The carbonated source region of Cenozoic mafic and ultra-mafic lavas from western Qinling: Implications for eastern mantle extrusion in the northeastern margin of the Tibetan Plateau

Abstract

Cenozoic Indo-Asian collision caused significant crustal shortening in central Tibet. The strike-slip faults around the Tibetan Plateau (TP) are generally attributed to extrusion tectonics, resulting from lower crust flow. Therefore, the mantle extrusion site corresponding to the Cenozoic elevation of the TP needs to be identified. This paper reports the petrology and geochemistry of Cenozoic mafic and ultra-mafic volcanic rocks in the Xiahe and Lixian areas, at the northeastern margin of the TP. Detailed analysis indicates a regular change in partial melting conditions and source regions of the volcanic rocks from west to east, revealing a Cenozoic eastward mantle extrusion in the eastern margin of the TP. The Xiahe volcanic rocks display ocean island basalt affinity with negative K anomalies and positive Nb and Ta anomalies. They are alkaline with extremely high Na2O/K2O ratios and relatively enriched Sr–Nd–Pb isotopic compositions, indicating that these basalts were derived from partial melting of carbonated pyroxenite. The Lixian picro-basalts are closely associated with igneous carbonatites. They have relatively high TiO2 (3.47–4.66%) and MgO (11.24–18.88%) contents and low SiO2 (41.14–44.82%) and Al2O3 (5.84–9.18%) contents. Based on the depleted Sr–Nd–Pb isotopic compositions, we propose that the Lixian picro-basalts may have originated from the partial melting of carbonated lithosphere mantle peridotites at relatively high pressure (>3GPa). Minor hornblendite in their source region can account for the high TiO2 and Na2O+K2O contents. Thus, we argue that these volcanic rocks were formed by episodic decompression melting of the carbonated mantle lithosphere during the eastward extrusion of the Tibetan lithosphere, in contrast to the conventional view that they were formed in a continental rift setting. Their partial melting and eruption processes may be closely related to the Cenozoic strike-slip fault activities in the northeastern margin of the TP.