Western Kunlun Orogenic Belt Research Articles

Petrogenesis of early Paleozoic syn-collisional granitoids and enclaves in Western Kunlun, Northwest China: Implications for the growth of continental crust

The Western Kunlun Orogenic Belt (WKOB), along the northwestern margin of the Qinghai–Xizang Plateau, was formed by the collision of Gondwana-derived terranes to the south and the Tarim Block to the north and was closely associated with closure of the Proto-Tethys Ocean during the late Neoproterozoic to early Paleozoic. We present a combined zircon UPb geochronology, whole-rock composition, and Sr–Nd–Hf isotopic study of syn-collisional granitoid plutons and mafic microgranular enclaves (MMEs) in the region. Zircon UPb dating yields ages of 443.8 ± 4.4, 451.9 ± 4.2, 462.9 ± 3.5, and 456 ± 4.2 Ma for the Tongayoupuagezi, Shanjie, and Pishigai plutons and MMEs from the Pishigai pluton, respectively. The granitoids are metaluminous to weakly peraluminous (A/CNK = 1.00–1.17) and belong to the high-K calc-alkaline series. They have (87Sr/86Sr)i ratios of 0.7058–0.7154, εNd(t) values of −8.78 to −0.93, and εHf(t) values of −19.72 to +6.87. The MMEs have variable SiO2 contents (45.7–60.2 wt%) and are more mafic than the host granitoids, but have similar Sr–Nd–Hf isotopic compositions to the host granitoids [(87Sr/86Sr)i = 0.7102–0.7110; εNd(t) = −6.57 to −3.56; εHf(t) = −7.17 to −1.81]. The MMEs are fragments of cumulates formed during the early stages of magma evolution. The granitoids were produced by the partial melting of a mélange source. The new data support the view that the Middle–Late Ordovician syn-collisional granitoids with MMEs distributed along the WKOB represent a magmatic response to terrane collision. This suggests that juvenile crustal growth in older orogenic systems, which occurs by arc addition, also involves some vertical addition during the final stage of orogenic collision. Our study suggests that mélange diaper melting is a key mechanism of crustal growth during the syn-collision stage in continental collision zones, associated with slab breakoff.

Provenance of Late Carboniferous–Triassic clastic rocks in the Western Kunlun orogenic belt, western China: Implications for the tectonic evolution of the Paleo-Tethys Ocean

The Western Kunlun orogenic belt in the northwest Tibetan Plateau is composed of the North Kunlun, South Kunlun, Tianshuihai, and Karakorum terranes, and has records about the tectonic evolution of the Paleo-Tethys Ocean. To provide new insights into the Paleo-Tethys orogeny in Western Kunlun, we present detrital zircon U-Pb and Lu-Hf isotopic data from the Late Carboniferous–Triassic sedimentary successions in the Tianshuihai terrane. Zircons with ages of 350–250 Ma are abundant in the Late Carboniferous–Late Permian samples and yielded εHf(t) values ranging from + 14.9 to −18.9. This indicates continuous arc-related magmatism in the Tianshuihai terrane during the Early Carboniferous–Late Permian. The Upper Permian Huangyangling Group primarily comprised Paleozoic detrital zircons predominantly sourced from the Tianshuihai terrane. Other strata contain higher proportion of Precambrian detrital zircons compared to the Huangyangling Group. This feature indicates that detritus from these strata were primarily originated from the North Kunlun, South Kunlun, and Tianshuihai terranes. We present a synthesis of provenance analysis, late Paleozoic–Triassic magmatic rocks, and Triassic metamorphic records associated with the Mazha–Kangxiwa suture zone. These data indicate that the Paleo-Tethys Ocean between the South Kunlun and Tianshuihai terranes opened at ca. 350 Ma and closed during the Middle Triassic, following continuous Carboniferous–Early Triassic subduction. Our study provides an important example of the provenance of clastic rocks in the orogen, which can be used to reconstruct information about eroded magmatic rocks and provide insights into the tectonic evolution of the orogen.

Geochemical Evidence of Plume Sources for High-MgO Lavas in the Western Kunlun Orogenic Belt

Abstract Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian–Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6–33.8 wt %) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2 ± 0.9 wt %. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite-dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg–Fe equilibria, Al-in-olivine and olivine–spinel thermometry and oxybarometer yield a mantle potential temperature of 1522–1556 °C and high oxygen fugacity of FMQ (fayalite-magnetite-quartz) + 0.93. The H2O contents in the picrite flows are estimated as 3.67 ± 1.0 wt %, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB (oceanic island basalt)-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd–O–Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.

Subduction polarity reversal facilitated by plate coupling during arc-continent collision: Evidence from the Western Kunlun orogenic belt, northwest Tibetan Plateau

Abstract Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (~2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530–487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485–430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ~10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.

