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The application of whole-rock compositions to trace magma evolution or crystal-melt equilibrium may be called into question when foreign crystals are incorporated into host magmas. To address this challenge, establishing the origins (orthocrysts, antecrysts, xenocrysts, etc.) of minerals with variable textures in magmatic plumbing systems is necessary. In this paper, we describe complex oscillatory zoning patterns of clinopyroxenes (Cpx) from the Miocene Majuangou (MJG) alkali basalts from West Qinling, China. Our detailed petrographic, mineralogical, and geochemical analyses unravel the origins of various Cpx crystals with distinct textural patterns, thereby providing information about magma storage, recharging and/or mixing, and transportation, as well as the reactions between crystals and melts/fluids. Based on textural patterns, Cpx may be divided into four types: normal (Type-1 Cpx), simple oscillatory (Type-2 Cpx), complex oscillatory (Type-3 Cpx), and grains that lack zoning (Type-4 Cpx, suggested to be orthocrysts). Through the textural characterization of Cpx, the comparison between different types of Cpx, and the relationships between Cpx major compositions from different lithologies, we concluded that Type-1–3 Cpx cores are antecrysts or xenocrysts with diverse origins: primitive magma (Type-1 Cpx cores), magma mush (Type-2 Cpx cores), and crustal granulite (Type-3 Cpx cores). The zoning patterns and the compositions of these Cpx crystals indicate at least three batches of magmatic activity, i.e., the Batch-1 low-Mg# magma (Mg#: 47.4–53.3), the Batch-2 primitive magma (Mg#: 57.2–64.5), and the Batch-3 low-Mg# host alkali magma (Mg#: 47.2–54.6). Cpx-melt thermobarometry demonstrates that at least two crustal magma reservoirs existed in the magma plumbing system at depths of 30.1 and 40.9 km. The antecrystic/xenocrystic Cpx cores were captured by, continued to grow in, and subsequently reacted with ascending K-rich melt/fluid. The spongy textures in Cpx cores/mantles are attributed to this reaction, which may be expressed as: Melt 1 (primitive or evolved) + K-rich melts/fluids + Cpx (CaMgSi2O6) = K-feldspar (KAlSi3O8) + ilmenite (FeTiO3) + Melt 2 (derivative). The products of this reaction (K-feldspar and ilmenite) filled the sieves in the spongy zones of Type-1–3 Cpx. This detailed investigation of compositional and textural features of Cpx antecrysts/xenocrysts suggests that the interactions between various interconnected magma reservoirs are widespread beneath the magmatic plumbing system. Our study emphasizes the importance of the incorporation of foreign crystals and the Cpx-melt/fluid reaction in magmatic plumbing system, which can significantly modify the whole-rock compositions and lead to the formation of spongy textures without the need for fractures and cracks in minerals. [ABSTRACT FROM AUTHOR]

Olivine grains from Songshugou mylonitized peridotite massif record δ18O both lower and higher than in pristine mantle samples in North Qinling Orogen, Central China. Olivines from dunites exhibit large variations in δ18O (4.03–7.07‰), and some porphyroclasts display negative correlations between δ18O and forsterite content {Fo; [100×Mg/(Mg+Fe2+)]}. The porphyroclast cores have low-δ18O values, indicating that they formed in the oceanic lithospheric mantle prior to subduction. We attribute low-δ18O values to seawater-peridotite interaction under high-temperature conditions. The porphyroclast rims and small olivines exhibit high-δ18O values. These features suggest that high-δ18O olivines formed during mylonitization in the exhumation process. Olivines reacted with 18O-rich melt/fluids released from subducted altered oceanic basalts and continental sediments at low temperature (<610–680 °C). The 18O-rich melt/fluids selectively affected porphyroclast rims and small olivine grains. Unlike the olivines in the dunites, the olivines and orthopyroxenes in the harzburgites show limited variations in δ18O (4.21–5.45‰ and 5.5–5.8‰, respectively), due to orthopyroxene exchange with melt/fluid at a slower rate than the coexisting olivine. The preservation of the low-δ18O signature in olivines indicates a short residence time (<20 Ma) for subducting peridotites to mantle depths. [ABSTRACT FROM AUTHOR]

