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The reworking of continental crust in collision zones is a key process in the chemical maturation of crustal rocks. However, the mechanisms that cause crustal reworking remain highly controversial. The compositionally diverse magmatic rocks along the Lhasa–Qiangtang collision zone provide a favorable opportunity for studying the mechanism of crustal reworking and maturation in continental collision zones. Here we report zircon U Pb ages and Hf isotopes, mineral major elements, and whole-rock geochemistry and Sr Nd isotopes for the Momu intrusive complex in the Lhasa–Qiangtang collision zone, central Tibet. Zircon U Pb ages reveal that the intrusive complex formed during the late Early Cretaceous (115–113 Ma). Diorites from the intrusive complex have high Mg# values (49.7–51.2) and initial 87Sr/86Sr ratios (0.7082 to 0.7083), and low ε Nd (t) values (−7.3 to −6.9), with negative to slight positive zircon ε Hf (t) values (−3.8 to +0.8), suggesting that these rocks were likely generated by the partial melting of the enriched lithospheric mantle. Granodiorites from the intrusive complex have high SiO 2 (64.4–67.9 wt%) and Al 2 O 3 (14.8–16.1 wt%) and low MgO (1.4–1.8 wt%) contents. These rocks are also characterized by high initial 87Sr/86Sr ratios (0.7091–0.7098) and whole-rock negative ε Nd (t) (−9.9 to −8.4) and zircon ε Hf (t) (−7.9 to −2.2) values. The associated granites have the highest SiO 2 (68.7–71.5 wt%) contents among the intrusive complex rocks, high initial 87Sr/86Sr (0.7089–0.7093) ratios, and negative whole-rock ε Nd (t) (−10.1 to −7.7) and zircon ε Hf (t) (−5.3 to −0.2) values. These geochemical characteristics indicate that the granodiorites were most likely derived from the partial melting of ancient continental crust and that the coeval granites were formed by the fractional crystallization of granodioritic magmas. Integrating our new results with data for the coeval magmatic rocks of the Lhasa–Qiangtang collision zone, we suggest that the Momu intrusive complex formed in a post-collisional slab breakoff setting. Asthenosphere upwelling caused by slab breakoff could have provided a substantial source of heat for crustal reworking, which gradually contributed to the maturation of continental crust in the Lhasa–Qiangtang collision zone. • The Momu intrusive complex of central Tibet formed at 115–113 Ma. • The diorites were derived by partial melting of modified lithospheric mantle. • The granodiorites and granites formed via partial melting of ancient crust. • Crustal maturation occurred by collision of the Lhasa and Qiangtang terranes. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Jiaqiong andesites and mafic dikes were formed at ca. 115 Ma. • The andesites were derived by partial melting of modified lithospheric mantle. • The mafic dikes were produced by asthenosphere–lithospheric-mantle interaction. • Post-collisional magmatism contributed to vertical growth of continental crust. Post-collisional magmatic rocks in central Tibet provide an opportunity to study the processes of continental crustal growth in a collision zone. Here, we report data for samples of andesites and coeval mafic dikes obtained from the Jiaqiong area of the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating shows that the andesites and mafic dikes were emplaced concurrently during the late Early Cretaceous (ca. 115 Ma). The andesites have high Mg# values (55.2–61.6) and initial 87Sr/86Sr ratios (0.7086–0.7088), negative whole-rock ε Nd (t) (−6.5 to − 6.1) and zircon ε Hf (t) (−5.1 to − 1.7) values, and elevated zircon δ18O (6.8‰–7.5‰) values, indicating that the andesites were probably derived by the partial melting of an ancient subduction-modified lithospheric mantle. The coeval mafic dikes have moderately high Mg# values (49.3–50.0) and initial 87Sr/86Sr ratios (0.7076–0.7082), and negative ε Nd (t) values (−5.4 to − 5.3), implying that they were derived from an enriched lithospheric mantle source. However, the zircon ε Hf (t) (−5.8 to + 1.6) and δ18O (5.2‰–6.3‰) values of these mafic dikes indicate the influence of depleted mantle components. Furthermore, results of Sr–Nd–Hf–O isotopic mixing modeling suggest that these mafic dikes could have been produced by interaction between asthenospheric and lithospheric mantle. Therefore, we suggest that the parental magmas of the mafic dikes were probably generated by interaction between asthenospheric and overlying metasomatized lithospheric mantle. Combined with data on nearby Cretaceous magmatic rocks and sedimentary strata, we propose that the studied magmatic rocks were formed in a post-collisional slab breakoff setting in central Tibet and have made a substantial contribution to the vertical growth of continental crust in this continental collision zone. [ABSTRACT FROM AUTHOR]

