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The distribution of Oligo−Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90°E and 84°E) and flattened subducted lithosphere (both 86°E and 81°E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 °C) were recorded in the Oligo−Miocene ultrapotassic-potassic rocks at 90°E and 84°E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86°E and 81°E. Besides, the melting depth of the Oligo−Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81°E and 90°E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction.

The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

A combination of U-Pb zircon ages and geochemical and Sr-Nd-Hf isotopic data are presented for the Early Paleozoic granodiorites from the Haoquangou and Baimawa plutons in order to probe the crustal thickness variation of the eastern North Qilian and the diachronous evolution of the North Qilian orogen. The granodiorites formed at 436–435 Ma and have high Sr/Y ratios (63–117). Elemental and isotopic data combined with geochemical modeling and comparisons with experimental data suggest that they were produced from the melting of relatively juvenile mafic rocks in the thickened lower crust. Together with other petrological and geochemical data and the calculation of variation in crustal thickness, this indicates that the eastern North Qilian experienced clear crustal thickening and thinning from the Late Ordovician to Late Silurian. Based on available data, we suggest that diachronous collision from east to west, which probably resulted in the distinct intensity of orogenesis between eastern and western North Qilian, can well account for the differential distribution of Early Paleozoic high Sr/Y magmatism and other geological differences between the eastern and western parts of the North Qilian. Our study also implies that diachronous collision may lead to, apart from distinct metamorphic, structural and sedimentary responses, the large differences in magmatism and deep crustal processes along the orogenic strike.

High Ba-Sr granitoids occur in a number of tectonic settings, and variable petrogenetic models have been proposed. Those from post-collisional settings are rarely studied and are the focus of this study. Zircon U-Pb geochronology and whole-rock elemental and Sr-Nd-Hf isotopic data are presented for a suite of high Ba-Sr granitoids from the southern margin of the Alxa block, NW China. U-Pb zircon dating shows that the Xijing high Ba-Sr granitoids were emplaced in two periods at ~440 Ma and ~410 Ma, and they are characterized by elevated Ba and Sr contents. The older ~440 Ma high Ba-Sr granitoids range from monzodiorite to quartz monzonite, and the younger ~410 Ma high Ba-Sr granites tend to be more evolved. Both suites have similar enriched Sr-Nd-Hf isotopic ratios, low Ba/Th and Sr/Th ratios, and high Th contents and Th/Ce ratios indicative of a sediment contribution. This is potentially linked to previous subduction of the North Qilian ocean slab. Major and trace element calculations suggest that the older ~440 Ma granitoids experienced up to ~60% fractionation at 4–6 kbar with a crystallizing assemblage dominated by amphibole and plagioclase. The younger ~410 Ma granites could in turn have been formed by a further 80% fractional crystallization at lower pressures (1–2 kbar). Trace element and Sr-Nd isotopic modeling suggest incorporation of ~5% slab-derived sediment into mantle magma source of these high Ba-Sr granitoids. We note that post-collisional granitoids with high Ba and Sr characteristics may also reflect a sediment contribution in their source region, and this may be a key aspect of why such granites plot in the field of post-collisional granites in the Rb vs Y + Nb diagram. Some sanukitoids with high Ba and Sr contents in the late Archean also appear to reflect sediment subduction and they hence may represent early stages of crustal recycling.

The southern margin of the Alxa Terrane is located between the North Qilian Orogenic Belt and the inner Alxa block, and Paleozoic mafic magmatism records the geodynamic evolution at different stages in the orogenic cycle. New zircon U−Pb ages and whole-rock geochemistry reveal three groups of mafic rocks in the southern margin of the Alxa block: (i) c. 480 Ma Xijing gabbros, characterized by high Ba/Th and low Th/La ratios with low eNd(t) (−5.7 to −4.9) values, implying their derivation from enriched mantle metasomatized by slab-derived fluids; (ii) c. 440 Ma Xijing and Jiling hornblende gabbros which have high Th/La and low Ba/Th ratios, suggesting that they were produced by partial melting of mantle modified by previously subducted sediments; (iii) c. 250 Ma Xijing pyroxenites, which are cumulates with relatively high eNd(t) (+5.2 to + 5.9), indicating that their host magmas were derived from relatively depleted mantle. Together with published results, these new data highlight that the c. 480 and c. 440 Ma mafic magmas were related to the northward subduction of the North Qilian oceanic slab and later post-collisional lithospheric delamination, respectively. The c. 250 Ma Xijing pyroxenites were related to younger lithospheric extension in the development of the Central Asian Orogenic Belt in the Alxa block. Supplementary material: Supplementary tables A1–A7 are available at https://doi.org/10.6084/m9.figshare.c.5287657

The petrogenesis of the Eocene (43–42 Ma) Nb-rich granitoid dykes from the Kohistan–Ladakh island arc provides insights into melting of the down-going Indian continental crust during the Indian-Eurasian continental collision. These Nb-rich granitoids (SiO2 = 53.8–72.3 wt%, Nb = 24.0–44.1 ppm) have high Sr/Y (41.2–76.8) and (La/Yb)N (15.6–36.8) ratios. Their geochemical and Sr–Nd–Hf isotopic compositions are distinct from those of the Kohistan–Ladakh basement (Eurasian continent), but similar to those of coevally metamorphic amphibolites (42–40 Ma) in the Nanga Parbat massif (Indian continent). This implies that the magma of the Nb-rich granitoids would be derived from partial melting of the subducted Indian continental plate. The biotites from the Nb-rich granitoids show high Mg# (up to 61) and Cr2O3 (up to 2.36 wt%) and low TiO2 (0–3.21 wt%). Some samples of the Nb-rich granitoids contain 2–3% phengites with SiO2 ranging from 48.33 to 51.74 wt% and calculated pressure of 1.6–0.6 GPa, indicating initial magma crystallization of the Nb-rich granitoids at high-pressure condition (depth > 55 km). We propose that partial melting of the subducted Indian continental crust occurred when it underthrusted into the Kohistan–Ladakh asthenosphere mantle and the resultant melts upward migrated and significantly modified the overlying lithosphere and the residual Indian continental crust sank into the deep mantle. Both the metasomatized lithospheric mantle and the residual Indian continental crust played a critical role in the formation of the Miocene ultrapotassic rocks in southwestern Tibet.

