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Major element, trace element, and Sr–Nd–Pb isotopic compositions of Cenozoic diabase in southeastern China provide insights into the nature of their mantle sources and processes. The diabases are alkaline in lithochemistry (Na2O + K2O = 4.37–5.19 wt.%) and have overall oceanic island basalt-like trace element patterns, without negative Nb–Ta anomalies. In addition, they are characterized by lower La/Nb (<1.5) and La/Ta (<22), and higher Ce/Pb (>15) and Nb/U (>30) ratios, indicating an origin in the asthenospheric mantle. The relatively lower143Nd/144Nd (0.512632–0.512648) and206Pb/204Pb (18.20–18.22), but intermediate87Sr/86Sr (0.7061–0.7063) ratios of the diabases are similar to enriched mantle type 1, suggesting crustal contamination or mixing with metasomatized lithsopheric mantle. However, the low Th and U contents and lack of correlations of Nd isotope compositions and MgO preclude significant crustal contamination. Alternatively, the moderate TiO2contents (2.01–2.09 wt.%) and high Cr concentrations (>240 ppm) suggest interaction between asthenosphere-derived melts and metasomatized lithospheric mantle. Petrological modelling suggests that the diabases were generated from a low degree (~3–5%) of partial melting of lherzolite with ~2–3% garnet. Jiangxi diabase was generated in a within-plate extensional regime, probably related to the far effect of the Himalaya–Tibetan orogen. [ABSTRACT FROM PUBLISHER]

Abstract: The Late Cenozoic geodynamics of the Alpine-Himalayan belt comprised the collision between continental-lithosphere plates and blocks and the effect of the Neo-Tethyan active residual asthenosphere, which reached the northern margin of the belt after the ocean had closed. From the late Eocene to the early Pliocene, strong deformation, lateral migrations of flaked plates, metamorphism, and magmatism (they all consolidated the crust) took place in the lithosphere with the participation of mobile asthenospheric components. In the Pliocene–Quaternary, the asthenosphere beneath the consolidated crust partly replaced the dense mantle lithosphere with remaining paleoocean mafic rocks, which subducted into the mantle. Phase transformations and deformations in the subducting metamafic slabs caused mantle earthquakes. The less compact metamafic rocks experienced metamorphic weakening under the effect of the asthenosphere and incorporated into the Earth’s crust. The upper-mantle and lower-crust weakening led to a drastic intensification of uplifting and the formation of mountain ranges. Recent volcanism is also attributed to the activity of the Neo-Tethyan asthenosphere. [Copyright &y& Elsevier]

The fate of the mantle lithosphere of the Indian Plate in the India–Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60° at 80–160 km depths to 150° at 160–220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust beneath NIL is very thick (58 ± 2 km), and the S velocity in the intermediate and lower crust is around 4.0 km s−1. This anomalously large velocity and thickness can be explained by scraping off the lower crust, when the Indian lithosphere underthrusts the Himalaya. [ABSTRACT FROM AUTHOR]