The petrogenesis of biotite granites, which may be metaluminous or peraluminous in composition, is a subject of debate. In this study, we present new zircon U-Pb geochronology and O-Hf isotopic, and whole-rock major and trace element, and Sr-Nd isotopic data for biotite granites in the Langla–Laqinpu area of northern Lhasa Terrane, central Tibet. The rocks are distributed along the eastern segment of the Bangong–Nujiang Tethys Ocean suture, and were emplaced during the late Early Cretaceous (ca.119 Ma). The biotite granites are peraluminous, and have high SiO 2 (67.5–72.4 wt%) and low MgO (0.57–1.20 wt%) and CaO (2.36–4.38 wt%) contents with variable K 2 O/Na 2 O ratios (0.77–1.32), and show geochemical affinity with the upper-continental crust, e.g., enrichment of Rb, Th, U, K and Pb, depletion of Nb, Ta, Ti, P, Eu, and Sr. They are characterized by relatively high initial 87Sr/86Sr ratios ((87Sr/86Sr) i = 0.7098–0.7137) and low ε Nd (t) values (−8.4 to −10.3), and high zircon δ18O values (8.29–13.47‰) and variable ε Hf (t) values (−0.02 to −21.36), indicating they were derived through partial melting of a crustal source consisting of metagreywackes with or without minor metaigneous rocks. Taking into account all available regional geological data, we suggest that the Bangong–Nujiang Tethys Ocean may undergo diachronous closure–although the eastern segment may have been closed by the late Early Cretaceous, its western and western–middle segments may still have been at least partially open and active at that time. The Langla–Laqinpu biotite granites were probably generated in post-collisional setting, following slab break-off after closure of the Bangong–Nujiang Tethys Ocean. • Cretaceous biotite granites were derived by partial melting of metagreywackes. • These biotite granites were formed in a post-collisional setting. • The Bangong–Nujiang Tethys Ocean may undergo diachronous closure. [ABSTRACT FROM AUTHOR]

As one of the most important parts of the eastern Tethys, the evolution of the Bangong–Nujiang Tethyan Ocean (BNTO) has important geological implications for paleogeographic reconstructions of the Tethyan Realm. However, the closure time of the BNTO remains highly debated. Here, we report geochronological, petrological and geochemical data from three newly identified undeformed stitching plutons that intrude a north–south-oriented section of ophiolitic mélanges in the middle–eastern parts of the Bangong–Nujiang suture zone (BNSZ), central Tibet. The stitching plutons comprise the Chaqu diorite–granite association, the Jiang Co granite, and the Konglong diorite porphyry. Secondary ion mass spectroscopy (SMIS) zircon U Pb dating suggests that these three plutons were emplaced at ca. 116–112 Ma. The stitching plutons show heterogeneous Sr–Nd–Hf–O isotopic compositions. The Chaqu diorites have initial 87Sr/86Sr ratios varying from 0.7062 to 0.7064 and ε Nd (t) values from −2.0 to −1.6, and slightly high zircon δ18O (6.2–6.8‰) and positive zircon ε Hf (t) (+2.6 to +7.4) values. The Chaqu granites show high initial 87Sr/86Sr (0.7064–0.7065) ratios and low ε Nd (t) (−2.3 to −2.1) values, and elevated zircon δ18O (6.2–7.3‰) values and variable zircon ε Hf (t) (−1.7 to +4.9) values, similar to those of the associated Chaqu diorites. The Jiang Co granites exhibit higher initial 87Sr/86Sr (0.7085–0.7094) ratios and negative ε Nd (t) (−8.33 to −8.27) values, and variable zircon δ18O (4.8‰ to 6.0‰) and ε Hf (t) (−6.9 to −2.0) values. The Konglong diorite porphyries have initial 87Sr/86Sr isotopic ratios ranging from 0.7069 to 0.7075 and negative ε Nd (t) values ranging from −4.2 to −4.1, and high zircon δ18O (9.0‰ to 9.8‰) and variable zircon ε Hf (t) (−7.3 to +0.7) values. Geochemical evidence indicates that they were derived from different magma sources that included varying proportions of mantle and