Mian-Lue oceanic crust and the latter collision between South Qinling Terrane and the South China Block along the Mian-Lue suture generated lithosphere-scale thermal anomalies to drive orogen-scale hydrothermal systems. The collision-related magmatism also provided heat source for regional ore-forming fluids in the Carlin-like gold deposits.Orogenic gold deposits such as Huachanggou, Liziyuan, and Baguamiao lie between the Shang-Dan and Mian-Lue sutures and are confined to WNW-trending brittle-ductile shear zones in Devonian and Carboniferous greenschist-facies metasedimentary rocks that were highly-deformed and regionally-metamorphosed. These deposits are typical orogenic gold deposits and formed within a Late Triassic age. The deposits show a close relationship between Au and Ag. Ores contain mainly microscopic gold, and minor electrum and visible gold, along with pyrite. The ore-forming fluids were main metamorphic fluids. Intensive tectonic movements caused by orogenesis created fluid-migrating channels for precipitation locations. Although some orogenic gold deposits occur adjacent to granitoids, mineralization is not synchronous with magmatism; that is, the granitoids have no genetic relations to orogenic gold deposits. As ore-forming fluids converged into dilated fractures during the extension stage of orogenesis, changes of physico-chemical conditions resulted in fluid immiscibility that played a key role in gold and sulfide deposition.The geochemical and mineralogical characteristics of the Carlin-type deposits in the western Qinling region are similar to those in the Carlin trend, Nevada, USA. Gold deposits such as La’erma and Jinlongshan occur mostly in the southeastern margin of the western Qinling regionic region whereas some deposits occur in its eastern part. These deposits are hosted in slightly metamorphosed Cambrian to Triassic sedimentary rocks, showing structurally- and stratigraphically-controlled features. The deposits mainly contain submicroscopic and microscopic gold in arsenian pyrite and arsenopyrite, with characteristic ore-forming elements of Au–As–Sb–Ba. The ore-forming fluids are early-stocked formation water and later-recharged meteoric water. Meteoric water apparently evolved in ore-forming fluids by circulation, indicating the extensional setting, and led to the deposition of Au and other elements in cool reactive permeable rocks at shallow levels, forming the disseminated ores.Carlin-like gold deposits occur between the Shang-Dan suture and the Fengxian-Zhen’an fault. The host rocks are mainly sedimentary rocks that underwent reconstruction through reworking by structural metamorphism. These deposits are structurally controlled by brittle-ductile shear zone and occur adjacent to granitoid plutons. The most important characteristic that differ to the orogenic and Carlin-type gold deposits is the genetic relationship with the synchronous magmatism. Gold occurs mainly as microscopic gold. Pyrite and arsenian pyrite can be recognized as gold-bearing minerals. The ore-forming fluids are main magmatic water mixed with metamorphic and/or formation water. Similar to orogenic gold deposits, fluid immiscibility caused the deposition of gold Carlin-like gold deposits.

Propagation tectonics and multiple accretionary processes of the Qinling Orogen

The Qinling Orogen was built through collision between the North China and South China Blocks. Previous detailed geological, geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono-lithostratigraphy, which are, from north to south, the southern sector of the North China Block, North Qinling Belt, South Qinling Belt and northern sector of the South China Block, separated by the Kuanping, Shangdan and Mianlue sutures. According to the petrology, geochemistry and geochronology of ophiolitic mélanges and related magmatic rocks, as well as the features of sedimentary units, we think that the North China Block, the North Qinling Belt and the South China Block were originally independent continental units while the South Qinling Belt had been the northern part of the South China Block. These units experienced three episodes of accretionary tectonic processes and amalgamation from south to north.The Neoproterozoic accretion took place along the Luonan-Luanchuan Fault and Kuanping ophiolitic mélange belt as a result of southward subduction and subsequent collision between the North Qinling and North China Blocks during ca. 1.0–0.8Ga related to the formation of the supercontinent of Rodinia. The Paleozoic accretion occurred along the Shangdan suture resulted from northward subduction of oceanic lithosphere in the Early Paleozoic and subsequent continental subduction in the Late Paleozoic. Late Triassic accretion took place along the Mianlue suture between the South Qinling and South China Blocks due to northward subduction of the Mianlue oceanic lithosphere during the Permian–Early Triassic and subsequent collision in the Late Triassic. After the Late Triassic collision along the Mianlue suture the whole Qinling Mountain range entered the phase of intense intracontinental deformation.

