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The Ailaoshan orogenic belt is one of the most significant orogenic belts in the southeastern margin of the Qinghai–Tibet Plateau. The widely developed magmatic rocks in this belt preserve the multi-stage tectonic evolution records of the South China Plate. As an important response to the Rodinia breakup tectonic event, the study of Neoproterozoic magmatic rocks in the area is of great significance for reconstructing the Neoproterozoic tectonic process of the Ailaoshan orogenic belt and the tectonic evolution of the South China Plate. Petrology, geochemistry, zircon U-Pb, and Lu-Hf isotopes of the Daping pluton in the Ailaoshan orogenic belt are studied in this paper. The Daping pluton is mainly divided into gabbros and granites. Gabbros and granites belong to the sub-alkaline series, which are relatively enriched in large ion lithophilic elements and depleted in high-field strength elements. The ΣREE contents of the gabbro are low with enrichment in LREEs and depletion in HREEs, and the degree of differentiation of light and heavy rare earth is low, with positive δEu and weak negative δCe anomalies. The ΣREE contents of the granite are low with enrichment in LREEs and depletion in HREEs, and the degree of light and heavy rare earth differentiation is high, with medium–weak negative δEu and weak positive δCe anomalies, suggesting an A2-type granite with A1-A2 transition characteristics. The weighted average age of the gabbro is 816.1 ± 4.1 Ma (MSWD = 0.11), with zircon εHf(t) values of −7.5–5.5. The magma source is a mixture of an ancient crust source and a new mantle source; the weighted average age of the syenogranite is 783.7 ± 8.1 Ma (MSWD = 1.4), with zircon εHf(t) values of −4.3–0.4. The magma source is mainly ancient crustal material (Ailaoshan Group), mixed with a small amount of mantle-derived material; the weighted average age of the monzogranite is 754.8 ± 6.1 Ma (MSWD = 3.0), with positive zircon εHf(t) values of 1.65–10.36. The magma source is a mixture of a large number of mantle-derived materials and a small number of crust-derived materials (Ailaoshan Group). The Daping pluton was formed in the transitional tectonic environment from post-collision to intraplate continental margin rift, corresponding to the Rodinia breakup process in the western margin of the South China Plate. [ABSTRACT FROM AUTHOR]

The Ailaoshan orogenic belt, located in the SE margin of the Qinghai–Tibet Plateau, is an important Paleo-Tethys suture zone in the eastern margin of the Sanjiang Tethys tectonic domain. The areas of Mojiang and Zhenyuan, located in the middle part of the Ailaoshan orogenic belt, are the key parts of the Ailaoshan Paleo-Tethys Ocean closure and collision orogeny. The rhyolites outcropped in the Mojiang area, and the granite porphyries outcropped in Zhenyuan area, are systematically studied for petrology, isotope geochemistry and geochronology. The Zircon U-Pb geochronology of rhyolites and granite porphyries give weighted average ages of 253.4 ± 4.2 Ma and 253.3 ± 2.0 Ma, respectively, both of which were formed in the late Permian period. The rhyolites belong to potassic calc-alkaline to subalkaline series. The patterns of the rare earth elements (REE) show a right-inclined seagull-type distribution, and the trace elements plot is right-inclined. The granite porphyries are high potassic calc-alkaline to subalkaline. The REE patterns show a right-inclined distribution, and the trace elements plot is right-inclined, which is consistent with the typical patterns observed in the crust. The peraluminous, highly differentiated and high ASI values suggest that rhyolites and granite porphyries are S-type granites. The zircon εHf(t) of the rhyolites range from −7.22 to −0.72, and two-stage Hf zircon model ages are (TDMC) 1771–2352 Ma, indicating that the magma source area is mainly crust-derived. The zircon εHf(t) of the granite porphyries range from −0.97 to 4.08, and two-stage Hf zircon model ages are (TDMC) 1336–1795 Ma, indicating that the magma is derived from a depleted mantle source and the partial melting of ancient crustal materials. The rhyolites and granite porphyries were possibly formed in the syn-collisional tectonic setting during the late Permian, and their ages limited the time of the final closure of the Ailaoshan Ocean and the initiation of collisional orogeny. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogenic Belt (EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism. Here we report the petrology, bulk geochemistry, zircon U–Pb dating and, Lu–Hf and Sr–Nd isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U–Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450–430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their ( 87 Sr/ 86 Sr) i values range from 0.7059 to 0.7085, ε Nd ( t ) values from –1.6 to –6.0 and the zircon ε Hf ( t ) values show large variations from +9.1 to –8.6 with Hf model ages ( T DM2 ) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas. [ABSTRACT FROM AUTHOR]