crustal components. The Chaqu diorites were derived by partial melting of an ancient metasomatized mantle source, and the Chaqu granites were formed by fractional crystallization of the Chaqu dioritic magmas. The Jiang Co granites were generated by partial melting of low δ18O high-temperature hydrothermally altered ancient continental crust. The Konglong diorite porphyries were likely generated by partial melting of high-δ18O enriched lithospheric mantle. In combination with data from adjacent ophiolites, magmatic rocks, and Cretaceous sedimentary rocks, we suggest that the stitching plutons were generated in a post-collisional setting induced by upwelling of asthenospheric mantle due to slab breakoff beneath the BNSZ. Our data provide an important new upper age limit for the time of collision between the Lhasa and Qiangtang terranes, i.e., the latest closure time for the BNTO. • Stitching plutons provide upper age limits on the timing of closure of the BNTO. • Contemporaneous plutons show heterogeneous Sr–Nd–Hf–O isotopic compositions. • Stitching plutons were formed by slab breakoff in a post-collisional setting. [ABSTRACT FROM AUTHOR]

High-silica granites are common in collisional orogenic belts. However, the petrogenesis of such granites and the geodynamic processes associated with their emplacement are debated. In this study, we present zircon U–Pb ages and Hf isotopes, and whole-rock geochemical and Sr–Nd isotopic compositions for biotite granites from the Jiang Co area, in the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating reveals that the Jiang Co granites were emplaced during the late Early Cretaceous (ca.114 Ma). These granites are characterized by high and variable SiO 2 (69.7–78.2 wt%) and K 2 O (4.7–8.4 wt%), and low MgO (0.09–1.25 wt%) and P 2 O 5 (< 0.1 wt%) contents. The samples are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, Ti, and Eu. These geochemical features suggest that the Jiang Co granites underwent extensive fractional crystallization. The high and variable initial 87Sr/86Sr (0.7066 to 0.7095) ratios and consistent ε Nd (t) (–8.2 to –9.2) values, along with T Nd 2DM ages of 1.66 to 1.58 Ga, and negative to positive zircon ε Hf (t) (−7.5 to +1.8) values, imply that these granites were formed by partial melting of the mature ancient crustal materials with minor involvement of mantle materials. Along with contemporaneous magmatic activity, we suggest that the Jiang Co granites were mainly generated by crustal melting in a post-collisional setting resulting from slab breakoff after the Lhasa–Qiangtang collision, and subsequently underwent a high degree fractional crystallization. • The Jiang Co granites were formed in the late Early Cretaceous (ca. 114 Ma). • These granitic magmas were derived from ancient crustal materials. • These granites were generated in a post-collisional setting. [ABSTRACT FROM AUTHOR]

Arc magmas are generated by complex geological processes in subduction settings. Due to the complexity of source materials and geological processes, the genesis of arc magmatic rocks can be difficult to disentangle. Here, we report on diorites from the Kenama area of the middle-eastern part of the Bangong–Nujiang suture zone in central Tibet. New SIMS and LA–ICP–MS zircon U Pb dating for three diorite samples indicates that they crystallized during the early Late Jurassic (ca. 161 Ma). The diorites can be divided into two groups based on whole-rock major element compositions and mineralogical characteristics: Group I diorites with relatively low SiO 2 (47.6–53.4 wt%), and high MgO (4.97–9.10 wt%) and amphibole contents (50–54 vol%); and Group II diorites with slightly higher SiO 2 (56.9–59.5 wt%), and low MgO (2.9–3.8 wt%) and amphibole contents (25–30 vol%). The Group I diorites exhibit relative enrichment in light rare earth element (LREE) ((La/Yb) N = 5.7–10.5) with slightly negative Eu anomalies, and are characterized by enrichment of large ion lithophile elements (LILEs; K, Rb, Ba, Th, and U), the depletion of high field strength elements (HFSEs; Nb, Ta, Zr, Hf, and Ti). The Group I diorites have relatively high initial 87Sr/86Sr ratios (0.7073–0.7094), variably negative ε Nd (t) values(−10.3 to −5.9), negative zircon ε Hf (t) values (−14.8 to–4.4), and slightly elevated δ18O values (6.5‰–7.3‰). The Group II diorites show similar trace element