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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]

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]

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]

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]