East Kunlun Orogenic Research Articles

The Giant Xiarihamu Ni-Co Sulfide Deposit in the East Kunlun Orogenic Belt, Northern Tibet Plateau, China

The Xiarihamu Ni-Co deposit, located in the East Kunlun orogenic belt, northern Tibet plateau, is the secondary largest Ni deposit in China and contains ~157 million metric tons (Mt) sulfide ores with average grades of 0.65 wt % Ni, 0.14 wt % Cu, and 0.013 wt % Co. The intrusion consists of gabbroic and ultramafic portions, the main orebody being hosted in the ultramafic portion. The zircons separated from the gabbronorite and websterite yield SHRIMP U-Pb ages of 405.5 ± 2.7 and 406.1 ± 2.7 Ma, indicating a genetic linkage with the Early Devonian regional magmatism (400–410 Ma). The main orebody includes two large ore pods comprising a pod of disseminated sulfides in the western portion grading into multiple ore sublayers to the east. The ore sublayers consist of net-textured and disseminated sulfides. The distribution and shape of the ore pods and sublayers of the orebody are conformable with the undulating roof and bottom of the ultramafic portion. The unusually high Ni/Cu ratios (~4–18) and extremely low tenors of platinum-group elements (PGE; <4 ppb Ir, <85 ppb Pt, and <115 ppb Pd) of the disseminated sulfides indicate a genetic relationship with low degrees of partial melting of a pyroxenite mantle source. The slightly higher Ir and Ru tenors suggest that the disseminated sulfides in the western portion of the main orebody were segregated from less evolved magma under higher mass ratios of silicate melt/sulfide liquid than the sulfides of the middle and eastern portions.

Two Contrasting Ophiolites in One Suture Zone: a Case Study from East Kunlun Orogenic Belt, North Tibetan Plateau

Two Contrasting Ophiolites in One Suture Zone: a Case Study from East Kunlun Orogenic Belt, North Tibetan Plateau

Mid-Neoproterozoic Tadong amphibolites at the junction of the East Kunlun and Western Qinling Orogens – a record of continental rifting during the break-up of Rodinia

ABSTRACTMid-Neoproterozoic rift-related Tadong amphibolites (TDA) are situated in the junction of the East Kunlun Orogen (EKO) and the West Qinling Orogen (WQO) in western China, and have important significance in the break-up of the Supercontinent Rodinia. The mineral assemblage of the TDA exhibits amphibolite facies metamorphism, and the protolith is basic volcanic rocks. Here, we present a detailed laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb age and geochemical data of TDA. LA-ICP-MS U–Pb zircon age determination indicates that the basic volcanics formed at ca. 743–711 Ma. Geochemically, TDAs are characterized by low SiO2 (46.06–50.18%), K2O (0.66–1.86%), and Mg# (40–45) and high TiO2 (1.9–3.5%), and are attributed to tholeiite basalts. Their chondrite-normalized REE patterns are characterized by moderate enrichments in LREEs (LREE/HREE = 4.98–5.51) with insignificant Eu anomalies (δEu = 0.99–1.12). The primitive mantle-normalized trace element diagrams are characterized by pronounced enrichment of high-field-strength elements (HFSEs, e.g. REE, Nb, Ta, Zr, Hf) with slightly positive anomalies of Ti, which is different from the subduction zone basalts and normal mid-ocean ridge basalts (NMORBs), but similar to the ocean-island basalts (OIBs) and Emeishan continental flood basalts (ECFBs), resembling the features of an OIB-like mantle source. Most samples exhibit high Nb/Ta (16.6–17.4), Nb/La (0.70–0.94), and positive ΔNb (ΔNb = 1.74 + log (Nb/Y) − 1.92*log (Zr/Y)) values. These geochemical features suggest that the Tadong basaltic rocks were most likely derived from an OIB-like mantle source and were the products of a lower degree partial melting of garnet lherzolite. Along with the tectonic discrimination diagrams, it is further inferred that these rocks were formed in a continental rifting setting. These results from our research confirm the existence of continental rifting in the junction of the EKO and the WQO in the mid-late Neoproterozoic, implying that the rifting is a response to the break-up of the Supercontinent Rodinia.