The geochronology of pegmatites in the Jiajika lithium deposit, western Sichuan, China: Implications for multi-stage magmatic-hydrothermal events in the Songpan-Ganze rare metal metallogenic belt

The Songpan-Ganze orogenic belt is a large-scale rare metal metallogenic belt in western China. Several large-scale granitic pegmatite-type lithium deposits are located in the Songpan-Ganze orogenic belt, which include the Jiajika Li deposit, one of the largest pegmatite-type Li deposits in China. The geochronological constraints of pegmatites and granite in Jiajika are still inadequate for revealing the relationships between mineralization and tectonothermal events. In this study, we performed columbite-group minerals (CGM) and cassiterite U–Pb dating on seven different pegmatite veins with different occurrences and deformation styles in the Jiajika deposit. These pegmatites yield CGM U–Pb ages of 213 to 208 Ma and cassiterite U–Pb ages of 199 to 192 Ma, regardless of their occurrences and deformation. Columbite-group minerals exhibit low Ta# (=Ta/(Ta + Nb)) cores and high Ta# rims, which were formed during the magmatic stage and the hydrothermal stage, respectively. The appearance of cassitertite is closely related with greisenization. Therefore, the CGM U–Pb ages, which were obtained from the low Ta# cores of CGM, are interpreted as magmatic crystallization age, while the cassiterite ages reflect another younger stage of magmatic-hydrothermal event. According to field observation, we consider that the occurrences and deformation of pegmatite veins are controlled by the development of various fracture systems formed during the ascent of the granitic pluton. Our results suggest long-lasting multi-stage magmatic-hydrothermal events in the Jiajika region. Geochronological data of other rare metal deposits located in the Songpan-Ganze orogenic belt and the Western Kunlun orogenic belt are comparable with those of the Jiajika Li ore deposit, indicating that the entire Songpan-Ganze rare metal metallogenic belt experienced long-lasting and multiple magmatic and fluid activities. The overlaps of the geochronological results of granites and pegmatites in the Songpan-Ganze rare metal metallogenic belt suggest that regional magmatism is closely related to Li mineralization. The enrichment of Li in the Songpan-Ganze rare metal metallogenic belt is possibly related to its unique geodynamic setting and the long duration of magmatic-hydrothermal activities.

Geochronology and skarn mineralogy of the Akesayi Fe deposit in the Western Kunlun, Xinjiang: Implications for mineralization process

The Akesayi deposit, located in the eastern part of the Tianshuihai terrane of the Western Kunlun Orogenic Belt, Xinjiang, China, is a recently discovered medium-sized magnetite deposit containing approximately 51 million tons of ore with an average grade of 41 wt%. This deposit is hosted by skarns at the contact between monzonitic granite, diorite porphyry, and medium to low-grade metamorphic clastic rocks of the Permian Huangyangling Group. The ore types include banded, disseminated, and massive ores. The metallogenic process can be divided into prograde skarn, retrograde skarn, oxide, quartz-sulfide, and carbonate stages. The metallic minerals include magnetite and pyrite, and the gangue minerals include olivine, garnet, hedenbergite, hornblende, actinolite, biotite, cummingtonite, chlorite, quartz, and calcite. The zircon U–Pb dating of biotite adamellite intrusion and schist together constrains the formation age of the host rocks in the Akesayi Fe deposit to 217–272 Ma. Rb–Sr dating of eight garnet samples yields an isochron age of 12.41 ± 0.31 Ma, which is almost identical to the 12.56 ± 0.50 Ma age obtained via Sm–Nd dating of eight garnet samples. Furthermore, zircon U–Pb dating yields ages of 12.76 ± 0.12 Ma and 12.37 ± 0.18 Ma for diorite porphyry and monzonitic granite near the Fe orebodies, respectively. The initial Sr isotopes of garnet ((87Sr/87Sr)i = 0.710901) prove that the diorite porphyrite ((87Sr/87Sr)i = 0.709283) is related to skarn mineralization. Thus, the Akesayi deposit is a skarn-type iron ore related to the middle Miocene diorite porphyry. The mineral assemblage indicates the extensive development of manganese-skarn in the Akesayi mining area. The manganese content of host rocks of the deposit is low, and the source of manganese in the skarn may mainly come from the hydrothermal fluids associated with Miocene diorite porphyry emplacement. The compositional variations between the cores and rims of single garnet grains and different stages of tephroite indicate the concentration characteristics of the hydrothermal system, particularly manganese content, which evidently changed during hydrothermal activity. The endmembers of garnet and pyroxene suggest that the ore-forming environment transformed from a relatively reduced acidic environment to an alkali-rich, high-oxygen fugacity environment. The widely developed chlorine-rich amphiboles and biotite indicate that the hydrothermal fluids were rich in chlorine and may have been related to the diorite porphyrite intrusion near the ore bodies. The iron mineralization of the Akesayi deposit is analogous to the Gangdese Miocene porphyry–skarn polymetallic mineralization, which occurred during the Miocene in a post-collisional tectonic setting. Considering the series of Miocene intrusives discovered in West Kunlun, we propose that Miocene skarn Fe mineralization events occurred in West Kunlun, and that this area has great prospecting potential.