Early Paleozoic tectonic evolution of the North Qinling orogenic belt: Evidence from geochemistry, phase equilibrium modeling and geochronology of metamorphosed mafic rocks from the Songshugou ophiolite

The Shangdan suture zone (SSZ) is the main collisional boundary between the North China Craton and the South China Craton, along which discontinuous Paleozoic ophiolites and subduction–accretion related volcanic arc assemblages occur. Here we report the petrology, geochemistry, geochronology and phase equilibria modeling of garnet amphibolite from the Songshugou ophiolite which is one of the largest ophiolite outcrops in the northern side of the SSZ. From petrological studies, we identify: (1) prograde stage, defined by garnet+clinopyroxene+calcic amphibole+ilmenite+rutile+epidote+plagioclase+quartz; (2) peak stage with garnet+clinopyroxene+ilmenite+rutile+quartz; and (3) retrograde stage with amphibole+plagioclase+titanite+ilmenite. Our pseudosection analysis defines stability of the peak assemblage at 750–850°C, 15–19kbar and traces a clockwise P–T path in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCFMASHTO), suggesting high pressure (HP) metamorphism. Subsequently, the rocks experienced rapid decompression and cooling. LA-ICP-MS U-Pb analyses of zircons from the garnet amphibolite yield a weighted mean 206Pb/238U age of 515±12Ma. This Early Paleozoic metamorphic age represents the emplacement time of the Songshugou ophiolite, and suggests that the HP metamorphism is possibly related to the northward deep subduction of the Shangdan oceanic crust in Early Paleozoic.

Episodic Triassic magmatism in the western South Qinling Orogen, central China, and its implications

The Qinling Orogen is located between the North China and South China cratons. The Mianlue Suture is the key boundary between the orogen and the South China Craton marking where the South China Craton subducted northward beneath the North China Craton during the late Triassic. Another older suture, the Shangdan Suture, divides the orogen into the North Qinling and South Qinling terranes. Our new geochronological data on eight granite plutons in the South Qinling Terrane indicate that the major phase of the Zhongchuan Granite is 219.5 ± 2.1 Ma, Luchuba is 217.1 ± 1.0 Ma, Baijiazhuang is 215.9 ± 3.0 Ma, Jiaochangba is 213.9 ± 1.7 Ma, Lvjing is 214.5 ± 2.1 Ma, Xiba is 215.6 ± 1.8 Ma, Wenquan is 217.3 ± 2.1 Ma and Fengxian is 240.9 ± 1.0 Ma. Compiled results reveal widespread magmatism began in the northwestern part of the South Qinling Terrane at ~240–250 Ma and episodically progressed to the southeast in the North Qinling Terrane. Magmatism was then widespread throughout the South Qinling Terrane during the Late Triassic. A new division of Triassic intrusions has been proposed across the Qinling Orogen based on critical geochemical and geochronological differences. First, the western part of the South Qinling Terrane, (western South Qinling suite), is significantly enriched in large ion lithophile elements, including Cs, Rb, U, Th and K, whereas the eastern part of the South Qinling Terrane, (eastern South Qinling suite), has moderate enrichment in these elements. Second, there is an obvious transition from calcium to potassium enrichment in the Late Triassic western‐South‐Qinling suite, whereas the Late Triassic eastern South Qinling suite appears to follow a curved trajectory starting from high Na through K to high Ca. Third, the Ba and Sr concentrations of the western South Qinling suite have marked troughs, whereas the eastern South Qinling suites have relative enrichment of the two elements. The source of the Triassic magma becomes increasingly juvenile towards the east‐southeast in the Qinling Orogen. Geochemical and isotopic data indicate that there are differences in the granites across the Chengxian–Huixian–Fengxian Fault within the western South Qinling Orogen, which highlight the tectonic significance of this northeast‐trending fault that is now recognized as a significant boundary within the orogen. The identification of such a large‐scale structure should attract some exploration attention, since this fundamental lineation is likely to be associated with some large mineral systems. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Crustal architecture of the Shangdan suture zone in the early Paleozoic Qinling orogenic belt, China: Record of subduction initiation and backarc basin development

The early Paleozoic Shangdan suture zone in the Qinling orogenic belt (China) marks the main tectonic boundary between the North China and South China blocks. The mafic rocks along this suture zone make up three distinct geochemical groups: (1) MORB-like; (2) boninite-like, and (3) island arc type. The MORB-like basalts display the highest values of TiO2 (0.86wt.%–1.99wt.%), LREE depletion and no negative Nb–Ta anomalies. However, their enrichment in LILEs relative to N-MORB indicates subduction influence in their melt source. The boninite-like rocks display lower TiO2 (0.09wt.%–0.41wt.%) and FeO* (4.3wt.%–10.25wt.%) contents, lower Ti/V ratios and total REEs, and high Cr (303–1495ppm) and Ni (102–383ppm) values. These geochemical features collectively indicate that magmas of the boninite-like rocks were derived from partial melting of a refractory mantle source. However, distinctly different ages of the boninite-like rocks imply that not all boninite-like rocks in the Shangdan suture zone formed in association with subduction initiation. The 524Ma boninite-like doleritic rocks record the subduction initiation magmatism, whereas the younger boninite-like dolerites with more depleted geochemical signatures represent a magmatic stage associated with backarc spreading around 474Ma. The arc-type lavas display strong negative Nb–Ta and Zr–Hf anomalies, characteristic of intra-oceanic island arc rocks. The combined geological, geochemical and regional tectonic features of the ophiolitic rocks along the Shangdan suture zone in the North Qinling orogenic belt provide a record of subduction initiation, island arc development and backarc rifting in the early Paleozoic, reminiscent of the modern IBM arc-trench system.