characteristics to Group I. They also have high initial 87Sr/86Sr ratios (0.7083–0.7087), uniformly negative ε Nd (t) values (−9.5 to −9.0), negative zircon ε Hf (t) values (−10.3 to −4.2), and elevated δ18O values (6.6‰–7.5‰). Amphiboles from Group I and Group II diorites have low Al 2 O 3 contents (3.6–9.9 wt% and 5.6–8.5 wt%, respectively), and formed at similar P–T conditions (751–871 °C and 69–226 MPa and 745–832 °C and 77–162 MPa, respectively). On primitive mantle-normalized spider diagrams, these low-Al amphibole grains have slightly convex upward REE patterns with distinctly negative anomalies in Pb, Sr, Eu, Zr, Hf, and Ti, suggesting that amphiboles from both groups crystallized from the similar arc magmas after plagioclase and magnetite crystallization. These mineralogical, geochemical and isotopic characteristics suggest that both groups of Kenama diorites probably originated from an enriched lithospheric mantle metasomatized by subducted oceanic sediment-derived melts. Their parental magmas may have similar geochemical characteristics and underwent varying degrees of accumulation and fractional crystallization. The Group I diorites were likely generated by accumulation of amphibole and fractional crystallization of olivine, clinopyroxene, and plagioclase from mafic magmas, and the Group II diorites were formed by the fractional crystallization of clinopyroxene and plagioclase from mafic magmas that were geochemically similar to the Group I diorites. In combination with regional geology, in particular adjacent ophiolites, high-magnesian andesitic rocks and Cretaceous sedimentary rocks, we conclude that all of the Kenama diorites were probably generated in an early Late Jurassic arc setting related to the subduction of the Bangong–Nujiang Tethys oceanic lithosphere. • Diorites in the Kenama area were emplaced at ca. 161 Ma. • Diorites originated from a lithospheric mantle metasomatized by sediment melts. • Formation of diorites was related to subduction of oceanic lithosphere. [ABSTRACT FROM AUTHOR]

High-MgO (>9 wt %) basaltic rocks can be primary magmas and are used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus, the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (>9 wt %) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at c. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt %), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (>3 GPa) in the presence of H2O and CO2. These geochemical features are different from those of the c. 30-Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that a heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (>9 wt %), evidence from mineral chemistry indicates that the whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and the subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of the mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations. [ABSTRACT FROM AUTHOR]

In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ18O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ18O mantle wedge, it has been difficult to decipher the contribution of high-δ18O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ18O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO2 (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallized before and after zircon remain almost constant. Interstitial zircons have δ18O (6.1–7.2‰) values similar to the earliest crystallized olivine (δ18O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ18O arc mantle modified by small amounts (<12%) of subducted sediments, and crystallized zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallization (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ18O zircons to crustal reworking may conceal recent crustal growth. [ABSTRACT FROM AUTHOR]

Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high 87Sr/86Sr (>0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya–Tibet orogen is one of the world's best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr–Nd–Pb–Hf–O isotopic compositions of Cenozoic potassic–ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U–Pb dating, coupled with published age results, constrain the timing of volcanism from ~8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr ≥ 0.7080; ε Nd ≤ −4.8; 206Pb/204Pb ≥ 18.704; ε Hf ≤ −2.6). Together with high zircon δ18O values (6.3–10.4‰), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India–Eurasia collision. Partial melting models assuming a hybridized mantle source that contains ~5% Indian continental crust suggest that the primary melts of the potassic–ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1–5%). The magma geochemistry is consistent with the model of mélange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably <50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities. [ABSTRACT FROM AUTHOR]