Identification of slab window process is important for understanding the nature of the accretionary orogenesis. In this study, we report detailed petrological, geochronological, geochemical, Sr–Nd isotopic, and mineral chemical data for two dyke-like gabbroic intrusions from the South Tianshan belt of Tajikistan, southwestern margin of the Central Asian Orogenic Belt. Both intrusions are composed of coarse- and fine-grained gabbros. U–Pb zircon dating shows that they were emplaced at 431±5 Ma. The gabbroic rocks show relatively large variation in elemental and isotopic compositions, with SiO2 of 40.62–53.97 wt.%, Sr of 333–1261 ppm, and εNdt of +2.5 to +5.8. Especially, the fine-grained gabbros show lower SiO2 and higher MgO but more evolved isotopes than the coarse-grained gabbros for each of the intrusions. All the rocks display OIB-like or transitional OIB-/E-MORB-like geochemical characteristics with no obvious Nb-Ta depletion, indicative of an intraplate affinity. Combined with their mineral chemical compositions, we suggest that these gabbroic rocks were generated by partial melting of asthenospheric mantle in the transitional spinel-garnet stability field, followed by different degrees of fractional crystallization of olivine, clinopyroxene, and plagioclase and mixing with carbonatitic melts. The available data indicate that roll-back of the subducting Turkestan oceanic slab occurred during the Late Ordovician to Early Silurian period. Asthenosphere upwelling due to the opening of slab window resulted from localized slab tearing during slab roll-back may have been responsible for the generation of the studied dyke-like gabbroic intrusions.

Porphyries, closely associated with the copper ore deposits in orogenic belts, are usually considered to have high oxygen fugacity and display high Sr/Y ratios. However, it is still ambiguous whether the high oxygen fugacity and the enrichment of copper are inherited from magma source or obtained by magmatic processes, and the linkage between the high Sr/Y magmas and the formation of porphyry Cu deposits remains unclear. To address these issues, an integrated study of zircon geochronology and oxygen fugacity, petrography, geochemical and Sr-Nd-Hf isotopic compositions for was carried our for the Shanyang porphyry groups from the South Qinling Orogen, Central China. The crystallization ages for the Shanyang porphyries range from ca. 152 to 140 Ma. Our results suggest that the Shanyang porphyry groups are high Ba-Sr granitoids, rather than adakitic rocks, and there is no inevitable connection between high Sr/Y magma and the formation of PCDs. Their parental magmas were derived from partial melting of enriched heterogeneous lithospheric mantle that had been metasomatized by fluid or melt released from the previous subducting slab. Through magma differentiation, the Shanyang porphyry magmas changed from the oxidation state (ΔFMQ +1 to +2) to the reduced state (ΔFMQ +1 to -0.5). The redox condition of magma may be very different from its source and can be shifted remarkably during magmatic evolution that caused by fractional crystallization of garnet in the deep crust and magma degassing in the shallow upper crust. Remelting of the early formed sulfides and gas-brine reactions could enrich copper in the exsolved volatile fluid. Furthermore, the periodic and long-lived magmatic-hydrothermal systems in the shallower magma reservoirs play a critical role in the formation of porphyry Cu deposits.

As an indicator of extensional tectonic events, MORB-like magmas could provide great insights into tectonic evolution of subduction zones. In this study, we carry out an integrated study of geochronology, geochemistry and Sr Nd isotopic compositions for the Early Paleozoic MORB–like Lashuixia gabbro in Hualong, southern Central Qilian block. U Pb zircon dating yields magma crystallization age of 506 ± 6 Ma for the gabbro. Geochemically, the gabbros display weakly fractionated REE patterns with (La/Yb)N = 1.17–1.61 and flat trace element patterns except for enrichment of large ion lithophile elements (LILEs) and depletion of Nb. Isotopically, the gabbros display pronouncedly positve eNd(t) values of +5.8 − +7.0, but their initial 87Sr/86Sr ratios ((87Sr/86Sr)i = 0.7046–0.7070) are slightly high because of post-magmatic alteration and metamorphism. Their geochemical and isotopic features indicate that several endmembers (i.e. MORB-sourced asthenospheric mantle, subcontinental lithospheric mantle and oceanic slab-derived fluid) were involved into their source region. Our modal batch partial melting calculation indicates that 7–17% partial melting of 90% asthenospheric mantle plus 10% subcontinental lithospheric mantle at spinel-stable depth accords well with most incompatible trace element compositions of the gabbros. In combination with regional data, we propose that rock association of arc-type pillow lava (ca. 494 Ma), Nb-enriched basalt (NEB)-like diabase (ca. 491 Ma), boninite (ca. 483 Ma) and later OIB-like basalt (ca. 468 Ma) in the Lajishan belt witnessed evolution of this belt from continental arc splitting to back-arc basin spreading due to rollback of the subducted North Qaidam-West Qinling Oceanic slab. We suggest that the Lashuixia gabbro, ca. 10 Ma earlier than the arc-type pillow lava, recorded the initial back-arc lithospheric extension. As with those in many modern arc-basin systems, MORB-like gabbro with overprinting of other endmembers in paleo-subduction zones could not only be an important indicator for back-arc extension, but also make a constraint on the time when the back-arc extension initiated.

The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66–71Grs14–17Prp9–12Sps3–5, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of ~4.9–6.3 kbar and ~755–820 ºC in the upper amphibolite facies to lowtemperature granulite facies, and retrograde cooling and decompression at conditions of ~2.5–3.1 kbar and ~525–545 ºC. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.