The scale and extent of isotopic heterogeneities in the Paleo-Asian oceanic mantle remain poorly constrained. Here, we report Os isotopic data for mantle peridotites from the Paleozoic Gangou and Kumishi ophiolites, which are preserved in the western Chinese Tianshan sutures (Xinjiang, China) and considered to sample the Paleo-Asian oceanic upper mantle. These heavily serpentinized peridotites display positive correlations between Al2O3, Ti, and Yb contents, indicating variable degrees of melt extraction from fertile mantle sources. However, enriched light rare-earth element patterns in the peridotites are inconsistent with simple residues of partial melting, thus reflecting the effect of post-melting melt percolation. Peridotites from both ophiolites show wide variation of Os isotopic compositions, several of which are characterized by highly unradiogenic 187Os/188Os values (0.117–0.118), reflecting Proterozoic melt depletion. Such feature is common in modern oceanic peridotites and most Phanerozoic ophiolites, meaning that Os-isotopically ultradepleted domains should be widespread in the Paleo-Asian oceanic mantle. The overall similarity of Os-isotope distribution between the Paleozoic Tianshan ophiolites and modern oceanic peridotites suggests that they derive from the same heterogeneous upper mantle reservoir. This might support the hypothesis that isotopically ultradepleted domains are inherent components of the convecting upper mantle and have not been sufficiently homogenized by convection. Alternatively, Re-depletion ages (TRD) for these ancient domains are broadly coincident with the major peaks in crustal zircon ages that are thought to reflect episodes of continental crust growth in the adjacent Yili–Central Tianshan microcontinents. This suggests that the ultradepleted mantle domains in some of the Tianshan ophiolites may derive from ancient subcontinental lithospheric mantle, which detached and were exhumed to form the substrate of ocean basins. [ABSTRACT FROM AUTHOR]

The Tianshan Belt, which lies in the southwest part of Central Asian Orogenic Belt (CAOB), is characterized by abundant ophiolites or ophiolitic mélanges, which are indicators of the existence of oceanic basin can be used for tracking the tectonic evolutionary history of the orogen. The Bindaban (BD) ophiolitic mélange is located in the Central Tianshan Belt, northwest China, which comprises mainly gabbros, diabases, basalts, basaltic lavas (greenschist), and cherts. This paper presents new zircon geochronology, whole‐rock major and trace elements, and Sr, Nd isotope data for mafic rocks in the BD ophiolitic mélanges. In the eastern part of BD ophiolitic mélanges, one normal (N)‐MORB‐type basalt has a U–Pb zircon age of 265.2 ± 1.8 Ma, and one enriched‐MORB‐type dolorite in the western part of BD ophiolitic mélanges has an age of 425.4 ± 3.9 Ma, respectively, indicating either an existing ocean basin from continental rifting to normal oceanic spreading with a long time span from late Silurian to Permain, or two separate ocean basins that formed in late Permian and late Silurian during the Paleo‐Tianshan Ocean evolution. In particular, the 265.2 ± 1.8 Ma age of N‐MORB‐type basalt from BD ophiolitic mélange suggests the youngest oceanic basin (late Permian) existed in southwest CAOB, which reveals that Paleo‐Asian Ocean might last to late Permian, therefore finally closed in or after the late Permian in the North Tianshan area. [ABSTRACT FROM AUTHOR]

