East Kunlun Orogenic Research Articles

Mo-Mg isotopes trace the role of serpentinite in generating arc magmatism

Serpentinite-derived components play an important role in chemical cycling in subduction zones. However, it is difficult to identify recycled serpentinite in subarc mantle due to the overprinting of other slab components, and how serpentinite contributes to arc magmatism is poorly understood. Here we report Mo and Mg isotopic compositions of Early Paleozoic Qushiang arc mafic rocks in the East Kunlun orogen. The Qushiang mafic rocks have variable δ98Mo values of −0.50‰ to 0.78% and δ26Mg values of −0.31‰ to −0.05‰. These features, combining with arc-type trace element distribution patterns, enriched SrNd isotopes, and variable zircon HfO isotopes, suggest the incorporation of different subducting components in their mantle source. Some mafic rocks show high δ98Mo values together with low Mo/Ce ratios and enriched SrNd isotopes, suggesting the contribution of subducted sediment (SSD)-derived melts. Additionally, light Mo isotopes are also identified in mafic rocks, which are closely related to depleted SrNd isotopes. These features are mainly inherited from the subducted dehydrated altered oceanic crust (SAOC)-derived melts. More importantly, the coexisting and significantly high δ26Mg and δ98Mo values of these rocks could not be explained by the SSD or SAOC components, but require serpentinite components in the subarc mantle source. Our study reveals that combined Mo and Mg isotopic studies are potential tools for tracing serpentinite recycling, and provides new insight into the mechanisms by which different subducting components contribute to arc magmatism.

Genesis of the Haidewula volcanic rock-hosted uranium deposit in the East Kunlun Orogen, northwestern China

The Haidewula deposit in Northwest China is a recently discovered volcanic-related uranium deposit. To determine the origin of this deposit, whole-rock geochemical, stable isotope, U–Pb isotope and trace element compositions of the pitchblende were obtained, and microthermometric analyses of the fluid inclusions were performed. The Haidewula deposit is located within the Phanerozoic volcanic basin and contains Silurian trachyte, rhyolite, rhyolitic tuff, and Triassic basalt. These Silurian volcanic rocks are characterized by high-K calc-alkaline properties and were intruded by Triassic dolerite, which served as a source for mineralization. The mineralized zones were primarily influenced by NE-NNE-trending and SE-dipping faults. Vein-like pitchblende is commonly associated with calcite, illite, quartz, pyrite, and fluorite. Geochronology indicated that uranium mineralization occurred during 235–230 Ma. The ore-forming fluids contained meteoric water with limited magmatic water and exhibited moderate to low temperatures (246–133 °C), moderate to low salinities (17.3–1.06 wt% NaCleqv) and Cl-bearing rocks. U precipitation was attributed to changes in physicochemical conditions resulting from fluid–rock interactions and fluid boiling. The uranium mineralization at Haidewula coincided with the evolution of the Tethys Ocean. During late Wenlock, an extensional environment, triggered by the closure of the Proto-Tethys Ocean, led to the extrusion of Haidewula rhyolite and trachyte. During the crustal extensional environment resulting from the subduction of the A’nyemaqen Ocean in the Middle Triassic, ore-forming fluids circulated and remobilized uranium from the Haidewula volcanic rocks. As a result of fluid–rock interactions and fluid boiling within brittle structures, the U-complexes destabilized and formed pitchblende at Haidewula.

Origin of granitoids and coeval mafic microgranular enclaves in the East Kunlun Orogen, northern Tibetan Plateau: Insights from petrography, U-Pb geochronology, geochemical and isotope analyses

Granitoids and coeval mafic microgranular enclaves (MMEs) provide important insights into petrogenesis and crustal evolution. In the eastern sector of the East Kunlun Orogen, the Halazha (HLZ) pluton contains abundant MMEs, recording magma mingling processes. The HLZ pluton comprises quartz diorites and granodiorites, whereas the MMEs have a gabbroic-dioritic composition. This paper provides a detailed description of the petrography and mineralogy of the HLZ pluton and MMEs, and presents new ages and geochemical data. The zircon U–Pb dating results indicate that quartz diorites and granodiorites are ranging from 243 ± 2 Ma to 244.8 ± 1.3 Ma and from 233.3 ± 1.1 Ma to 233.6 ± 1.5 Ma, respectively. The zircon ages of the coeval MMEs are identical to the host rocks, i.e., 244.5 ± 1.1 Ma − 244.5 ± 1.2 Ma, and 233.7 ± 1.3 Ma, respectively. Both quartz diorites and granodiorites contain hornblende, display metaluminous characteristics with average A/CNK values of 0.93 and 0.99, respectively, and thus belong to I-type granite. Both of them are enriched in large-ion lithophile elements such as Rb and Ba and depleted in high-field-strength elements such as Nb and Ta. In addition, they both have low εHf(t) values, ranging from −5.6 to −1.7 and −5.4 to −1.79, respectively. Results suggest that HLZ pluton is derived from magmas formed by the partially melting juvenile mafic lower crust. Quartz diorites experienced hornblende accumulation for its low SiO2 contents. Whereas, the high Sr/Y ratio of granodiorites attributes to the fractional crystallization of apatite. Notably, the crystallization pressure and temperature (average 3.43 kbar and 764 °C) of the hornblende xenocrysts in MMEs is close to it of the hornblende phenocryst in quartz diorite (average 3.68 kbar and 766 °C). This similarity indicates that MMEs in the quartz diorites resulted from magmatic mingling between the mafic magma and the host rock within the deep crust. The hornblende xenocrysts present in MMEs of the granodiorites contain a higher number of inclusions, reveal higher crystallization pressures (average 2.73 kbar) and temperatures (average 825 °C) compared to their host rocks (average 1.35 kbar and 702 °C). These results suggest that MMEs in granodiorites underwent two distinct magmatic mixing processes: the magmatic mixing of mafic magma in the deep crust and then during the upwelling process after this mixed mafic magma entered the granodiorites.

