The high‐pressure and low‐temperature metamorphic rocks constitute the important parts of the Longmu Co‐Shuanghu suture zone (LSS) in Tibet, providing a natural window for unveiling the tectonic evolution of the Palaeo‐Tethys Ocean. In this contribution, geochemical studies, including the whole‐rock geochemistry, Sr‐Nd isotopes and zircon U–Pb geochronology, were performed on the Pianshishan garnet amphibolites and retrograded eclogites, to reconstruct the genesis of the protoliths and to reveal tectonic evolution of the Longmu Co‐Shuanghu Palaeo‐Tethys Ocean (LSPTO). The garnet amphibolites show characteristics similar to the retrograded eclogites, representing different degrees of retrograde metamorphism of the Pianshishan eclogites. The protoliths of garnet amphibolites and retrograded eclogites are alkaline basalts, with geochemical characteristics similar to ocean island basalts (OIB) without Nb, Ta or Ti negative anomalies. Furthermore, geochemical data imply that the protoliths were formed by fractional crystallization of the magma derived from partial melting of garnet lherzolites without crustal contamination. The parental magma was probably generated in a relatively depleted but heterogeneous asthenosphere mantle source. Their protoliths erupted as a seamount within the LSPTO. Garnet amphibolites and retrograded eclogites yielded zircon U–Pb weighted mean ages of 240.2 ± 1.7 Ma and 239.1 ± 2.7 Ma, respectively. Combining with previous studies, we suggest that the LSPTO remained open in the Pianshishan area during the Middle Triassic, and the whole ocean might be gradually closed during the Early to Late Triassic. The small remnant ocean basin during the Middle Triassic can well explain a rapid evolution of the Pianshishan eclogite‐type rocks, that is, from generation to eclogite‐facies metamorphism and then to exhumation‐related retrograde metamorphism within ~19 Myr.
The genesis of Late Triassic granitoids in the central Qiangtang is significant for deducing the evolution of the Palaeo‐Tethys Ocean in Qiangtang Block. Herein, we present a comprehensive study of zircon U–Pb ages, whole‐rock geochemistry and zircon Lu–Hf isotopes for the mafic microgranular enclaves (MMEs) and host granodiorites from the Shuanghu area, in order to evaluate their petrogenesis and geodynamic implications. New zircon U–Pb dating show identical formation ages for host granodiorites (207.1 ± 2.2 Ma and 206.6 ± 3.8 Ma) and MMEs (205.4 ± 2.9 Ma and 209.2 ± 2.2 Ma). The Shuanghu granodiorites contain amphibole and biotite and are characterized by moderate SiO2 (64.74–66.40 wt%) and K2O contents (2.48–3.62 wt%) as well as low A/CNK (0.69–1.00), indicating I‐type granite affinity. Its enriched zircon Hf isotopes (εHf(t) = −12.04 to −5.09) indicate that they were derived from the ancient mafic lower crust. The MMEs have a similar mineral assemblage, emplacement ages, trace elemental and zircon Hf isotopic compositions with the host granodiorites, indicating the cognate origin. These MMEs represent autoliths captured by latter ascending host magmas in the middle‐upper magma chamber. The highly variable Mg# (37–53), Cr (16.9–190 ppm) and Ni (5.71–67.2 ppm) in the Shuanghu MMEs and host granodiorites indicate that the mantle‐derived magma provided not only the heat but also the mafic components. In combining with previous studies, we suggest that the Shuanghu granodiorites and MMEs were formed in the post‐collision stage, which were caused by the asthenosphere upwelling related to the slab breakoff.
The North Qinling Orogenic Belt (NQOB) is a composite orogenic belt in central China. It started evolving during the Meso‐Neoproterozoic period and underwent multiple stages of plate subduction and collision before entering intra‐continental orogeny in the Late Triassic. The Meso‐Cenozoic intra‐continental orogeny and tectonic evolution had different responses in various terranes of the belt, with the tectonic evolution of the middle part of the belt being particularly controversial. The granites distributed in the Dayu and Kuyu areas in the middle part of the NQOB can provide an important window for revealing the geodynamic mechanisms of the NQOB. The main lithology of Dayu and Kuyu granites is biotite monzogranite, and the zircon U–Pb dating yielded diagenetic ages of 151.3 ± 3.4 Ma and 147.7 ± 1.5 Ma, respectively. The dates suggest that the biotite monzogranite were formed at the end of the Late Jurassic. The whole‐rock geochemistry analysis shows that the granites in the study areas are characterised by slightly high SiO2 (64.50–68.88 wt%) and high Al2O3 (15.12–16.24 wt%) and Na2O (3.55–3.80 wt%) contents. They are also enriched in light rare earth elements, large ion lithophile elements (e.g. Ba, K, La, Pb, Sr), and depleted in high field strength elements (HFSEs) (e.g. Ta, Nb, P, Ti). Additionally, the granites demonstrated weakly negative‐slightly positive Eu anomalies (δEu = 0.91–1.19). Zircon Lu–Hf isotopic analysis showed ɛHf(t) = −6.1–−3.8, and the two‐stage model age is T2DM(crust) = 1.5–1.6 Ga. The granites in the study areas are analyzed as weak peraluminous high‐K calc‐alkaline I‐type granites. They formed by partial melting of the thickened ancient lower crust, accompanied by the addition of minor mantle‐derived materials. During magma ascent, they experienced fractional crystallisation, with residual garnet and amphibole for a certain proportion in the magma source region. Comprehensive the geotectonic data suggest that the end of the Late Jurassic granite magmatism in the Dayu and Kuyu areas represents a compression‐extension transition regime. It may have been a response to multiple tectonic mechanisms, such as the late Mesozoic intra‐continental southward subduction of the North China Craton and the remote effect of the Paleo‐Pacific Plate subduction.
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