Early Cretaceous high-Mg adakitic rocks from central Eastern China provide important insights into the thinning mechanism of the over-thickened lithosphere in the Yangtze Block (YB) as well as the North China Block (NCB). The Tanlu fault (TLF), located between the North China and Yangtze Blocks, and has been considered as a prominent pathway of magmas and fluids that resulted in lithosphere thinning of the YB during the Mesozoic–Cenozoic. Here we report the petrology, whole-rock geochemistry, zircon U–Pb geochronology, in situ Hf isotopes, and whole-rock Sr–Nd–Pb isotopes of four high-Mg adakitic intrusions along the TLF in northeastern Langdai. These adakitic intrusions consist of monzodiorite, quartz monzonite porphyry, and quartz monzodiorite. Zircon LA-MC-ICPMS analyses of five samples yield weighted mean 206 Pb/ 238 U ages of 127.58 ± 0.80, 126.90 ± 0.81, 120.71 ± 0.64, 122.75 ± 0.57, and 129.2 ± 1.1 Ma, indicating their emplacement during the Early Cretaceous. The intrusions have intermediate SiO 2 (53.18–65.48 wt%) and high potassium (K 2 O = 3.07–3.95 wt%; Na 2 O/K 2 O = 1.02–1.26) and are classified as shoshonitic to high-K calc-alkaline series. They are characterized by high MgO (1.80–7.35 wt%), Mg # (50–65), Sr (591–1183 ppm), Ni (20.3–143.0 ppm), and Cr (51.40–390.0 ppm) contents, high (La/Yb) N (11.60–28.33) and Sr/Y (27.9–113.5) ratios, and low Y (7.79–22.4 ppm) and Yb (0.60–2.01 ppm) contents, comparable with high-Mg adakites. The samples are enriched in light rare earth elements but depleted in heavy rare earth elements and high field strength elements with slightly negative to positive Eu anomalies ( δ Eu = 0.81–1.30), resembling the features of high-Mg adakitic rocks. Their whole-rock ε Nd( t ) = −16.2 to −15.0, initial ( 87 Sr/ 86 Sr) i = 0.7060–0.7074, low radiogenic Pb ( 206 Pb/ 204 Pb( t ) = 16.208–16.509, 207 Pb/ 204 Pb( t ) = 15.331–15.410, and 208 Pb/ 204 Pb( t ) = 36.551–36.992), and zircon ε Hf( t ) = −36.6 to −16.6 suggest magma derivation from a continental crustal source. The geochemical and isotopic features, in combination with existing geological data, suggest that the intrusions are high-Mg adakites and, by comparison with contemporaneous intrusions in Eastern China, were likely to have been formed by partial melting of over-thickened basaltic lower crust following the delamination of eclogitic lithosphere during the Late Jurassic to Early Cretaceous. It is possible that the TLF played a key role in lithospheric delamination and thinning and in the generation of the high-Mg adakitic rocks. [ABSTRACT FROM AUTHOR]

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO tot and Cr 2 O 3 of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb) N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu ∗ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε Nd ( t ) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc. [ABSTRACT FROM AUTHOR]