RECOGNITION OF A 600-KM-LONG LATE TRIASSIC RARE METAL (Li-Rb-Be-Nb-Ta) PEGMATITE BELT IN THE WESTERN KUNLUN OROGENIC BELT, WESTERN CHINA

AbstractThe rising demand of strategic metals, especially lithium, necessitates discovery of new resources to meet the global supply chain. Recently, several pegmatite-hosted rare metal (Li-Rb-Be-Nb-Ta) deposits have been discovered in the Western Kunlun orogenic belt, making it a new world-class rare metal resource (estimated ~7 Mt Li2O and 0.16 Mt BeO). Understanding the metallogenesis of this belt is critical to further evaluate the rare metal potential. In this study, columbite-tantalite (coltan) and monazite from rare metal pegmatites and zircon from potential parental granites were collected from five representative rare metal pegmatite deposits in the western, middle, and eastern parts of the Western Kunlun orogenic belt for U-Pb geochronology. The results indicate that despite the distances of the sampling localities in different parts of the Western Kunlun orogenic belt, the ages of pegmatite-hosted rare metal mineralization fall in a narrow range of ca. 208–204 Ma. These rare metal pegmatites are temporally and spatially related to adjacent postorogenic granites emplaced following the closure of the Paleo-Tethys Ocean. The compositional characteristics of K-feldspar, biotite, and muscovite of the granites and pegmatites, along with regional mineralogical and textural zonation of the pegmatites, suggest that the rare metal pegmatites were derived from the volumetrically much more important, highly fractionated granitic intrusions. We propose that, in combination with the data from previous studies, the 218–204 Ma interval represents a newly recognized rare metal metallogenic period linked with granitic intrusions in the Western Kunlun orogenic belt, revealing a 600-km-long late Triassic rare metal pegmatite belt composed of multiple ore fields formed in a similar metallogenic setting. These results emphasize the importance of identifying fertile, Late Triassic to Early Jurassic granitic intrusions for rare metal pegmatite exploration. Furthermore, combined with recent studies on the Songpan-Ganzi rare metal pegmatite belt along the eastern segment of the Paleo-Tethys, this study further highlights the great potential of rare metal resources in this global tectonic zone.

The geology, magnetite geochemistry, and oxygen isotopic composition of the Akesayi skarn iron deposit, Western Kunlun Orogenic Belt, Xinjiang, northwest China: Implications for ore genesis

The Akesayi skarn iron (Fe) deposit is hosted by the Huangyangling Group within the Tianshuihai Terrane of the Western Kunlun orogenic belt, Xinjiang, China, and consists of several orebodies that formed along contacts between Miocene syenogranite, diorite porphyry dykes and late Paleozoic carbonate rocks. Manganese-bearing skarn minerals are common within the deposit. The ore-forming process can be divided into five stages: Prograde skarn stage (I), retrograde skarn stage (II), oxide stage (III), quartz-sulfide stage (IV), and carbonate stage (V), with this evolution being typical of Fe skarns. Magnetite, which formed mainly during the retrograde skarn and oxide stages, includes banded (BO), disseminated (DO), and massive (MO) magnetite. Backscattered-electron images indicate that magnetite within the massive ores can be further classified as primary (MO-1) and secondary (MO-2) magnetite.Magnetite within all ore types exhibits a strongly negative correlation between Si and Fe3+ contents, indicating that Si was incorporated into intra-crystalline sites of magnetite by substitution of Si4+ for Fe3+. Positive correlation among Al, Mn, Mg, and Si contents indicate that these elements behaved similarly during magnetite formation. The δ18OV-SMOW values of MO-1 magnetite range from 4.3‰ to 4.7‰, higher than magmatic magnetite (1‰–4‰), while the BO and DO magnetite have lower δ18OV-SMOW values of 1.7‰ to 2.4‰ and 1.8‰ to 2.5‰ respectively. Its anomalous oxygen isotopic composition indicates that most magnetite in the Akesayi Fe deposit precipitated from evolved ore-forming fluids formed by reaction between country rocks and primary magmatic fluids during the early stages of mineralization, with input from meteoric water during the later stages of mineralization. Trace-element patterns indicate that the different types of magnetite share a common source, with variations in the magnetite geochemistry amongst the different ore types of the Akesayi Fe deposit reflecting variations in the extent of fluid-rock interaction, dissolution-reprecipitation, mineral precipitation, temperature, and oxygen fugacity (ƒO2). The different types of magnetite of the Akesayi skarn Fe deposit form distinguishable populations in (Ca + Al + Mn)-(Ti + V) and (Ni/(Cr + Mn))-(Ti + V) discrimination diagrams, with the range of compositions possibly indicating more variable magnetite compositions in skarn deposits than previously thought.

U-Pb zircon geochronology and geochemistry of metavolcanics and associated iron ores of the magnetite-rich BIF deposits in the Western Kunlun orogenic belt: Constraints on the depositional age, origin and tectonic setting

Large amounts of magnetite-rich banded iron formation (BIF) deposits, located in the Western Kunlun orogenic belt (WKOB), are hosted within the greenschist- to amphibolite-facies metamorphosed Bulunkuole complex. The iron (Fe) orebodies of these BIF deposits are interlayered with biotite-quartz schist and amphibolite, indicating that they are temporally closely associated. Analyses of the contents and ratios of major and trace elements indicate that the protoliths of the amphibolite should be basalt and andesite. LA-ICP-MS U-Pb dating of zircons from three representative amphibolite samples suggests that the studied BIF deposits were formed in the Early Cambrian (516–520 Ma). Based on regional geology and geochemistry of the amphibolite, we propose that these BIF deposits were formed in a back-arc basin setting. A wide range of Ce and Eu anomalies for Fe ores indicate that the studied BIF deposits were formed in both oxygenated surface seawater and deeper suboxic to anoxic seawater. Linking the tectonic setting with geochemical characteristics of bulk-rock Fe ores, we propose Fe and Si are predominantly seafloor-derived, with pulsed contribution of high- to low-T hydrothermal fluids that were closely associated with the extension of back-arc basin. In addition, we postulate that periodically upwelled metalliferous hydrothermal fluids were subsequently oxidized by surface seawater and mixed with detrital components, which ultimately led to the rhythmic precipitation of the Fe-rich and Si-rich layers.