Neoproterozoic intrusions are widespread in the northwestern margin of the Yangtze Block, and their petrogenesis was closely related to the assembly and breakup of Rodinia supercontinent. A combined study of zircon U–Pb dating, geochemistry and Sr–Nd–Hf isotopes was carried out for three types of intrusive rocks from the Hannan region, including the Beiba gabbros, the Tianpinghe and Taojiaba I-type granites and the Tiechuanshan and Huangguan A-type granites. LA-ICP-MS zircon U–Pb dating results show that the studied intrusions have crystallization ages ranging from ca. 880 to 770 Ma. The ca. 880 Ma Beiba gabbros have variable contents of MgO, Al2O3, Cr and Ni and extremely low high field strength elements (HFSE) concentrations that indicate a cumulate origin. These gabbros display subduction-related geochemical signatures with whole-rock initial 87Sr/86Sr (ISr) = 0.704–0.705, eNd(t) = −0.4 to +0.6 and zircon eHf(t) = +6.9 to +8.4, suggesting that they were derived from the partial melting of an enriched lithospheric mantle that had been modified by slab-derived materials. The ca. 860 Ma Tianpinghe I-type granites have high ISr (> 0.704) and negative eNd(t) (−5.6 to −3.5) values and zircon eHf(t) = −1.9 to +1.6, indicating their sources were mixtures of Neoproterozoic juvenile mafic crust and minor Paleoproterozoic crustal components, whereas the ca. 770 Ma Taojiaba I-type granites have relatively low ISr (

Early Paleozoic boninites occur in the Central Qilian orogenic belt, Northwest China. Their petrogenesis provides insights into lithosphere process and tectonic evolution of the Qilian block. In this paper, we carry out a study of geochronological, geochemical and Sr-Nd isotopic compositions for the Early Paleozoic boninites in the Lajishan area of the Central Qilian block. The Lajishan boninites (∼483 Ma) have high Al2O3/TiO2 (36.7–64.7) and CaO/TiO2 (31.1–49.6) ratios, and high MgO (7.86–10.47 wt%), Cr (439–599 ppm) and Ni (104–130 ppm) contents, indicating that the boninites result from a refractory mantle source. They are depleted in high field-strength elements (HFSE) and enriched in large ion lithophile elements (LILE), coupled with slightly high initial 87Sr/86Sr values of 0.7059–0.7074 and low eNd(t) values of −1.05 to +2.66, indicating that the mantle source was metasomatized by subducted slab-derived components. We found that an assemblage of low-Ca group and high-Ca group boninites occurred in the Lajishan belt. The high-Ca group boninites were derived from relatively fertile mantle with slightly higher melting degree, whereas the low-Ca group boninites were generated by partial melting of more refractory mantle wedge peridotites with slightly lower melting degree. The assemblage of low-Ca group and high-Ca group boninites reveals that the low-Ca group boninites were generated by the further melting of the more refractory mantle source after the segregation of the high-Ca group boninitic magmas in response to the back-arc basin opening. In the light of reported boninites worldwide, a diagram of Zr/Y vs. CaO/Al2O3 is used to identify boninites in fore-arc and back-arc regions. We suggest that the Lajishan boninites represent the products of back-arc basin development in response to the northward subduction of the Qaidam-West Qinling ocean slab.

The Lajishan belt of the Central Qilian block was a back-arc basin during Early Paleozoic. The basaltic magmatism and temporal evolution in this basin provide an opportunity to study the development of back-arc basin in an active continental margin. In this study, we carry out an integrated study of geochronological, geochemical and Sr-Nd isotopic compositions for the Early Paleozoic arc-like and OIB-like basalts. The Lajishan arc-like basalts are enriched in large ion lithophile element (LILE) and show negative Nb and Ta anomalies whereas the OIB-like basalts have high LILE abundances and show positive Nb and Ta anomalies. The arc-like basalts have initial 87Sr/86Sr values of 0.7050–0.7054 and eNd(t) values of +0.51–+2.63, and the OIB-like basalts have initial 87Sr/86Sr values of 0.7049–0.7050 and eNd(t) values of +0.66–+1.57. The geochemical and Sr-Nd isotopic compositions suggest that the arc-like basalts are derived from partial melting of a depleted mantle source metasomatized by slab-derived components at shallow depth levels, and the OIB-like basalts also originated from a metasomatized mantle wedge source. U-Pb zircon dating yielded the ages of ~494 ± 4 Ma for the arc-like basalts and ~468 ± 6 Ma for the OIB-like basalts. We argue that the arc-like basalts are products of back-arc extension before the back-arc rifting initiated in earlier stage, resulting from the northward subduction of the Qaidam-West Qinling oceanic slab, while the OIB-like basalts represent products of further back-arc spreading in response to rollback of the Qaidam-West Qinling oceanic lithospheric slab. The association of arc-like and OIB-like basalts in the Lajishan belt records the development of back-arc basin from initial rifting to subsequent spreading, offering insight into how basaltic magmatism generates in the formation of back-arc basin in subduction zone setting.

The role of the mantle in generating ultrahigh-temperature metamorphism and peraluminous S-type granites, and the extent of crust−mantle interaction are topics fundamental to our understanding of the Earth's evolution. In this study we present geochronological, geochemical, and Sr–Nd–Hf isotopic data for dolerites and mafic volcanic rocks from the Darongshan granite complex belt in western Cathaysia, South China. LA–ICP–MS U–Pb zircon analyses yielded magma crystallization ages of ca. 250–248 Ma for the dolerites, which are coeval with eruption of the mafic volcanic rocks, ultrahigh-temperature metamorphism, and emplacement of S-type granites in the Darongshan granite complex belt. The mafic volcanic rocks are high-K calc-alkaline or shoshonitic, enriched in Th, U, and light rare earth elements, and depleted in Nb, Ta and Ti. The dolerites are characterized by high Fe2O3tot (11.61–20.39 wt%) and TiO2 (1.62–3.17 wt%), and low MgO (1.73–4.38 wt%), Cr (2.8–10.8 ppm) and Ni (2.5–11.4 ppm). Isotopically, the mafic volcanic rocks have negative whole-rock eNd(t) values (−6.7 to −9.0) and high ISr values (0.71232 to 0.71767), which are slightly depleted compared with the dolerite samples (eNd(t) = −10.3 to −10.4 and ISr = 0.71796 to 0.71923). Zircons in the dolerites have eHf(t) values of −7.6 to −10.9. The mafic volcanic rocks are interpreted to have resulted from the partial melting of an enriched lithospheric mantle source with minor crustal contamination during ascent, whereas the dolerites formed by late-stage crystallization of enriched lithospheric mantle-derived magmas after fractionation of olivine and pyroxene. The formation of these mantle-derived mafic rocks may be attributed to transtension along a NE-trending strike-slip fault zone that was related to oblique subduction of the Paleo-Pacific plate beneath South China. Such underplated mafic magmas would provide sufficient heat for the generation of ultrahigh-temperature metamorphism and S-type granites, and act as a mafic end-member for S-type granite genesis.