Abstract: Cretaceous magmatism in southern Lhasa subterrane, Tibetan plateau has been investigated for many years and a series of models have been proposed to illustrate their petrogenesis and geodynamic implications. But rare work has been done on the Cretaceous magmatism in central Lhasa subterrane. Here we report the petrology, zircon in situ U–Pb geochronology, Hf isotopes, trace element, and whole-rock elements and Sr–Nd isotopic geochemical data of the host granodiorites, and gabbroic and dioritic enclaves in Mamba area, central Lhasa subterrane. Zircon U–Pb dating for a Mamba host granodiorite yields a crystallization age of ~84Ma, with in situ Hf isotopic analyses for 18 spots of the same zircons of εHf (t) ranging from −7.5 to −0.3. A dioritic enclave (85.2Ma) is coeval with the host granodiorite and shows similar zircon Hf isotopic compositions (εHf (t)=−4.0 to +0.2). Mamba granodiorites (SiO2 =66.6–67.5wt.%) and dioritic enclaves (SiO2 =53.9–57.6wt.%) are high-K calc-alkaline, and a gabbroic enclave is shoshonitic (K2O=2.81%). All these samples are metaluminous, and enriched in large ion lithophile elements (LILEs, such as Rb, Ba, K, U, Th) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and Zr). The host granodiorites are enriched in light rare earth elements (REEs), depleted in heavy REEs with weakly negative Eu anomalies (δEu=0.86–0.88), with high Al2O3 (15.0–15.7wt.%), high Sr/Y ratio (58.1–68.3) and Sr (680–755ppm), and low Y (10.8–13.0ppm) abundance, suggesting adakitic affinities. Mamba adakitic granodiorites, gabbroic and dioritic enclaves exhibit homogeneous Sr isotopes ((87Sr/86Sr)i =0.7066–0.7067, 0.7073, and 0.7067, respectively) and Nd isotopes (εNd(t)=−5.7 to −4.4, −4.0, and −3.6, respectively). These geochemical features allowed us to conclude that the adakitic host granodiorites and mafic (gabbroic–dioritic) enclaves were derived from magma mixing between ancient thickened lower crust and enriched fluid-metasomatized mantle. The distance between Mamba and the suture zone was more than 200km when the intrusives emplaced at ~85Ma, which implies that these rocks cannot be resulted from the mid-ocean ridge subduction. Combining of the intra-plate environment indicated by the gabbroic enclave of this study, the presence of the coeval bimodal igneous rocks in the similar latitude in central Lhasa subterrane, and other records in late Cretaceous sedimentary basin, the Mamba ~85Ma magmatism were attributed to the back-arc extension of Neo-Tethyan Ocean. [Copyright &y& Elsevier]

The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 (71.68%-72.47%), K2O (4.73%-5.54%), total alkali (K2O + Na2O = 8.21%-8.53%), K2O/Na2O ratios (1.36-1.94) and low P2O5 (0.13%-0.17%), with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, Sr, Ti, P and Eu. They are highly fractionated, slightly pet aluminous I-type granite. The two samples of the Kejie pluton give a large variation of εHf( t) values (-5.04 to 1.96) and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock (801°C-823°C) show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization. [ABSTRACT FROM AUTHOR]

Abstract: Lincang granite is a batholith located in the Sanjiang region and is an important research subject for understanding subduction and collision during the Paleo-Tethyan period. It is widely exposed in the Lincang Terrane and extends south into Burma. Based on various petrological and geochemical investigations performed from south to north across the Lincang granite, a new set of data, which includes zircon chronological and Hf isotopic data, is presented to discuss the origin of the Lincang granite and its tectonic significance. The Lincang granite is a peraluminous, high-K calc-alkaline body with sub-parallel REE patterns and a strong negative Eu anomaly. This anomaly is characteristic of a post-collision peraluminous S-type granitic batholith. The 200–230Ma formation age of the Lincang granite was determined using LA-ICP-MS zircon U–Pb dating. Thus, it has been confirmed that the granite formed during the late Triassic period, and the formation process lasted for approximately 30Ma. Geochemical and isotopic compositions indicate that the primary magma of Lincang granite most likely originated from a crustal source, and possibly underwent an assimilation–fractionation crystallization (AFC) process during its emplacement. The Lincang granite formed during the continental collision between the Baoshan–Gengma Terrane and the Lanping–Simao Terrane after the northeast subduction of the Paleo-Tethyan Oceanic Plate. Therefore, the late Triassic Lincang granite is important evidence for the closure of the Paleo-Tethyan Ocean. [Copyright &y& Elsevier]

Abstract: Ultrapotassic lavas having distinct geochemical compositions (K2O/Na2O >2, K2O >3%, and MgO >3%) are common and widespread on the Tibet Plateau, where they are closely linked to N–S-trending normal faults. The Tibetan ultrapotassic rocks range in age from ~8 to 24 Ma, slightly older than the spatially associated potassic rocks (10–22 Ma). These lavas consist mainly of trachyte, trachyandesite, basaltic trachyandesite, phonolite and tephriphonolite. They have high light rare earth element (LREE) and large ion lithophile element (LILE) concentrations, but are low in high field strength elements (HFSE). They are characterized by having extremely radiogenic Sr (87Sr/86Sr(i ) =0.710719 to 0.736451) and Pb isotopes (206Pb/204Pb=18.449–19.345, 207Pb/204Pb=15.717–15.803, 208Pb/204Pb=39.443–40.168) with unradiogenic Nd isotopes (ε Nd(0 ) =−7.6 to −15) and old Nd model ages (T DM =1.3–2.1 Ga), similar in character to the Himalaya crystalline basement. Their isotopic character is interpreted to reflect subduction of the Indian plate beneath the Lhasa terrane, leading to a highly contaminated mantle source. Delamination of the subducted oceanic/continental materials may have played an essential role in the genesis of the ultrapotassic rocks in the Lhasa terrane. The available geological, geochemical and geophysical data favor a model in which the Indian plate was subducted under southern Tibet. [Copyright &y& Elsevier]

