Geochronology,geochemistry and tectonic implications of a new ophiolitic mélange in the northern West Junggar,NW China

The West Junggar, located in the southernmost part of the Central Asian Orogenic Belt (CAOB), is a key region for understanding the Paleozoic evolution of the CAOB. Issues of the timing of initial subduction and tectonic unit connections in northern West Junggar still remain controversial. In this study, we report a new ophiolitic mélange named the E'min ophiolitic mélange in northern West Junggar. The tectonic blocks in the E'min ophiolitic mélange are mainly composed of serpentinized peridotite, serpentinite, gabbros, pillow basalts, and cherts, with a matrix consisting of highly deformed serpentinites. A gabbro exhibits a zircon SHRIMP U-Pb age of 476 ± 2 Ma, and the zircon grains have δ¹⁸O values similar to those of mantle zircons. Those basalt samples display depletions of light rare earth element (REE) relative to heavy REEs. They exhibit weak enrichment of Ba and Th, and moderate depletion of Nb and Ta. The basalts display similar geochemical characteristics to that of fore–arc basalts in the present-day fore–arc setting. The gabbros exhibit high MgO and compatible element contents, but low TiO₂, total REE and high field strength element (HFSE) contents. They exhibit light REE depletion, enrichment in large-ion lithophile elements, and depletion of HFSEs. The boninite-like geochemical patterns of the gabbros indicate that they were formed in a subduction-related environment, and were derived from an extremely depleted mantle source infiltrated by subduction-derived fluids and/or melts. The E'min ophiolitic mélange has a geochemical make-up similar to those of suprasubduction-zone (SSZ)-type ophiolites formed in a forearc setting. Hence, we propose that the E'min ophiolitic mélange formed in a forearc setting and may represent the initial subduction in northern West Junggar. Based on geochronological data, we propose that the E'min ophiolite, together with the Kujibai, Hoboksar and Hongguleleng ophiolites, formed during a similar period and comprise a huge E–W trending ophiolitic belt.     

Zircon U–Pb ages and tectonic implications of Paleozoic plutons in northern West Junggar,North Xinjiang,China

North Xinjiang, Northwest China, is made up of several Paleozoic orogens. From north to south these are the Chinese Altai, Junggar, and Tian Shan. It is characterized by widespread development of Late Carboniferous–Permian granitoids, which are commonly accepted as the products of post-collisional magmatism. Except for the Chinese Altai, East Junggar, and Tian Shan, little is known about the Devonian and older granitoids in the West Junggar, leading to an incomplete understanding of its Paleozoic tectonic history. New SHRIMP and LA-ICP-MS zircon U–Pb ages were determined for seventeen plutons in northern West Junggar and these ages confirm the presence of Late Silurian–Early Devonian plutons in the West Junggar. New age data, combined with those available from the literature, help us distinguish three groups of plutons in northern West Junggar. The first is represented by Late Silurian–Early Devonian (ca. 422 to 405 Ma) plutons in the EW-striking Xiemisitai and Saier Mountains, including A-type granite with aegirine–augite and arfvedsonite, and associated diorite, K-feldspar granite, and subvolcanic rocks. The second is composed of the Early Carboniferous (ca. 346 to 321 Ma) granodiorite, diorite, and monzonitic and K-feldspar granites, which mainly occur in the EW-extending Tarbgatay and Saur (also spelled as Sawuer in Chinese) Mountains. The third is mainly characterized by the latest Late Carboniferous–Middle Permian (ca. 304 to 263 Ma) granitoids in the Wuerkashier, Tarbgatay, and Saur Mountains.As a whole, the three epochs of plutons in northern West Junggar have different implications for tectonic evolution. The volcano-sedimentary strata in the Xiemisitai and Saier Mountains may not be Middle and Late Devonian as suggested previously because they are crosscut by the Late Silurian–Early Devonian plutons. Therefore, they are probably the eastern extension of the Early Paleozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China. It is uncertain at present if these plutons might have been generated in either a subduction or post-collisional setting. The early Carboniferous plutons in the Tarbgatay and Saur Mountains may be part of the Late Paleozoic Zharma–Saur volcanic arc of the Kazakhstan block. They occur along the active margin of the Kazakhstan block, and their generation may be related to southward subduction of the Irtysh–Zaysan Ocean between Kazakhstan in the south and Altai in the north. The latest Late Carboniferous–Middle Permian plutons occur in the Zharma–Saur volcanic arc, Hebukesaier Depression, and the West Junggar accretionary complexes and significantly postdate the closure of the Irtysh–Zaysan Ocean in the Late Carboniferous because they are concurrent with the stitching plutons crosscutting the Irtysh–Zaysan suture zone. Hence the latest Late Carboniferous–Middle Permian plutons were generated in a post-collisional setting. The oldest stitching plutons in the Irtysh–Zaysan suture zone are coeval with those in northern West Junggar, together they place an upper age bound for the final amalgamation of the Altai and Kazakhstan blocks to be earlier than 307 Ma (before the Kaslmovian stage, Late Carboniferous). This is nearly coincident with widespread post-collisional granitoid plutons in North Xinjiang.     

