Partial Melting of Thickened Tibetan Crust: Geochemical Evidence from Cenozoic Adakitic Volcanic Rocks

Abstract

Major and trace elements and Sr-Nd-Pb isotopic data are presented for the Late Paleogene to Neogene age (9.4-26.9 Ma) adakitic volcanic rocks from the northern Qiangtang region, Tibetan Plateau. Lacking Eu depletion, this group of rocks exhibits high SiO2 (>58%), Sr (>350 ppm) and La/Yb (>22) ratios, and low Y (<18 ppm) and Yb (<1.5 ppm). Samples plot within the adakite field on the Sr/Y versus Y diagram. Isotopic compositions of this group rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.706365 to 0.708156; 208Pb/204Pb = 38.955 to 39.052; 207Pb/204Pb = 15.651 to 15.672; 206Pb/204Pb = 18.679 to 18.839), and non-radiogenic Nd (143Nd/144Nd = 0.512411 to 0.512535; εNd = -2.01 to -4.43). These geochemical features suggest magma generation by partial melting of an eclogitic mafic lower crust beneath the Tibetan Plateau. On the basis of Mg#, the Qiangtang "adakites" can be divided into two groups—a high-Mg group (Mg# ≥0.45) and a subordinate, low-Mg group (Mg# ≤0.45). These groups are attributed to convective thinning of the enriched lithospheric mantle beneath the Qiangtang region and southward intracontinental subduction of the Kunlun-Qaidam block along the Jinshajiang suture (JRS) zone, respectively. Our study indicated that both convective thinning of the mantle lithosphere and intracontinental subduction played important roles in the Cenozoic magmatism of northern Tibet.