Abstract: The relationship between the breakup of eastern Gondwanaland and the Kerguelen plume activity is a subject of debate. The Cona mafic rocks are widely exposed in the Cona area of the eastern Himalaya of south Tibet, and are studied in order to evaluate this relationship. Cona mafic rocks consist predominantly of massive basaltic flows and diabase sills or dikes, and are divided into three groups. Group 1 is composed of basaltic flows and diabase sills or dikes and is characterized by higher TiO2 and P2O5 content and OIB-like trace element patterns with a relatively large range of ɛNd(T) values (+1.84 to +4.67). A Group 1 diabase sill has been dated at 144.7±2.4 Ma. Group 2 consists of gabbroic sills or crosscutting gabbroic intrusions characterized by lower TiO2 and P2O5 content and “depleted” N-MORB-like trace element patterns with relatively higher, homogeneous ɛNd(T) values (+5.68 to +6.37). A Group 2 gabbroic diabase dike has been dated at 131.1±6.1 Ma. Group 3 basaltic lavas are interbedded with the Late Jurassic–Early Cretaceous pelitic sediments; they have compositions transitional between Groups 1 and 2 and flat to slightly enriched trace element patterns. Sr–Nd isotopic data and REE modeling indicate that variable degrees of partial melting of distinct mantle source compositions (enriched garnet–clinopyroxene peridotite for Group 1 and spinel-lherzolite for Group 2, respectively) could account for the chemical diversity of the Cona mafic rocks. Geochemical similarities between the Cona mafic rocks and the basalts probably created by the Kerguelen plume based on spatial–temporal constraints seem to indicate that an incubating Kerguelen plume model is more plausible than a model of normal rifting (nonplume) for the generation of the Cona mafic rocks. Group 1 is interpreted as being related to the incubating Kerguelen plume–lithosphere interaction; Group 2 is likely related to an interaction between anhydrous lithosphere and rising depleted asthenosphere enriched by a “droplet” originating from the Kerguelen plume, while Group 3 may be attributed to thermal erosion resulting in the partial melting of lithosphere during the long-term incubation of a magma chamber/pond at a shallow crustal level. The Cona mafic rocks are probably related to a progressively lithospheric thinning beneath eastern Gondwanaland from 150–145 Ma to 130 Ma. Our new observations seem to indicate that the Kerguelen plume may have started its incubation as early as the latest Jurassic or earliest Cretaceous period and that the incubating Kerguelen plume may play an active role in the breakup of Greater India, eastern India, and northwestern Australia. [Copyright &y& Elsevier]

Abstract: The distribution of peraluminous granites in Tibet is treated on the basis of the tectonic zones in which they occur, their spatial and temporal distribution, the peak of magmatic activity and the volume of magma intruded. Magmatic activity, with the intrusion of peraluminous granites, was initiated during the Early Jurassic and culminated in the middle Miocene, especially between 20 and 10Ma. Rock types include tourmaline, muscovite and two-mica granites. Magmatic activity in the Gangdise Belt migrated from the east to west and from the south to the north. Episodes of tectonic evolution for the lithosphere of the Qinghai–Xizang (Tibet) Plateau, deduced from peraluminous granite intrusion are: (1) Latest Triassic to Early Jurassic (208–157Ma), representing the subduction phase of the Bangong Co–Nu Jiang oceanic zone; (2) Late Jurassic to Early Cretaceous (157–97Ma), representing the subduction and collision phases of the Bangong–Nu Jiang oceanic zone; (3) Late Cretaceous to early Paleocene (97–65Ma), representing the subduction and initial collision phases of the Yarlung Zangbo oceanic zone ; (4) Paleocene to Eocene (65–40Ma), representing the major collisional stage of the Yarlung Zangbo Oceanic zone and the formation of crust-derived granites; and (5) Oligocene to Recent, representing an intense intracontinental convergence phase. [Copyright &y& Elsevier]