Geochemistry and geochronology of early Palaeozoic seamount in Western Kunlun orogenic belt and the tectonic implications

ABSTRACT The Western Kunlun orogenic belt (WKOB) located south of the Tarim Basin and the north-western margin of the Tibetan Plateau, was previously considered a complex orogenic belt that closed along the Kudi-Qimanyute suture zone (KQSZ) and Mazar–Kangxiwar suture zone (MKSZ) from north to south during Proto- and Paleo-Tethys. The MKSZ between the South Kunlun and Tianshuihai terranes was interpreted as the southern boundary of the WKOB that formed during the subduction of the Paleo-Tethys oceanic crust. However, the evolution of the MKSZ in the Proto-Tethys Ocean remains controversial. We newly recognized seamount formation with pillow basalts and carbonate cap from Dongguashan group on Tianshuihai terrane. The pillow basalts had geochemical features of typical oceanic island basalts (OIBs). Zircon U–Pb dating revealed that this basalt had a crystallization age of 465 ± 6.6 Ma, with a gap of more than 10 Ma between the pillow basalts and fossils in the seamount. This implied that the basalt base reached the carbonate compensation depth. Accordingly, the seamount depositional age was restricted to the Late Ordovician. Detrital zircon showed that part of the clastic unit at the top of the Dongguashan group originated from the South Kunlun and Tianshuihai terranes, suggesting that the analysed sediments probably formed in the remnant of the Proto-Tethys Ocean and were deposited on the top of or accreted into the seamount during oceanic crust subduction. This discovery provides robust evidence of the MKSZ undergoing an evolution with Proto-Tethys. Moreover, our results supported the approach that an accretionary wedge, including the Late Ordovician seamount in the southern MKSZ, should be considered part the WKOB.

Petrogenesis of Early Cambrian granitoids in the western Kunlun orogenic belt, Northwest Tibet: Insight into early stage subduction of the Proto-Tethys Ocean

Abstract The west Kunlun orogenic belt, located on the northwest margin of the Tibetan Plateau, represents a crucial tectonic junction between the central Asia and Tethys domains. Its evolution was closely related to the Paleozoic subduction and closure of the Proto-Tethys Ocean, which was formed by the breakup of the Rodinia supercontinent following the Neoproterozoic. However, the early evolution of Proto-Tethys oceanic subduction (e.g., subduction initiation timing, polarity, and process) remains controversial. The source of the Early Cambrian granitoids is also unclear. To explore these questions, four Cambrian plutons (i.e., two Tianshuihai monzogranites and south Kunlun diorite and monzogranite) were chosen for geochronological and geochemical studies. Zircon U-Pb dating reveals that these plutons formed at ca. 533–513 Ma and thus represent the oldest arc-related magmatism in the west Kunlun orogenic belt. The Tianshuihai monzogranites have positive εNd(t) values (+0.76 to +1.34) and zircon εHf(t) values of +0.25 to +6.42, with low δ18Ozrn values of +5.11‰ to +7.38‰, suggesting that their source includes juvenile material. These rocks are weakly peraluminous and have relatively old Hf model ages of 1.09–1.48 Ga. Mass balance calculations show that the Tianshuihai monzogranites were derived from partial melting of Mesoproterozoic meta-igneous rocks with the addition of 22% of juvenile material. The south Kunlun monzogranites in this study are weakly peraluminous, and their lowest εNd(t) values are –9.24 to –9.27 and zircon εHf(t) values are –7.80 to –11.2. The oldest Hf model ages are 1.97–2.18 Ga, and the highest zircon δ18Ozrn values are +8.11 to +9.73‰. Their isotopic compositions are different from those of the magmas derived from partial melting of just Paleoproterozoic and Mesoproterozoic basement rocks but can be produced by a mixing source of 32% meta-igneous rock and 68% meta-sedimentary rock. The south Kunlun diorites are characterized by high Sr contents and relatively high Sr/Y (52–63) ratios but low Y, Yb, Cr, and Ni contents, like those of the thickened continental crust-derived adakites. Their Sr-Nd–Hf-O isotopic compositions indicate that their parental magma was derived from a Mesoproterozoic metaigneous basement in the garnet stability field. Based on the newly identified, oldest island arc magmatic records in the west Kunlun orogenic belt, the subduction initiation of the Proto-Tethys oceanic slab must have occurred prior to the Early Cambrian (>533 Ma). Our results, with previously published data, show that the west Kunlun orogenic belt was in an extensional setting during the Early Cambrian and that the magmatism migrated northeastward along the axis of the south Kunlun terrane between 533 Ma and 513 Ma. Therefore, considering the spatial and temporal distribution and petrogenesis of the Early–Middle Cambrian plutons in the west Kunlun orogenic belt, we propose that the Early Cambrian magmatism was most plausibly triggered by asthenospheric upwelling in response to the rollback of southward-subducted Proto-Tethys oceanic slab.