This paper presents an integrated study of petrology, geochronology, and geochemical and Sr–Nd–Hf isotopic compositions for the Jishishan medium- and fine-grained diorites and the Bamishan gabbroic and quartz dioritic dykes from the eastern Qilian orogen, to address their petrogenesis and the lithospheric evolution of the Qilian orogen. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon analyses show that they formed at c. 472–432 Ma. Geochemical and isotopic data suggest that the Jishishan diorites were derived from amphibole-bearing garnet-facies lithospheric mantle metasomatized by slab-derived fluids, whereas the Bamishan gabbroic dyke was derived from phlogopite-bearing garnet-facies lithospheric mantle enriched by sediment-derived melts. The Bamishan quartz dioritic dykes are high-potassic calc-alkaline and were predominantly derived from mafic lower crust. Based on available data, we suggest that the Jishishan diorites were generated during northward subduction of the South Qilian oceanic slab, whereas the Bamishan mafic–intermediate dykes formed in a post-collisional setting related to slab break-off. Moreover, we propose an integrated model for the Early Paleozoic tectonomagmatic evolution of the eastern Central Qilian, in which fluid-induced mantle metasomatism and melting characterized the oceanic subduction stage, whereas melting of varied sources (including subducted oceanic crust and overlying sediments, sediment melt-modified lithospheric mantle and the continental crust) was involved at the post-collisional stage. Supplementary material: Analytical methods, whole-rock geochemical and Sr–Nd isotopic compositions and zircon U–Pb and Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3975447

Post-collisional granitoids are widespread in the North Qilian and southern margin of the Alxa block and their petrogenesis can provide important insights into the lithospheric processes in a post-collisional setting. This paper carries out an integrated study of U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions for five early Paleozoic intrusive plutons from the North Qilian to southern margin of the Alxa block. The geochronological and geochemical results show that their magmatism can be divided into three periods with distinct geochemical features. The early-period intrusive rocks (~ 440 Ma) include the Lianhuashan (LHS) and Mengjiadawan (MJDW) granodiorites. Both of them display high Sr/Y ratios (52–91), coupled with low Y and HREE contents, implying that they were derived from partial melting of thickened lower crust, with garnet in the residue. The middle-period intrusive rocks (~ 430 Ma), including the MJDW quartz diorites and Yangqiandashan (YQDS) granodiorites, are high-K calc-alkaline with low Sr/Y values. The geochemical and isotopic data suggest that they are generated from partial melting of lower crust without garnet in the residue. The late-period intrusive rocks (414–422 Ma), represented by the Shengrongsi (SRS) and Xinkaigou (XKG) plutons, are A-type or alkali-feldspar granites. They are possibly derived from partial melting of felsic crustal material under lower pressure condition. Our data show decreasing magma crystallization ages from MJDW pluton in the north and LHS pluton in the south to the SRS and XKG plutons in the central part of the study area. We suggest that such spatial and temporal variations of magmatic suites were caused by lithospheric delamination after the collision between the Central Qilian and the Alxa block. A more plausible explanation is that the delamination propagated from the margin part of the thickened lithosphere to inward beneath the North Qilian and southern margin of the Alxa block.

The Central Asian Orogenic Belt (CAOB) formed mainly in the Paleozoic due to the closure of the Paleo-Asian oceanic basins and accompanying prolonged accretion of pelagic sediments, oceanic crust, magmatic arcs, and Precambrian terranes. The timing of subduction–accretion processes and closure of the Paleo-Asian Ocean has long been controversial and is addressed in a geochemical and isotopic investigation of mafic rocks, which can yield important insight into the geodynamics of subduction zone environments. The Xilingol Complex, located on the northern subduction–accretion zone of the CAOB, mainly comprises strongly deformed quartzo-feldspathic gneisses with intercalated lenticular or quasi-lamellar amphibolite bodies. An integrated study of the petrology, geochemistry, and geochronology of a suite of amphibolites from the complex constrains the nature of the mantle source and the tectono-metamorphic events in the belt. The protoliths of these amphibolites are gabbros and gabbroic diorites that intruded at ca. 340–321 Ma with positive eHf (t) values ranging from + 2.89 to + 12.98. Their T DM1 model ages range from 455 to 855 Ma and peak at 617 Ma, suggesting that these mafic rocks are derived from a depleted continental lithospheric mantle. The primitive magma was generated by variable degrees of partial melting of spinel-bearing peridotites. Fractionation of olivine, clinopyroxene and hornblende has played a dominant role during magma differentiation with little or no crustal contamination. The mafic rocks are derived from a Late Neoproterozoic depleted mantle source that was subsequently enriched by melts affected by slab-derived fluids and sediments, or melts with a sedimentary source rock. The Carboniferous mafic rocks in the northern accretionary zone of the CAOB record a regional extensional event after the Early Paleozoic subduction of the Paleo-Asian Ocean. Both addition of mantle-derived magmas and recycling of oceanic crust played key roles in significant Late Carboniferous (ca. 340–309 Ma) vertical crustal growth in the CAOB. Amphibolite–facies metamorphism (P = 0.34–0.52 GPa, T = 675–708 °C) affected these mafic rocks in the Xilingol Complex at ca. 306–296 Ma, which may be related to the crustal thickening by northward subduction of a forearc oceanic crust beneath the southern margin of the South Mongolian microcontinent. The final formation of the Solonker zone may have lasted until ca. 228 Ma.