Abstract: The leucogranites in the Baoshan Block of the Tethyan belt in Western Yunnan, are composed mainly of two-mica granite with subordinate muscovite granite. Here we present zircon U–Pb ages from four intrusions that show ages of 448–476Ma suggesting that these rocks were emplaced during the Ordovician. The leucogranites are high-K calc-alkaline and strongly peraluminous, with K2O/Na2O>1 and A/CNK=1.12–1.54. These rocks are enriched in large-ion lithophile elements (LILEs) and light rare-earth elements (LREEs) [(La/Yb)N =1.13–32.4] and Pb, and are depleted in high field-strength elements (HFSEs). They show similar chondrite-normalized REE patterns, with negative Eu anomalies (Eu/Eu*=0.03–0.46). A wide range of zircon εHf(t) values (−9.6 to −2.6) and varying Hf-isotope crustal model ages (2.1–1.6Ga) are also observed. The geochemical signatures indicate that the leucogranites are S-type granites derived mainly from the anatexis of ancient crustal materials. The ages, geochemistry and tectonics in the Baoshan Block and the Lhasa Terrane are closely comparable, suggesting that the Baoshan Block might represent part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean. The Pinghe granites of the early phase in the Baoshan Block which are coeval with the Cambrian magmatism (ca. 492Ma) identified in the central and southern Lhasa subterranes can be interpreted as products of the slab break-off associated with the subduction of proto-Tethyan oceanic lithosphere. However, the late leucogranite stocks are analogous to the North Himalayan leucogranites, which formed in a short-lived extensional setting caused by the slab break-off associated with the subduction–collision system. [Copyright &y& Elsevier]

The tectonic transition from oceanic subduction to continental collision is a fundamental process during orogenesis. Yet, the nature of this transition and its deep geodynamic processes for the southern Tibetan Plateau remain contentious. This study presents zircon U-Pb-Hf isotopes, whole-rock geochemistry, and Sr Nd isotopes for a quartz diorite intrusion and bimodal dykes from Dagze, combined with compiled data from the southern Lhasa subterrane, to reveal the geodynamic processes of late-stage of Neo-Tethyan subduction. Most of Dagze samples were emplaced between 66 and 64 Ma, while muscovite granite aplite dykes were emplaced between 64 and 60 Ma. They exhibit relatively uniform, depleted Sr Nd isotopes, with ε Nd (t) values ranging from +2.2 to +4.0, positive zircon ε Hf (t) values, and young depleted mantle model ages. Both exhibit compositional characteristics of subduction-related arc magma, marked by the enrichment of large ion lithophile elements and the depletion of high field strength elements. The mafic dykes with relatively low SiO 2 , high MgO, Cr, Ni, possibly originated from a fluid-metasomatized depleted mantle. The quartz diorite intrusion with low MgO, Cr, and Ni was generated by partial melting of sub-arc lithospheric mantle and experienced subsequent fractional crystallization of amphibole, clinopyroxene and orthopyroxene. The granitic dykes originated from partial melting of Gangdese juvenile crust and experienced fractional crystallization dominated by plagioclase. We suggest that the slab rollback and continental collision conjointly controlled the evolution of the latest Cretaceous to Paleocene Himalayan–Tibetan orogeny. Slab rollback of Neo-Tethys commenced around 70 Ma, triggering the partial melting of depleted mantle and juvenile crust, and this process resulted in the formation of Dagze diorites and bimodal dykes with arc affinity. • Diorite intrusion and mafic dykes originated from a fluid-metasomatized depleted mantle. • Granitic dykes originated from Gangdese juvenile crust and experienced fractional crystallization. • Slab rollback of the Neo-Tethys oceanic slab commenced around 70 Ma. [ABSTRACT FROM AUTHOR]