Heterogeneous sources of the Triassic granitoid plutons in the southern Qinling orogen: An E‐W tectonic division in central China

The Qinling orogen is an important orogenic belt formed by the collision between the North and South China blocks along the Mianlue suture during the Triassic. The orogen is customarily divided into the western and eastern Qinling terranes. However, the boundary has long been a matter of debate. There are many Triassic granitoid plutons in the orogen, especially in South Qinling, along the southern part of the Shangdan suture. Systematic analysis of U‐Pb ages and Hf isotopes in zircons frxom six granitoid plutons, including Guangtoushan, Gaoqiaopu, Xiba, Laocheng, Dongjiangkou, and Zhashui from west to east, allows us to trace their formation ages and source regions. All plutons yield ages that vary only from 218 to 211 Ma. However, zircon Hf‐isotope data for these Triassic plutons cluster in two distinct groups. Granitoids from the western segment (i.e., Guangtoushan, Gaoqiaopu, and Xiba) of the South Qinling belt have negative εHf(t) values (−20.9 to −5.2) and relatively old two‐stage Hf model ages (1.58 to 2.57 Ga). In contrast, those from the eastern segment (i.e., Laocheng, Dongjiangkou, and Zhashui) show higher εHf(t) values (−5.4 to +6.8) and younger two‐stage Hf model ages (0.82 to 1.60 Ga). Integrating these analyses with published Sr‐Nd‐Pb‐Hf isotopic data, we suggest that the division between the western and eastern South Qinling segments is located near the Taibai‐Chenggu line, between the Baoji‐Chengdu railway and 108°E longitude. The integrated data suggest that the western South Qinling segment separated from the North China block during the Paleoproterozoic and early Mesoproterozoic and then switched into continental convergence with the eastern South Qinling segment (northern margin of the Yangtze block) during the late Mesoproterozoic to early Neoproterozoic; finally, the two segments amalgamated during the late Neoproterozoic.

Timing of Paleozoic amalgamation between the North China and South China Blocks: Evidence from detrital zircon U–Pb ages

LA-ICP-MS U–Pb ages of detrital zircons from clastics of the undated fore-arc sedimentary unit (FAS) along the Shangdan Suture and the Middle–Upper Devonian Liuling Group in the South Qinling belt are used to establish the maximum depositional age and provenance of these tectonic units which were deposited on both sides of the Shangdan suture zone between the North China Block (NCB) and South China Block (SCB). The new data and geological evidence show that the FAS was deposited in a fore-arc basin with an exclusive source of the clastics in the North Qinling Belt (NQB). The depositional age of FAS is limited by the youngest U–Pb 455Ma-ages of detrital zircons from clastics and the intrusive age of 435±7Ma of mafic dykes between the Late Ordovician–Early Silurian, while the NCB was still separated from the SCB by the Shangdan Ocean. However, detrital zircon U–Pb ages from six samples of the Middle Devonian Liuling Group indicate sources in both NQB and SCB suggesting pre-Middle Devonian collision of NCB and SCB. All results indicate deposition of the FAS in a forearc setting upon an active continental margin during Late Ordovician–Early Silurian, while the Middle–Upper Devonian Liuling Group represents a marine foreland basin after closure of the Shangdan Ocean. Together with the unconformity between Middle Devonian and pre-Devonian strata, this reveals still evolving subduction and accretion on the southern side of the NQB during the Ordovician-Early Silurian, and the Early Devonian collision between the NCB and SCB.