Fayalite- and ferrosilite-bearing felsic igneous rocks are a distinctive category of A-type granitic magmas, and the origins of these rocks have proved controversial. In addition, the shallow-crust high-silica rhyolitic magmas responsible for such rocks are considered to have higher viscosities than their plutonic equivalents, which casts doubt on whether fractional crystallization could have occurred during the evolution of these high-silica melts. Here, we report a study of earliest Early Cretaceous (145–142 Ma) fayalite- and ferrosilite-bearing rhyolitic porphyries (FFBRPs) and closely associated biotite-bearing rhyolitic porphyries (BBRPs) from the Baishuizhai area, Guangzhou, Guangdong Province, South China. The Baishuizhai suite provides an excellent opportunity to establish the magmatic origins and evolutionary processes responsible for the FFBRPs. Rocks from the Baishuizhai suite have SiO 2 contents of 73.2–77.7 wt%, Na 2 O + K 2 O contents of 7.83–9.42 wt%, and Fe 2 O 3 T contents of 1.52–2.97 wt%. They have A-type granite features including mineral assemblages (euhedral fayalite and ferrosilite in addition to anhedral amphibole and biotite) and geochemical characteristics [e.g., high 10,000 × Ga/Al values (3.7–4.7), high FeOT/MgO ratios (13.0–34.8), and high Zr (245–514 ppm) and Nb (56.4–96.5 ppm) contents]. The rocks have slightly enriched in situ plagioclase Sr [(87Sr/86Sr) i = 0.7070–0.7076] and whole-rock Nd [ε Nd (t) = −2.3 to −2.0] compositions and depleted zircon Hf isotopic compositions [ε Hf (t) = −0.9 to +7.0] relative to chondrite, as well as higher zircon δ18O values (6.3‰–7.8‰) and intermediate in situ plagioclase Pb isotopic compositions [(206Pb/204Pb) i = 18.368–18.727, (207Pb/204Pb) i = 15.488–15.673, and (208Pb/204Pb) i = 38.049–38.801] compared with depleted mantle. We infer that the primary magmas of this rock suite were formed by mixing of mantle-derived magmas with subordinate (20%–50%) metasedimentary-rock-derived magmas. Fayalite and ferrosilite in the FFBRPs were most likely crystallized from dry, hot, and reduced magma in a lower (<16.5 km depth) chamber. The lower magma gradually evolved during ascent to a water-enriched magma in an intermediate chamber at a shallow depth (3–4 km), forming the FFBRPs (elevation: 129–168 m). Finally, the residual melts migrated upward and resided in an upper (<3 km depth) chamber, forming the BBRPs (elevation: 196–824 m). According to the established temporal–spatial distribution of Early Cretaceous A-type granites and adakitic rocks in South China, we suggest that the Paleo-Pacific plate underwent diachronous rollback, starting during the earliest Early Cretaceous. This rollback enhanced the degree of crust–mantle interaction in the Guangzhou area. [Display omitted] • The Baishuizhai suite was formed at earliest early Cretaceous (145–142 Ma). • The Baishuizhai suite was mainly derived from mantle. • The Paleo-Pacific plate underwent diachronous rollback during the Early Cretaceous. [ABSTRACT FROM AUTHOR]

Amphibole is thought to be a key control on the differentiation of arc magmas but the fractionation of this mineral remains enigmatic because it is rarely present in erupted lavas. This study presents detailed petrological, mineralogical, and geochronological data for newly identified Late Triassic (217–215 Ma) mafic intrusive rocks from the Gangdese arc of southern Tibet. These gabbros host an amphibole-rich cumulate enclave and clinopyroxene crystal clots, including two mineral assemblages in chemical disequilibrium: (1) amphibole and Type-1 clinopyroxene within the enclave and clots; and (2) small crystals of Type-2 clinopyroxene, plagioclase, and biotite within the host. Textural and geochemical evidence indicate that the enclave and clots represent captured fragments of amphibole- and clinopyroxene-dominated lower crustal crystal mushes, respectively, whereas