Tectono-thermal events in East Kunlun, Northern Tibetan Plateau: Evidence from zircon U–Pb geochronology

The widely distributed high-grade gneisses in the East Kunlun Orogenic Belt (EKOB) are keys to understand the Precambrian tectonic evolution of the Northern Tibetan Plateau. In this study, new LA-ICP-MS zircon U–Pb ages from paragneiss and schist of the Proterozoic Jinshuikou Group and quartzite of the Proterozoic Binggou Group are reported in an attempt to evaluate the Neoproterozoic and Paleozoic tectono-thermal events of the EKOB. These geochronologic data can be classified into 4 groups: Group 1 ages ranging from 2243Ma to 3701Ma are represented by inherited zircons from protolith and confirm the existence of Eoarchean to Paleoproterozoic continental nucleus in the source region of the Jinshuikou Group. Group 2 ranging from 928Ma to 1849Ma yields lower intercept ages of 0.9–1.0Ga which represent the Neoproterozoic tectono-thermal event. This event, similar to that of the northern margin of Qaidam, might be a response to the assembly of Rodinia. Group 3 ranges from Neoproterozoic to early Paleozoic with lower intercept ages which are identical to the weighted mean ages of Group 4. These two age groups confirm the tectono-thermal event related to Paleozoic oceanic subduction. Moreover, based on the youngest age of 2.2Ga in Group 1 and the upper intercept age of 1.8Ga in Group 2, the depositional timing of the Jinshuikou and Binggou groups can be defined as Paleoproterozoic and Mesoproterozoic, respectively.

Zircon U–Pb dating, geochemistry and Sr–Nd–Pb–Hf–O isotopes for the Nan'getan granodiorites and mafic microgranular enclaves in the East Kunlun Orogen: Record of closure of the Paleo-Tethys

The East Kunlun Orogen in the Northern Qinghai–Tibet Plateau is an ideal region to investigate the geodynamic processes of magmatism related to the closure of the Paleo-Tethys Ocean. Here, we report petrology, zircon U–Pb geochronology, whole-rock geochemistry and multiple isotope data from granodiorites and the associated dioritic enclaves in a dominant Nan'getan granitoid in the East Kunlun Orogen. Zircon U–Pb ages indicate that the host granodiorites and dioritic enclaves were synchronously emplaced at ~243Ma. The granodiorites are medium- to high-K calc-alkaline, metaluminous (A/CNK=0.93–0.98), with high Al2O3 content (15.28%–16.10%), Mg# (47–49), very low Sr/Y ratios (127–217), high abundances of incompatible elements (Y=3.87–8.36ppm, Nb=3.04–5.71ppm, Th=3.04–5.71ppm), low (87Sr/86Sr)i (0.7050–0.7079), negative whole-rock εNd(t) (−8.2 to −5.8), (206Pb/204Pb) 243Ma of 18.520 to 18.772, (207Pb/204Pb) 243Ma of 15.611 to 15.650, (208Pb/204Pb) 243Ma of 38.227 to 38.528, δ18OSMOW=6.8‰–9.1‰, εHf(t) of −1.2 to +2.4. The dioritic enclaves (SiO2=51.08%–56.29%) have Mg# values of 48–51, with negative Eu anomalies (δEu=0.59–0.79), low (87Sr/86Sr)i (0.7058–0.7080), negative whole-rock εNd(t) (−8.2 to −5.8), (206Pb/204Pb) 243Ma of 18.376 to 18.809, (207Pb/204Pb)243Ma of 15.606 to 15.661, (208Pb/204Pb)243Ma of 38.244 to 38.540, δ18OSMOW=5.6‰–10.0‰, εHf(t) of −3.5 to +1.7. These isotopic features of arc-type rocks from the East Kunlun suggest that the parental magmas of the Nan'getan granodiorites and the dioritic enclaves originated from an enriched lithospheric mantle. The Nan'getan granitoids might have recorded the northward subduction of the Paleo-Tethys ocean lithosphere following the initial collision of the Bayan Har–Songpan Ganzi–East Kunlun terrane and the closure of the Paleo-Tethys Ocean at ~243Ma.

Geochemistry, zircon U–Pb ages and Sr–Nd–Hf isotopes of an Ordovician appinitic pluton in the East Kunlun orogen: New evidence for Proto-Tethyan subduction

