Cenozoic volcanic rocks were recently discovered during full-coring kilometer-scale major scientific drilling in the Xisha Islands, northwestern South China Sea. A systematic mineralogical study of these samples was performed for this paper. The results show that the volcanic rock samples are basaltic pyroclastic. The major elements demonstrate that the clinopyroxenes are diopside and fassaite, which contain high Al 2 O 3 (5.33-11.2 wt. %), TiO 2 (2.13-4.78 wt. %) and wt. %). Clinopyroxenes have high REE abundances (104-215 ppm) and are strongly enriched in LREE (LREE/HREE = 3.56-5.14, La/Yb N = 2.61-5.1). Large-ion lithophile elements show depleted characteristics. Nb/Ta shows obvious fractionation features: Nb is lightly enriched, relative to primitive mantle, but Ta is heavily depleted, relative to primitive mantle. The parental magma of the basaltic pyroclastic rocks belongs to a silica-undersaturated alkaline series, characterized by a high temperature, low pressure, and low oxygen fugacity. The Al IV content increases with decreasing Si concentration. The Si-unsaturated state causes Si-Al isomorphic replacement during the formation of clinopyroxene. The electric charge imbalance caused by the replacement of Si by Al is mainly compensated by Fe 3+ . The clinopyroxene discrimination diagrams show that the parental magma formed in an intraplate tectonic setting environment.The mineral composition of igneous rocks depends on the chemical composition and crystallization environment of magma. Changes in mineral composition and structure are direct evidence of changes in the diagenetic environment and material composition. Therefore, the characteristics of mineral composition can be used to explore mineral petrogenesis and the magma evolution mechanism [18]. Clinopyroxene is one of the main rock-forming minerals of mafic-ultramafic rocks and plays a major role in the generation and subsequent differentiation of magma [16,[19][20][21][22][23][24]. Clinopyroxene composition mainly depends on primary magma characteristics, the crystallization environment, and the tectonic setting of magmatism [25][26][27][28][29][30][31][32]. Relative to other minerals in mafic-ultramafic rocks, clinopyroxene is considered to be the main carrier for most trace elements and holds a unique further record of magma history [33,34]. The mineral chemistry of relict clinopyroxene in igneous rocks can reflect the characteristics of the primary magma well and has been widely used to study the nature of the original magma and complicated processes that have affected the lithospheric mantle [21,[35][36][37][38][39][40][41][42]. Furthermore, clinopyroxenes in altered basalts are of interest, because they often survive in a relatively pristine form, after the matrix and other minerals have been converted to secondary minerals. The major-and minor-element geochemistry of clinopyroxene is varied and provides a filtered view of the composition of the magma from which the clinopyroxene crystallized, lending it the potential to provide insight into t...
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During the late Mesozoic, the East Asian continent underwent a complex tectonic history due to multiple episodes of plate convergence. How the crust responds to the multiple plate convergence in the North China Craton (NCC) remains unclear. Here we undertook field geological investigations and fault-slip vectors analysis of the Shangyi Basin in the western Yanshan fold and thrust belt, northern margin of the NCC. Combined with new geochronological data, we delineate three phases of intracontinental deformation in the area: 1) NW-SE compression during the Late Jurassic to earliest Cretaceous (ca. 151–141 Ma); 2) NW-SE extension during the middle–late Early Cretaceous (ca. 135–110 Ma); and 3) NE-SW compressional deformation later than 110 Ma. The early NW-SE compression controlled the present bulk architecture of the basin, and the subsequent two tectonic events only caused limited reworking of the previous structures. Through balanced cross-section restoration, we estimate the horizontal shortening ratio of the crust in the study area is over 27% due to the NW-SE compression. Moreover, the contribution of tectonic shortening from the north side of the basin is greater than that from the south side. NW-SE compressional deformation is consistent in time with the episode B of the Yanshanian movement (Yanshanian B), which may be influenced by the subduction of the Paleo-Pacific plate beneath East Asia and the closure of the Mongol-Okhosk Ocean. Subsequent NW–SE extension is likely to be associated with the destruction of the NCC during the Early Cretaceous. Extension may result from the roll-back of the Paleo-Pacific plate and post-orogenic collapse of the Mongol-Okhotsk belt. The last NE-SW compressional event may be linked to the remote effect of the final collision between the Qiangtang and Lhasa terranes.
Summary
The Qinling–Dabie orogenic belt, which contain the arc-shaped Dabbashan orocline and is the world's largest belt of HP/UHP metamorphic rocks, formed by a long-term complex amalgamation process between the North China Block and the Yangtze Block. To understand the collision processes and tectonic evolution, we constructed a three-dimensional (3D) S-wave velocity model from the surface to a depth of ∼120 km in the eastern Qinling-Dabie orogenic belt and its adjacent region by inverting 5–70 s phase velocity dispersion data of Rayleigh waves extracted from ambient noise data. Our 3D model reveals low velocities in the middle–lower crust and high velocities in the upper mantle beneath the orogenic belt, suggesting the delamination of the lower crust. Our results support a two-stage exhumation model for the HP/UHP rocks in the study area. First-stage exhumation was caused by the slab breaking away from the subducted Yangtze Block during the Early–Middle Triassic. Partial melting of the lithospheric mantle caused by slab breakoff–related asthenospheric upwelling weakened the lithospheric mantle beneath the orogenic belt, and continued convergence of the two continental blocks led to further thickening of the lower crust. Such processes promoted lower-crust delamination, which triggered the second-stage exhumation of the HP/UHP rocks. In the Dabbashan orocline, two deep-rooted high-velocity domes, that is, Hannan–Micang and Shennong–Huangling domes, acted as a pair of indenters during the formation stage. High-velocity lower crust was observed beneath the Dabbashan orocline. In addition, our 3D model reveals that high-velocity lithospheric mantle extends from the Sichuan Basin to the Dabbashan orocline, with a subhorizontal distribution, providing strong support for the high-velocity lower crust. We also observed the destruction of lithospheric mantle beneath the Yangtze Block; the destruction area is bounded by the North–South Gravity Lineament, suggesting that the destruction mechanism of the Yangtze Block may be similar to the North China Block.
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