small host crystals formed by the shallow crystallization of evolved melts extracted from these crystal mushes. The compositional variations between the two types of clinopyroxene also reveal this melt segregation process. Amphibole with similar rare earth element patterns to associated Type-1 clinopyroxene inclusions suggest that the former formed through peritectic reactions consuming clinopyroxene. Enhanced amphibole fractionation may also have occurred as a result of the presence of high-density clinopyroxene inclusions. This indicates that the fractionation of amphibole together with precursory clinopyroxene inclusions within the deep crust controls the early chemical evolution of mafic arc magmas, a process that is consistent with the geochemical variations in lavas from three other active arc volcanoes. The associated cumulates can efficiently delaminate back into the mantle, ultimately leading to the generation of SiO 2 -rich continental crustal material. • Late Triassic mafic rocks represent early products of Indus–Yarlung Zangbo Tethyan subduction. • The amphibole-rich enclave and clinopyroxene crystal clots were derived by crystal fractionation. • The fractionation of amphibole and clinopyroxene precursors occurs in the deep arc crust. • This fractionation controls the early chemical evolution of arc magmas. [ABSTRACT FROM AUTHOR]

The nature and pre‐Cenozoic evolution history of crust in southern Lhasa, which is crucial for our understanding of Indo‐Asian continental collision and Tibetan uplift during the Cenozoic, remains controversial due to a "missing" pre‐Mesozoic magmatic record. In this contribution, we report petrological and geochemical data for newly identified Paleozoic bimodal magmatism in the Zhengga area of southern Tibet. The magmatism comprises Late Devonian‐Early Carboniferous (366–353 Ma) amphibolite and two‐mica gneissic granite. The protoliths of the Zhengga amphibolite were gabbro and diorite with low SiO2 and high MgO, Cr, and Ni contents with high εNd(t) values of +3.3 to +8.0, variable and positive zircon εHf(t) of +0.9 to +11.2, and low zircon δ18O of 5.7 ± 0.2‰. These protoliths are proposed to have formed by decompression melting of asthenosphere during intracontinental back‐arc extension. In contrast, the granite has relatively high SiO2 and low MgO contents with much lower εNd(t) of −8.6 to −7.3, variable and negative zircon εHf(t) of −10.4 to −1.3, and high zircon δ18O of 9.4 ± 0.2‰ values and was most likely derived from an ancient metasedimentary source. This magma subsequently underwent recharge with minor amounts mafic magma followed by fractional crystallization of K‐feldspar in middle‐upper crust (~10–20 km) magma chambers. Using our new data, in combination with Nd‐Hf isotopes, we present the first comprehensive picture of crustal evolution in southern Lhasa. The southern Lhasa sub‐block is likely to have been a microcontinent that underwent extensive Phanerozoic crustal reworking and growth, rather than a Mesozoic‐Early Tertiary juvenile accretionary arc terrane. Key Points: Late Paleozoic bimodal rocks comprising asthenosphere‐derived mafic rock and metasediment‐derived granite have been discovered in southern TibetThe Lhasa was once a microcontinent within Paleo‐Tethyan Ocean and the bimodal magmatism was caused by northward subduction of oceanic slabThe microcontinent represented by the southern Lhasa Block was transformed into juvenile terrane by Phanerozoic crustal growth and reworking [ABSTRACT FROM AUTHOR]

A-type granites have been the focus of considerable research due to their distinctive major- and trace-element signatures and tectonic significance. However, their petrogenesis, magmatic source and tectonic setting remain controversial, particularly for aluminous A-type granites. The earliest Cretaceous (ca. 140 Ma) Comei granite in the eastern Tethyan Himalaya is associated with coeval oceanic island basalt (OIB)-type mafic lava, and has A-type granite geochemical characteristics including high 10,000 × Ga/Al (up to 6), FeO total /MgO (4.6–6.1) and (Na 2 O + K 2 O)/Al 2 O 3 (0.50–0.61) ratios but low CaO (0.6–1.6 wt%) and Na 2 O (1.8–2.6 wt%) contents. The Comei granite also has variable peraluminous compositions (A/CNK = 1.00–1.36) along with zircon δ 18 O, ε Nd (t) and initial 87 Sr/ 86 Sr values of 8.2‰ to 9.3‰, − 13.0 to − 12.4 and 0.7238 to 0.7295, respectively. This range of compositions can be interpreted as the interaction between high-temperature upwelling OIB type basaltic magmas and a shallow crustal (< 5 kbar) metapelitic source. The Comei granite and coeval OIB type basaltic rock could represent the earliest stage (145–140 Ma) of a large igneous event in eastern Tethyan Himalaya, which may well have been triggered by pre-breakup lithospheric extension prior to the arrival of the Kerguelen plume head. [ABSTRACT FROM AUTHOR]