Metamorphic evolution and geological significance of Early Palaeozoic high‐pressure granulites from the East Kunlun (NW China)

AbstractGranulites and eclogites are useful for revealing the thermal and tectonic evolution of orogens. Early Palaeozoic granulites and associated eclogites in the East Kunlun Orogenic Belt (EKOB) display contrasting metamorphic age. Such asynchronous granulite–eclogite associations have rarely been reported, and the geological significance of their existence remains to be further explored. In this study, petrological and geochronological techniques were used to investigate two felsic and two mafic granulites collected from the Qingshuiquan area in the eastern section of the EKOB. These rocks record similar P–T paths, which are characterized by a peak stage within suprasolidus and high‐pressure (HP) granulite facies conditions (750–832°C and 10.1–12.0 kbar), followed by an initial decompression and cooling stage to subsolidus conditions (600–748°C and 6.5–8.6 kbar), and then a stage of further retrogression under greenschist facies conditions. The protoliths to these granulites are of volcanic and sedimentary origin and suggested to be a component of the continental basement unit. Metamorphic P–T paths indicate that these rocks experienced peak metamorphism at a depth of ~40 km, then cooling and uplift to a depth of ~25 km, and eventually experienced low‐grade retrogression at shallow crustal levels. Cathodoluminescence images and compositional data demonstrate that the zircons in these rocks are of metamorphic origin and they crystallized at or near peak conditions. SIMS U–Pb dating of representative zircon grains yield concordant metamorphic ages of c. 490–520 Ma, with a peak value of 505 Ma on the probability density curve. These ages are similar to other 480–530 Ma ages typically retrieved from EKOB granulites and associated rocks, and are markedly older than the 400–450 Ma ages retrieved from eclogites and their host rocks. The HP granulites and eclogites of the EKOB do not show overprinting relationships. Such asynchronous characteristics imply that the two rock types formed in distinct tectonic settings and at different stages of a protracted subduction–collision process. The studied granulites are suggested to have formed in the root of a continental arc during a stage of Proto‐Tethys Ocean subduction. The formation of the eclogites could be attributed to subsequent deep continental subduction.

Petrogenesis of Paleozoic trachyte and rhyolite in the Haidewula area, East Kunlun Orogenic Belt, and their implications for uranium mineralization

Zircon U–Pb-Hf isotopic compositions, mineralogical chemistry, and whole-rock geochemistry analyses were performed on rhyolite and trachyte from the Haidewula deposit in the East Kunlun Orogen Belt (EKOB). These analyses aimed to elucidate the genetic relationship between trachyte and rhyolite and to investigate the role of these volcanic rocks in uranium mineralization. Zircon U–Pb isotope dating indicates that the Haidewula rhyolite and trachyte were contemporaneous with emplacement ages of 421 Ma to 428 Ma, respectively. The trachyte samples exhibit high K2O/Na2O and Fe# [FeO/(FeO + MgO)] ratios, and belong to metaluminous, ferroan volcanic rock; the rhyolite samples display high A/CNK and Fe# ratios, elevated high field strength element contents, and can be classified as strongly peraluminous A2-type magma. Trace and major elemental geochemical signatures demonstrate that the Haidewula trachyte originated from the high-temperature partial melting of a mafic lower crust, followed by fractional crystallization of plagioclase, pyroxene, magnetite, and/or K-feldspar. On the other hand, the rhyolite was likely generated by high-temperature melting of granodiorite in a low water and mid-crust pressure environment, followed by fractional crystallization of feldspar and ilmenite. The occurrence of A-type volcanic rocks at Haidewula suggests that the initial timing of crustal extension in the EKOB was not later than 428 Ma. Overall, Paleozoic igneous rocks in the EKOB revealed that a post-collisional extensional regime, resulting from the rewelding of the microcontinents in eastern Gondwana, was responsible for intensive magmatism during the Late Silurian period. In comparison to other volcanic complexes hosting uranium deposits in eastern China, the relatively U-rich rhyolite and trachyte had the potential to contribute to the formation of U mineralization. In particular, the rhyolitic tuffaceous U-rich glassy volcanic rocks in the Haidewula deposit may represent favorable U-source rocks for mineralization, with uranium released through volcanic glass devitrification processes and leaching by oxidizing hydrothermal fluids.

