Subduction Of Paleo-Asian Ocean Research Articles

Late Paleozoic slab rollback events in the southeastern part of the Central Asian orogenic belt: Implications for Paleo-Asian Ocean subduction and continental crust growth

The Central Asian orogenic belt is considered to be the largest Phanerozoic accretive orogenic belt on Earth. The late Paleozoic magmatic rocks in central Inner Mongolia are crucial for understanding continental crust growth and the tectonic evolution of the southeastern part of the Central Asian orogenic belt. We present comprehensive geochemical, isotopic, and geochronological data from three late Paleozoic magmatic units in the Mandula area, west of the Solonker suture zone. Zircon U-Pb dating indicates that these rocks formed during the late Carboniferous (316−304 Ma). The Mandula high-Mg diorites exhibit high MgO (3.9−6.5 wt%), high Mg# (61−69), and depleted Nd-Hf isotopic compositions, generated through interaction between a metasomatized mantle and slab melts with the overlying sediments. The Mandula granodiorites display adakite geochemical characteristics with high Sr/Y mass ratios (29−52), high MgO (1.7−2.2 wt%), and high Mg# (52−54), formed by partial melting of the oceanic slab with the addition of overlying sediment. Mafic microgranular enclaves have consistent ages, Sr-Nd-Hf isotope compositions, and hornblende crystallization temperature-pressure conditions with their host granodiorite, formed from a cognate magma associated with the host granodiorites through cumulate. We propose that two phases of slab rollback took place during the late Paleozoic southward subduction-accretion of the Paleo-Asian Ocean. The first phase corresponded to the transformation of low- to medium-angle slab subduction, while the second phase led to subduction-related extension. Considering the tectonic-magmatic evolution, crustal maturity, and thickness variations in the late Paleozoic southeastern part of the Central Asian orogenic belt, we propose that prolonged subduction and slab rollback promoted continental crust growth. The Central Asian orogenic belt coincides temporally and spatially with the Phanerozoic Pangea cycle, suggesting that continuous subduction and supercontinent amalgamation significantly contributed to continental crust growth.

Initial oceanic subduction triggered by arc–continent collision: An example from the convergence between the Paleo-Asian Ocean and the North China Craton

The driving mechanisms for initial oceanic subduction beneath a passive continental margin remain unclear. The Paleozoic convergence history between the North China Craton (NCC) and the Paleo-Asian Ocean provides an ideal opportunity to investigate the driving mechanisms of the onset of oceanic subduction. Our detrital zircon U–Pb dating results yield Middle Devonian to Permian ages (391–273 Ma) in Paleozoic strata within China's Aoji region, specifically in the Bainaimiao arc belt situated between the NCC and Central Asian Orogenic Belt (CAOB). Detrital zircon age spectra, εHf(t) values, and two-stage model ages of the Devonian to Permian sediments suggest zircon contributions from both the NCC and CAOB. Combined with data from various regions along the Bainaimiao arc belt, findings suggest that the Bainaimiao island arc was accreted to the northern margin of the NCC through arc–continent collision during the late Silurian, rendering the NCC margin passive until the end-Devonian. At this point, southward subduction in the Paleo-Asian Ocean beneath the NCC commenced and continued into the Permian, transforming the northern margin of the NCC from passive to active. This initial subduction (ca. 359 Ma) in the Paleo-Asian Ocean resulted from collision-induced magmatism and deformation due to the protracted convergence and also led to a reversal in subduction polarity. Subsequently, in response to Paleo-Asian Ocean subduction, the northern margin of the NCC underwent rapid uplift during the Carboniferous, which led to widespread exposure of the NCC basement rocks in the late Carboniferous.

Multiple ridge subduction processes in the southern Altaids: Implications from clinopyroxene chemistry and Sr–Nd–Hf isotopes of late carboniferous Nb-enriched, magnesian diorite-andesites in West Junggar, NW China

Magnesian (Mg# > 45) and/or Nb-rich (Nb > 6 ppm) dioritic-andesitic rocks have been identified in the Mayile area of southern West Junggar, Northwest China. A systematic petrological, geochronological, whole-rock geochemical, Sr–Nd–Hf isotope, and clinopyroxene chemistry study was conducted to constrain the petrogenesis, mantle source, and P–T conditions of magmas as well as the tectonomagmatic processes in the southern Paleo-Asian Ocean subduction zone. Mayile dioritic-andesitic rocks generated in the Late Carboniferous (306–312 Ma) have a high SiO2 content (52.3–63.2 wt%) and high Mg# (46–66) as well as sanukitic-like low LaN/YbN and Sr/Y ratios. They have high Nb contents (6.27–11.6 ppm) and Nb/U (9–18) ratios and are Nb-enriched sanukitic high-Mg andesites (HMAs). Clinopyroxene thermobarometry indicates that the parental magma of the Mayile HMAs experienced a high-temperature (1137–1199 °C) and shallow-level (0.8–0.17 GPa) partial melting in a mantle wedge. They have εNd(t) (+5.14 to +6.42), εHf(t) (+8.8 to +14.4), and zircon εHf(t) (+9.4 to +15.8) values and a slightly high (87Sr/86Sr)t ratio ranging from 0.703214 to 0.704140 relative to normal mid-ocean ridge basalt (N-MORB). The results suggest that West Junggar was dominated by continuous records of sanukitic rocks hinting at multiple ridge subduction processes in the southern Altaids. We reconstructed the subduction history of the Junggar Ocean in the Carboniferous-Permian from these particular observations in West Junggar, and gained insight into the gradual evolution of the tectonic setting, which transitioned from normal intraoceanic subduction to hot subduction of the young oceanic lithosphere from the Early to Late Carboniferous, with subsequent ridge subduction occurring mainly in the Late Carboniferous.