The time duration and evolution of the major North Tianshan Ocean (also the Junggar Ocean) in the Chinese Tianshan orogenic collage remain poorly constrained. Here we report new geochronological and geochemical data of Carboniferous volcanic rocks documented from the North Tianshan accretionary complex in the Gangou area (Xinjiang, NW China). The studied rocks include basaltic andesites and andesites, with one andesite yielding a zircon U–Pb age of 332 ± 4 Ma. This reflects nearly synchronous eruption of the volcanics with the formation of host Early Carboniferous Yamansu Formation. These rocks are mostly tholeiitic in compositions, with a few samples transitioning to calcalkaline. All the samples show enriched LREE and LILE but depleted HFSE, a ubiquitous subduction signature observed in supra‐subduction zone volcanics. The positive εNd(t) (+5.2 to +6.7) and low initial 87Sr/86Sr (0.7038 to 0.7040) in these rocks suggest derivation from a MORB‐depleted peridotite source. However, the overall low MgO contents (and Mg#) in these rocks compared with typical high‐Mg andesites indicate that they are less likely formed via simple melting of metasomatized mantle wedge peridotites. Instead, partial melting of mafic‐ultramafic metasomes (e.g., pyroxenites) formed by slab melts/fluids‐peridotite interactions may explain these rocks. The studied rocks share many geochemical similarities with those Early Carboniferous volcanic rocks widely documented from the Chinese Western Tianshan and those volcanics from typical continental arcs (e.g., the Gangdese arc). Therefore, we suggest the volcanic rocks, combining with recent results from the North Tianshan ophiolites, tend to support a continued southward subduction of the major North Tianshan Ocean from the Cambrian to the Carboniferous periods. [ABSTRACT FROM AUTHOR]

The formation of the Shiquanhe–Yunzhug–Namu Tso ophiolite mélange zone (SNMZ) within the Lhasa Terrane, Tibetan Plateau, is key to understanding the Mesozoic tectonic evolution of this terrane, which remains controversial. We show that the Yunzhug ophiolite in the central segment of the SNMZ formed at ~ 150 Ma, based on U–Pb dating of zircons from a gabbroic sample in a well-developed sheeted dike complex. Geochemically, these mafic rocks are dominated by E-MORB-type compositions, along with minor amounts of rocks with P-MORB-type compositions. The samples also exhibit high ε Nd (t) values and lack negative Nb and Ta anomalies. Data for all the samples plot within the MORB array on a Th/Yb–Nb/Yb diagram. Therefore, these mafic rocks most likely formed in either a slow spreading oceanic setting or an embryonic ocean, and not in a back-arc basin as has been previously assumed. Taking into account the regional geology, we propose that the Yunzhug ophiolite is part of a distinct ophiolitic belt and represents material formed in an embryonic ocean within the Lhasa Terrane, which provides new insights into the Jurassic tectonic evolution of the Lhasa Terrane. [ABSTRACT FROM AUTHOR]

The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re-Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re-Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial Os/Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γ(t) values (+20 to +202). In contrast, the Re-Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial Os/Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γ(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re-Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts. [ABSTRACT FROM AUTHOR]

Abstract: It is generally believed that Miocene adakitic rocks in the Lhasa terrane (block) of Southern Tibet were derived from a thickened lower crust, and that their compositions will therefore reflect the geochemical characteristics of their lower-crustal source(s). Adakitic rocks in the western part of the Lhasa terrane possess geochemical and Sr–Nd isotopic characteristics that are clearly different from those in the eastern part of the Lhasa terrane; the former display a more enriched Nd–Sr isotopic signature together with an older model age relative to the latter, suggesting that the compositions of the lower crust under the western and eastern parts of the Lhasa terrane also differ. In addition, the adakitic rocks in the Himalayas on the southern side of the Indus–Yalu suture (IYS) have similar geochemical characteristics and Sr–Nd isotopic compositions to the adakitic rocks of the western Lhasa terrane, but differ from rocks in the eastern Lhasa terrane and from igneous rocks derived from crust beneath the Himalayas. We suggest that the lower (and/or middle) crustal materials beneath the western Lhasa terrane probably extend to the southern side of the IYS due to channel flow. If mid-lower crustal flow has occurred below Southern Tibet, the southeastward-moving anatectic mid-lower crustal material was probably derived from below the western part of the Lhasa terrane rather than the eastern. Moreover, the emplacement ages of adakitic rocks from Mayum, on the southern side of the IYS, imply that southeastward ductile flow of mid-lower crust beneath western Lhasa may have occurred as early as 17Ma. [Copyright &y& Elsevier]