Genesis of the Hacipushanbei Pb-polymetallic deposit in the East Kunlun Orogen, NW China: Constraints from geochronology and in situ trace-elements and sulfur isotopes of sulfides

A recent mineral exploration programme in the East Kunlun Orogen (EKO) identified the new Hacipushanbei Pb-polymetallic deposit. The ores are hosted in faults in granitic rocks and are marked by high Pb grades up to 22.5 %. U–Pb dating of calcites associated with mineralization yields an intersection age of 208 ± 9 Ma, indicating a late Triassic mineralization event. Fluid-inclusion analysis reveals that the ore-forming fluids are characterized by medium-to-high temperature, medium-to-low salinity, and low density. Fluid boiling or immiscibility during ore-formation is also noted. Three sub-generations of pyrite (Py1-1, Py1-2, and Py2) are recognized in the early mineralization stages. In situ trace element analysis shows low Co and Ni contents in Py1-1 and Py1-2 but high contents in Py2. Conversely, Au and As concentrations are the highest in Py1-2. Variations in As contents are systematically coupled with Au. Additionally, sphalerite in the second stage contains higher amounts of Co, Cu, Cd, and In, via a wide range of isomorphic elemental substitutions of X4+(Sn4+ and Ge4+) + X3+(In3+, Sb3+, and Ga3+) + X+(Ag+ and Cu+) ↔ 4Zn2+ and the associated In enrichment. The average δ34S values of in situ and single minerals of Py1-1, Py1-2, Py2, Sp2, Ccp2, and Gn3 are 2.3 ‰, 3.3 ‰, 2.6 ‰, 2.4 ‰, 2.2 ‰, and 0.9 ‰, respectively. These values display a trend of initial increase and then decrease, consistent with thermodynamic fractionation. These data collectively indicate that the ore-forming fluid originated from a deep-seated magma chamber, and subsequent fluid immiscibility led to the formation of high-grade ores. Given the conspicuous association of Pb-polymetallic deposits with magmatic rocks in the EKO, we envisage that our genetic model can be applied, to some extent, to evaluate the potential of some mineral exploration targets.

Petrogenesis and tectonic implication of the Middle Triassic pegmatite from the Mokeri area, eastern segment of the East Kunlun Orogen (China)

The newly discovered pegmatite, along with associated syenogranite and gabbroic diorite in the Mokeri area of the East Kunlun Orogen (EKO), were comprehensively investigated utilizing geochronological, whole rock and mineral chemical analysis. Zircon U-Pb dating results indicate that the pegmatite and syenogranite were emplaced almost simultaneously at 236 ± 4 Ma and 236 ± 2 Ma, respectively, while the gabbroic diorite was emplaced at 240 ± 3 Ma. The Mokeri pegmatite and syenogranite are chemically classified as high-K calc-alkaline, weakly peraluminous I-type granites. They exhibit similar geochemical compositions with high SiO2 (73.70 – 78.57 wt%), low MgO (0.04 – 0.11 wt%), Fe2O3T (0.23 – 2.00 wt%) and TiO2 contents (0.02 – 0.20 wt%), as well as low Nb/Ta (3.81 – 12.05) and Zr/Hf ratios (12.64 – 31.04), indicating their common crustal magma source. Geochemical and mineralogical evidences suggest an evolutionary relationship between the Mokeri syenogranite and pegmatite, characterized by higher SiO2 and Na2O contents, as well as elevated Rb/Sr and Rb/Ba ratios, alongside lower Fe2O3T, MgO and TiO2 concentrations in the pegmatite. The pegmatite exhibits a high differentiation index (DI) of 94 (mean value), which signifies a continuous sequence of magma evolution in conjunction with the moderately differentiated syenogranite (DI = 90). Plagioclase components in the pegmatite (An7 to An18) are more evolved than those in syenogranite (An19 to An31). Garnet, derived from two pegmatite dykes, belongs to almandine-spessartine series and exhibits a progressive enrichment in manganese with increasing distance from the parental body—syenogranite. The Mokeri gabbroic diorite exhibits compositional depletion of Nb, Ta, and Ti contents and enrichment of Sr, Ba, and Pb contents, resembling the observed patterns in the island arc basalts (IAB). The elevated ratios of Th/Yb, Ba/Zr and Rb/Y suggest it derivates from an enriched lithospheric mantle modified by subducted slab-fluid. The Mokeri pegmatite-syenogranite-gabbroic diorite association was formed during a transitional phase between syn-collision and post-collision extension (240 – 236 Ma), indicating the termination of collision between the East Kunlun Terrane and Bayan Har Terrane at 236 Ma. These magmatic events likely represent a tectonic-magmatic response to slab break off during the post-collision.