Cambrian-Early Ordovician intrusive bodies in Dunhuang Block: Records of Paleo-Asian Ocean subduction and implications for regional Early Paleozoic tectonic evolution

一直以来，敦煌地块缺少1.6~0.46Ga的地质记录，从而严重制约了对该地块在新元古代和早古生代期间地质构造演化的全面认识。通过15万区域地质调查，本次工作在敦煌地块东北缘新发现了寒武纪-早奥陶世小宛山岩体、截山子岩体和小宛南岩体等多个中酸性侵入体，测得其LA-ICP-MS锆石U-Pb年龄分别为517±3Ma、480±3Ma和473±3Ma，由此厘定出敦煌地块目前古生代最古老的侵入岩体及早奥陶世侵入岩体。通过这些岩体的岩石岩相学、岩石地球化学特征、岩石成因及大地构造环境分析，表明它们属于与洋壳俯冲消减作用有关的活动陆缘环境下形成的富钠质I型花岗岩。其中，小宛山花岗闪长质岩体是在低压低温条件下由玄武质地幔楔部分熔融而成，同时受到俯冲流体的强烈交代；截山子岩体和小宛南岩体则是在高压低温富水条件下，由新生镁铁质洋壳发生部分熔融并受到地幔楔强烈混染而形成，虽然它们均属于（类） O型埃达克岩，但其部分熔融的压力及其残留矿物组合不同。上述研究揭示敦煌地块北缘早古生代517Ma就存在俯冲作用，且至少持续了44Myr。结合区域地质资料和以往研究成果，敦煌地块北缘早古生代洋陆转换过程可分为三个阶段：（1）晚震旦世-早寒武世（574~518Ma），敦煌地块北缘被动陆缘演化阶段；（2）寒武纪第二世-早奥陶世（517~471Ma），敦煌地块北缘活动陆缘演化阶段，期间，古亚洲洋南支洋分别向敦煌地块和石板山地块/马鬃山地块发生双向俯冲消减；（3）中奥陶世-早泥盆世（464~412Ma），敦煌地块与石板山地块/马鬃山地块碰撞造山阶段，期间古亚洲洋南支洋闭合。

Lipid Biomarker and Stable Isotopic Profiles through Late Carboniferous–Early Triassic of the Deepest Well MS-1 in the Junggar Basin, Northwest China

The Carboniferous–Triassic period was an important stage of global sea–land transformation, with coal formation in the Carboniferous, biological extinction at the end of the Permian, and global drought in the Triassic. The MS-1 well in the Mosuowan High of the Junggar Basin is the deepest well drilled in Northwestern China. In this paper, we investigate the sedimentary environment and climate evolution of the Mosuowan area in the central Junggar Basin during the Late Carboniferous–Early Permian by the petrothermal, lipid biomarker, and isotopic composition of mud shale core samples, and explore the tectonic–climatic events and Central Asian orogenic belt evolution driving the sedimentary environment. The study shows that the organic matter from the Upper Carboniferous to the Lower Permian is at a mature stage, but biomarkers maintained the primary information although the organic matter was subjected to thermal evolution. In the late Carboniferous period (Tamugan Formation), the study area was a closed remnant sea with a relatively humid climate, triggering lush terrestrial vegetation and high organic carbon content in the sediments, which had the potential to evolve into natural gas. During the Xiazijie Formation of the Middle Permian, tectonic activity shifted to the subsidence period, and the salinity of the water decreased after a large input of fresh water. The lake basin area expanded, and the content of aquatic organisms continued to increase. As the Lower Permian stratigraphy is missing, this sea–land transition seems to jump. The low and upper Urho Formations of the Middle–Upper Permian are a deltaic foreland deposit, and geochemical indicators show an overall lake retreat process with a continuous increase in organic matter content of terrestrial origin. The lithologic assemblage of the Triassic Baikouquan Formation is braided river deltaic sedimentation with migration of deposition centers of the lake basin. In conclusion, the Late Carboniferous–Early Permian period was influenced by global changes, Paleo-Asian Ocean subduction, and continental splicing, which resulted in a continuous increase in terrestrial organic matter, water desalination, and oxidation-rich sediments in the Mosuowan region, but the P–T biological mass extinction event was not recorded.