The appinite–granite association has been found in various tectonic regimes related to recent subduction, arc-continent or continent–continent collision and post-collision (orogen), and appinites generally originate from recently subduction-modified lithospheric mantle. We conducted a study on a rarely reported appinite–granite association formed in an intracontinental regime, the Zhangjiafang–Qinglongshan complex (ZQC), in which the appinites were derived from an ancient sub-continental lithospheric mantle (SCLM). The ZQC is located in the western Wugongshan domain, and consists of basaltic to intermediate appinites and granitoids. Ten dated samples (including massive and gneissoid granitoids, hornblende gabbro, and diorite) give zircon 206Pb/238U ages ranging from 444 ± 3 Ma to 452 ± 4 Ma, indicating that these various lithologies were emplaced synchronously. The basaltic appinites show radiogenic 87Sr/86Sri (0.71016–0.71431) and negative eNd(t) (− 6.1 to − 8.9) and zircon eHf(t) (− 4.2 to − 7.5) values. Combined with regional geological background, an origin from the Neoproterozoic metasomatised SCLM can be inferred for the appinites in the Wugongshan domain. The granitoids are peraluminous and almost high-K calc–alkaline to shoshonitic. They exhibit a wide range of isotopic compositions (87Sr/86Sri = 0.70828–0.71857, eNd(t) = − 6.2 to − 10.5, zircon eHf(t) = − 9.5 to − 26.6). Some of the granitoids display the most evolved Sr–Nd isotopic signatures among all the studied lithologies, which are consistent with those of the middle to upper crust, suggesting a pure crustal origin. Other granitoids show relatively mafic composition and less evolved isotopic signature. The intermediate appinites have intermediate chemical compositions between those of the basaltic appinites and granitoids, and similar Sr–Nd isotopic compositions to those of the basaltic appinites that have relatively evolved composition and isotopic signature. The petrographical and the geochemical studies, as well as magma mixing and mingling between mafic magma and felsic magma observed in field work, indicate that the ZQC was formed by several complex magmatic processes including assimilation and fractional crystallization (AFC) of a basaltic magma derived from an enriched mantle, magma mixing and mingling, fractional crystallization of a crust-derived felsic magma. The contribution of the Neopreoterozoic subduction-modified lithospheric mantle to the early Paleozoic granitoids in South China has been poorly constrained due to its evolved isotopic signatures (such as negative eNd(t) and eHf(t) values, high initial ratio of 87Sr/86Sr) and few studies on the early Paleozoic basaltic–felsic rock association. Our work here suggests that the Wugongshan appinite–granite association was formed in an intracontinental orogen, and the ancient enriched mantle may have had a significant chemical contribution to the formation of some of the Paleozoic peraluminous granites in South China.

Early Paleozoic peraluminous granites are abundant in the eastern part of the Qilian orogen, northeastern margin of the Tibetan Plateau. A combined study involving geochronology, whole-rock geochemical and Sr–Nd–Hf isotopic compositions for three Early Paleozoic peraluminous granitic plutons (Jishishan, Ledu and Shichuan plutons) from the eastern Qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas. These granitic plutons have magma crystallization ages of 455–427 Ma and are moderately to strongly peraluminous (A/CNK = 1.03–1.18). Geochemical and Sr–Nd–Hf isotopic data indicate that they consist substantially of crust-derived melts. The Jishishan and Ledu peraluminous granites were dominantly produced by partial melting of Precambrian orthogneisses. The Shichuan monzogranites, with low HREE contents (e.g., Yb = 0.80–1.83 ppm) and slightly negative eNd(t) (− 5.3 to − 2.3) and positive eHf(t) (+ 1.6 to + 3.4), could be derived from immature crustal materials. Relatively high average zircon saturation temperatures (> 750 °C for each pluton), obvious negative Eu anomalies (Eu/Eu* = 0.28–0.80) and low Pb/Ba ratios (0.03–0.16) for the Jishishan, Ledu and Shichuan granites are consistent with crustal melting involving biotite breakdown under fluid-absent conditions. Our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions, while the processes such as mixing with mantle-derived magma, fractional crystallization, restite unmixing and peritectic assemblage entrainment were insignificant (or only play secondary roles) in their genesis. Late Ordovician to Middle Silurian crustal anatexis in the eastern Central Qilian was probably linked with slab break-off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post-collisional setting.

The petrogenesis of high Sr/Y-type magmas is still open to debate. Usually, such magmas could result from melting under high-pressure settings (> 12 kbar). In this paper, we gave an example that some high Sr/Y-type magmas could originate from melting of crustal materials at pressure of 10–12 kbar. We carried out a study of petrogenesis for Devonian high Sr/Y granites from the Beidashan batholith (397–411 Ma), southwestern Alxa Block, Northwest China. The Beidashan granites have SiO 2 (69.21–74.60 wt.%) and Al 2 O 3 (14.01–16.20 wt.%) with A/CNK ratios of 0.99–1.08. According to their trace element compositions and whole-rock zirconium saturation temperatures ( T Zr ), the Beidashan granites can be divided into two groups: Group I ((Dy/Yb) N = 1.2–3.0, Eu/Eu* = 0.77–1.3, T Zr = 761–856 °C), resulted from fluid-absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of ~ 12 kbar, and Group II ((Dy/Yb) N = 0.76–2.16, Eu/Eu* = 1.7–5.3, T Zr = 651–785 °C), formed by fluid-present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of ~ 10 kbar. Both Group I and Group II show high Sr/Y and (La/Yb) N features. They show I Sr = 0.7134–0.7180, e Nd ( t ) = − 6.61 to − 9.71, T 2DM = 1.7–1.9 Ga; e Hf ( t ) = − 5.6 to − 9.9 and T DM C = 1.7–2.0 Ga, indicating that the Beidashan high Sr/Y granites were derived from melting of crustal basement materials. Our results suggest that some high Sr/Y-type granites formed under relatively lower pressure conditions (~ 10–12 kbar), and they could not be an indicative of partial melting of thickened crust.

Early Paleogene granitoids in Southern Lhasa subterrane have been widely investigated and many petrogenesis and geodynamic models have been proposed in the past few years. However, contemporaneous granitoids in the Motuo tectono-magmatic belt, southeast Lhasa terrane, are still limitedly studied. Here we present the petrology, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotope data of the Damu and 52 K quartz–monzonite in the Motuo area. LA-ICP-MS U–Pb zircon dating shows that they have magma crystallization ages of 49 and 69 Ma, respectively. The Damu quartz–monzonite (SiO2 = 63.76–68.33 wt.%) is high-K calc-alkaline (K2O = 2.54–4.02 wt.% with K2O/Na2O = 0.59–1.09) and metaluminous to weakly peraluminous (A/CNK = 0.99–1.07). The 52 K quartz–monzonite (SiO2 = 61.12–66.12 wt.%) shows slightly higher K2O contents (3.80–5.28 wt.% with K2O/Na2O = 1.03–1.45) and metaluminous series (A/CNK = 0.96–1.00). The analyzed samples are characterized by high Ba (850–2573 ppm), Sr (534–986 ppm) contents, and fractionated REE patterns ((La/Yb)N = 22–72 and (Sm/Yb)N = 4.55–8.24). These geochemical features are comparable with those of high Ba–Sr granite. They display weakly evolved Sr–Nd–Hf compositions (whole-rock (87Sr/86Sr)0 = 0.7068 to 0.7086, eNd(t) = − 4.20 to − 3.41, and zircon eHf(t) = − 5.2 to − 0.9). Geochemical and Sr–Nd–Hf isotopic data reflect that the Damu and 52 K quartz–monzonite represent residual magma from AFC processes of lithospheric mantle-derived mafic melts. The over-thickened lower crust in the eastern Lhasa terrane had been delaminated during ca. 83–70 Ma, which led to the replacement of ancient lithospheric mantle by the juvenile lithospheric mantle. The juvenile mantle wedge in the study area was suspected to be metasomatized by melts that were derived from the foundering arc root, rather than the subducted sediments. Thus, the early Paleogene high Ba–Sr magmas from the SE Lhasa terrane may provide evidence for recycling of continental lithosphere by density destruction.