Appinite is commonly derived from a mantle source in subduction zones and thus holds a key to constrain the tectonic evolution of ancient orogens. This study presents chronological, mineralogical and geochemical data for one appinitic pluton from the south Tethyan suture zone in the East Kunlun orogen, Northern Tibetan Plateau. The pluton is predominantly composed of hornblende-rich mafic appinites, with minor amounts of granodiorite. Zircon U–Pb age of the granodiorite (466Ma) is identical to the mafic appinites (447–450Ma). The mafic appinites are commonly hornblende diorites, which comprise large amounts of magnesio-hornblende [Mg/(Fe+Mg)=0.61–0.68] and andesine (An43–54). The hornblende diorites have low contents of SiO2 (48.62–54.95wt.%), high contents of total FeO (7.90–12.84wt.%) and MgO (4.32–11.89wt.%) and moderate values of Mg# [Mg#=molar 100∗Mg/(Mg+Fe); 49–69]. Their geochemistry displays: slight enrichment of light rare earth elements ((La/Yb)N=1.89–6.84) and flat heavy rare earth elements ((Ga/Yb)N=1.49–2.22); enrichment in large ion lithophile elements and depletion in high field strength elements; less-enriched isotopic compositions with initial 87Sr/86Sr ratios of 0.70536–0.70617, εNd(t) of 0.79–3.02 and zircon εHf(t) of 8.73–12.82. The associated granodiorites comprise plagioclase (45–50vol.%, An=26–39), quartz (15–20vol.%), K-feldspar (5–10vol.%), ferrohornblende [2–5vol.%, (Mg/(Fe+Mg)=0.45–0.49] and epidote (1–5vol.%). The epidotes have pistacite components ranging from 23 to 28. The granodiorites exhibit calc-alkaline character, and have rare earth and trace element patterns similar to the hornblende diorites. The geochemical compositions and simulations suggest that the parental magma of the mafic appinites was generated by partial melting of one depleted mantle source which was metasomatised by subducted sediment-derived felsic melts (ca. 20–25%). Fractional crystallization of clinopyroxene, hornblende, magnetite and titanite has driven the appinitic magma to cogenetic granodiorite. The Late Ordovician mafic appinites indicate that there did exist Proto-Tethyan subduction along the South Kunlun suture zone, and the subduction would add significant volumes of sediment-derived melts into the depleted mantle wedge, resulting into metasomatism and melting with the formation of appinitic magma.

Petrogenesis and tectonic significance of the late Triassic mafic dikes and felsic volcanic rocks in the East Kunlun Orogenic Belt, Northern Tibet Plateau

We present zircon U–Pb ages and geochemical data on the late Triassic mafic dikes (diabase) and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in the East Kunlun Orogenic Belt (EKOB). These rocks give a small age window of 228–218Ma. The mafic dikes represent evolved alkaline basaltic melts intruding ~8–9Myrs older and volumetrically more abundant A-type granite batholith. Their rare earth element (REE) and multi-element patterns are similar to those of the present-day ocean island basalts (OIBs) except for a weak continental crustal signature (i.e., enrichment of Rb and Pb and weak depletion of Nb, Ta and Ti). Their trace element characteristics together with the high 87Sr/86Sr (0.7076–0.7104), low εNd(t) (−2.18 to −3.46), low εHf(t) (−2.85 to −4.59) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks are characterized by high LREE/HREE (e.g., [La/Yb]N of 5.71–17.00) with a negative Eu anomaly and strong depletion in Sr and P, resembling the model upper continental crust (UCC). Given the high 87Sr/86Sr (0.7213–0.7550) and less negative εNd(t) (−3.83 to −5.09) and εHf(t) (−3.06 to −3.83) than the UCC plus the overlapping isotopes with the mafic dikes and high Nb–Ta rhyolites, the felsic volcanic rocks are best interpreted as resulting from melting-induced mixing with 45–50% crustal materials and 50–55% mantle-derived mafic melts probably parental to the mafic dikes. Such mantle-derived melts underplated and intruded the deep crust as juvenile crustal materials. Partial melting of such juvenile crust produced felsic melts parental to the felsic volcanic rocks in the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are associated with post-collisional extension and related orogenic collapse. Such processes are probably significant in causing asthenospheric upwelling, decompression melting, induced melting of the prior metasomatized mantle lithosphere and the existing crust. This work represents our ongoing effort in understanding the origin of the juvenile crust and continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.

Petrogenesis and geodynamic implications of the Mid-Triassic lavas from East Kunlun, northern Tibetan Plateau