Abstract: It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks of the Tibetan Plateau were generated by partial melting of enriched mantle. This paper reports new data regarding the elemental and isotopic compositions of the KonglongXiang volcanic rocks (KVR; K2O/Na2O>2 and low MgO contents) in the middle–western Lhasa terrane, Tibetan Plateau. Ar–Ar isotope dating of a sample from the KVR yields an age of 21.3Ma, indicating that the KVR lavas erupted during the Early Miocene. The KVR consists mainly of trachyte and rhyolite. A comparison of the KVR with Cenozoic potassic volcanic rocks of the Tibetan Plateau and the results of melting experiments involving K-rich volcanic rocks, as well as other K-rich volcanic rocks from throughout the world, reveals that the KVR trachytes originated from a mafic lower crust containing K-rich minerals rather than from a mantle source. The KVR rhyolites were probably derived from the middle–upper crust. Based on age data, compositional data, and the regional tectonic setting, we propose that the formation of the KVR and associated N–S grabens was triggered by Early Miocene E–W regional extension in the area of the southern Tibetan Plateau. [Copyright &y& Elsevier]

Abstract: The Anzishan ophiolite, a typical ophiolitic block of early Carboniferous age in the Mian-Lue suture zone of the Qinling Mountains, central China, consists of amphibolites/metabasalts, gabbros and gabbroic cumulates. All of these rocks, as well as those in the Hunshuiguan-Zhuangke (HZ) block, have compositions similar to normal MORB and back-arc basin basalts (BABB) with high εNd( t ) values, indicating that they were derived from a depleted mantle source. The Mian-Lue suture zone also contains blocks of other lithologies, e.g., rift volcanic rocks in the Heigouxia block and arc volcanic rocks in the Sanchazi block. Although they are in fault contact with each other, the presence of these different blocks in the Mian-Lue suture zone may represent a complete Wilson cycle, from initial rifting to open ocean basin to final subduction and continent-continent collision, during the late Paleozoic-early Triassic. In this region, the North and South China Cratons were separated by Paleo-Tethys at least until the early Carboniferous, and final amalgamation of both cratons along the Qinling orogenic belt took place in the Triassic. [Copyright &y& Elsevier]

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high &epsiv;Nd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin–island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian. [Copyright &y& Elsevier]

[Display omitted] • Pyrite have markedly heavier Mo isotopes (up to ~2‰). • Fractionation crystallization exert insignificant influence on Mo isotopes fractionation. • Significant Mo isotopic fractionation occurs between pyrite and molybdenite in Pulang porphyry Cu deposit. The behavior of molybdenum isotopes in magmatic–hydrothermal systems is poorly known. Late Triassic andesitic rocks from the Disuga and Lannitang areas, and contemporaneous porphyry–skarn Cu polymetallic deposits (e.g., Pulang, Xuejiping, Yaza, Hongshan, and Langdu), from the Zhongdian region, southeastern Tibetan Plateau, China, present an ideal opportunity to study Mo isotope fractionation in magmatic–hydrothermal systems. In this study, we report Mo isotopic compositions, analyzed by multi-collector–inductively coupled plasma–mass spectrometry, of ore-forming porphyries and hydrothermal chalcopyrite, pyrite, and molybdenite from the Pulang deposit, and coeval barren andesites from the Disuga and Lannitang areas. The δ98Mo values of the Pulang porphyries (−0.59‰ to −0.05‰) are lower than those of the Disuga and Lannitang andesites (−0.29‰ to −0.03‰). In the Pulang deposit, whole-rock δ98Mo values are lower (−0.59‰ to −0.05‰) than those of pyrite (+0.73‰ to +2.09‰). Chalcopyrite has lower δ98Mo values (−0.16‰ to +1.06‰), than pyrite, but similar δ98Mo values to molybdenite (−0.18‰ to +0.06‰). We infer, tentatively, that fractional crystallization was not a major control on the Mo isotopic composition of the Pulang magmatic–hydrothermal system. Heavy Mo isotopes partitioned into pyrite relative to molybdenite, and 98Mo is thought to partition into Mo6+-bearing species relative to Mo4+-bearing species. Therefore, if MoO 4 2 − was the predominant Mo species in exsolved hydrothermal fluids, then formation of pyrite from these fluids explains the high δ98Mo values of pyrite. We infer that the chalcopyrite from the Pulang deposit contains micron-scale molybdenite inclusions, which partially explains the comparable δ98Mo values of chalcopyrite and molybdenite. Pyrite has a strong influence on Mo isotope fractionation, at least in the magmatic–hydrothermal system described here. [ABSTRACT FROM AUTHOR]