Structural control on the Baiyinha'er gold mineralization (Southern Central Asian Orogenic belt) related to Early Paleozoic Paleo-Asian Oceanic subduction

Structural control on the Baiyinha'er gold mineralization (Southern Central Asian Orogenic belt) related to Early Paleozoic Paleo-Asian Oceanic subduction

Early Paleozoic subduction fingerprints of the Paleo-Asian ocean in easternmost Central Asian Orogenic Belt (CAOB): Identification of the oldest Alaskan-type complex in the CAOB

Abstract The early-stage subduction records of the Paleo-Asian ocean are poorly preserved in the eastern segment of the Central Asian Orogenic Belt (CAOB), which hinders constraints on the evolution of the whole CAOB. This study presents new age data and zircon Hf-O isotopes as well as bulk-rock geochemistry of the Wuxing mafic-ultramafic complex in the Xingkai massif in northeastern China, which has been identified as Alaskan-type complex with aspects of field occurrence, petrological and mineral assemblages, and mineral chemistry in our recently published work. The results indicate that the complex formed mainly between 517 Ma and 510 Ma with a lithological sequence of Sanying clinopyroxenite (517 Ma), then Sanying gabbro (514 Ma), and Erying hornblendite (513 Ma), and finally Erying hornblende clinopyroxenite (510 Ma). The lithological formation sequence is consistent with the intrusive relations between lithological phases and their irrelevant major element compositions and variable trace element patterns of the bulk rocks. The εHf(t) values of zircon in two samples in this study vary from from +4.45 to +7.61 and from −11.8 to +4.42, respectively and tend to be more depleted with age. These features suggest that the Wuxing complex was a product of long-term arc magmatism and experienced significant ancient crustal assimilation in early-stage magmas and negligible contamination in later ones. The presence of 1222 Ma and 706 Ma inherited zircon grains implies existence of Proterozoic basement in the Xingkai massif and its continental arc setting in Cambrian. The Wuxing complex is the oldest Alaskan-type complex found so far in the entire CAOB and is a good witness of the Paleo-Asian oceanic subduction in the easternmost CAOB. The older age of the Wuxing complex compared to the regional Hongqiling intrusion is also compatible with its Alaskan-type nature and platinum-group element mineralization, which are distinct to the Permian-Triassic Ni-Cu sulfide deposit-hosting maficultramafic intrusions in the southern CAOB.

Early Permian magmatism above a slab window in Inner Mongolia, North China: Implications for the Paleo-Asian Ocean subduction processes and accretionary crustal growth

Identifying magmatic rock associations in a subduction zone is substantial for understanding the related geodynamic evolution. The final closure time of the Paleo-Asian Ocean and the exact subduction processes have continuously been controversial, hindering our recognition of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). Here we investigated the Early Permian gabbros-granodiorites from Xi Ujimqin in the northern Inner Mongolian region. The gabbros have slightly depleted light rare-earth elements [(La/Yb)N = 0.8] and flat heavy rare earth elements (HREE) [(Gd/Yb)N = 1.1] that are comparable to those of N-MORB. They show depletion in Nb and Ta, positive whole-rock εNd(t) values of +7.7 to +8.7 and zircon εHf(t) values of +8.7 to +10.6. These mafic rocks are interpreted to be products of partial melting of depleted mantle that had been metasomatized by slab-derived fluids, with subsequent fractional crystallization and minor crustal contamination. These granodiorites chemically resemble typical adakites and have MORB-like whole-rock SrNd isotopic compositions (87Sr/86Sri = 0.7028–0.7029; εNd(t) = +8.2–+8.5) and positive zircon εHf(t) (+13.4–+15.9) values, suggesting that they originated from the subducted slab-melts and reacted with mantle wedge peridotite. UPb zircon dating shows emplacement of N-MORB-type gabbros and adakitic granodiorites at ∼297 Ma and ∼290 Ma, respectively. Our new data indicate the presence coeval slab-derived adakites and slab fluid-metasomatized asthenosphere-derived N-MORB-type rocks, indicating that subduction lasted until at least the Early Permian. Such an association along with the extension-related magmatism in northern Mongolia recorded the northward ridge subduction of the Paleo-Asian Ocean during the Late Carboniferous–Early Permian. This model is consistent with the spatial distribution and the ages of magmatic activity with positive εNd(t)–εHf(t) values from this region. Ridge subduction and induced slab windows probably played a key role in Paleozoic crustal growth of CAOB, and by inference in the other accretionary orogens throughout Earth history.