Lying in the northern margin of the Tibetan Plateau, the East Kunlun Orogenic Belt (EKOB) is characterized by widespread of the late Permian to Late Triassic magmatic rocks. In order to better understand magma genesis and evolution during the waning stage of the Paleo-Tethyan oceanic subduction and subsequent collision, we present zircon U–Pb dating and Lu–Hf isotopes, whole-rock major and trace elements, and Sr–Nd isotope data for the Triassic volcanic lavas in the Haishigou area of the EKOB, northern Tibet. Lithologically, the Haishigou volcanic lavas are mainly composed of dacites and rhyolites. The LA–ICP–MS zircon U–Pb analyses for rhyolites have shown that the Haishigou volcanic rocks formed during the Middle Triassic with ages of ca. 244–245Ma. The Haishigou volcanic lavas actually belong to part of the Middle Triassic Naocangjiangou Formation, rather than the Late Triassic Elashan Formation. Geochemically, Haishigou volcanic lavas have SiO2=60.31–76.19wt% and K2O=2.60–4.18wt%, placing them in high-K calc-alkaline series. These lavas are characterized by enrichment in some large-ion lithophile elements (e.g., Rb, K and Pb) and light rare earth elements and depletion in some high field strength elements (e.g., Nb, Ta, and Ti), with geochemical affinities to those rocks forming in a continental or an oceanic arc setting. All the volcanic rocks exhibit high initial 87Sr/86Sr ratios (0.70614–0.70841) and moderately negative εNd(t) values (−5.9 to −4.3) that imply a continental rather than oceanic type magma source. The rhyolites in the Haishigou volcanics exhibit moderately negative to slightly positive εHf(t) values (−4.2 to 1.4). Combined with their zircon Hf two-stage model ages of 1187–1538Ma and whole-rock Nd two-stage model ages of 1.37–1.38Ga, it can be inferred that the crustal growth of East Kunlun occurred during the Mesoproterozoic, making them similar in age to the lower crust metamorphic basement beneath the EKOB (i.e., the Xiaomiao Group). We suggest that the Haishigou dacites were generated by partial melting of the mafic lower crust beneath the EKOB with addition of a mantle-derived mafic component and that the rhyolites were produced by fractional crystallization from a dacitic parent. Taking into account the Late Permian to Triassic geological record from the EKOB and surrounding regions, we argue that the Middle Triassic volcanic rocks in the Haishigou area erupted during the northward subduction of the Paleo-Tethyan oceanic plate. Consequently, the timing of closure of the Paleo-Tethyan Ocean just south of the EKOB is no earlier than the Middle Triassic.

Evolution process of the Late Silurian–Late Devonian tectonic environment in Qimantagh in the western portion of east Kunlun, China: Evidence from the geochronology and geochemistry of granitoids

The East Kunlun Orogenic Belt has undergone a composite orogenic process consisting of multiple orogenic cycles and involving many types of magmatic rocks spread over the whole district. However, due to bad natural geographical conditions and complex superimposed orogenic processes, most of the Caledonian orogenic traces were modified by the late tectonic uplift and denudation, so these rocks are poorly studied. Multiperiodic magmatic activity during the Late Silurian (approximately 420 Ma)–Late Devonian (approximately 380 Ma) exists in the Qimantagh area. We obtained 5 zircon U–Pb ages from the Late Silurian–Late Devonian granitoids in the Qimantagh area. Those ages are 420.6 ± 2.6 Ma (Nalingguole biotite monzogranite), 421.2 ± 1.9 Ma (Wulanwuzhuer potassium granite), 403.7 ± 2.9 Ma (Yemaquan granodiorite), 391.3 ± 3.2 Ma (Qunli granite porphyry), and 380.52 ± 0.92 Ma (Kayakedengtage granodiorite). These granitoids belong to the sub-alkaline, high-K calc-alkaline, metaluminous or weakly or strongly peraluminous series. The rocks are right oblique types, having overall relative LREE enrichment and HREE depletion, though rocks from different times may exhibit different degrees of Eu anomalies or overall moderate Eu depletion. The rocks are rich in large ion lithophile elements (LILE), such as Rb, Th, and K, and high field strength elements (HFSE), such as Zr and Hf, and are depleted in Ba, Nb, Ta, Sr, P, Eu, and Ti. The rocks have complex composition sources. The Late Silurian granitoids are mainly crust-derived. Most of the Devonian granitoids are crust-mantle mixed-source and only some parts of them are crust-derived, especially the Middle Devonian granitoids. Those mid-acidic and acidic intrusive rocks are formed in a post-collision tectonic setting, lithosphere delamination may have occurred in the Early Devonian (407 Ma), and the study area subsequently experienced an underplating of the mantle-derived magma at least until the Late Devonian (380 Ma).