A rare earth element and Nd isotopic investigation into the provenance and deposition of the Dahongliutan banded iron formation and associated carbonates, NW China: Implications on Neoproterozoic seawater compositions

Rare earth element and Nd isotopic data for the recently discovered Neoproterozoic Dahongliutan banded iron formation (BIF) and associated carbonates are employed to infer aspects of the provenance and depositional environment, including ambient seawater compositions. The geochemical evidence is suggestive of deposition of the Dahongliutan marble in an epicontinental shallow–marine environment with riverine influx and low–T hydrothermal input, with restricted exchange to the open ocean. In contrast, the Dahongliutan BIF formed in a more open setting, with high–T hydrothermal input and old continental materials addition. The clearly distinct but negative εNd (0.593 Ga) values indicate that Neoproterozoic seawater at Dahongliutan could be spatially inhomogeneous with respect to 143Nd/144Nd ratios, owing to inefficient vertical and horizontal mixing of seawater closely related to the riverine Nd input derived from the weathering of different types and ages of continental crusts of the Western Kunlun orogenic belt and the Tarim Block. The Nd isotopic mass balance calculations of the Dahongliutan BIF indicate that most (>68%) of the Nd were derived from continental source materials transported by river water. In conjunction with previously published geochemical data of Neoproterozoic BIFs worldwide, we evaluate the chemical compositions of seawater where they formed, represented by a seawater reservoir strongly influenced by continentally derived freshwaters (>10%) mixed with open ocean and minor (<5%) high–T hydrothermal fluids as indicated by Sm/Yb and Eu/Sm ratios. By comparing the Nd isotopic features of Neoproterozoic BIFs, it is concluded that similar to modern oceans, the Neoproterozoic ocean was not well–mixed with respect to its Nd isotopic composition and continental source materials transported by river water dominated the great majority of Nd in Neoproterozoic seawater.

Geochronology, petrogenesis and tectonic significance of Dahongliutan pluton in Western Kunlun orogenic belt, NW China

The Dahongliutan granitic pluton, in the eastern part of the West Kunlun orogenic belt, provides significant insights for studying the tectonic evolution of West Kunlun. This paper presents a systematic study of LA-ICP-MS zircon U-Pb age, major and trace elements, Sr-Nd-Hf isotopes, and the first detailed Li isotope analysis of the Dahongliutan pluton. LA-ICP-MS zircon U-Pb dating shows that the Dahongliutan granites were emplaced in the Late Triassic ((213±2.1) Ma). Geochemical data show relatively high SiO2 contents (68.45 wt%–73.62 wt%) and aluminum saturation index (A/CNK=1.11–1.21) indicates peraluminous high-K calc-alkaline granite. The Dahongliutan granites are relatively high in light rare earth elements (LREE) and large ion lithophile elements (LILEs) (e.g., Rb, K, Th), and relatively depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P, Ti). The eNd(t) values range from −8.71 to −4.73, and (87Sr/86Sr)i=0.7087–0.71574. Zircons from the pluton yield 176Hf/177Hf values of 0.2826181 to 0.2827683, and eHf(t) values are around 0; the two-stage Hf model ages range from 0.974 to 1.307 Ga. The δ7Li values are 0.76‰–3.25‰, with an average of 2.53‰. Isotopic compositions of the pluton suggest a mixed trend between the partial melting of the Middle Proterozoic ancient crustal material and a juvenile mantle-derived material. This study infers that the Dahongliutan rock mass is formed in the post-collisional extension environment, when the collision between South Kunlun and the Tianshuihai terranes results in the closure of the Palaeo-Tethys. The mantle-derived magma results in partial melting of the lower crust.

Geochemical attributes of magmatic apatite in the Kukaazi granite from western Kunlun orogenic belt, NW China: Implications for granite petrogenesis and Pb-Zn (-Cu-W) mineralization