[Display omitted] • Monzonite was formed from melts derived from a mantle wedge after fractionation. • The adakitic features of granite are formed by amphibole-dominated fractionation. • Nenjiang ocean experienced slab rollback at ∼375 Ma, and closed at ∼320 Ma. Disputes regarding the subduction process, closure time, and the location of the Nenjiang Ocean have arisen due to limited exposure of late Paleozoic magmatic rocks in the Xing'an block. In this study, we present new zircon U-Pb age, zircon Hf isotope, zircon trace elements and whole rock geochemistry to reveal the evolution process of the Nenjiang Ocean in the late Paleozoic. The Heihe monzonite samples originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids and experienced fractional crystallization of plagioclase and amphibole. The Heihe granites show adakitic affinity, with low contents of compatible elements and U-shaped chondrite-normalized REEs patterns, indicating that the adakitic characteristics of the Heihe granites were formed by amphibole-dominated fractional crystallization. Geochemical modeling shows that the Heihe granite can be generated at a degree of 30–75 % amphibole-dominated fractional crystallization. The geochemical characteristics and genetic type of Heihe granites, associated with coeval sedimentary strata, imply extensional tectonic setting in late Devonian-early Carboniferous. By analyzing the spatiotemporal distribution and geochemical data of the 300–400 Ma magmatic rocks in the Xing'an block, and by conducting locally weighted regression fitting, we propose a geodynamic model. According to this model, the Nenjiang oceanic slab began to rollback at approximately 375 Ma, resulting in the production of the Heihe monzonite and granite. Subsequently, the closure of the Nenjiang Ocean, and collision between the Xing'an block and the Songnen block occurred at around 320 Ma. [ABSTRACT FROM AUTHOR]

The northern Guizhou area, located near the southwestern margin of the Yangtze Block and south of the Sichuan Basin, is a promising area for shale gas exploration and development. The discovery of a natural gas accumulation in the Lower Silurian Songkan Formation in the study area is the first discovery of natural gas in this stratum in China, marking an exciting breakthrough in natural gas exploration in northern Guizhou area. This paper presents an investigation of this accumulation using gas geochemistry, including gas composition, carbon and hydrogen isotopes, in context with geological information available for the study area. Our results indicate that the natural gas accumulation in Songkan Formation is probably sourced from the Lower Silurian Wufeng-Longmaxi shale. The δ 13 C(CH 4 ) values range from − 33.9‰ to − 33.2‰, the δ 13 C(C 2 H 6 ) values range from − 37.0‰ to − 36.2‰, and the δ 2 H(CH 4 ) values range from − 157‰ to − 144‰. These results indicate that the gases are of thermogenic origin and are oil-derived. Furthermore, isotopic rollovers seen in the natural gas is similar to that seen in the natural gas from the local Wufeng-Longmaxi shale, which is identified as carbon exchange at high temperature. The δ 13 C(CO 2 ) values range from − 20.8 to − 17.1‰, suggesting that carbon dioxide was mainly generated by thermogenic processes. The fracture investigation indicates that the large development of horizontal fractures in calcareous mudstone is one of the reasons for the high yield of natural gas in Songkan Formation. The petrophysical parameters suggest that limestone with low porosity and permeability effectively prevents natural gas escaping and seals natural gas into calcareous mudstone with horizontal fractures. [ABSTRACT FROM AUTHOR]

The lithium isotopic compositions of adakitic rocks and K-rich volcanic rocks in southern Tibet range from + 1.3‰ to + 7.5‰ and − 4.9‰ to + 3.5‰, respectively. The subduction of the Indian Plate beneath the Lhasa Terrane means that traditional lithium isotopic compositions of various reservoirs are unable to explain the aforementioned δ 7 Li data. Therefore, it is necessary to determine the Li isotopic compositions of the different geological endmembers underneath southern Tibet. Here, we report the lithium isotopic composition and concentration of Indian upper and lower continental crust. On the basis of whole-rock major and trace element data and Sr–Nd–Pb isotope data, leucogranites from Luozha and Longzi are considered as representative of Indian upper crust, whereas two-mica granites from Quedang and Dala and granulites and gneisses from Nyalam are considered as representative of Indian lower crust. The Li concentration of Indian upper crust varies from 23 to 45 ppm with a mean of 34 ppm, consistent with a weighted mean Li concentration for the upper crust of 35 ± 11 ppm. In contrast, the Li abundance of Indian lower crust is estimated to be 33–84 ppm with a mean of 58 ppm, much higher than the average Li concentration for the lower crust of ~ 8 ppm because of the high modal abundance of biotite and muscovite. The Li isotopic compositions (+ 0.9‰ to + 5.6‰) of Indian upper continental crust are relatively heavy compared with the average Li isotopic composition of upper continental crust. On the other hand, the Li isotopic compositions of Indian lower continental crust (− 4.4‰ to − 0.1‰) are lighter than those of Indian upper continental crust. The isotopically heavy signature of Indian upper crust is produced by high-δ 7 Li fluids released from the Indian lower crust slab, whereas the lighter signature of Indian lower crust is generated by the partial melting of residual Indian lower crust slab during metamorphic dehydration of Indian lower crust. [ABSTRACT FROM AUTHOR]