Zircon U–Pb ages, geochemistry, and Sr–Nd–Pb isotopic compositions of Middle Triassic granodiorites from the Kaimuqi area, East Kunlun, Northwest China: implications for slab breakoff

The Qimantage area of Northwest China lies in the western part of the East Kunlun Orogenic Belt, and is dominated by late Permian to Late Triassic granitoids. Among these, the Middle Triassic granitoids are mainly distributed south of the North Kunlun Fault, and consist of two main granitic assemblages: the Kaimuqi assemblage in the east and the Mositu assemblage in the west. To better constrain the Indosinian tectonic evolution of this area, we present data on the geochronology, geochemistry, and petrology of ore-bearing granodiorites from the Kaimuqi area in eastern Qimantage. The granodiorite samples have porphyritic or fine-grained textures. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating yields emplacement ages of 238–242 Ma, interpreted here as the result of the Middle Triassic magmatism. The granodiorites are mostly of the high-K calc-alkaline series, and are enriched in light rare earth elements, depleted in heavy rare earth elements such as Nb, Ta, P, and Ti, and have weak negative Eu (Eu/Eu*) anomalies. The Kaimuqi granodiorites have lower SiO2 and Sr contents, and higher Na2O/K2O ratios than the Mositu granodiorites. They also show initial 87Sr/86Sr ratios of 0.712151–0.715436, εNd(t) values of −7.4 to −6.3, and two-stage Nd model ages of 1.53–1.61 Ga. Together with their radiogenic Pb isotopic ratios for 206Pb/204Pb(t) (18.271–18.622), 207Pb/204Pb(t) (15.637–15.651), and 208Pb/204Pb(t) (38.452–37.870), these data indicate both mantle and crustal contributions to the source of the granodiorites. Field investigations show that Middle Triassic granitoids in both the Mositu and Kaimuqi assemblages contain large numbers of mafic microgranular enclaves, which supports an interpretation of mantle and crustal magmatic mixing. Based on a comparison of these results with data from coeval granites in the Mositu assemblage, we propose that the Middle Triassic granitoids in the Qimantage area were produced at ca. 240 Ma, as a result of the end of subduction and the initiation of collision during the Variscan–Indosinian orogeny. Magma mixing may be interpreted as the result of slab breakoff in a subduction zone environment, which led to fluid metasomatism and induced partial melting of an enriched lithospheric mantle, resulting in the formation of voluminous granitic magma.

Crustal thickening prior to 220 Ma in the East Kunlun Orogenic Belt: Insights from the Late Triassic granitoids in the Xiao-Nuomuhong pluton

The East Kunlun Orogenic Belt (EKOB) played an important role in plate tectonics, magma generation, and crustal evolution. Late Triassic granodiorites and their mafic micro-granular enclaves (MMEs) from Xiao-Nuomuhong in the EKOB were studied for geochemistry and geochronology to constrain their petrogenesis. Zircon LA-ICP-MS dating indicates that the Xiao-Nuomuhong granodiorites are coeval with their MMEs (∼222Ma). The granodiorites are high-K calc-alkaline rocks that are enriched in Rb, Th, U and LREE, and depleted in Cr, Ni and HFSE, with high Sr/Y ratios (82.2–85.3) and geochemically resemble the lower crust-derived adakites. The MMEs are also high-K calc-alkaline rocks, with high Al2O3 (16.8–18.8wt.%), low Mg# (30–40), Nb, Zr and Hf, with weak negative Eu anomalies (Eu/Eu#=0.8–0.9). We suggest the MMEs are mafic magmatic globules that were injected into the felsic host magma. The adakitic rocks from the Xiao-Nuomuhong pluton were generated by partial melting of thickened crust, while the primitive compositions of the MMEs were most likely from the lithospheric mantle beneath the EKOB. The Late Triassic Xiao-Nuomuhong pluton is important evidence that crustal thickening in the EKOB occurred prior to 220Ma. The pluton is interpreted as the result of mixing between thickened lower crust-derived melts and lithospheric mantle-derived mafic melts and the protracted magmatic response to the break-off of the Paleo-Tethys oceanic slab at ∼232Ma.

Reworking of old continental lithosphere: an important crustal evolution mechanism in orogenic belts, as evidenced by Triassic I-type granitoids in the East Kunlun orogen, Northern Tibetan Plateau

