Origin of Mesozoic high-Mg adakitic rocks from northeastern China: Petrological and Nd-Sr-Os isotopic constraints

Abstract

Mesozoic high-Mg dioritic rocks from northeastern China have been suggested to have an adakitic origin, attributed to the equilibration of partial melts from delaminated mafic crust (eclogite) with mantle peridotite. Here we present petrological and Nd-Sr-Os isotopic data that counter this delamination model for the formation of these rocks, and instead propose a process of magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle in a post-kinematic setting. The magma mixing process is supported by (1) euhedral overgrowths of high-Ca plagioclase and high-Mg pyroxene over low-Ca plagioclase and low-Mg pyroxene, respectively, and (2) highly radiogenic Os isotopic compositions, and negatively correlated Nd and Sr isotopic ratios. Our proposed model is probably generally applicable to the mode of origin and tectonic settings of high-Mg adakitic magmas.