Mafic intrusive rocks are widely exposed in the Motuo tectono-magmatic belt, southeast Lhasa terrane. LA–ICP-MS U–Pb zircon dating shows that they have magma crystallization ages of 69 and ca. 50 Ma. These mafic intrusive rocks are characterized by variable SiO 2 (44.60–57.60 wt.%), high Al 2 O 3 (17.19–20.86 wt.%), and low MgO (1.85–5.38 wt.%) with Mg# of 31–55. Their chemical composition is comparable with low-MgO high-Al basalts to basaltic andesites. They are enriched in LILEs (Rb, Ba, K) and LREE and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with weakly evolved Sr–Nd–Hf compositions (whole-rock ( 87 Sr/ 86 Sr) 0 = 0.7064 to 0.7086, e Nd ( t ) = − 3.41 to + 1.22, and zircon e Hf ( t ) = − 3.8 to + 6.4). The mafic rocks were derived from partial melting of metasomatized lithospheric mantle. Geochemical and Sr–Nd–Hf isotopic data show that they were insignificantly modified during magma emplacement. We provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern Lhasa terrane. Sr–Nd isotopic composition of the Motuo Late Cretaceous–Eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane. The foundered ancient materials may subsequently re-fertilize the lithospheric mantle not only in the eastern Lhasa terrane but also in the surrounding areas.

The slab dynamic processes, such as roll-back during oceanic subduction and break-off during collision orogenesis, would affect the movement of mantle material and the interaction between crust and mantle, accompanied by significant amounts of magmatism. Providing accurate depictions for these processes are extremely difficult, especially in the case of ancient orogens. Here, we present an integrated study of petrology, geochronology, geochemistry and Sr–Nd–Hf isotopes for Early Paleozoic granitoids from the Duojielaka, Ledu and Binglingsi plutons of the southeastern Qilian orogen. Our LA-ICP-MS U Pb zircon data show that these granitoids were emplaced at 476–432 Ma. The petrological and geochemical data suggest that the ~476 Ma Duojielaka porphyritic syenogranites formed by feldspar-dominated fractionation from relatively K-rich basaltic rock-derived adakitic parental magma. The ~454 Ma Duojielaka biotite monzogranites are highly fractionated I-type granites and were derived from basement orthogneisses mixed with minor mafic rocks. The Ledu fine- and medium-grained quartz diorites (~446 and ~452 Ma) are high-K calc-alkaline to shoshonitic and were produced by partial melting of relatively old K-rich mafic crust followed by minor mixing with mantle-derived magmas. The ~432 Ma Binglingsi biotite monzogranites with adakitic characteristics most likely originated by partial melting of thickened mafic lower crust. Based on the new data, in combination with previously published data, we suggest that the roll-back of the South Qilian oceanic slab occurred during the Middle Cambrian to early Middle Ordovician. The Duojielaka porphyritic syenogranites represented crustal anatexis in an arc front associated with slab roll-back. The Late Ordovician–early Middle Silurian intrusive rocks probably resulted from diachronous slab break-off in the post-collisional stage. The rocks record the opening of the break-off window from east to west and the gradual heat propagation from the central part to the southern and northern margins of the eastern Central Qilian due to the widening of break-off window and asthenospheric upwelling. We propose that complex geodynamic processes and their thermal effects can be traced by a comprehensive consideration of the petrogenetic, temporal and spatial evolution of related magmatism.

Porphyries, closely associated with the copper deposits worldwide, are usually considered to have high oxygen fugacity and high Sr/Y ratios. However, it is still ambiguous whether the high oxygen fugacity and the enrichment of chalcophile elements are inherited from magma source or obtained by magmatic processes, and the linkage between the high Sr/Y magmas and the formation of porphyry Cu deposits remains unclear. To address these issues, we performed an integrated study of zircon geochronology and oxygen fugacity, petrography, geochemical and Sr-Nd-Hf isotopic compositions for the Shanyang porphyry groups at an intra-continental setting from the South Qinling Orogen, Central China. The Shanyang porphyries can be divided into six groups in terms of their spatial distribution with the crystallization ages ranging from ca. 152 to 140 Ma. They are characterized by high Sr and Ba contents, consistent with the geochemical features of high Ba-Sr granitoids. They show a continuous evolution trend from mafic to felsic samples and mafic samples have high Mg#, Cr, and Ni contents, indicating that they were evolved from mantle-derived basaltic magmas. Most samples display homogeneous ISr (0.704 to 0.706), eNd(t) (−6 to −1.5) and zircon eHf(t) (−4 to +2) values, suggesting that they were derived from an enriched lithospheric mantle with minor crustal contamination. Geochemical results suggest that they were generated by low degree melting of a heterogeneous lithospheric mantle at spinel facies that had been modified by previous slab-derived fluids. These porphyry bodies were derived from multiple magma reservoirs within their distinct plumbing systems and had undergone polybaric differentiation. With magma differentiation, the Shanyang porphyry magmas changed from an oxidized state (ΔFMQ +1 to +2) to a reduced state (ΔFMQ +1 to −0.5). The high (Dy/Yb)N ratios indicate crystallization of garnet had cryptically occurred in the deep crust, which induced sulfide saturation at an earlier stage. This process could result in the deficiency of chalcophile elements and elevate the oxygen fugacity in the residual melts from a relatively reduced parental mafic magma. Degassing in the shallow crust would lead to a decrease in the ƒO2 of the felsic magmas. Thus, some other processes, such as, re-melting of early formed sulfide and gas-brine reactions, are required to concentrate chalcophile elements in the felsic magmas. Our study implies that intra-crustal processes could play a more important role in the enrichment of chalcophile elements and that the oxygen fugacity of the magma system can change during magma evolution. The observations of the Shanyang porphyry groups are very helpful for understanding similar porphyry systems globally. We propose that porphyries worldwide formed in the thickened crust with long-lived magmatic hydrothermal systems in the shallower magma reservoirs are more prone to form porphyry Cu deposits.