The Junggar orogenic collage in Central Asia is characterized by the accretion of multiple Paleozoic intra-oceanic arc terranes developed within the Junggar Ocean, yet magmatic records relating to the construction of these ancient oceanic arcs are largely disrupted. Here, we report new petrological and geochemical data of mafic-ultramafic rocks documented from the Early Cambrian Saleinuohai ophiolitic mélange in southern West Junggar. The mafic rocks include dolerites and basalts, which show geochemical features (e.g., lower TiO 2 and Zr relative to MORB, depleted LREE, and high ε Nd(t)) resembling those of forearc basalts from the Izu-Bonin-Mariana arc. Thus, they may represent the upper section of newly-accreted oceanic crust formed during subduction initiation, upon which a nascent arc is soon built. In contrast, the ultramafic rocks include dunite, wehrlite, and clinopyroxenite, which display cumulate textures with a crystallization order of olivine-clinopyroxene-plagioclase. Similar cumulate sequences are commonly observed in island-arc environments, presumably crystallized under a relatively hydrous and high-pressure condition. The calculated equilibrium melts for these cumulates further resemble those adakitic rocks previously reported from the Saleinuohai ophiolitic mélange. Thus, they may represent the basal section of mature island arc crust that formed during intracrustal fractionation. Our data, together with previous results, provide a complete magmatic record of the development of an Early Cambrian intra-oceanic arc within the Junggar Ocean. • Forearc basalts from West Junggar ophiolitic mélanges indicate subduction initiation. • Ultramafic cumulates from the same mélanges represent the basal part of a mature arc. • A complete magmatic record of an Early Cambrian intra-oceanic arc was described. [ABSTRACT FROM AUTHOR]

Continental convergent margins are commonly considered as significant sites for understanding crustal reworking and growth, but the geodynamic processes involving crustal growth remain highly debatable. Triassic intermediate–felsic intrusive rocks are widespread in the northwestern segment of the West Qinling orogenic belt on the NE Tibetan Plateau. These rocks well document the evolution of one branch of the Paleo-Tethys oceans (i.e., the A'nyemaqen Ocean) in East Asia. Here we present a comprehensive study of zircon U Pb and Lu Hf isotopic data, along with whole-rock elemental and Sr Nd isotopic data to decipher the origins of intermediate–felsic intrusions and crustal evolution of the West Qinling in the Triassic. Two episodes of magmatism were detected in this study, i.e., the early high-Mg diorite porphyry–granodiorite–granodiorite porphyry assemblage (Episode #1, ~244–230 Ma), followed by a later monzogranite–granodiorite–granite porphyry assemblage (Episode #2, ~228–203 Ma). These two episodes of magmatism are interpreted as products related to oceanic subduction and continental collision, respectively. In Episode #1, the mafic magmatic enclaves were mainly mantle-derived and were indicative of magma mixing. The high-Mg diorite porphyries were likely derived from partial melting of the mantle wedge metasomatized by sediment-derived melts, whereas the coeval granodiorites and granodiorite porphyries mainly originated from partial melting of amphibolites with significant addition of mantle-derived components. By contrast, the granodiorite porphyries in Episode #2 probably originated from the lower crust-sourced melts with insignificant mantle contribution. The contemporaneous granodiorites and monzogranites were generated by partial melting of the crustal amphibolites or metagreywackes. The proportion of positive zircon ε Hf (t) values decreased from Episode #1 to Episode #2, indicating a decreasing tendency of the mantle or juvenile component contribution. Our study suggests that the addition of mantle-derived magmas to the continental crust during oceanic subduction processes played an important role in crustal growth. [Display omitted] • The studied intermediate-felsic plutons were emplaced at 244–203 Ma in West Qinling. • The juvenile components decreased from the oceanic to the continental collision. • These plutons record continental crust growth during oceanic subduction stage. [ABSTRACT FROM AUTHOR]