Final-Stage Magmatic Record of Paleo-Asian Oceanic Subduction? Insights from Late Permian to Early Triassic Intrusive Rocks in the Yanbian Area, Easternmost Central Asian Orogenic Belt

This paper reports new zircon LA–ICP–MS U–Pb and Hf isotope data, and whole-rock major and trace element data for Late Permian to Early Triassic intrusive rocks in the Yanbian area, NE China. These data provide new insights into the timing of the final subduction of the Paleo-Asian Ocean beneath the North China Craton. The zircon U–Pb age data indicate that a suite of Late Permian to Early Triassic intrusive rocks related to subduction is present within the Yanbian area. The Late Permian intrusive rocks consist of diorites while the Early Triassic granites and hornblende gabbros constitute a geochemically bimodal igneous rock association. Furthermore, the Early Triassic granites show the geochemical characteristics of shoshonitic rocks. All the rocks are characterized by enrichment in LILEs and LREEs, and depletion in HREEs and HFSEs, suggesting they formed in a subduction setting. Zircons from the Early Triassic gabbros have εHf(t) values and TDM2 ages of +7.6 to +10.7 and 735–1022 Ma, respectively, suggesting that they formed from a primary magma generated by the partial melting of lithospheric mantle material that had been previously modified by subduction-related fluids. The Late Permian diorites have εHf(t) values and TDM2 ages of +0.5 to +9.5 and 853 to 1669 Ma, respectively, while they have high contents of Al2O3, Fe2O3, and low contents of SiO2, Cr, and Ni, indicating Late Permian diorites should derive from the mantle and are influenced by some crustal material. Early Triassic granitic rocks have a wide range of εHf(t) values and TDM2 ages of −4.8 to +9.4 and 852 to 2136 Ma, respectively. Their zircons imply that the Early Triassic granites could be mainly derived from partial melting of the crust, with minor contribution of the crustal material of an ancient crust. The Early Triassic bimodal intrusive rocks in Yanbian area, combined with the regional geologic information; therefore, record a final post-subduction extensional environment due to the break-off of the previously subducted slab.

Early–Middle Permian southward subduction of the eastern Paleo-Asian Ocean: Constraints from geochronology and geochemistry of intermediate-acidic volcanic rocks in the northern margin of the North China Craton

The northern margin of the North China Craton (NCC) resulted from complicated subduction-accretion process of the eastern Paleo-Asian Ocean (PAO), thus late Paleozoic magmatic records in this region could place conclusive constraints on the final-stage subduction processes of the eastern PAO. Here we present petrological, geochronological, geochemical, and in situ zircon Hf isotopic data of the late Paleozoic intermediate-acidic volcanic rocks, and compiled age and geochemical data of the Permian magmatic rocks along the northern margin of the NCC, with the aim of elucidating their ages, petrogenesis and tectonic implications. Zircon UPb ages indicate that these rocks formed in the early–middle Permian (275–267 Ma). Lithologically, these volcanic rocks are composed mainly of andesites, with minor rhyolites and dacites. The studied andesites contain 53.94–64.87 wt% SiO2, 1.86–3.05 wt% MgO and 0.47–3.56 wt% K2O, and belong to low-K calc-alkaline to shoshonite series. Coupled with variable zircon εHf(t) values of −14.57 to +12.57, we propose that the parental magma was derived from partial melting of heterogeneous lithospheric mantle resulted from metasomatism of subduction-related fluids. In contrast, the rhyolite-dacite suites are distinguished by higher SiO2 (65.94–74.77 wt%) and lower MgO (0.15–1.60 wt%) contents, indicating an affinity with highly fractionated I-type granite originated by partial melting of lower crust. These intermediate-acidic rocks are characterized by enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; e.g., Nb, Ta, P, and Ti), suggesting that they formed in an active continental margin setting. These geochemical features and lithological assemblage, together with the East-West-trending Permian magmatic belt with arc affinities, indicate that their generation was related to southward subduction of the Paleo-Asian oceanic lithosphere beneath the NCC. Synthesized data from this and previous studies lead us to favor a double-sided, early–middle Permian subduction model, with the PAO not closed prior to the middle Permian.

Geochemistry, geochronology and Hf isotope of granitoids in the northern Alxa region: Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial. The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa. In this paper, we present new zircon U–Pb ages, Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula Tectonic Belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region. The zircon U–Pb data could be grouped as two phases: Late Devonian granite and diorite (ca. 373–360 ​Ma), and Late Carboniferous granodiorite (ca. 318 Ma). The Late Devonian granites and diorites are metaluminous to slightly peraluminous, with A/CNK and A/NK ratios of 0.90–1.11 and 0.95–2.19, respectively. The Late Devonian diorites are characterized by high MgO, Cr and Ni contents and MgO# values, together with variable εHf(t) values from –1.0 to +1.3 and old TDM2 ages varied from 1283 ​Ma to 1426 ​Ma, indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust. Even though the Late Devonian granites yielded most positive and minor negative εHf(t) values between –1.1 to +5.7 (three grains are negative) with two-stage model ages (TDM2) of 1003–1438 Ma, they display low MgO, Cr and Ni contents and MgO# values, suggesting that they were mainly derived from juvenile crustal materials, mixed with a small amount of ancient crust. The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series, with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and 1.75 to 1.90, respectively. These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle, as evidenced by their high εHf(t) values (+11.6 to +14.1) and young TDM2 ages (427 ​Ma to 586 ​Ma), as well as high Mg# values, and MgO, Ni and Cr contents. Our data, along with available sedimentary evidence and previous researches, indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting. The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean, but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.