Late Cretaceous (~ 106–76 Ma) adakite-like intrusive rocks in the middle-eastern Gangdese belt occur in an E–W trending belt paralleling the Indus–Yarlung suture, south Tibet. Their petrogenesis and geodynamic processes have been a subject of debate. We report here U–Pb zircon ages, geochemical and Sr–Nd–Hf isotopic data for adakite-like intrusive rocks as well as the normal arc rocks (gabbros and gabbroic diorites) in the middle Gangdese belt. LA-ICPMS U–Pb zircon analyses yielded an identical age of ~ 88 Ma for two adakite-like rocks, which are slightly younger than the gabbro and gabbroic diorite (ca. 94–90 Ma). Both the adakite-like rocks and the normal arc rocks have similar whole-rock Sr–Nd and zircon Hf isotope compositions, indicating that they have been derived from a common source. Similarly, the adakite-like and normal arc intrusive rocks in the eastern Gangdese belt also show similar Sr–Nd–Hf isotope compositions. In the middle-eastern Gangdese belt, the > 85 Ma Late Cretaceous intrusive rocks consist of a magma series from gabbro to granodiorite, including both normal arc rocks and adakite-like rocks. These rocks overlap in space and time that conform to a normal arc differentiation trend. In terms of major and trace elements, they also show a clear evolution from the normal arc magmatic into adakitic field. Thus, we suggest that these > 85 Ma Late Cretaceous intrusive rocks were ultimately derived from melting of the hydrated mantle wedge and the adakite-like rocks can be generated in normal arc magmas by amphibole-dominated fractionation. Taking into accounting for the spatial and temporal distribution of the Cretaceous magmatic rocks in the Lhasa terrane, we prefer a model of early Late Cretaceous rollback following Early Cretaceous low-angle oceanic slab subduction. At intermediate pressure and H2O-rich conditions, fractionation of amphibole changes the major and trace element compositions of arc magmas, and will efficiently drives basaltic composition to andesitic composition in arc magmas.

An integrated study involving whole-rock and Sr–Nd–Hf isotope geochemistry and zircon geochronology and trace element combined with detailed field investigation was carried out for the composite Meiwu batholith in the West Qinling orogenic belt of central China to probe the origins of its compositional diversity and its emplacement history. The batholith is composed of quartz diorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves and minor tonalitic enclaves in the granodiorite. The crystallization age of the batholith is ∼240–245 Ma. Geochemical and Sr–Nd–Hf isotopic data indicate that the magmas that formed the quartz diorite and the mafic enclaves were derived by partial melting of enriched lithosphere mantle, followed by variable degrees of hybridization with crustal magmas in deep crustal hot zones. These initially heterogeneous, hybrid magmas successively intruded into the upper crust and coalesced into a large magma chamber. Zircon trace element and Hf isotopic compositions suggest that the outer fine-grained part of the quartz diorite pluton crystallized from a less differentiated magma as a result of rapid cooling and thus preserved its initial heterogeneities, whereas the inner medium-grained part of the quartz diorite pluton crystallized from a convecting, isotopically homogeneous magma that had undergone advanced magmatic differentiation. The Sr–Nd–Hf isotope compositions of the mafic magmatic enclaves are strikingly similar to those of the host granodiorite, implying their isotopic equilibration. The tonalitic enclaves have high Sr/Y ratios and most probably represent magmas derived from partial melting of thickened mafic lower crust. The Sr–Nd–Hf isotope data suggest that the granodiorite and biotite granite were dominantly derived from isotopically heterogeneous crustal sources. However, the granodiorite also has relatively high Mg#, Cr, and Ni, indicating a minor contribution from a mantle source. The granodiorite was constructed incrementally from a number of discrete melt batches that were generated by partial melting of mafic lower crust under variable water fugacity. These melt batches did not assemble into a large magma chamber and thus preserved their source chemical features. The granodiorite magma was also replenished by mafic and high Sr/Y magmas, resulting in abundant and compositionally diverse magmatic enclaves. The biotite granite formed by the successive accumulation of discrete magma pulses generated by dehydration melting of mafic lower crust under water-absent conditions. These magma pulses coalesced into a small single magma chamber where they underwent fractional crystallization. The various rock types exhibit distinct geochemical variations, indicating that the Meiwu batholith was constructed from multiple injections of magma over a protracted period. Fractional crystallization, assimilation, magma mixing and/or mingling occurred during magma ascent and at the emplacement level. Distinct magma sources played a primary role in controlling the chemical diversity of the igneous bodies at pluton and batholith scale.

Voluminous Early Paleozoic intrusive rocks occur in the Qilian orogenic belt, northeastern margin of the Tibetan Plateau. Their petrogenesis can provide insights into the tectonic evolution of Qilian and its adjacent areas. This paper carries out an integrated study of U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions for the Bamishan and Heishishan plutons in the eastern Qilian orogen. Both the Bamishan and Heishishan plutons consist of mafic, intermediate and felsic intrusive rocks. U–Pb zircon dating yielded the magma crystallization ages of 459–449 Ma for the Bamishan pluton and 447–438 Ma for the Heishishan pluton. Their geochemical and Sr–Nd–Hf isotopic data show that the parental magmas of the Bamishan monzogranites and quartz diorites were generated by slab (MORB and overlying sediments) melting. The Bamishan hornblende gabbros have extremely high Cr (1159–1163 ppm), Ni (220–353 ppm) and Mg# (73–74), indicating a cumulate origin. The Heishishan granodiorite–granite has adakitic geochemical signatures and they were produced by partial melting of the mafic lower crust. The Heishishan hornblende gabbro and quartz diorites were derived from the enriched lithospheric mantle metasomatized by subduction-related fluid/melt. In light of the available data, we propose that there was an Early Paleozoic ocean basin between the Central-South Qilian and Qaidam–West Qiling belts. The northward closure of this basin and subsequent continental collision and post-collisional processes resulted in the generation of the Early Paleozoic magmatism in the eastern Central Qilian. The Late Ordovician to Early Silurian (~ 460–440 Ma) intrusive rocks in the eastern Central Qilian formed in a post-collisional setting. A slab break-off model may account for the magma generation.