Ferroan granitoids are typically generated in the extensional tectonic settings, and thus record important geodynamic processes related to crustal and mantle evolution. However, their petrogenesis remains controversial. In this study, we investigated newly identified ferroan granites in the Lake Cona region of the southern Qiangtang subterrane (SQT), central Tibet. We present zircon U–Pb age and Hf–O isotope, whole-rock major and trace element and Sr–Nd isotope, and mineral major element data for these rocks. The Lake Cona granites consist of biotite monzogranite and granite porphyry. Zircon U–Pb dating indicates these rocks were generated in the late Late Cretaceous (ca. 75 Ma). The biotite monzogranites are metaluminous to weakly peraluminous, and characterized by relatively high SiO 2 (67.1–68.6 wt%), total alkali (10.4–10.8 wt%), Th (31–46 ppm), light rare earth element (LREE), and Zr + Nb + Ce + Y (421–730 ppm) contents, and high Fe-numbers (0.83–0.91; FeOt/[FeOt + MgO]). These rocks can be classified as alkalic ferroan granites. However, compared with the typical ferroan granitoids generated under relatively anhydrous conditions, the biotite monzogranites have higher Al 2 O 3 (15.9–17.0 wt%) contents, lower 10000 × Ga/Al (2.1–2.5) ratios, and small negative Eu anomalies, suggesting that they were generated by brine-assisted (a fluid with low water activity) crustal anatexis. The biotite monzogranites have (87Sr/86Sr) i = 0.7065–0.7069, ε Nd (t) = −2.3 to −1.9, δ18O = 6.0–6.6‰, and ε Hf (t) = +2.1 to +9.3, indicative of a magma source that was a mixture of juvenile and ancient crustal components. The granite porphyries also have high contents of total alkalis (9.9–10.2 wt%) and Fe-numbers (0.87–0.90), and similar Sr–Nd–Hf–O isotopic compositions as the biotite monzogranites, but have higher SiO 2 and lower TiO 2 , MgO, TFe 2 O 3 , P 2 O 5 , CaO, and REE contents, and larger negative Eu anomalies. These features suggest that the granite porphyries represent the highly evolved biotite monzogranitic magmas that underwent differentiation in the shallow crust. The Lake Cona ferroan granites formed in a post-collisional extensional setting that had additional heat input from the deep mantle. Unlike the early Late Cretaceous that was dominated by calc-alkaline magmatism in the SQT, the occurrence of alkali ferroan granites in the late Late Cretaceous may record the transformation from a post-collisional to an intraplate tectonic regime in the central Tibet. • The Lake Cona alkali ferroan granites formed at ca.75 Ma. • Ferroan granites generated by brine-assisted anatexis. • A Campanian transition from a post-collisional to intraplate setting [ABSTRACT FROM AUTHOR]

Crustal reworking in the collisional zones is commonly considered as an important process to result in continental crust differentiation and maturation. However, the reworking mechanism remains unclear. Here, a combined study of zircon SIMS U–Pb age, whole-rock major and trace element, Sr–Nd–Hf and zircon Hf–O isotopic geochemistry was carried out for the Duoma–Namuqie (DM–NMQ) granite porphyry intrusions, including DM, NMQ I, II and III intrusions, in southern Qiangtang, central Tibet. Zircon SIMS U–Pb dating yielded ages of 79–76 Ma, suggesting that they were formed in the later stage of Late Cretaceous. The DM granite porphyries are metaluminous to peraluminous with slightly low SiO 2 (69.2–71.4 wt%) and high MgO (0.67–0.79 wt%) contents. They are depleted in high-field-strength elements (HFSEs) and enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), and show weakly negative Eu anomalies. The NMQ III granite porphyries show similar elemental features to the DM granite porphyries, but have relatively high SiO 2 (69.8–72.2 wt%) and low MgO (0.25–0.28 wt%) contents. Both of them belong to I-type granites. The NMQ I