Late Silurian–early Devonian adakitic granodiorite,A-type and I-type granites in NW Junggar,NW China: Partial melting of mafic lower crust and implications for slab roll-back

Late Silurian–early Devonian magmatism of the NW Junggar region in the Central Asian Orogenic Belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes. In this study, we report results of Zircon U–Pb ages and systematic geochemical data for late Silurian–early Devonian largely granitic rocks in NW Junggar, aiming to constrain their emplacement ages, origin and geodynamic significance. The magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks, e.g. adakitic granodiorite, K-feldspar granite, syenitic granite, gabbro and rhyrolite. U–Pb zircon ages suggest that the granitoids and gabbros were emplaced in the late Silurian–early Devonian (420–405 Ma). Adakitic granodiorites are calc-alkaline, characterized by high Sr (407–532 ppm), low Y (12.2–14.7 ppm), Yb (1.53–1.77 ppm), Cr (mostly < 8.00 ppm), Co (mostly < 11.0 ppm) and Ni (mostly < 4.10 ppm) and relatively high Sr/Y (31–42) ratios, analogous to those of modern adakites. K-feldspar granites and rhyolites are characterized by alkali- and Fe-enriched, with high Zr, Nb and Ga/Al ratios, geochemically similar to those of A-type granites. Syenitic granites show high alkaline (Na₂O + K₂O = 8.39–9.34 wt.%) contents, low Fe^# values (0.73–0.80) and are weakly peraluminous (A/CNK = 1.00–1.07). Gabbros are characterized by low MgO (6.86–7.15 wt.%), Mg^# (52–53), Cr (124–133 ppm) and Ni (84.7–86.6 ppm) contents. The geochemical characteristics of the gabbroic samples show affinity to both MORB- and arc-like sources. All granitoids have positive εNd(t) (+ 3.9 to + 6.9) and zircon εHf(t) (+ 9.8 to + 15.2) values and low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7035–0.7043), with young T_DM(Nd) (605–791 Ma) and T_DM(Hf) (425–773 Ma) ages, suggesting significant addition of juvenile material. The adakitic granodiorites probably resulted from partial melting of mafic lower crust, leaving an amphibolite and garnet residue. The K-feldspar granites, rhyolites and syenitic granites probably formed from partial melting of the Xiemisitai mid-lower crust, while the gabbroic intrusion was probably generated by interactions between asthenospheric and metasomatized lithospheric mantle. Voluminous plutons of various types (adakites, A-type granites, I-type granites, and gabbros) formed during 420–405 Ma, and their isotopic data suggest significant additions of juvenile material. We propose that a slab roll-back model can account for the 420–405 Ma magmatic “flare up” in NW Junggar as well as an extensional setting.     

Keketuohai mafic–ultramafic complex in the Chinese Altai,NW China: Petrogenesis and geodynamic significance

Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409 ± 5 Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO₂, K₂O and Na₂O contents, negative covariations between P₂O₅, TiO₂ and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive ε_Nd(t) values (0 to + 2.7) and slight enrichments in light rare earth elements (e.g., La/Yb_N = 0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg# = 66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.     

Petrogenesis of the Wudaogou intermediate–mafic complex,NW China: zircon U–Pb dating,whole-rock and isotope geochemistry,and geological implications