Middle–late permian I-type granitoids from the Diaobingshan region in the northern margin of the North China Craton: insight into southward subduction of the Paleo–Asian Ocean

ABSTRACT An integrated study of the petrology, zircon U–Pb geochronology, whole–rock geochemistry, and in situ zircon Hf isotopic compositions of middle–late Permian granitoids from the Diaobingshan region in the northern margin of the North China Craton (NCC) has been implemented, with the aim of investigating their geochronological framework, petrogenesis, and tectonic significance. The studied granitoids are widely distributed on the southern side of the Changchun–Yanji suture zone and comprise mainly monzogranite and syenogranite, with minor quartz monzonite. New zircon U–Pb data measured by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) yield emplacement ages of 262–256 Ma, reflecting middle–late Permian magmatism. Geochemically, the rocks contain 60.53–73.33 wt.% SiO2, 12.60–18.35 wt.% Al2O3, 3.50–8.10 wt.% K2O, 0.18–0.91 wt.% TiO2, and 0.22–1.01 wt.% MgO, and exhibit weakly peraluminous (A/CNK = 0.86–1.01) and high-K calc–alkaline and shoshonitic characteristics, indicating I-type granite affinities. They are also characterized by low Sr and high Yb contents, reflecting low-pressure magma source conditions with residual plagioclase (i.e. amphibolite facies). These granitoids also exhibit geochemical characteristics typical of arc–type magmatic rocks, with enrichment in large ion lithophile elements (LILEs; e.g. Rb, Ba, and K) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs; e.g. Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating that they were formed in a subduction-related setting. Moreover, magmatic zircons from the studied rocks have relatively homogeneous Hf isotopic compositions (εHf(t) = – 1.4 to +15.1) and corresponding two–stage model (TDM2) ages of 1839–348 Ma, indicating that they were generated by partial melting of Paleoproterozoic–Phanerozoic accreted lower crust, with minor contributions of ancient crustal materials. Their geochemical characteristics and specific lithological assemblage, together with the temporal–spatial distribution of Permian granitic magmatism, indicate that the formation of middle–late Permian granitoids in the northern margin of the NCC was related to southward Paleo–Asian oceanic subduction. Data from this and previous studies lead us to favour a double-sided, early–late Permian subduction model, with the Paleo–Asian Ocean closing after the late Permian.

The Timing of the Paleo-Asian Oceanic Closure: Geochemical Constraints from the Jigede Gabbro in the Alxa Block

This paper investigates the Jigede gabbros from the Shalazhashan tectonic belt, Alxa Block. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating reveals that they were emplaced in the Middle Permian (ca. 262 Ma). All the gabbro samples collected from the intrusion exhibit low contents of TiO2 (0.24–0.37 wt %) and $${\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{{\text{3}}}^{{\text{T}}}$$ (4.87–5.41 wt %), but high levels of MgO (11.15–12.15 wt %), CaO (11.45–12.75 wt %), Al2O3 (14.18–17.08 wt %) and high Mg# (0.81–0.83). Relative to HREEs and LILEs, the gabbros are enriched in LREEs and depleted in Th, Nb, Ce, Zr, Hf, and Ti, with positive Eu, Sr and U anomalies. In contrast to MORB, the Jigede intrusion has higher initial 87Sr/86Sr values of 0.7046 to 0.7054 and lower eNd(T) of + 1.8 to + 4.8. Elemental and isotopic data suggest that the intrusion was likely to have been produced by partial melting of a shallow lithospheric mantle source modified by slab-derived fluids, with subsequent cumulation of plagioclase. Thus, the Jigede intrusion was emplaced in a back-arc setting, in response to the Paleo-Asian oceanic subduction. In addition, the final closure of the Paleo-Asian Ocean did not occur prior to the Middle Permian.