and II granite porphyries are relatively differentiated with high SiO 2 (76.4–77.9 wt%), low MgO (0.11–0.16 wt%) contents, and significantly negative Eu anomalies. Extremely low LREE contents and notable REE tetrad effect can be observed in the NMQ II granite porphyries. The NMQ I and II intrusions are fractionated I-type granites. The whole-rock Sr–Nd–Hf and zircon Hf–O isotope data exhibit an enriched trend from the DM granite porphyries ((87Sr/86Sr) i = 0.7049–0.7050, ε Nd (t) = 1.2–1.6, ε Hf (t) whole-rock = 9.9–12.0, ε Hf (t) zircon = 7.4–11.5, δ18O = 6.2–7.1‰) to the NMQ III granite porphyry ((87Sr/86Sr) i = 0.7058–0.7067, ε Nd (t) = −0.9 to −1.1, ε Hf (t) whole-rock = 7.0–7.4, ε Hf (t) zircon = 2.7–9.5, δ18O = 7.2–7.6‰) to the NMQ I and II granite porphyries (ε Nd (t) = −3.0 to −4.3, ε Hf (t) whole-rock = 5.3–6.6, ε Hf (t) zircon = 1.5–7.4, δ18O = 6.6–9.2‰). We suggest that the DM–NMQ granite porphyries were mainly derived by partial melting of the juvenile mafic lower crust with variable volumes of ancient crustal components involved in their magma sources. The way in which these granite porphyries were formed provides evidence for the crustal differentiation and maturation in a post-collisional extensional setting triggered by the far-field rollback of the subducted Neo-Tethys oceanic slab • The Duoma–Namuqie granite porphyries were formed at 79–76 Ma. • The Duoma–Namuqie granite porphyries were mainly derived by juvenile crust reworking. • The Duoma–Namuqie granite porphyries were formed in a post-collisional setting. [ABSTRACT FROM AUTHOR]

The South China Block (SCB) is considered to have undergone extensive reworking of continental crust. However, the mechanisms of this crustal reworking are unclear. Here, we report on Late Jurassic (162–155 Ma) two-mica granites and associated diorites and syenite porphyries from the Huolushan–Longyandong area of Guangzhou City, southeastern China. The two-mica granites have high SiO 2 contents (69.6–73.6 wt%) and weakly peraluminous to strongly peraluminous compositions. These rocks have enriched Sr–Nd–Hf isotopic compositions ((87Sr/86Sr) i = 0.7094–0.7145; ε Nd (t) = −10.5 to −9.3; ε Hf (t) = −12.7 to −4.2). The Huolushan two-mica granites have variable zircon O isotopic compositions (δ18O = 6.8‰–10.4‰), whereas the Longyandong two-mica granites have homogeneous compositions (δ18O = 8.6‰–9.9‰). The Huolushan syenite porphyries have moderate SiO 2 (59.6–60.1 wt%) and high alkaline (K 2 O + Na 2 O = 9.4–9.8 wt%) contents. Their Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr) i = 0.7105–0.7119; ε Nd (t) = −9.9 to −9.8; ε Hf (t) = −14.1 to −5.9; δ18O = 6.5‰–10.6‰) are similar to those of the Huolushan two-mica granites. The Huolushan diorites have moderate SiO 2 contents (59.9–60.4 wt%) and are high-K calc-alkaline and metaluminous with Mg# values of 43.2–43.8. Their Sr Nd isotopic compositions ((87Sr/86Sr) i = 0.7106–0.7109, ε Nd (t) = −8.8 to −8.6) are less enriched than those of the two-mica granites and syenite porphyries. In addition, the diorites have high zircon δ18O values (7.8‰–8.9‰) and a wide range of zircon ε Hf (t) values (−13.4 to −5.6). We propose that the Huolushan two-mica granites were derived from a metagreywacke-dominated hybridized source with minor juvenile crustal rocks and that the Longyandong two-mica granites were generated by partial melting of metagreywackes. The Huolushan syenite porphyries were formed by the mixing of metasedimentary-rock-derived felsic magmas and minor enriched-mantle-derived alkaline magmas. The Huolushan diorites were most likely derived by partial melting of mafic lower crust, with the dioritic magmas undergoing assimilation and fractional crystallization (AFC) during ascent. All of these intrusive rocks were likely formed in an extensional intraplate setting. The mantle-derived magmas, which could have been triggered by lithospheric extension, underplated the middle–lower crust and provided heat for crustal reworking and materials for late Mesozoic crustal growth. • The Huolushan–Longyandong granitoids were formed at Late Jurassic (162–155 Ma). • The Huolushan–Longyandong granitoids were mainly derived from crust. • Middle crust reworking of the South China could be heated by mafic magmas. • Crust reworking of the South China accompanied by vertical crustal growth. [ABSTRACT FROM AUTHOR]