The Wudaogou plutonic complex is located in the eastern Yanbian area of Jilin Province and consists of hornblende gabbros, gabbroic diorites, and quartz diorites that contain abundant dioritic microgranular xenoliths. Zircon U–Pb dating of gabbroic and quartz diorites yielded weighted mean ²⁰⁶Pb/²³⁸Pb ages of 263.5 ± 5.1 Ma (N = 12, mean squared weighted deviation (MSWD) = 0.78, probability = 0.66) and 262.0 ± 5.6 Ma (N = 10, MSWD = 0.50, probability = 0.87), respectively. These units are characterized by high Na₂O/K₂O (0.33–0.77) ratios and Al₂O₃ (15.05–18.91 wt%) concentrations and are large ion lithophile element (LILE) (light rare earth element (LREE), Rb, Ba, K, etc.) enriched and high field strength element (HFSE) (Nb, Ta, P, Ti) depleted. They also have initial ⁸⁷Sr/⁸⁶Sr values of 0.70192–0.70420 and ?_Nd(t) values of +1.9 to +4.7 with two-stage model ages (T_DM2) of 653–878 Ma. These characteristics indicate that these rocks formed from calc–alkaline magmas derived from partial melting of a mixture of juvenile crust formed attending the Neoproterozoic subduction of the Palaeo-Asian oceanic crust and lower crustal material. The dioritic xenoliths have whole-rock compositions that are similar to their host rocks, but with negative ?_Nd(t) values (?1.6 to ?4.3) and older T_DM2 ages (1166–1382 Ma), further indicating that this magmatic event involved older crustal material. Combining these data with existing knowledge of the crustal evolution of this area, we conclude that this complex formed in a post-collisional extensional setting during closure of the Palaeo-Asian Ocean.     

40Ar–39Ar age and geochemistry of subduction-related mafic dikes in northern Tibet,China: petrogenesis and tectonic implications

The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende ⁴⁰Ar–³⁹Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO₂ = 46.55–55.75%, MgO = 2.80–7.38%, Mg^# = 36–61, and (Na₂O + K₂O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their I_Sr and ?_Nd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.     

Petrogenesis and timing of emplacement of porphyritic monzonite,dolerite, and basalt associated with the Kuoerzhenkuola Au deposit,Western Junggar,NW China: implications for early Carboniferous tectonic setting and Cu–Au mineralization prospectivity

ABSTRACT

The Kuoerzhenkuola epithermal Au deposit is located in the northern part of the West Junggar region of NW China and is underlain by a recently discovered porphyritic monzonite intrusion that contains Cu–Au mineralization. Zircon LA-ICP-MS U–Pb dating of this intrusion yielded an age of 350 ± 4.7 Ma. The porphyritic monzonite is calc-alkaline and is characterized by high concentrations of Sr (583–892 ppm), significant depletions in the heavy rare earth elements (HREE; e.g. Yb = 0.96–2.57 ppm) and Y (10.4–23.3 ppm), and primitive mantle-normalized multi-element variation diagram patterns with positive Sr and Ba and negative Nb and Ti anomalies, all of which indicate that this intrusion is compositionally similar to adakites elsewhere. The composition of the porphyritic monzonite is indicative of the derivation from magmas generated by the melting of young subducted slab material. The area also contains Nb-enriched basalts that are enriched in sodium (Na₂O/K₂O = 1.20–3.90) and have higher Nb, Zr, TiO₂, and P₂O₅ concentrations and Nb/La and Nb/U ratios than typical arc basalts. The juxtaposition of adakitic rocks, Nb-enriched basalts, and dolerites in this region suggests that the oceanic crust of the expansive oceans within the West Junggar underwent early Carboniferous subduction. Magnetite is widespread throughout the Kuoerzhenkuola Au deposit, as evidenced by the volcanic breccias cemented by late hydrothermal magnetite and pyrite. In addition, the zoned potassic, quartz-sericite alteration, and propylitic and kaolin alteration in the deeper parts of the porphyritic monzonite are similar to those found in porphyry Cu–Au deposits. These findings, coupled with the mineralogy and geochemistry of the alteration associated with the Kuoerzhenkuola Au deposit, suggest that the mineralization in this area is not purely epithermal, with the geology and geochemistry of the porphyritic monzonite in this area suggesting that a porphyry Cu–Au deposit is probably located beneath the Kuoerzhenkuola Au deposit.     

Geochemical Characteristics,Petrogenesis of the Sanukitic Dikes in the Miaoergou Area of West Junggar,Xinjiang, NW China and Their Geological SignificanceEI北大核心CSCD