Apatite is a significant accessory mineral which is ubiquitous in Kukaazi granitic bodies and provides vital geological information concerning petrogenetic as well as metallogenic attributes. Particularly, igneous apatite is important to delineate nature of parental magma and its characteristics. Here we present new EPMA and LA-ICP-MS data on magmatic apatite of Kukaazi pluton to explore its potential in petrogenesis as well as mineralization and this deposit is widely exposed in western Kunlun, NW China. Our results reveal that the apatite can be characterized as fluorapatite and it reflects the parental magma characteristics of Kukaazi granite. The apatite exhibits high fluorine content, right inclined distribution pattern with enrichment of light rare earth elements (LREEs), a prominent moderate negative Eu anomaly and this anomaly is possibly resulting from plagioclase crystallization prior to apatite. The negative correlation and variation of δCe and δEu together with the moderate negative Eu anomaly suggests that the parental magma of Kukaazi granite have moderate oxidizing nature. The apatite also reveals Kukaazi pluton with adakitic affinity containing higher Sr/Y and Eu/Eu* ratios. This implies that the fluorapatite can be robust tool for distinguishing the adakite plutons and may preserve original Sr/Y and Eu/Eu* ratios as a result of no influence of hydrothermal alteration. Our results also indicate that apatite Cl/F ratio is a potential indicator to track the content of magmatic volatiles and pointer for mineralization. It is inferred that Cl/F ratio in the parental magma of Kukaazi pluton is low, attributable to source control and discrepancy in the Cl/F values can be ascribed to variability of degassing. Halogen data indicates low Cl, fairly high F and low Cl/F ratios which implies that the parental magma of Kukaazi granite is produced in consequence of partial melting of lower crust material. The negative correlation of F and Cl suggests crystallization of Kukaazi apatite from hydrous parental magma. Besides, apatite Eu/Eu* vs. Cl content is significant as proxy record to discriminate mineralized rock from unmineralized. Based on Eu/Eu* vs. Cl plot, we propose that the Kukaazi granite is mineralized. Furthermore, variation and positive correlation between F/Cl ratios and F contents, increase in fluorine concentration, (La/Yb) N and δEu possibly support process of magmatic differentiation. Based on apatite geochemistry, we identified two types of ore deposits specifically Mo-W and Cu deposit in Kukaazi area. In comparison, the apatite of Mo-W deposit is distinguished by slightly higher fluorine and Y, lesser Sr and δEu values however, copper deposits are characterized by higher Sr, δEu values and lesser F, Y. These elements of apatite are prime prolific indicators and proxies for distinguishing fertile granite from barren one. Thus, in view of these findings, we confirm that apatite is a potential pointer of petrogenesis, metallogenesis, mineralization and ore varieties.

Early Palaeozoic sub-arc chromitite-bearing peridotite in the Kudi ophiolite on the westernmost Tibetan Plateau

ABSTRACTA chromite deposit was discovered in the Kudi ophiolite in the Palaeozoic western Kunlun orogenic belt. Chromite forms elongated (<2 m in width) and banded chromitite bodies (<0.1 m in width for each band) in dunite and podiform chromitite bodies (<1.5 m in width) in harzburgite. Dunite is classified into two types. Type I dunite hosting massive and banded chromitites shows low Fo in olivine (88.1–90.9), moderate Cr# [=Cr/(Cr + Al), 0.47–0.56] in chromite, and a positively sloped primitive mantle-normalized platinum group elements (PGE) pattern, suggesting that it is a cumulate of a mafic melt. Harzburgite and type II dunite show olivine with high Fo (>91.1) and chromite with moderate to high Cr# (0.44–0.61), and flat to negatively sloped primitive mantle-normalized PGE patterns, indicating that they are residual mantle peridotite after partial melting. Chromite in all three types of chromitites has relatively uniform moderate values Cr# ranging from 0.43 to 0.56. Massive chromitite contains euhedral chromite with high TiO2 (0.40–0.43 wt.%) and has a positively sloped primitive mantle-normalized PGE pattern, suggesting that it represents a cumulate of a melt. Rocks containing disseminated and banded chromite show overall low total PGE, < 117 ppb, and a negatively sloped primitive mantle-normalized PGE pattern. Chromite grains in these two types of occurrences are irregular in shape and enclose olivine grains, suggesting that chromite formed later than olivine. We suggest that chromite-oversaturated melt penetrated into the pre-existing dunite and crystallized chromite. The oxygen fugacity (fO2 values of chromitites and peridotites are high, ranging from FMQ+0.8 (0.8 logarithmic unit above the fayalite-magnetite-quartz buffer) to FMQ+2.3 for chromitites and from FMQ+0.9 to FMQ+2.8 for peridotites (dunite and harzburgite). The mineral compositions and high fO2 values as well as estimated parental magma compositions of the chromitites suggest that the Kudi ophiolite formed in a sub-arc setting.

Geochemistry and origin of the Neoproterozoic Dahongliutan banded iron formation (BIF) in the Western Kunlun orogenic belt, Xinjiang (NW China)