There is a strong genetic relationship between the petrogenesis of I-type granitoids and the evolution of continental crust in orogenic belts. This study of I-type granitoids in the East Kunlun orogen, Northern Tibetan Plateau, shows that reworking of old continental lithosphere is an important key to this genetic relationship. The East Kunlun has numerous Triassic granitic plutons that are related to subduction of the Palaeo-Tethyan ocean and terrane collision in the early Mesozoic. U–Pb analysis of zircons from these Triassic granitoids indicates that the granitic magmatism lasted from 249 to 223 Ma. Based on elemental and isotopic compositions and their petrogenesis, the magmatism can be divided into three groups. (1) Group 1 consists of quartz diorites and granodiorites (241–249 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit typical subduction-related chemical characteristics. They were derived from lower crust mainly composed of Precambrian metabasaltic basement rocks with different degrees of involvement of mantle material. (2) Group 2 consists of granitic porphyries and syenogranites (231–238 Ma), which are high Rb/Sr, metaluminous to weakly peraluminous high-K alkali-calcic I-type granitoids, showing characteristics of typical pure crustal-derived granitoids. They were derived from partial melting of a Mesoproterozoic metagreywacke source in the lower crust. (3) Group 3 consists of porphyry granodiorites ( c . 223 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit the typical geochemical characteristics of adakites (e.g. high La/Yb and Sr/Y ratios and low Y and Yb contents). Their high K 2 O and low Mg# with evolved Sr–Nd–Hf isotopic compositions indicate that they were most probably derived from thickened mafic lower continental crust, which underwent partial melting induced by underplated hot mafic magma. Combining the present work with previous studies, we propose that the subduction of the Palaeo-Tethyan ocean lasted from 278 to 241 Ma, and the collision between the Bayan Har terrane and the East Kunlun occurred at 231–238 Ma, whereas the group 3 granitoids most probably formed in a post-collisional setting. Overall, all the studied I-type granitoids were derived from partial melting of old continental lower crust with minor addition of lithospheric mantle material; thus reworking of old continental lithosphere is an important mechanism for the evolution of orogenic crust. Supplementary materials: Analytical methods, zircon U–Pb data, geochemical data, and Sr–Nd–Hf isotope data for the granitoids are available at www.geolsoc.org.uk/SUP18758 .

Geochemical constraints on the petrogenesis of granitoids in the East Kunlun Orogenic belt, northern Tibetan Plateau: Implications for continental crust growth through syn-collisional felsic magmatism

Early Triassic syn-collisional granitoids with mafic magmatic enclaves (MMEs) crop out along the entire East Kunlun Orogenic belt (EKOB) at the northern margin of the Tibetan Plateau. They are andesitic in composition and enriched in light rare earth elements (LREEs) with a flat heavy REE (HREE) pattern. Their average composition resembles that of the bulk continental crust. The enclosed MMEs have the same mineralogy as their host granitoids, but contain a greater mode of mafic minerals (amphibole and biotite), and thus have higher HREE abundances. Zircon U–Pb dating shows that both the granitoid hosts and MMEs have the same crystallization age of ~250Ma and indistinguishable bulk rock Sr–Nd–Pb–Hf isotope compositions (ISr of 0.7080–0.7116, varying 206Pb/204Pbi of 18.53–19.32, essentially constant εNd(t) of −5.3 to −2.1 and a small range of positive εHf(t), mostly 1.7–5.2). The complete isotopic overlapping between the granitoid hosts and the MMEs is understood to reflect that the MMEs are disintegrated cumulates formed at an early stage of the granitoid magma evolution within the same magmatic system. The isotopic data set reveals that the granitoids are variably evolved melts produced by partial melting of the subducted Paleo-Tethyan oceanic crust with terrigenous sediments under amphibolite-facies conditions in response to the continental collision.

Geochemical Features, Age, and Tectonic Significance of the Kekekete Mafic‐ultramafic Rocks, East Kunlun Orogen, China

Abstract:The Kekekete mafic‐ultramafic rocks are exposed in the Kekesha‐Kekekete‐Dawate area, which are in the eastern part of the East Kunlun Orogenic Belt It outcrops as tectonic slices intruding tectonically in the Paleoproterozoic Baishahe Group and the Paleozoic Nachitai Group. The Kekekete mafic and ultramafic rocks is located near the central fault in East Kunlun and lithologically mainly consists of serpentinite, augite peridotite, and gabbro. The LA‐ICP‐MS zircon U‐Pb age of the gabbro is 501 ± 7 Ma, indicating that Kekekete mafic‐ultramafic rocks formed in the Middle Cambrian. This rock assemblage is relatively poor in SiO2 and (Na2O+K2O) but rich in MgO and SFeO. The chondrite‐normalized REE patterns of the gabbro dip slightly to the right; the primitive mantle and MORB‐normalized spidergrams of trace elements show enrichment of large‐ion lithophile elements (Cs, Rb, Ba, etc.) and no differentiation of high field strength elements. The general dominance of E‐MORB features and the geochemical characteristics of OIB suggest that the Kekekete mafic‐ultramafic rocks formed in an initial oceanic basin with slightly enriched mantle being featured by varying degrees of mixing of N‐MORB depleted mantle and a similar‐OIB‐type source. From a comprehensive study of the previous data, the author believes that the tectonic history of the East Kunlun region was controlled by a geodynamic system of rifting and extension in the late stages of the Neoproterozoic to early stages of the Early Paleozoic and this formed the paleo‐oceanic basin or rift system now represented by the ophiolites along the central fault in East Kunlun, the Kekekete mafic‐ultramafic rocks and Delisitan ophiolite.