Polyphase Exhumation of the East Kunlun Orogenic Belt: Evidence from Modern River Detrital Zircon and Apatite Fission Track Dating

Abstract The East Kunlun Orogenic Belt (E-KOB) stands out as one of the most prominent basin-mountain geomorphic features in the northern interior of the Tibetan Plateau. It records a series of accretion-collision events from the Mesozoic to the Cenozoic. In particular, with the uplifting of the Tibetan Plateau, the E-KOB experienced intracontinental deformation and exhumation in the Cenozoic. Clarifying the exhumation history of the E-KOB is crucial to define the growth time and mechanism of the Tibetan Plateau. In this study, we apply detrital zircon fission-track (ZFT) and apatite fission-track (AFT) analyses on modern river sands in order to constrain the regional exhumation history of the eastern E-KOB. Four peak ages have been identified and interpreted as results of rapid exhumation correlated with intracontinental deformation. Two older peak ages at 144.7–141.0 and 114.6–82.1 Ma are in good accordance with the collision time of the north-south Lhasa-Qiangtang Block along the Bangong-Nujiang suture zone and the subsequent progressive deformation stage toward the north. Peak age at 60.9–45.3 Ma is coeval with the initial timing of the India-Asia collision. The youngest peak age at 25.1–18.3 Ma matches well with the extensive outward and upward growth of the Tibetan Plateau during the Oligocene to Miocene time. The Cretaceous and early Cenozoic rapid exhumations suggest that the E-KOB has been involved in the intracontinental deformation induced by collisions of the Lhasa-Qiangtang and India-Asia from the south. It implies that the northern Tibetan Plateau likely has been elevated or was a structural high before the Eocene. In addition, some of our detrital samples show a younger ZFT peak age than the AFT peak age. We attributed this data bias to the contribution of hydrodynamic sorting and/or lithological difference. The combination of ZFT and AFT dating has advantages in eliminating interfering age signals in detrital thermochronology.

Petrogenesis and Geodynamic Implications of Cretaceous Nb-Enriched Mafic Dykes in the East Kunlun Orogen, Northern Tibet Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd Isotopes

There is a magmatic lull period in the East Kunlun orogen (EKO) during the Jurassic to the Cretaceous. However, due to the lack of records of magmatic activity restricts our understanding of the late Mesozoic magmatic-tectonic evolution of the EKO. Herein, an integrated study of geochronology, whole-rock geochemistry and Sr-Nd isotopes were conducted for the Cretaceous mafic dykes in the EKO, Northern Tibet Plateau, to reveal their petrogenesis and geodynamic implications. LA-ICP-MS Zircon U-Pb dating reveals that the studied mafic dykes comprising diabase and diabase porphyry emplaced at ca. 80.9 ± 0.8 Ma. The Cretaceous mafic dykes have low contents of SiO2 (46.36 wt.%~47.40 wt.%) but high contents of MgO (6.79 wt.%~7.38 wt.%), TiO2 (1.91 wt.%~2.13 wt.%), Nb (12.4~18.3 ppm) and Nb/U ratio (31~39), resembling Nb-enriched mafic dykes. They exhibit chondrite-normalized rare earth element (REE) and primitive mantle-normalized trace element patterns, remarkably similar but not identical to the oceanic island basalts (OIB). The moderate REE fractionation ((La/Yb)N = 3.55~5.37), weak negative Eu anomalies (δEu = 0.87~0.97) and relative enrichment of Rb, Ba, K, as well as high contents of Cr and Ni and slightly depleted Sr-Nd isotopes (εNd(t) = −0.18~1.33), suggest that the studied dykes originate from a partial melting of spinel lherzolite and a little of garnet which was previously modified by subducted sediments. Combined with other evidence, we propose that the studied Cretaceous Nb-enriched mafic dykes in the Northern Tibet Plateau were formed in the intraplate setting, which may be a partial melting of the enriched mantle in the lower lithosphere caused by the activity of the East Kunlun strike-slip fault.

Subduction and retreat of Permian oceanic plates in the East Kunlun Orogenic Belt: evidence from mafic-ultramafic intrusive rocks

ABSTRACT The East Kunlun Orogenic Belt, an important part of the Central Orogenic Belt in China, preserved wealthy geological information for the tectonic evolution of the Proto- and Paleo-Tethys Ocean. Several Permian mafic-ultramafic rocks were identified in the Qimantagh area, EKOB, and zircon U-Pb dating, zircon Hf isotope, geochemistry and whole-rock Sr-Nd isotopes were studied in this paper. The results suggest that the formation age of the Xiyingzhaogou intrusion is 280 ± 4 Ma. The rocks show the characteristics of continental-arc calc-alkaline basalt. The whole-rock εNd(t) value of gabbro in Xiyingzhaogou intrusion is −4.2~-2.6 and the εHf(t) value of zircon is 2.77 ~ 5.82, indicating the characteristics of enriched or weakly depleted mantle; The formation age of Yingzhaogou intrusion is 263 ± 4 Ma. The geochemical characteristics of the Yinzhaogou rocks are consistent with those of continental-arc tholeiite. The whole-rock εNd(t) value of gabbro in Yingzhaogou intrusion is 0.73 ~ 0.92, and the εHf(t) value of zircon is 8.33 ~ 13.50, indicating the characteristics of depleted mantle. We believe that the Xiyingzhaogou intrusion was formed in the Early Permian and originated from the mantle wedge metasomatized by subduction fluid. The Yingzhaogou intrusion was formed in an extensional continental-arc environment caused by the retreat of the subduction plate. Asthenospheric mantle and fluid metasomatic lithospheric mantle were jointly involved in the mantle source. This study confirmed that the subduction of the Paleo-Tethys oceanic slab should had started before the Early Permian and a wide magma arc formed due to flat subduction. The retreat of the subduction plate led to the extension of the East Kunlun region, resulting in the significant enhancement of magmatism in the Middle Permian.