The Late Permian highly fractionated I-type granites from Sishijia pluton in southestern Inner Mongolia, North China: A post-collisional magmatism record and its implication for the closure of Paleo-Asian Ocean

Whole-rock major and trace elements, zircon UPb geochronology and Hf isotopic data have been implemented for the felsic rocks from the Sishijia pluton in southeastern Inner Mongolia, North China, in order to determine their petrogenesis and futher constrain the termination of Paleoasian Oceanic subduction in southeastern Central Asian Orogenic Belt. Petrographic observation indicates that Sishijia granitoids mainly consist of syenogranites and porphyritic granites, with the major mineral compositions of alkaline feldspar, quartz, plagioclase and biotite, without any alkaline melanocratic or peraluminous minerals. Miarolitic structures are widely developed in these granitoids, indicating for the characteristics of shallow emplacement depth and rapid condensation. Zircon LA-ICP-MS UPb dating yields weighted mean ages of 256.2 ± 1.4 and 250.9 ± 1.2 Ma for syenogranites, and 251.6 ± 1.4 Ma for porphyritic granites, representing a Late Permian magmatism record. These granitoids exhibit weakly peraluminous nature (A/CNK = 1.03–1.08) with high SiO2 (76.50–81.92 wt%) and K2O (3.64–5.56 wt%) concentrations and differentiation index values (D.I. = 93.8–97.2). Chondrite-normalized REE patterns of these samples display enrichment of LREE with high ratios of (La/Yb)N (12.29–30.34) and significant negative Eu anomalies (δEu = 0.15–0.53). In mantle-normalized multielement variation diagrams, they are all characterized by typical depletions of Ba, Nb, Ta, Sr, P, and Ti and enrichments of Rb, Th, U, and K. Petrological and geochemical features of the pluton indicate a highly fractionated I-type affinity. Zircon Hf isotope analyses show variably negative εHf(t) values ranging from −13.7 to −4.0 and corresponding two-stage model ages varying of 2148–1540 Ma, indicating that they were derived from an Paleo- to Mesoproterozoic crustal source with minor juvenile materials. In combination with previous geological evidences, it is reasonable to propose that the Sishijia granitoids emplaced in a post-collisional extension regime during the transformation from pre-collision to post-orogenic, which symbolize a Late Permian final closure of Paleo-Asian Ocean in southeastern segment of the Central Asian Orogenic Belt.

Late Permian intermediate and felsic intrusions in the eastern Central Asian Orogenic Belt: Final-stage magmatic record of Paleo-Asian Oceanic subduction?

The Central Asian Orogenic Belt (CAOB), which formed by closure of the Paleo-Asian Ocean, is one of the largest Phanerozoic accretionary orogens on Earth. However, the timing of final closure of the Paleo-Asian Ocean remains debated. Here we present new zircon UPb dating, oxygen fugacity estimates, and whole-rock geochemical and SrNd isotopic data from intermediate and felsic intrusive rocks in the Chunhua area of NE China to constrain their petrogenesis and assess the tectonic implications for the evolution of the eastern CAOB. Zircon UPb dating indicates that the intrusive rocks formed in the late Permian (257–261 Ma). Based on their chemical compositions, two groups of rocks can be identified. Group Ι consists mainly of dioritic rocks (e.g., gabbroic diorites, diorites and tonalites) with relatively low SiO2 but high MgO contents and Mg# values. Petrological and geochemical characteristics show that Group Ι rocks were formed by mixing of mantle- and crust-derived magmas. Mafic microgranular enclaves within Group Ι rocks have high MgO contents (up to 9 wt%) and depleted isotopic compositions (εNd(t) = +4.3 to +7.2), and probably represent mafic end-member magmas derived from a metasomatized lithospheric mantle, which subsequently were injected into the felsic crustal magmas. In contrast, Group ΙI rocks are mostly granodiorites characterized by high SiO2 and low Mg# values, with high Sr/Y and La/Yb ratios. These rocks were probably generated by partial melting of mafic lower crust with little or no addition of mantle materials. Trace element modeling suggests that intracrustal differentiation was dominated by hornblende fractionation and caused the high Sr/Y and La/Yb signatures of Group ΙI rocks. Their low (87Sr/86Sr)i (0.70345–0.70485) and positive εNd(t) values (+3.8 to +4.6) suggest predominantly juvenile sources, typical of newly underplated mafic lower crust. The low oxidation states (FMQ–1.1 to FMQ–7.9) for both Group Ι and Group ΙI rocks, as indicated by Ce4+/Ce3+ ratios in zircon, imply reducing conditions for the lithospheric mantle and the overlying lower-crustal source rocks. The Chunhua intrusions show petrological and geochemical characteristics of arc-type igneous rocks, including abundant euhedral hydrous minerals, calc-alkaline affinities, and enrichment in large ion lithophile elements (e.g., Rb, Ba, and Pb) and depletion in high field strength elements (e.g., Nb, Ta, and Ti), indicating that they formed in a subduction-related setting. Given the occurrences of Early Triassic syn-collisional granites (235–252 Ma) in the area, the late Permian intrusions (257–261 Ma) likely represent the final stages of magmatism related to subduction of Paleo-Asian Ocean, signifying that final closure of the Paleo-Asian Ocean in the easternmost CAOB occurred during the end-Permian.

