Petrogenesis of the Paleogene potassium-rich volcanic rocks in the western Yangtze Craton, southeastern Tibetan Plateau

Abstract

Paleogene potassium-rich volcanic rocks (PVRs) are widespread in the southeastern Tibetan Plateau, providing an opportunity to illustrate the architecture of the lithosphere and further constrain the development of the SE Plateau. Their petrogenesis, together with the geodynamic mechanism, remains in dispute. Mg isotopic compositions of the PVRs in western Yangtze Craton, along with new elemental and Sr-Nd-Pb isotopic data, are utilized to elucidate their petrogenesis and reveal the plausible metasomatic and genetic mechanisms. The PVRs give a compositional spectrum from basaltic trachyandesite to trachyte and are characterized by high large ion lithophile elements concentrations, negative anomalies in high field strength elements, and enriched Sr-Nd-Pb isotopes, suggesting an origin of enriched lithospheric mantle. The correlations among varying major oxides and fractional crystallization simulation results jointly reveal fractional crystallization of olivine and clinopyroxene in the magmatic process. The less-evolved PVRs present similar major oxide contents (e.g., Ti, Na, K) to those melts originated from carbonated peridotite + silicate sediment. Apart from enriched Sr-Nd-Pb isotopes, they also show high La/Yb ratios (11.73–58.95), low Ti/Ti* (0.08–0.71), Hf/Sm (mostly <0.70), and varying δ 26 Mg values (−0.13 to −0.36‰). Combined with the Sr-Nd-Mg isotope numerical simulations, it points out that the mantle source of the PVRs was replenished by a hybrid carbonate-silicic sediment, which was plausibly caused by the Neoproterozoic seafloor subduction. These volcanic rocks also present relatively high K 2 O (3.29–7.50 wt%) and K 2 O/Na 2 O ratios (1.00–2.85). Besides, varying elevated Rb/Sr and low Ba/Rb ratios, together with the La/Yb-Yb non-modal batch melting result, suggest that the enriched mantle source recorded by these PVRs is predominantly constituted by a hybrid phlogopite-bearing garnet-spinel lherzolite. Constrained by petrology studies and seismic tomography, the generation of the PVRs is proposed to be closely associated with the delamination of the thickened lithosphere and thereafter asthenosphere upwelling. • Paleogene K-rich volcanic rocks in the western Yangtze Craton are variably evolved. • These volcanic rocks are sourced from phlogopite-bearing garnet-spinel lherzolite. • Recorded mantle source was replenished by a hybrid carbonate-silicic sediment.