The Neoproterozoic (593–532Ma) Dahongliutan banded iron formation (BIF), located in the Tianshuihai terrane (Western Kunlun orogenic belt), is hosted in the Tianshuihai Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Iron oxide (hematite), carbonate (siderite, ankerite, dolomite and calcite) and silicate (muscovite) facies are all present within the iron-rich layers. There are three distinctive sedimentary facies BIFs, the oxide, silicate–carbonate–oxide and carbonate (being subdivided into ankerite and siderite facies BIFs) in the Dahongliutan BIF. They demonstrate lateral and vertical zonation from south to north and from bottom to top: the carbonate facies BIF through a majority of the oxide facies BIF into the silicate–carbonate–oxide facies BIF and a small proportion of the oxide facies BIF.The positive correlations between Al2O3 and TiO2, Sc, V, Cr, Rb, Cs, Th and ∑REE (total rare earth element) for various facies of BIFs indicate these chemical sediments incorporate terrigenous detrital components. Low contents of Al2O3 (<3wt%), TiO2 (<0.15wt%), ∑REE (5.06–39.6ppm) and incompatible HFSEs (high field strength elements, e.g., Zr, Hf, Th and Sc) (<10ppm), and high Fe/Ti ratios (254–4115) for a majority of the oxide and carbonate facies BIFs suggest a small clastic input (<20% clastic materials) admixtured with their original chemical precipitates. The higher abundances of Al2O3 (>3wt%), TiO2, Zr, Th, Cs, Sc, Cr and ∑REE (31.2–62.9ppm), and low Fe/Ti ratios (95.2–236) of the silicate–carbonate–oxide facies BIF are consistent with incorporation of higher amounts of clastic components (20%–40% clastic materials). The HREE (heavy rare earth element) enrichment pattern in PAAS-normalized REE diagrams exhibited by a majority of the oxide and carbonate facies BIFs shows a modern seawater REE signature overprinted by high-T (temperature) hydrothermal fluids marked by strong positive Eu anomalies (Eu/Eu∗PAAS=2.37–5.23). The low Eu/Sm ratios, small positive Eu anomaly (Eu/Eu∗PAAS=1.10–1.58) and slightly MREE (middle rare earth element) enrichment relative to HREE in the silicate–carbonate–oxide facies BIF and some oxide and carbonate facies BIFs indicate higher contributions from low-T hydrothermal sources. The absence of negative Ce anomalies and the high Fe3+/(Fe3+/Fe2+) ratios (0.98–1.00) for the oxide and silicate–carbonate–oxide BIFs do not support ocean anoxia. The δ13CV-PDB (−4.0‰ to −6.6‰) and δ18OV-PDB (−14.0‰ to −11.5‰) values for siderite and ankerite in the carbonate facies BIF are, on average, ∼6‰ and ∼5‰ lower than those (δ13CV-PDB=−0.8‰ to +3.1‰ and δ18OV-PDB=−8.2‰ to −6.3‰) of Ca–Mg carbonates from the silicate–carbonate–oxide facies BIF. This feature, coupled with the negative correlations between FeO, Eu/Eu∗PAAS and δ13CV-PDB, imply that a water column stratified with regard to the isotopic omposition of total dissolved CO2, with the deeper water, from which the carbonate facies BIF formed, depleted in δ13C that may have been derive from hydrothermal activity.Integration of petrographic, geochemical, and isotopic data indicates that the silicate–carbonate–oxide facies BIF and part of the oxide facies BIF precipitated in a near-shore, oxic and shallow water environment, whereas a majority of the oxide and carbonate facies BIFs deposited in anoxic but Fe2+-rich deeper waters, closer to submarine hydrothermal vents. High-T hydrothermal solutions, with infusions of some low-T hydrothermal fluids, brought Fe and Si onto a shallow marine, variably mixed with detrital components from seawaters and fresh waters carrying continental landmass and finally led to the alternating deposition of the Dahongliutan BIF during regression–transgression cycles.The Dahongliutan BIF is more akin to Superior-type rather than Algoma-type and Rapitan-type BIF, and constitutes an additional line of evidence for the widespread return of BIFs in the Cryogenian and Ediacaran reflecting the recurrence of anoxic ferruginous deep sea and anoxia/reoxygenation cycles in the Neoproterozoic. In combination with previous studies on other Fe deposits in the Tianshuihai terrane, we propose that a Fe2+-rich anoxic basin or deep sea probably existed from the Neoproterozoic to the Early Cambrian in this area.

Constraints of molybdenite Re–Os and scheelite Sm–Nd ages on mineralization time of the Kukaazi Pb–Zn–Cu–W deposit, Western Kunlun, NW China

The Kukaazi Pb–Zn–Cu–W polymetallic deposit, located in the Western Kunlun orogenic belt, is a newly discovered skarn-type deposit. Ore bodies mainly occur in the forms of lenses and veins along beddings of the Mesoproterozoic metamorphic rocks. Three ore blocks, KI, KII, and KIII, have been outlined in different parts of the Kukaazi deposit in terms of mineral assemblages. The KI ore block is mainly composed of chalcopyrite, scheelite, pyrrhotite, sphalerite, galena and minor pyrite, arsenopyrite, and molybdenite, whereas the other two ore blocks are made up of galena, sphalerite, magnetite and minor arsenopyrite and pyrite. In this study, we obtained a molybdenite isochron Re–Os age of 450.5 ± 6.4 Ma (2σ, MSWD = 0.057) and a scheelite Sm–Nd isochron age of 426 ± 59 Ma (2σ, MSWD = 0.49) for the KI ore block. They are broadly comparable to the ages of granitoid in the region. Scheelite grains from the KI ore block contain high abundances of rare earth elements (REE, 42.0–95.7 ppm) and are enriched in light REE compared to heavy REE, with negative Eu anomalies (δEu = 0.13–0.55). They display similar REE patterns and Sm/Nd ratios to those of the coeval granitoids in the region. Moreover, they also have similar Sr and Nd isotopes [87Sr/86Sr = 0.7107–0.7118; eNd(t) = −4.1 to −4.0] to those of such granitoids, implying that the tungsten-bearing fluids in the Kukaazi deposit probably originate from the granitic magmas. Our results first defined that the Early Paleozoic granitoids could lead to economic Mo–W–(Cu) mineralization at some favorable districts in the Western Kunlun orogenic belt and could be prospecting exploration targets.