Regional Tectonic Transformation in East Kunlun Orogenic Belt in Early Paleozoic: Constraints from the Geochronology and Geochemistry of Helegangnaren Alkali‐feldspar Granite

Abstract:The Helegangnaren feldspar granite exposed in the eastern part of East Kunlun, is characterized by high concentrations of SiO2 and alkaline, low abundances of Fe, Mg and Ca, metaluminous‐weak peraluminous. Trace elements analysis shows that the granite is depleted extremely in Ba, Sr and Eu, and rich in some large‐ion lithophile elements and high field strength elements. Besides, the granite has high Ga contents, the values of 104(Ga/Al) vary from 2.50 to 2.77, which is mainly greater than the lower limit of A‐type granites (2.6), and is higher than the I‐ and S‐type granites’ average (2.1 and 2.28, respectively). Rare earth element (REE) is characterized by relatively high fractionations of light REE (LREE) and heavy REE (HREE) (LREE/HREE=9.3–13.60, (La/Yb)N=10.92–18.02), pronounced negative Eu anomalies (δEu=0.08–0.13), and exhibits right‐dipping gull pattern. Major elements, rare elements and trace elements features show the granite is ascribed to A‐type granite and A2 subtype in tectonic genetic type. They are plotted into post‐collision or within‐plate area in a variety of tectonic discriminations. Geological and geochemical data comprehensively suggest that the granite is formed in a post‐collision extensive tectonic setting. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb dating yields a weighted mean age of 425 Ma, belonging to Middle Silurian, which is similar to the age of the post‐collision geological events in the region. The differences of magmatic rocks in formation age, rocks assemblage and rocks series systematically indicate that the regional tectonic stress regime in the East Kunlun orogenic belt experienced a major transformation from compress to extension in Middle Silurianin, and the Helegangnaren feldspar granite intruded in the early stage of tectonic transformation.

The thickness and structural characteristics of the crust across Tibetan plateau from active-sources seismic profiles

The Tibetan plateau as one of the youngest orogen on the Earth was considered as the result of continent-continent collision between the Eurasian and Indian plates. The thickness and structure of the crust beneath Tibetan plateau is essential to understand deformation behavior of the plateau. Active-source seismic profiling is most available geophysical method for imaging the structure of the continental crust. The results from more than 25 active-sources seismic profiles carried out in the past twenty years were reviewed in this article. A preliminary cross crustal pattern of the Tibetan Plateau was presented and discussed. The Moho discontinuity buries at the range of 60–80 km on average and have steep ramps located roughly beneath the sutures that are compatible with the successive stacking/accretion of the former Cenozoic blocks northeastward. The deepest Moho (near 80 km) appears closely near IYS and the crustal scale thrust system beneath southern margin of Tibetan plateau suggests strong dependence on collision and non-distributed deformation there. However, the ∼20 km order of Moho offsets hardly reappears in the inline section across northern Tibetan plateau. Without a universally accepted, convincing dynamic explanation model accommodated the all of the facts seen in controlled seismic sections, but vertical thickening and northeastern shorten of the crust is quite evident and interpretable to a certain extent as the result of continent-continent collision. Simultaneously, weak geophysical signature of the BNS suggests that convergence has been accommodated perhaps partially through pure-shear thickening accompanied by removal of lower crustal material by lateral escape. Recent years the result of Moho with ∼7 km offset and long extend in south-dip angle beneath the east Kunlun orogen and a grand thrust fault at the northern margin of Qilian orogen has attract more attention to action from the northern blocks. The broad lower-velocity area in the upper-middle crust of the Lhasa block was once considered as resulted from partially melted rocks. However the low normal νP/νS ratio and the Moho stepwise rise fail to support significant partial melting in the middle-lower crust of the central-northern Tibetan plateau. Furthermore, the lower-velocity of crust occasionally disappears, and/or local thinned exhibits their non-stationary spatial distribution.

U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution

LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 +5.0/−6.0 Ma, and the other orthogneiss sample contains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma, 895 Ma and 657 Ma for the magmatic protolith, and metamorphic recrystallized rims with peak 206Pb/238U ages of 559 +12/−17 Ma and 516 ±13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 +10/−8.0 Ma, 516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Precambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen, the Altyn Tagh belt, north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.