NS-trending dikes which contain dioritic enclaves widely occur in the Miaoergou pluton, West Junggar, Xinjiang. The dikes are composed of quartz diorite and quartz diorite porphyrite. LA-ICP-MS zircon U-Pb ages of the quartz diorite and diorite are 298.0±3.7 Ma and 299.4±2.5 Ma, respectively, corresponding to the end of the Late Carboniferous to beginning of the Early Permian. The dikes and enclaves have similar geochemical properties with island arc features. They are calc-alkaline, with moderate SiO2 (53.58% to 57.89%), high MgO (3.09% to 4.83%, Mg# values ranging from 44.69 to 54.12), TiO2 (1.17% to 1.66%), Cr (51.24×10-6 to 126.1×10-6), Ni (35.91×10-6 to 57.55×10-6) contents and K/Na ratios (0.35 to 0.70). Moreover, all samples are enriched in large-ion lithophile elements (LILEs: e.g. K, Rb, Ba and U) and light rare earth elements, but strongly depleted in high field strength elements (HFSEs: e.g. Nb, Ta and Ti), with insignificant Eu anomalies (δEu=0.67 to 1.08). In contrast, the dikes and enclaves in the Miaoergou pluton show geochemical signatures similar to those of the Cenozoic sanukitoids in Setouchi volcanic belt of SW Japan and the sanukitoids in the Hatu area, West Junggar. The source of the dikes might be the depleted mantle previously metasomatized by fluids released from subduction slabs. These sanukitic dikes may be generated by interaction of the mantle wedge with fluids derived from dehydration of the subducting oceanic slab, resulting in 2% to 5% partial melting of amphibole-spine peridotite. The identification of the sanukitic dikes in the Miaoergou pluton, together with previous studies, suggest that the southern West Junggar region was still dominated by subduction-related island arc setting at the beginning of the Early Permian, and multi-stage subduction-accretionary orogeny may account for the difference of subduction duration between the north and the south of West Junggar. © 2018, Science Press. All right reserved.     

Geodynamic Setting and Tectonic Model of Porphyry Copper–gold Deposits in West Junggar, NW China

Please refer to the attachment(s) for more details     

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

Age and geochemistry of the Neoproterozoic granitoids in the the Songnen–Zhangguangcai Range Massif, NE China: Petrogenesis and tectonic implications

<正>1 Introduction The Songnen–Zhangguangcai Range Massif(SZRM)crops out over an extensive part of NE China and was thought to contain Precambrian crystalline basement material,as evidenced by the presence of what appears to bePaleoproterozoicbasementmaterialwithin exploration drillholes(Pei et al.,2007).An alternative view is that the basement within the SZRM is     

Zircon U–Pb geochronology and geochemistry of the post-collisional volcanic rocks in eastern Xinjiang Province,NW China: implications for the tectonic evolution of the Junggar terrane

We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial ⁸⁷Sr/⁸⁶Sr values (0.7034–0.7042) and positive ε_Nd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive ε_Nd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).     

Middle Devonian volcanic rocks in the Weibao Cu–Pb–Zn deposit,East Kunlun Mountains,NW China: Zircon chronology and tectonic implications

The Weibao copper–lead–zinc deposit, located in the eastern part of the Qimantagh area, East Kunlun Orogenic Belt (EKOB), consists of three skarn ore blocks known as Weixi, Main and Weidong from west to east. The mineralization within the Weibao Cu–Pb–Zn deposit is hosted by the Mesoproterozoic Langyashan Formation. In this study, we describe for the first time basaltic lavas that intruded into this host sequence and chronological, isotopic, major and trace element data of these volcanic rocks are presented here to constrain their eruption age as well as the tectonic setting. Two basaltic lava samples yield sensitive, high-resolution ion-microprobe (SHRIMP) U–Pb zircon ages of 393.0 ± 5.5 Ma–392.0 ± 5.0 Ma, indicating that volcanic rocks in the Weibao deposit erupted in Middle Devonian. The majority of the volcanic rocks have compositions characterized by high potassium, light rare earth element (LREE)-enriched patterns in chondrite-normalized rare earth elements (REE) diagrams, and evident enrichment of Rb, Ba and K and depletion of Th, U, Nb and Ta contents in primitive mantle-normalized patterns, although the degrees of enrichment and depletion are variable. These characteristics of major and trace element data highlight a hornblende-dominated fractionation during ascent of magmas. The εHf(T) values of zircons are relatively scattered and slightly enriched, ranging from −2.6 to +7.5. Modelling the features of the major, trace and isotopic element data indicates a hybrid origin involving combined depleted mantle (and hence asthenospheric mantle) and melts and/or fluids inherited from an early subduction event. Besides, these geochronological and geochemical data, together with previously published data in the EKOB, suggest that the Weibao basaltic lavas formed in a post-collisional setting, and that the Qimantagh area underwent strong interactions between mantle and crust in Early Paleozoic–Middle Devonian.     