Analytical Methods

Table 1. Major element (wt.%) and trace element (ppm) data.

Table 2. Whole-rock Sr–Nd isotopic data.

Whole-rock geochemistry, Sr–Nd isotope and zircon U–Pb isotope data are reported for seven granitoid intrusions from the eastern Lhasa terrane. Our zircon U–Pb data exhibit three periods of magmatism: 164–125 Ma, 83 Ma, and 71–45 Ma. Granitoids from the Middle Jurassic to Early Cretaceous (164–125 Ma) display evolved Nd isotope composition, with eNd(t) = − 8.6 to − 15.5 and T2DM = 1.6 to 2.2 Ga. In contrast, the Late Cretaceous (83 Ma) granitoids with adakitic characteristics (high Sr/Y ratios) display less evolved Nd isotopic composition, with eNd(t) = − 0.3 to − 3.0 and T2DM = 0.9 to 1.1 Ga. Geochemical and Sr–Nd isotopic data indicate that the Middle Jurassic to Early Cretaceous granitoids were derived from partial melting of Proterozoic crustal basement and the Late Cretaceous adakitic granitoids were derived from partial melting of over-thickened lower crust. Geochemical and Sr–Nd isotopic data of granitoids from the Latest Cretaceous to Eocene period (71–45 Ma) reveal that they result from diverse magma sources including both juvenile and reworked mature crustal materials. Our compilation of new and published data from the eastern Lhasa terrane show a marked variation in Sr/Y ratios and a step change in Sr–Nd isotope compositions during the Late Cretaceous. We suggest that the Middle Jurassic to Early Cretaceous granitoids resulted from the northward Neo-Tethyan ocean slab subduction. The increasing Sr/Y ratios from the Middle Jurassic to Late Cretaceous granitoids and northeastward migration of arc magmatism in the eastern Lhasa terrane during the interval ca. 125–95 Ma are attributed to the shallowing angle of subduction of the Neo-Tethyan ocean slab. The Late Cretaceous magmatism in the eastern Lhasa terrane probably resulted from the Neo-Tethyan mid-ocean ridge subduction and subsequent delamination of the arc root. The Latest Cretaceous to Eocene granitoids could be interpreted as a magmatic response to roll-back and/or break-off of the subducted Neo-Tethyan slab.

Understanding the processes of tectonic erosion and crustal relamination during continental collision has important implications for the growth and differentiation of the continental crust. The discrepancy in isotopic compositions between the pre- and syn-collision magmatic rocks from the Gangdese belt in south Tibet provides an opportunity for studying these processes during the India-Asian collision. The Nyingchi granites and Confluence hornblende gabbros from the eastern Gangdese belt have zircon U-Pb ages of ca. 50 Ma. The Nyingchi granites have high Sr/Y and (Dy/Yb)N ratios, indicating that the magma was generated under eclogite-facies conditions. Their Sr-Nd-Pb-Hf isotopic compositions require significant incorporation of ancient supracrustal materials from the Gangdese belt and the Indian continent. The Confluence hornblende gabbros display arc-like trace element patterns but have enriched Sr-Nd-Pb-Hf isotopic compositions compared with those from the Jurassic-Cretaceous arc magmatic rocks, indicating significant input of ancient components into their mantle sources. The occurrence of the Cenozoic felsic metamorphic rocks in the lower crust of the Gangdese belt allows us to propose that the Nyingchi high Sr/Y granites were derived from partial melting of relaminated crustal materials which were removed from the Gangdese belt by tectonic erosion and the subducted Indian continent. The Confluence gabbros were sourced from lithospheric mantle which was metasomatized by inputs from relaminated crustal materials derived from the Gangdese belt and the subducted Indian continent. The estimated tectonic erosion rate is 150–188 km3/km/my, indicating significant crustal loss occurred during continental collision. Our study demonstrates that tectonic erosion and crustal relamination play an important role in the refinement of the composition of continental crust during continental collision.

The Precambrian history of the Lhasa Terrane provides important context for understanding the India–Asia continental collision and formation and evolution of the Tibetan Plateau. However, it has not yet been well constrained, due to the scarcity of Precambrian outcrops. In this study, we present geochronological, geochemical, and Lu–Hf isotopic data for granitoid gneisses from the Bomi Complex in the eastern Lhasa Terrane. Our new data, together with those from the literature, reveal three episodes of magmatic activity at ca. 1866–1782 Ma, ca. 1343–1250 Ma, and ca. 824 Ma, and two metamorphic events at ca. 1117 Ma and ca. 625–600 Ma. While the Mesoproterozoic gneiss complexes suggest a transition from early rift-related to late arc signatures, the other two episodes of gneiss formation have arc signatures. We suggest that during the Paleoproterozoic and Middle Neoproterozoic, the Bomi Complex had an affinity with the Lesser and Greater Himalaya terranes along the northern margin of India, respectively. During the Mesoproterozoic and late Neoproterozoic, the Bomi Complex might be correlated with the contact zone between the Indian Archean cratons and the Eastern Ghats Belt in southeastern India. These magmatic and metamorphic events indicate that the Lhasa Terrane was located along the northern margin of the Indian continent during its early evolution. We propose the Precambrian evolution of the Lhasa Terrane was as follows: (1) the Lhasa Terrane and Lesser Himalayan sequence represent a Paleoproterozoic continental arc along the northern margin of the Indian continent; (2) the eastern Lhasa Terrane was affected by the Grenvillian-age Eastern Ghats orogeny during the Mesoproterozoic; (3) the Lhasa Terrane and Greater Himalayan Crystalline Complex formed a middle Neoproterozoic Andean-type orogen along the margin of the Rodinia supercontinent; and (4) the Lhasa Terrane experienced the Pan-African Orogeny, which might have marked the initial assembly of East Gondwana.

The Qinling–Dabie–Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U–Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite–granodiorite pluton in the middle part of the West Qinling orogen. U–Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244–242 Ma for the Xiekeng pluton. Geochemical and Sr–Nd–Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.