Genesis and tectonic setting of an Early Palaeozoic skarn in the East Kunlun Orogenic Belt, China: constraints from adakitic rocks and garnet megacrysts

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) in the northern Tibet Plateau has undergone a long multi-stage history of drifting, accretion, and collision of the Proto- and Palaeo-Tethyan oceans. It has been addressed a trench-arc-back-arc basin system of the Proto-Tethys in the E-KOB, and the tectonic history of this major ocean has been well constrained by previous studies. However, the evolution of the back-arc basin still remains unclear. In this study, the Yugusayi monzodiorite, quartz-bearing syenite, and gradually contacted skarn rocks characterized by garnet megacrysts, in the North Qimantagh Belt were investigated. To reveal the tectonic setting and skarnization mechanism, geochemical and geochronological studies were conducted on the intrusions and garnet skarn, and the elemental compositions of the garnet megacrysts were also analysed. These monzodioritic rocks have high MgO (6.87–8.31 wt.%), Sr and La contents, low Y and Yb contents, and relatively high La/Yb (31–55) and Sr/Y (41–51) ratios, resembling to high-Mg adakites. They display right-sloping chondrite-normalized REE patterns, enriched in LILEs, and depleted in Ti with significant Nb-Ta troughs, which are similar to those of continental arc rocks. Zircons from monzodiorite and skarn samples have contemporaneous U-Pb ages of 459 ± 3 Ma and 454 ± 4 Ma with εHf(t) values of − 8.53 to − 4.64 and − 7.7 to + 12.8, respectively. In addition, the chemical zoning garnet megacrysts are composed mainly of grossular and andraditic grossular, and are characterized by spoon-shaped REE patterns. Based on our new data and the regional geology, it can be concluded that the Yugusayi intrusive rocks formed in a subduction-related tectonic setting and resulted in the skarnization. These data not only provide key constraints on the initial subduction of the Qimantagh back-arc basin’s oceanic crust, but also offer a unique opportunity to explore the Caledonian skarnization in the Qimantagh Belt.

Separation of Au and Sb mineralization in the Qukulekedong intrusion-related deposit, East Kunlun Orogen (NW China): Evidence from fluid inclusions, H–O isotopes, and quartz geochemistry

Separation of Au and Sb mineralization from different hydrothermal stages is a common phenomenon in many Au–Sb deposits, but its cause remains enigmatic. The Qukulekedong intrusion-related Au–Sb deposit in the East Kunlun Orogen has undergone four hydrothermal stages: (1) barren quartz vein, (2) quartz vein with disseminated arsenopyrite–pyrite alteration halo, (3) stibnite–quartz ± native gold vein, and (4) calcite–quartz vein. Stages 2 and 3 formed disseminated Au and vein-type Sb ± Au ore, respectively. Here, internal texture and trace elements in quartz, fluid inclusions (Fls) microthermometry, and H–O isotopes were analyzed to unravel the separation mechanism of the Au and Sb ± Au mineralization at Qukulekedong. Stage 2 quartz has euhedral growth zones with elevated Ti-Al-Li-K-Sb and varying As contents. Stage 2 FIs have medium homogenization temperature (203–307 °C) and salinity (12.2–17.8 wt% NaCl equiv.), and are CH4-rich. They have δ18OH2O = 9.5–10.8 ‰ and δDH2O = −93.4 to −80.8 ‰. In contrast, stage 3 quartz has lower Ti-Al-Li-K and varying Sb contents. Stage 3 FIs show phase separation (coexistence of V-, L- and V-L-type FIs), and are of lower temperature (141–164 °C) and salinity (2.1–10.7 wt% NaCl equiv.), and CO2– and CH4-bearing. They have δ18OH2O = 3.3–5.2 ‰ and δDH2O = − 95.4 to − 89.6 ‰. Considering that the disseminated Au ore is Sb-rich and its pyrite has fine stibnite inclusions, we suggest that the disseminated Au and vein-type Sb ± Au mineralization shared a common Au- and Sb-rich initial magmatic fluids. The disseminated Au mineralization may have formed by fluid–rock interactions. Meanwhile, the vein-type Sb ± Au mineralization was likely caused by the phase separation and cooling led by dilational activities and meteoric water injection.