The origin and response of zircon in eclogite to metamorphism during the multi-stage evolution of the Huwan Shear Zone, China: Insights from Lu–Hf and U–Pb isotopic and trace element geochemistry

The Huwan Shear Zone (HSZ) is an eclogite bearing transpressive wrench zone located along the Shangdan Suture that juxtaposes the Paleozoic Qinling and Mesozoic Hong'an–Dabie orogenic terrains. The region preserves a complex history that bridges the gap between adjacent orogenic terrains. Simultaneous in-situ trace element, U–Th–Pb and Lu–Hf‐isotope analysis of zircon grains from samples of the Xiongdian and Sujiahe eclogite identify a late Carboniferous to early Permian period of high pressure metamorphism, ca. 283 to 306Ma. Zircon grains are observed to respond to metamorphic overprint via a two stage process: (1) An initial prograde stage of fluid catalyzed interface coupled dissolution–reprecipitation, involving exsolution of a non-ideal solid solution thorite (ThSiO4) end member and loss of highly incompatible components (LREE and Pb), (2) A second stage of coupled zircon dissolution, coarsening, and new rim growth in equilibrium with garnet at high pressure conditions.We identify Proterozoic whole rock Sm–Nd and zircon grain Lu–Hf isotopic evidence which challenges the traditional interpretation that the Xiongdian and Sujiahe eclogite formed in response to early Paleozoic mantle melting and oceanic crust generation. We argue the Huwan Shear Zone contains no conclusive evidence of early/middle Paleozoic oceanic crust, but rather Proterozoic crustal components analogous to those found in the Northern Qinling Terrain and associated with formation of the Shangdan Suture. We present a simpler geodynamic model involving continuous convergence and accretion of terrains onto the southern margin of the North China Block during the Paleozoic Qinling and Mesozoic Dabie orogenies.

Thermochronological constraints on two-stage extrusion of HP/UHP terranes in the Dabie–Sulu orogen, east-central China

Along the Qinling–Dabie–Sulu orogenic belt in China crops out the world's largest terrane composed of ultrahigh-pressure (UHP) metamorphic rocks. Differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high-pressure (HP) and UHP slices in different parts of the belt. The western part of the Dabie orogen (western Dabie terrane) holds a key to understanding of the transition from oceanic to continental subduction. This paper reports geochronological results to test a two-stage tectonic model for the exhumation of HP/UHP rocks in western Dabie. This model involves two different stages and types of extrusion for exhumation of the HP/UHP rocks in east-central China. Mica Ar/Ar ages, ranging from 241 to 231 Ma, indicate a general middle Triassic cooling probably driven by early upward extrusion during the collision between the North and South China Blocks. Late Triassic–Early Jurassic cooling was associated with later eastward extrusion, ranging from 200 to 184 Ma. The second event is recorded also in mica in the region that was not affected by later deformation and magmatism. The lateral movement along lithosphere-scale faults resulted in the eastward extrusion of the HP–UHP metamorphic terrane, which was followed, in the Late Triassic–Early Jurassic time, by a major compressive event. These two extrusion events are correlative with the two stages of Triassic exhumation of the western Dabie HP–UHP rocks, respectively. Wintin the framework of the Qinling–Dabie–Sulu orogenic belt, it is suggested for western Dabie that the subduction/exhumation of blueschist-facies unit is related to the Mianlue suture, whereas the subduction/exhumation of HP/UHP eclogite-facies units is related to the Shangdan suture.

Palaeozoic tectonics and evolutionary history of the Qinling orogen: Evidence from geochemistry and geochronology of ophiolite and related volcanic rocks

The tectonic framework and the evolutionary history of the Qinling orogenic belt are keys for understanding the convergent processes between the North China and South China blocks. The widely exposed ophiolitic and subduction-related volcanic melange along the Shangdan and Erlangping belts provides important constraints on the tectonic evolutionary processes of the Qinling orogen. The melange in the Shangdan zone is predominantly composed of ultramafic and mafic rocks that can be divided into three geochemical groups: (1) N- MORB type; (2) E-MORB type; and (3) island-arc/active continental margin-related basalts. The samples with N- and E-MORB affinity are characterized by depletion or slight enrichment of LREE without fractionation of HFSE and no negative Nb–Ta anomaly. The island-arc/active continental margin-related basalts are typically depleted in Nb–Ta and Ti. It is inferred that the melange within the Shangdan suture represents remnants of an oceanic crust and associated volcanics. An age for the melange can be constrained by a U–Pb zircon age of 517.8±2.8Ma, obtained from the gabbro within the E-MORB-type ophiolite in the Yanwan area.To the north of the suture zone, the Erlangping melange consists similarly of ultramafic and mafic rocks, andesites and rhyolites. The mafic and andesitic rocks exhibit strong depletion of Nb–Ta and Ti indicating a subduction-related affinity. However, their depletion in Nb–Ta is weaker than that of the island-arc/active continental margin-related volcanic rocks. Taken together, the two ophiolitic melange zones indicate the existence of an early Palaeozoic Shangdan Ocean that was associated with a back-arc basin on the northern North Qinling Island arc terrane, separating the South China from the North China blocks.

U-Pb and 40Ar/ 39Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, 40Ar/ 39Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The 40Ar/ 39Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite 40Ar/ 39Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.