Petrogenesis of an early Eocene gabbro–granite complex in Kachang (SW Yunnan) and its implications for Eocene magmatism in the Tengchong terrane of SW China

Major and trace element, zircon U–Pb and Hf-isotope data are reported for mafic intrusions and host granite from the Kachang pluton (North Yingjiang of SW Yunnan) in the Tengchong Terrane, in order to investigate their sources, petrogenesis and tectonic implications. The zircon U–Pb age of the mafic rocks (~55 Ma) is identical to that of the host granite (56.7 ± 0.6 Ma). The mafic rocks have high MgO concentrations (up to 13.43 wt.%) at low SiO₂ contents (low to 42.73 wt.%) and slight negative to positive εHf(t) values (?2.26 to +0.59). They are enriched in LILE and LREEs and depleted in HFSEs, which can be explained as melts derived from a enriched mantle, with some crustal contamination. The host granite have high SiO₂ contents (69.18–72.65 wt.%), highly negative εHf(t) values (?9.08 to ?5.14), suggesting mainly derived from an ancient crustal source. Field observations, geochronology, geochemistry and zircon Hf isotopic compositions point to a complex petrogenesis, where enriched mantle- and crust-derived magma mixing was coupled with crystal fractionation, thus explaining the genetic link between mafic and felsic magmas, result of mafic magma upwelling triggered by the subduction rollback of the Neotethyan slab. Our new data, along with the data reported (especially zircon U–Pb dating and Lu–Hf isotope data) in the Tengchong Terrane, indicate that the spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern and central Tibet.     

Petrogenesis and Tectonic Implications of Middle Ordovician Ocean Island Basalts from the Chagantaolegai Ophiolitic Mélange in Junggar, NW China

Seamount accretion is one of the most significant accretionary orogenic processes in the Central Asian Orogenic Belt, but there are few paleo-seamounts reported from and debate on the tectonic evolution of the Junggar Ocean still exists. In this study, we present geochronological, mineralogical, geochemical and isotopic data for basalts from the Chagantaolegai ophiolitic mélanges in Junggar. Zircon U-Pb dating on one basalt yielded a weighted mean 206 Pb/238 U age of 469 ± 7 Ma, which suggests that it formed in the Middle Ordovician. All rock samples belong to alkaline basalt and show similar geochemical characteristics, displaying high TiO2(～3 wt%),(La/Yb)N(17.6–19.0), ΣREE(232–289 ppm) and enrichment in Nb and Ta, which implies an ocean island basalt(OIB) affinity. Based upon positive εN d(t)(+4.16 to +4.23), ΔNb(0.20–0.22) and low initial 87 Sr/86 Sr(0.70425 to 0.70452) and Zr/Nb(3.35–3.57), we suggest that the Chagantaolegai OIB samples were likely derived from a fertile mantle source related to plume. The OIB rock assemblage, chert and marble in the southern part of the Chagantaolegai ophiolitic mélange indicates that a Middle Ordovician seamount was accreted to the Boshchekul-Chingiz arc due to the northward subduction of the Junggar-Balkhash Ocean.     

Detrital Zircon U-Pb Geochronology and Provenance of the Hebukesaier Formation in the Shaerbuerti Mountains,Northern West Junggar:Implication for Devonian Subduction of the Junggar–Balkhash Ocean

Limited Devonian magmatic record in northern West Junggar leads to contrasting models on its tectonic evolution. In this study, we conducted LA-ICP-MS U-Pb dating on detrital zircons of two sandstones from the Hebukesaier Formation in the Shaerbuerti Mountains. Detrital zircons with oscillatory zoning are characterized by high Th/U(> 0.3) and low La/Yb(< 0.15), indicating their magmatic origin. The youngest zircon ages of two samples are 402 ± 2 Ma and 406 ± 2 Ma, respectively, suggesting ...     

Structures and detrital zircon ages of the Devonian–Permian Tarbagatay accretionary complex in west Junggar,China: imbricated ocean plate stratigraphy and implications for amalgamation of the CAOB