Geochronology, geochemistry, and Sr–Nd–Pb–Hf isotopes of the Zhunsujihua granitoid intrusions associated with the molybdenum deposit, northern Inner Mongolia, China: implications for petrogenesis and tectonic setting

The Zhunsujihua porphyry molybdenum deposit, located in northern Inner Mongolia of China that belongs to Central-Asian Orogenic Belt (CAOB), is the only Mo deposit formed in the late Carboniferous in this area so far. Its mineralization is mainly restricted to the Zhunsujihua granitoid intrusions, which are composed of the main granodiorite (GD) and crosscutting, virtually coeval minor syn-ore leucogranite (LG) and diorite porphyry (DP) dykes. LA-ICP-MS zircon U–Pb dating yields crystallization ages of 300.0 ± 2.0, 299.3 ± 2.0, and 299.0 ± 2.6 Ma for the GD, LG, and DP, respectively. The major and trace element lithogeochemical data show that the GD and LG are metaluminous to weakly peraluminous, high-K calc-alkaline series with I-type granite characteristics, strongly oxidized, with low concentrations of Ba, Nb, Sr, P, and Ti and elevated K and Rb contents, indicating typical arc magmatic features. The LG is a product derived by extensive fractional crystallization of a parental magma similar to the GD as evident from the lower Eu/Eu*, Nb/Ta, Zr/Hf, and T Zr. The moderately altered DP exhibits high concentrations of K, Rb, Cs, LREE, Y, and low Sr/Y, with a positive e Nd (300 Ma), which indicates a mantle or juvenile source associated with an arc setting. The Sr–Nd–Hf isotope data show low I Sr (0.70406–0.70461) and moderate e Nd (300 Ma) (−0.9 to 1.5) for the GD and LG, and relatively high e Hf (300 Ma) values (−3.6 to +11.2) for the GD, suggesting the magma mainly originated from the juvenile lower crust that was derived from depleted mantle, with a minor component of ancient continental crust. Lead isotope data have characteristics of a lower crust source with minor contamination by upper crustal material. Combined with previous research, the Zhunsujihua granitoid intrusions developed in an intracontinental volcanic arc (Uliastai) associated with northward subduction of the Paleo-Asian Ocean plate during late Carboniferous to early Permian; this suggests that the subduction of Paleo Asian Ocean may have continued to late Carboniferous, and the Hengenshan basin is probably closed during the early Permian. Fractional crystallization is the main evolutionary process of the felsic magma, which has played an important role in the Mo mineralization. The coeval DP may have provided additional heat for the extended evolution of the crystallizing felsic magma chamber.

Re–Os and U–Pb geochronology of the Shazigou Mo polymetallic ore field, Inner Mongolia: Implications for Permian–Triassic mineralization at the northern margin of the North China Craton

The recently discovered polymetallic Shazigou Mo–W–Pb–Zn ore field is located at the northern margin of the North China Craton. This integrated metallogenic system is comprised of quartz vein mineralization in three deposits: Shazigou Mo–W, Jindouzishan Pb–Zn and Mantougou Pb–Zn. The total reserves are estimated to be 50kt Mo, 626t WO3, 244kt Pb and 150kt Zn. Molybdenite Re–Os dating of five quartz vein-type ores yielded a mean model age of 243.8±1.6Ma (MSWD=0.81) and hydrothermal zircons yielded a concordant U–Pb age of 245±2.6Ma (MSWD=0.65). These results suggest that the mineralization was formed in the early Triassic and could be related to Paleo-Asian Ocean subduction. Microthermometry and quartz fluid inclusion compositions indicate that fluids related to the Mo–W mineralization were mainly derived from magmatic sources and precipitated under relatively high temperature (280–340°C) and salinity conditions (6–9wt% NaCl equiv.), whereas subsequent Pb–Zn mineralization-related fluids may have been modified by metamorphic and meteoric waters. The discovery of the Shazigou ore field suggests conditions may be favourable for more extensive mineralization in the western Xilamulun Mo metallogenic belt at the northern margin of the North China Craton.

Different sources involved in generation of continental arc volcanism: The Carboniferous–Permian volcanic rocks in the northern margin of the North China block

New zircon U–Pb dating results on the Carboniferous–Permian volcanic rocks in the northern margin of the North China block (NCB) indicate their eruption during the Early Carboniferous to Late Permian from 347±3Ma to 258±1Ma and a slight decrease of the upper limits of the volcanic sequences from west to east. They have a main rock association of basalt, basaltic andesite, andesite, dacite, rhyolite, tuff, and tufaceous sandstone. Most of them have calc-alkaline compositions and exhibit variable SiO2 contents from 48.2wt.% to 77.1wt.%. There is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams, indicating that they are not bimodal in composition. The Carboniferous–Permian volcanic rocks exhibit subduction-related geochemical features such as negative Nb and Ta anomalies of mafic to intermediate rocks on primitive mantle-normalized diagrams, indicating they were formed in an Andean-type continental arc during southward subduction of the Paleo-Asian oceanic plate beneath the northern NCB. However, their wide range of whole-rock Sr–Nd and zircon Hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks. Geochemical and Sr–Nd–Hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust; however, most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust. There is an increasing input of crustal materials from the Carboniferous to Permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences. The results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments, mantle wedge and arc continental crust could be involved in their generation. For the Permian andesite, dacite and rhyolitic volcanic rocks, there is a striking correlation between their Sr–Nd–Hf isotopic compositions and those of the underlying basement. Sr–Nd–Hf isotopic differences between the Permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the North China craton suggest that mixture and homogenization of the magmas in the melting–assimilation–storage–hybridization (MASH) zone within this continental arc was incomplete, and incomplete homogenization of magmas is likely a common feature of many MASH zones within continental arcs. Our results on the volcanic rocks also show that the final closure of the Paleo-Asian Ocean along the Solonker Suture Zone has not been finished prior to the Middle Permian and the decease of the Paleo-Asian oceanic subduction beneath the northern NCB in its middle-western parts is a little earlier than that in its eastern part.