Early Paleozoic back-arc basin in the East Kunlun Orogen, northern Tibetan Plateau: Insight from the Wutumeiren ophiolitic mélange

The Early Paleozoic tectonic architecture of East Kunlun Orogen in the northern Tibetan Plateau is crucial to deciphering the tectonic evolution of the Proto-Tethys ocean. The Wutumeiren ophiolitic mélange located in the Qimantagh-Xiangride suture between the North Qimantagh and Central Kunlun belts represents essential records of the Proto-Tethyan tectonic domain. It comprises serpentinite, dolerite, basalt and black chert with minor siltstone. The serpentinites possess high MgO, low SiO2 and ΣREE contents, and are characterized by slight depletion of mid-REE, showing ophiolitic ultramafic affinities. Both dolerites and basalts exhibit tholeiitic compositions with high Mg# numbers, low (Na2O + K2O)/SiO2 ratios and ΣREE contents. In comparison, the dolerites show flat patterns of REE, while the basalts display slight enrichment of LREE. All these features suggest they were generated from an E-MORB-type mantle source by different degrees of partial melting. The cherts exhibit distinctive high SiO2 and low ΣREE contents, and show depletion of LREE with negative Ce anomalies and high Y/Ho rations, indicating they are typical abyssal cherts. Together with the comparable basements on both sides of the suture, a back-arc basin setting is suggested for these ophiolitic components. The SHRIMP zircon U–Pb age of 416 ± 34 Ma for the basalt, and the youngest age of 490.6 ± 6.6 Ma for the black siltstones interbedded with cherts signal the back-arc basin from ca. 490–416 Ma. We thus infer a west-Pacific-type active continental margin in the East Kunlun Orogen associated with the northward subduction of the Proto-Tethys oceanic crust in the Early Paleozoic.

Origin and tectonic setting of middle–late Triassic Lalingzaohuo granitoids in the western East Kunlun Orogen, northern Tibetan Plateau

ABSTRACT Triassic granitoids are widespread in the East Kunlun Orogen (EKO), with Middle Triassic granitoids being dominant and Late Triassic granitoids being subordinate. This paper presents new whole-rock geochemical and Sr – Nd isotope, and zircon U – Pb age and Lu – Hf isotope data for eight granitoids in the Lalingzaohuo area, western EKO, northern Tibetan Plateau. These granitoids yield Middle and Late Triassic ages of 245–239 and 223–220 Ma, respectively. The granitoids from the Lalingzaohuo area have features typical of I-type granites and belong to the high-K calc-alkaline series. They exhibit variable rare earth and trace element patterns, and Sr – Nd – Hf isotopic compositions, indicative of different petrogenetic processes. The Middle Triassic samples have higher ISr values of 0.71440–0.71572 and lower εNd(t) values of −6.18 to −4.26 than the Late Triassic samples (ISr = 0.71139–0.71348; εNd(t) = –3.23 to −1.51), which are consistent with I-type granitoids derived from the lower crust. Geochemical and isotopic data indicate that the two stages of I-type granitoids had a significant mantle contribution. The Middle Triassic granitoids in the EKO, particularly the western EKO, have adakitic characteristics and formed by partial melting of lower continental crust. The T2–T3 granites formed by oceanic slab breakoff related to subduction and mantle convection, and the Late Triassic granites formed by lithospheric thinning and asthenospheric upwelling after slab breakoff.

Geodynamic evolution in the post-collisional stage of the East Kunlun Orogenic Belt: constraints from the Late Triassic intermediate–felsic igneous rocks

The Late Triassic igneous rocks associated with post-collision are widely distributed in the East Kunlun Orogenic Belt (EKOB), but their specific dynamic mechanism and evolutional process are still controversial. To address these key issues, we investigated the geochronology and geochemistry of intermediate–felsic igneous rocks from the EKOB, including trachyandesite, rhyolite, porphyritic monzogranite, syenogranite, granite porphyry and crystalline tuff. Zircon U–Pb isotopes of these rocks indicate that they were formed between 231.1 and 215.8 Ma (i.e. Late Triassic). These igneous rocks have moderate to high SiO2 (60.29–79.56 wt%) and low mantle compatible element contents (e.g. Co: 0.37–12.62 ppm; Ni: 1.55–15.59 ppm), suggesting that continental crustal-derived material played an important role in their formation. Porphyritic monzogranite (231.1 Ma), syenogranite (228.1 Ma), trachyandesite (227.4 Ma) and rhyolite (215.8 Ma) have Nb/U (1.41–4.71) and Ce/Pb (1.48–6.19) ratios like that of the crust, with εHf(t) values (–1.31 to 2.26) and old two-stage model (TDM2) ages of 1340–1119 Ma, suggesting that they originated from the partial melting of a Mesoproterozoic crust with minor mantle material involved in their source. Crystalline tuff (224.6 Ma) and granite porphyry (222.3 Ma) have Nb/U (1.93–3.81) and Ce/Pb (0.30–3.18) ratios, negative εHf(t) values (–7.04 to −5.12) and old TDM2 ages (1703–1581 Ma) closer to those of crust, suggesting that they were derived from the partial melting of a Paleo–Mesoproterozoic continental crust without addition of mantle material. Based on our new data and published data, the Late Triassic igneous rocks from the EKOB can be divided into three stages, 236–227, 226–218 and 216–208 Ma, corresponding to slab break-off, lithospheric mantle delamination and thickened lower crust delamination, respectively.