ABSTRACT

The Tarbagatay Complex, located in northwest Junggar, is situated tectonically between the Zharma–Saur arc to the north and the Tacheng terrane and the Boshchekol–Chingiz arc to the south. This Complex belt is variably composed of ophiolitic mélange, sedimentary mélange, and coherent units of turbidites and shallow water sediments. These rocks crop out in fault-bound slices with fault-parallel asymmetric folds. Both the lithologies and deformation features of the Tarbagatay Complex suggest an accretionary origin generally with a top-to-the-south tectonic vergence, suggesting N-dipping subduction beneath the Zharma–Saur arc. The presence of a former ocean is indicated by the Ordovician ophiolite mélanges and related marine fossils. The time duration of the Tarbagatay Complex can be bracketed by detrital zircon ages of turbidites and shallow water sediments with a lower limit of major peak ages of 350–370 Ma, and an upper limit of middle Permian indicated by detrital zircon ages of 262.3 Ma. Based on these data, we suggest that the subduction of the Tarbagatay Ocean likely started in the Late Devonian and lasted until the middle Permian. Taking into account the formation of the northern part of the Kazakhstan orocline, which has a similar temporal-spatial framework, we propose a tectonic model for the western CAOB that involves accretion and amalgamation from the Ordovician to the middle Permian.     

Detrital zircon geochronology of Devonian quartzite from tectonic mélange in the Mianlue Suture Zone,Central China: provenance and tectonic implications

ABSTRACT

Devonian quartzite occurs as blocks within a phyllite matrix in Puziba area of the Mianlue Suture Zone (MLSZ) in central China. The depositional time of the quartzite is younger than 425 Ma (mainly Early Devonian), constrained by the zircon U–Pb geochronology data from the quartzite, cross-cutting relationships with granite, and palaeontology evidence. The detrital zircons in the quartzite show typical magmatic features with four main age peaks at: 2676–2420 Ma (11.6% of the population), 1791–1606 Ma (4.8%), 997–817 Ma (26.5%), and 597–425 Ma (17.5%). In combination with the zircon εHf(t) values, we propose that the quartzite in the MLSZ was sourced from Neoproterozoic and Palaeozoic magmatic and sedimentary rocks in the South Qinling Block and the South China Block (particularly from the Bikou Terrane), with minor contributions from Archaean and Palaeoproterozoic magmatic units from both of the South and North China blocks. The blocks of quartzite, slate, marble, metasandstone, and chert blocks in the phyllite matrix in the Puziba area show a typical block-in-matrix texture in a tectonic mélange, and provide significant evidence from sedimentary rock blocks rather than ophiolite or volcanic rock for the existence of the MLSZ.     

LA-ICP-MS Zircon U-Pb Age of Newly Discovered Hatu Tectonic Mélange in the West Junggar,Xinjiang, NW China

<正>Objective The West Junggar, situated in the southwestern segment of the Central Asian Orogenic Belt, is considered to be an important area for Phanerozoic crustal growth owing to the excellent exposures of diverse rock types and multiple generations of structures and magmatic rocks. Recently, a new tectonic mélange has been identified in the southern West Junggar during geological mapping at a scale of 50000. This work presents new geochronological data for     

Geochronology and Sr–Nd–Hf isotopic composition of the granites, enclaves, and dikes in the Karamay area, NW China: Insights into late Carboniferous crustal growth of West Junggar

New whole-rock major and trace elements,and zircon U-Pb and Hf-Nd isotope compositions are reported for the Karamay dikes,enclaves,and host granites in the West Junggar,NW China.Zircon U-Pb dating of the Karamay pluton yields an age of 300.7 ± 2.3 Ma for the enclave and 300.0 ± 2.6 Ma for the host granite,which was intruded by dike with an age of 298 Ma.The host granites exhibit relatively low SiO_2 contents and A/CNK and Ga/Al ratios,low initial ~(87)Sr/~(86)Sr ratios(0.703421—0.703526) and positive εHf(t)(5.5—14.1) and εNd(t)(7.3—8.1) values with a young model age,suggesting that they are I-type granites and were mainly derived from a juvenile lower crustal source.The enclaves and dikes belong to an andesitic calc-alkaline series and have high MgO concentrations at low silica content and positive εNf(t)(7.6-13.2,14.2-14.9) and εNd(t)(6.8-8.3,~6.9) values.They are enriched in LILEs(Rb,Ba and U) and LREE and depleted in HFSEs(Nb and Ta) with insignificant negative Eu anomalies,indicating that the melts were derived from an enriched lithospheric mantle modified by subducted oceanic crust-derived melts and minor fluids,followed by fractional crystallization.The Karamay host granites and enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and lithospheric mantle-derived magmas,and were intruded by the unmixed dikes subsequently.The upwelling mantle through a slab window in an island arc environment might have triggered partial melting of the lithospheric mantle and its subsequent interaction with the granitic magma,further suggesting that the ridge subduction played an important role in the crustal growth of West Junggar.