Melilite-derived mineral inclusions in chromite from the Gaositai complex: Implications for an extensional tectonic setting in Early Permian at the north North China Craton

The Early Permian mafic-ultramafic concentrically zoned Gaositai intrusion at Chengde, on the northern margin of the North China Craton (NCC), is a cumulative complex emplaced along a giant fracture that penetrates deeply into the continental lithosphere. Melt inclusions are present in chromite crystals from the inner dunite and chromitite zones of the Gaositai complex. The melt inclusions have experienced post-trap crystallization and resulted in multiple mineral phases, including melilite, garnet, phlogopite, magnesite and apatite, which can indicate the liquidus minerals of the primitive magma. The characteristics of the melilite+melanite+clinopyxene assemblage indicate that the primary parental magma was highly undersaturated and derived from an alkali-rich mantle source. The crystallization of phlogopite, magnesite and apatite suggests a primary magma rich in K, H2O and CO2. When compared with experimental data, the primary magma of the Gaositai intrusion is concordant with a kamafugite magma originating from partial melting of enriched mantle with H2O and CO2 at pressures greater than 2.7 GPa. This magmatic process would have been related to extensional thinning of the continental lithosphere. The Gaositai primary magmas have high Nb/La ratios, which are similar to those of ocean island basalts, but different from arc-related magmas. This suggests that the northern margin of the NCC was not an active continental margin of the Paleo-Asian Ocean subduction zone during the Early Permian: an extensional tectonic setting during the emplacement of the Gaositai intrusion is more likely.

Pyroxenite and peridotite xenoliths from Hexigten, Inner Mongolia: Insights into the Paleo-Asian Ocean subduction-related melt/fluid–peridotite interaction

The in situ major, trace-element and Sr-isotopic compositions of the peridotite and pyroxenite xenoliths from the Hexigten region in the Xing-Meng orogenic belt (XMOB) were examined to evaluate the influences and contributions of the Paleo-Asian Oceanic slab subduction on the lithospheric mantle transformation. Pyroxenes in the Type 1 pyroxenite exhibit low and variable Mg# (67–85) and relatively high 87Sr/86Sr ratios (0.7036–0.7053), indicating that they were formed by assimilation and fractional crystallization processes during a basaltic underplating event. The peridotite and Type 2 pyroxenite xenoliths sampled the lithospheric mantle and recorded subduction-related metasomatism. The mineral chemistries of the Type 1 peridotite suggest that the lithospheric mantle beneath this area suffered 1–15% melt extraction. Clinopyroxene (Cpx) in some Type 1 peridotites are characterized by high (La/Yb)N coupled with marked depletions in high field strength elements (HFSE) (Nb, Ta, Zr, Hf and Ti) and negative correlations between the low Ti/Eu (Nb/La) and 87Sr/86Sr ratios (0.7037–0.7055), suggesting metasomatism by subduction-related CO2-rich fluids. Olivine (Ol) and orthopyroxene (Opx) in the Type 2 peridotite are characterized by a relatively low Mg# but high Ni contents. In addition to the normal incompatible element-depleted Opx, Opx with enrichments in Rb, Ba, Th, U, Nb, Ta and LREE were observed, as well. The Mg# of incompatible element-depleted Opx exhibits weak zonations (i.e., decreasing from the cores to the rims). Cpx and Opx of the Type 2 pyroxenite exhibit similarly high Mg# and Ni contents. Rb, Ba, Th, U, Nb, Ta and LREE contents and 87Sr/86Sr ratios of the Cpx increase from the cores to the rims. Moreover, Opx in the Type 2 peridotite and Cpx in the Type 2 pyroxenite exhibit increased Nb/Ta ratios and Ni contents relative to those in the Type 1 peridotites. These observations collectively suggest a rutile-bearing eclogite-derived silicic melt–peridotite reaction as the origin for the Type 2 peridotite and pyroxenite. Considering the geological setting, it is suggested that the melt/fluid–peridotite interactions were caused by the Paleo-Asian Ocean subduction, which could have contributed significantly to the transformation of the lithospheric mantle beneath the northern margin of the NCC, as well.