A combined methodology for reconstructing source-to-sink basin evolution, exemplified by the Triassic Songpan–Ganzi basin, central China

Source-to-sink evolution of a basin is a key to understand sedimentary processes, especially in a complex regional orogenic setting. Detrital zircon populations can be traced from their primary sources to their depositional settings. The resulting interpretations are enhanced by calculation of the adjacent orogen's paleoaltimetry, which provides additional insights into paleogeography. In this study, we present a combined methodology which aims to reconstruct source-to-sink evolution by the analysis of detrital zircon age distribution in sandstones, together with the calculation of paleo-elevation of surrounding orogens based on the chemical compositions of coeval magmatic rocks. We test the method using detrital zircon U–Pb geochronological data sets from the Triassic Songpan–Ganzi basin in central China, combined with whole-rock geochemical data from intermediate-composition magmatic rocks in adjacent crustal blocks. Application of the combined methodology supports a syn-collisional basin model for the formation of the Triassic Songpan-Ganzi basin in preference to a continental back-arc basin. The clastic sediments, mainly deep-marine turbidites, accumulated in a remnant Paleotethyan Ocean that was surrounded by the converging North China Block, South China Block, East Kunlun Orogenic Belt and the Qiangtang Block. The North China Block and the North Qaidam Block were major proto-sources of detrital zircons to the basin, contributing on average 12 % and 15 %, respectively. Triassic magmatic rocks in the East Kunlun and Qiangtang regions were major sources of igneous zircons, up to 68 % for the former and up to 56 % for the latter. Despite being located at a calculated elevation of ca. 4000 m, the Qinling Orogenic Belt contributed only ca. <10 % of the zircons, mostly restricted to the eastern depocenter of the basin. In contrast, supply from the North Qiangtang Block, despite its calculated lower elevation (1000–3000 m), accounts for 2–10 % of the detrital zircons in the basin, suggesting high erosion rates of this block. The minimal supply of zircons from the South China Block, restricted to 3–6 % in the central and western depocenters, is inconsistent with the zircon abundances predicted in the alternative back-arc basin model of the Songpan–Ganzi basin.

Origin of Quartz Diorite and Mafic Enclaves in the Delong Gold-Copper Deposit and Evaluation of the Gold-Copper Mineralization Potential

The Triassic Paleo-Tethyan magmatic belt in the East Kunlun Orogen (EKO) hosts a small number of porphyry-skarn deposits. The controls of these deposits, especially those in the eastern EKO, are poorly understood. In this contribution, we report new petrological, zircon U-Th-Pb-Hf isotopic, whole-rock elemental with Sr-Nd isotopic, and mineral chemistry data of the Delong quartz diorite and mafic enclaves to constrain their petrogenesis and metal fertility. The quartz diorite and mafic enclaves are emplaced in the Late Triassic (ca. 234 Ma). They are medium-K, metaluminous, enriched in large-ion lithophile elements (e.g., Rb, Ba, Th) and light rare earth elements (e.g., La, Ce, Nd), and relatively depleted in high field strength elements (e.g., Nb, Ta, Ti, P) and heavy rare earth elements (e.g., Gd, Er, Tm, Yb). The quartz diorite show similar (87Sr/86Sr)i (0.712584~0.713172) and more depleted εNd(t) (−6.4~−5.7) and εHf(t) (−2.3~+2.6) to those of mafic enclaves ((87Sr/86Sr)i = 0.712463~0.713093; εNd(t) = −6.4~−6.0; εHf(t) = −9.4~−4.8). Geochemical compositions of zircon, amphibole, and biotite yield high water content (5.3 wt.%~6.9 wt.% and 6.1 wt.%~7.3 wt.% based on amphibole, respectively) and high redox state for both the quartz diorite and mafic enclaves. These data, together with petrography, indicate the Delong intrusion was formed by mingling of magmas from enriched mantle and lower continental crust with juvenile materials. The oxidized and water-rich features of these magmas denote they have potential for porphyry Cu (±Au ± Mo) deposits, as do some Triassic magmatic rocks in the eastern EKO that show similar geochemical and petrographic characteristics with the Delong intrusion.

Early Paleozoic subduction imprints of the Proto-Tethys Ocean: Evidence from the Appinite–Diorite–Granodiorite complex in East Kunlun, Northern Tibet