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The East Kunlun Orogen in the Northern Qinghai–Tibet Plateau is an ideal region to investigate the geodynamic processes of magmatism related to the closure of the Paleo-Tethys Ocean. Here, we report petrology, zircon U–Pb geochronology, whole-rock geochemistry and multiple isotope data from granodiorites and the associated dioritic enclaves in a dominant Nan'getan granitoid in the East Kunlun Orogen. Zircon U–Pb ages indicate that the host granodiorites and dioritic enclaves were synchronously emplaced at ~ 243 Ma. The granodiorites are medium- to high-K calc-alkaline, metaluminous (A/CNK = 0.93–0.98), with high Al2O3 content (15.28%–16.10%), Mg# (47–49), very low Sr/Y ratios (127–217), high abundances of incompatible elements (Y = 3.87–8.36 ppm, Nb = 3.04–5.71 ppm, Th = 3.04–5.71 ppm), low (87Sr/86Sr)i (0.7050–0.7079), negative whole-rock εNd(t) (− 8.2 to − 5.8), (206Pb/204Pb) 243Ma of 18.520 to 18.772, (207Pb/204Pb) 243Ma of 15.611 to 15.650, (208Pb/204Pb) 243Ma of 38.227 to 38.528, δ18OSMOW = 6.8‰–9.1‰, εHf(t) of − 1.2 to + 2.4. The dioritic enclaves (SiO2 = 51.08%–56.29%) have Mg# values of 48–51, with negative Eu anomalies (δEu = 0.59–0.79), low (87Sr/86Sr)i (0.7058–0.7080), negative whole-rock εNd(t) (− 8.2 to − 5.8), (206Pb/204Pb) 243Ma of 18.376 to 18.809, (207Pb/204Pb)243Ma of 15.606 to 15.661, (208Pb/204Pb)243Ma of 38.244 to 38.540, δ18OSMOW = 5.6‰–10.0‰, εHf(t) of − 3.5 to + 1.7. These isotopic features of arc-type rocks from the East Kunlun suggest that the parental magmas of the Nan'getan granodiorites and the dioritic enclaves originated from an enriched lithospheric mantle. The Nan'getan granitoids might have recorded the northward subduction of the Paleo-Tethys ocean lithosphere following the initial collision of the Bayan Har–Songpan Ganzi–East Kunlun terrane and the closure of the Paleo-Tethys Ocean at ~ 243 Ma.

Precise timing of the Early Paleozoic metamorphism and thrust deformation in the Eastern Kunlun Orogen

In Dulan County, Qinghai Province NW China, the arc volcanic sequences in the northern side of the Central Fault of the East Kunlun were metamorphosed progressively from upper greenschist facies in the south to epidoteamphibolite facies in the north. High-angle thrust deformation was developed synchronously with the peak metamorphim and superimposed with later low-angle striking-slip deformation. Zircon U-Pb dating yields a concordant age of (448± 4) Ma for the metavolcanics. Syn-kinematic hornblende and muscovite separated from the high-angle thrusting belt give 40Ar–39Ar plateau age of (427± 4) Ma and 408 Ma, respectively. These results precisely constrain the timing of the closure of early Paleozoic volcanic basin (Proto-Tethys) over the eastern portion of the East Kunlun Orogen, and the thrust tectonic slice had a cool rate of ca. 9°C/Ma.

40Ar-39Ar and U-Pb ages of metadiorite from the East Kunlun Orogenic Belt: Evidence for Early-Paleozoic magmatic zone and excess argon in amphibole minerals

Single-grain zircon U-Pb and amphibole40Ar-39Ar dating have been conducted on a deformed and metamorphosed diorite in the East Kunlun Orogenic Belt, which intruded into the middle Proterozoic Kuhai Group exposed in the south of Xiangride region, Dulan County, NW Qinghai Province. The zircon gives a concordant U-Pb age of (446.5±9.1) Ma. The amphibole yields Ar plateau age of (488.0±1.2) Ma and an isochronal age of (488.9±5.6) Ma. Age results of both stepwise released Ar and conventional K-Ar analysis are remarkably higher than that of zircon U-Pb, suggesting that the amphibole contains excess argon and the amphibole plateau age cannot be taken as the timing of metamorphism or deformation. The zircon age is interpreted to be crystallization age of the diorite pluton, which suggests that an Early-Paleozoic magmatic zone indeed existed in the East Kunlun Orogenic Belt stretching along the region south to the Golmud, Normuhong and Xiangride.