Understanding the geological history of the Lhasa Terrane prior to the India–Asia collision (~ 55 ± 10 Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~ 18–10 Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183–174 Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr–Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I–S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Sandaowanzi volcanic rocks were originated from delaminated lower crust. • The early Cretaceous paleo-Pacific slab rollback caused lithospheric delamination. • The geochemical characteristics of magmatic zircons can imply the magma fertility. The complex and diversified geodynamic mechanism brought massive magmatic activity and gold mineralization in the early Cretaceous in the Great Xing'an Range. The Sandaowanzi gold deposit has a close temporal-spatial correlation with the early Cretaceous magmatism. However, the genetic relationship between the Early Cretaceous volcanic rocks and Sandaowanzi gold mineralization is not well constrained. In addition, the metallogenic geodynamic background of the Sandaowanzi gold deposit remains controversial. Here, the zircon U-Pb geochronology, zircon trace elements, in-situ zircon Hf isotopes, and whole-rock geochemistry of the Sandaowanzi ore-hosting volcanic rocks are reported. The Sandaowanzi volcanic rocks, including andesite and dacite, show variable contents of MgO, Cr, Ni, and adakitic affinity, and are proposed to originate from delaminated lower crust that interacted with lithospheric mantle metasomatized by subduction-related sediments. Besides, it may have experienced fractional crystallization of olivine, clinopyroxene, and amphibole during the early stage of magmatic evolution. Their petrogenesis, combined with other geological evidence, indicates that the Sandaowanzi volcanic rocks were formed during the slab rollback process of the Paleo-Pacific Ocean. Finally, the elemental and isotopic signatures of zircons in the Sandaowanzi magmatic rocks imply that magmas with high water content, high oxygen fugacity, and a high proportion of mantle material contribution produced during 121–123 Ma have the highest mineralization potential, which is consistent with the Rb-Sr isochron ages of ore minerals. The geochemical characteristics of zircons from magmatic rocks in ore deposits provide effective indices for constraining the magma fertility. [ABSTRACT FROM AUTHOR]

Ore deposits in the Southern Lhasa Terrane that formed prior to the India-Eurasia collision are rare. Among them, the formation of the Kelu and Sangbujiala skarn Cu-Au deposits in the Zedong area is associated with a magmatic "flare-up" in the Late Cretaceous (∼95 Ma). However, the tectonic setting, geochemistry, and petrogenesis of the mineralization-related magma of these deposits are not well understood. The biotite granodiorite is the dominant phase in the Kelu and Sangbujiala skarn Cu-Au deposits, and this study reports a set of biotite granodiorite veins (Zircon U-Pb age: 94.1 ± 0.4 Ma) without skarn alteration intruding into the cumulated hornblendite of the Late Jurassic Zedong Terrane. The rocks are adakitic, and have primitive Sr-Nd-Pb isotope compositions ((87Sr/86Sr) i = 0.7040, εNd (t) = +3.1, (206Pb/204Pb) t = 18.457, (207Pb/204Pb) t = 15.564, (208Pb/204Pb) t = 38.215) and zircon εHf (t) values (+9.7 ∼ +13.5). The trace elements of the rocks indicate that they are derived from the magma that originated from the partial melting of the mantle wedge strongly influenced by slab-derived fluids and sediments, and underwent the fractional crystallization of hornblende and plagioclase. The biotite granodiorite magma is rich in water and Cl with high oxygen fugacity (≈ ΔNNO+2 ∼ ΔNNO+4). It crystallizes at low pressure (≈ 157–200 MPa) in a closed system, and the crystallization temperature is inferred to be about 708–733 °C and 588 °C in the early and late stages, respectively. Based on the analysis of zircon data from this study and previous papers, we find that the number of magmatic zircons in the Late Cretaceous Southern Lhasa Terrane began to increase intensively at 95 Ma, accompanied by a leap in εHf (t) values and crystallization temperatures, as well as a sharp decline of (Ce4+/Ce3+) ratios. Those zircons with high εHf (t) values and crystallization temperatures, and low (Ce4+/Ce3+) ratios disappeared at 89 Ma. Therefore, we suggest that the magmatic "flare-up" in the Late Cretaceous (95–89 Ma) Southern Lhasa was triggered by mid-ocean ridge subduction, while the flat subduction occurred during 109–95 Ma and after 89 Ma. • Mid-ocean ridge subduction occurred during 95–89 Ma in the Southern Lhasa Terrane. • Mineralization-related magma during 95–89 Ma is derived from hydrous mantle wedge. • The magma is rich in water and Cl, with high oxygen fugacity. [ABSTRACT FROM AUTHOR]

Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ 26 Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ 26 Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ 26 Mg values (− 0.46 to − 0.26‰ and − 0.41 to − 0.17‰, respectively) significantly lighter than the mantle (− 0.25 ± 0.07‰, 2SD). No correlation of δ 26 Mg with either SiO 2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ 26 Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87 Sr/ 86 Sr ratios (0.70814 to 0.71105) and negative correlation between δ 26 Mg and δ 18 O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120 km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources. [ABSTRACT FROM AUTHOR]

The magnitude of crustal contamination in intra-continental OIB-like basalts is commonly difficult to assess in terms of major, trace-element and Pb–Sr–Nd isotopes. However, the Re–Os isotope system is a very sensitive tracer of the crustal component during formation of basaltic rocks. Here we report new Sr–Nd–Pb–Os geochemical and isotopic data from the Cretaceous Duofutun Volcanic Rocks (DVRs) of West Qinling, central China. The DVRs exhibit low Os abundances (4.46–42.02 ppt) with Re abundances ranging from 45.58 to 812.98 ppt, variably high Re/Os (3–126), and high initial 187 Os/ 188 Os ratios (0.2112–0.6784). We propose that the degree of partial melting is responsible for the first-order control on the variation of Os concentrations. Even the least radiogenic sample still possesses a higher initial 187 Os/ 188 Os ratio than the primitive upper mantle, implying a non-negligible contribution from high Re/Os and 187 Os/ 188 Os reservoir(s). Both the crust and pyroxenite-rich mantle are reliable candidates, but the latter cannot be reconciled with the Os mass balance between pyroxenite and ambient peridotite. Hence, we contend that these rocks were contaminated by crustal materials en route to the surface. Assimilation/fractional crystallization modeling manifests the observed isotopic variations can be produced by minor assimilation of the Proterozoic mafic lower crust. In addition, the intra-continental OIB-like basalts show a relatively wider range of 187 Os/ 188 Os ratios in comparison with MORB and OIB, confirming the continental crust can have a higher concentration of radioactive Os, which is induced by time-integrated growth. [ABSTRACT FROM AUTHOR]

Post-collisional potassic and high Sr/Y magmatism in the Lhasa terrane provides critical constraints on the timing and mechanism of subduction of Indian lithosphere and its role in the uplift of the Tibetan Plateau. Here, we report whole-rock geochemistry, mineral geochemistry, zircon U Pb ages, and in situ zircon Hf isotope ratios for the Nyemo pluton, a representative example of such magmatism. The Nyemo pluton is composed of high Sr/Y host rocks and coeval shoshonitic mafic microgranular enclaves (MMEs). Whole-rock compositions of the host rocks and MMEs form linear trends in Harker diagrams, consistent with modification of both end-members by magma mixing. Although the main high Sr/Y phase of the pluton formed by partial melting of the lower crust of the thickened Lhasa terrane, the MMEs display abnormally enriched light rare earth elements, low whole-rock ε Nd (t) and low zircon ε Hf (t) that suggest derivation from low degree melting of hydrous and enriched mantle. Based on the occurrence of shoshonitic magma and high La/Yb and high Sr/Y with adakitic affinity host rocks around 30 Ma, the Nyemo pluton is best explained as a record of onset of extension that resulted from convective removal of the mantle lithosphere beneath Tibet in the Oligocene. • High Sr/Y host rocks of Nyemo pluton are derived from thickened lower crust. • Coeval shoshonitic enclaves were produced from enriched lithospheric mantle. • Lithospheric mantle convective removal, onsets of extension may have occurred during Oligocene. [ABSTRACT FROM AUTHOR]