The East Kunlun Orogenic Belt (EKOB), on the northern margin of the Tibetan Plateau, forms a part of the western segment of the orogenic system in Central China. The Paleozoic tectonic evolution of the EKOB, along the western margin of Gondwana, is closely related to Paleozoic subduction and closure of the Proto-Tethys and is key to the palinspastic reconstruction of the Central Asia and Tethys domains. Numerous studies have been conducted on the Early Paleozoic granitoids in the EKOB. However, a convincing magmatic proxy to indicate the timing of oceanic subduction during the Early Paleozoic is yet to be well constrained, partially because the arc geochemical signature of the calc-alkaline granitoids is not a reliable indicator of oceanic subduction. Herein, we present new zircon U–Pb ages, whole-rock major and trace element analyses, and Nd–Pb and zircon Hf isotopic data for Early Ordovician appinite (mafic), diorite (intermediate), and granodiorite (felsic) intrusive rocks in the Dundegou (DDG) area of the EKOB, northern Tibet. The zircon U–Pb ages suggest this complex was emplaced at approximately 477–472 Ma. Abundant 557–510 Ma zircon xenocrysts and/or inherited zircons were found in the granodiorite and appinites, suggesting two earlier magmatic episodes during the late Ediacaran–Cambrian. Appinites and diorites are characterized by depletion in high field strength elements, with negative initial εNd(t) values (appinites: −6.5 to −9.4; diorites: −8.2 to −9.1) and negative zircon εHf(t) values (appinites: −2.5 to −10.3; diorites: −1.0 to −9.1). These features suggest that appinites and diorites were most likely derived from the partial melting of enriched mantle wedge peridotites with the addition of sediment-derived melts from the sedimentary veneer of the subducting slab, along with variable degrees of fractional crystallization and a limited extent of crustal contamination. The granodioritic magma was contributed by (a) the ancient mafic lower crust-derived magma, (b) the derivative felsic magma from a dioritic parent, and (c) a minor mantle-derived magma component, resulted in the formation of granodiorites with highly unradiogenic initial Nd and Hf isotopic compositions (εNd(t) values of −8.5 and εHf(t) values from −0.6 to −7.3). Our study suggests that the subcontinental lithospheric mantle beneath the EKOB is likely heterogeneous in terms of its isotopic composition. The DDG complex is the oldest appinite-bearing complex found so far in the EKOB and a good indicator of proto-Tethys oceanic subduction.

Eclogite in the East Kunlun Orogen, northwestern China: A record of the Neoproterozoic breakup of Rodinia and early Paleozoic continental subduction

An early Paleozoic eclogite belt extends discontinuously for ∼500 km in the East Kunlun Orogen, northwestern China, which provides insight into the evolution of the Proto-Tethys Ocean. However, the nature and age of the eclogite protoliths and subsequent metamorphism are unclear. Here, we report mineral chemical, zircon, and titanite geochronological; whole-rock major and trace elemental; and Sr-Nd isotopic data for eclogites from the Xiarihamu area in the western East Kunlun Orogen. At a minimum, these eclogites underwent peak eclogite-facies metamorphism at 27−28 kbar and 630−650 °C, post-peak decompression, and retrograde amphibolite-facies overprinting. Geochemically, eclogites have characteristics typical of enriched mid-oceanic-ridge basalts and oceanic-island basalts. Inherited magmatic zircon cores yielded a protolith age of 828 Ma, and metamorphic zircons yielded peak and post-peak decompression ages of 443 Ma and 422 Ma, respectively. A titanite U-Pb age of 413 Ma constrains the timing of later amphibolite-facies overprinting. The protoliths of the Xiarihamu eclogites were the products of continental rift-related basaltic magmatism, which was generated during the initial breakup of Rodinia. The protoliths experienced eclogite-facies metamorphism in a continental subduction/collisional setting during the early Paleozoic. The formation of the Xiarihamu eclogites suggests that the Proto-Tethys Ocean had closed by 443 Ma. Given the spatio-temporal relationship between the Xiarihamu magmatic Cu-Ni sulfide deposit and the eclogites studied, the former may have formed in a post-collisional extensional setting. The eclogites and magmatic Cu-Ni sulfide deposit from the Xiarihamu area jointly reveal the evolution of the East Kunlun Orogen from Neoproterozoic supercontinent breakup to early Paleozoic tectonic transitions from collisional convergence to post-collisional extension.

Granulite facies metasedimentary rocks: Insight into the formation of the East Kunlun continental arc

The metamorphism and dynamic processes of the Early Paleozoic continental margin arc in the East Kunlun orogen are still unclear. This study investigated the metasedimentary rocks with granulite facies in the Qingshuiquan area in the eastern section of the East Kunlun orogen. The kyanite is not found in the pelitic granulite. The phase equilibrium modeling indicates a middle-pressure type with metamorphic conditions of 0.7 to 0.8 GPa and 800 to 840°Cfor pelitic granulite. UPb dating of zircons from metasedimentary rocks yielded metamorphic ages of 490-480 Ma and detrital zircons ages of 1700-1500 Ma. REE patterns of the metasedimentary rocks are enriched LREEs and distinct negative Eu anomalies, the εNd values vary from −1 to −16. These results imply that the Precambrian supracrustal rocks were buried into the middle-lower crust level and granulite facies metamorphism occurred in Late Cambrian-Early Ordovician.Synchronous (520-480 Ma) magmatic rocks in Qingshuiquan and adjacent areas indicate that subduction of the Proto-Tethyan oceanic crust led to the formation of arc magmatic rocks. Meanwhile, it induced some sedimentary rocks to underthrust or return flows into the middle-lower crust of the upper plate and underwent granulite facies metamorphism.