Provenance and tectonic setting of siliciclastic rocks associated with the Neoproterozoic Dahongliutan BIF: Implications for the Precambrian crustal evolution of the Western Kunlun orogenic belt, NW China

The Late Neoproterozoic Dahongliutan BIF is associated with siliciclastic rocks in the Tianshuihai terrane of the Western Kunlun orogenic belt (WKO), NW China. The sedimentary rocks have various weathering indices (e.g., CIA=57–87, PIA=61–96 and Th/U=4.85–12.45), indicative of varying degrees of weathering in the source area. The rocks have trace element ratios, such as Th/Sc=0.60–1.21 and Co/Th=0.29–1.67, and light rare earth element (LREE) enriched chondrite–normalized REE patterns, suggesting that they were mainly sourced from intermediate and felsic rocks. Available U–Pb ages of detrital zircon from these rocks reveal that the detrital sources may have been igneous and metamorphic rocks from the WKO and the Tarim Block. Our study suggests that the Dahongliutan BIF and hosting siliciclastic rocks may have deposited in a setting transitional from a passive to active continental margin, probably related to the Late Neoproterozoic–Early Cambrian seafloor spreading and subduction of the Proto–Tethys Ocean. U–Pb dating of 163 detrital zircons defines five major age populations at 2561–2329Ma, 2076–1644Ma, 1164–899Ma, 869–722Ma and 696–593Ma. These age groups broadly correspond to the major stages of supercontinent assembly and breakup events widely accepted for Columbia, Rodinia and Gondwana. Some zircons have TDM2 model ages of 3.9–1.8Ga and negative εHf(t) values, suggesting that the Archean to Paleoproterozoic (as old as Eoarchean) crustal materials were episodically reworked and incorporated into the late magmatic process in the WKO. Some Neoproterozoic zircons have TDM2 model ages of 1.47–1.07Ga and 1.81–1.53Ga and positive εHf(t) values, indicating juvenile crustal growth during the Mesoproterozoic. Our new results, combined with published data, imply that both the Tianshuihai terrane in the WKO and the Tarim Block share the same Precambrian tectonic evolution history.

Geochemistry and petrogenesis of the early Palaeozoic appinite-granite complex in the Western Kunlun Orogenic Belt, NW China: implications for Palaeozoic tectonic evolution

AbstractThe Datong pluton, the largest early Palaeozoic granitoid in the Western Kunlun Orogenic Belt (WKOB) in NW China, is a typical appinite-granite complex. It consists of diorites, quartz diorites, monzodiorites, quartz monzodiorites, monzonites, quartz monzonites, syenites, granodiorites and monzogranites. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded crystallization ages of 459 ± 3 Ma for the quartz monzonites and 452 ± 5 Ma for the monzogranites (Late Ordovician). The rocks possess a wide range of SiO2 (56.0–73.4 wt %), MgO (0.17–4.55 wt %) and Mg no. (25–60), with high K2O (2.83–5.29 wt %) contents, exhibiting high-K calc-alkaline to shoshonitic traits. They are characterized by enrichments in large-ion lithophile elements (LILEs) and light rare Earth elements (LREEs), as well as depletions in high-field-strength elements (HFSEs). The rocks have initial 87Sr/86Sr ratios of 0.7086–0.7185, negative εNd(t) values of –3.72 to –1.79 and εHf(t) values vary from –1.6 to +4.7. These features are modelled to show that they were most likely derived from an enriched lithospheric mantle source and that fractional crystallization with minor crustal contamination was involved in their petrogenetic process. Considering the distribution and chronology of the Palaeozoic intrusions – such as Kegang, Bulong, Qiukesu, Yierba, North Kudi, Dongbake, Buya, Ayilixi and Warengzilafu granitoid plutons with ages of c. 420–530 Ma – in conjunction with the Palaeozoic metamorphic overprinting in the WKOB, we propose a divergent double-subduction model to explain the destruction of the Proto-Tethys Ocean and suggest that the Datong pluton was likely emplaced in a post-collisional setting following the termination of subduction in response to slab break-off.

Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2–69.3wt.%), and high Al2O3 (15.3–17.1wt.%), total alkali (Na2O+K2O=8.07–10.2wt.%) and ratios of K2O/Na2O (0.77–1.83). They plot in “shoshonite” field, and show high abundances of LILE including LREE ((La/Yb)n=15.4–26.2; mean 20.2) with pronounced negative anomalies of Nb–Ta–P–Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu=2Eun/(Smn+Gdn)=0.68–0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3–72.0wt.%), similar Al2O3 (14.7–17.6wt.%), and slightly lower total alkalis (6.57–9.14wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80–17.2ppm) and Yb (0.63–1.59ppm), and high Sr/Y (>40). U–Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at ~444Ma to ~443Ma, and a separate small adakitic plug at ~462Ma. The mean εHf(t) values of zircon range from −1.6 to −0.94 (n=14) with TDM2 of ~1.5Ga for shoshonitic rocks and εHf(t) values from −1.8 to +0.72 (n=12) with TDM2 of 1.4 to 1.5Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092–0.7100 and −3.9 to −3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099–0.7134 and −3.6 to −3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources.Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende.