Origin of mafic enclaves from the Taihang Mesozoic orogen, north China craton

Abstract

The origin of mafic enclaves in intermediate and felsic granitoids has long been debated. In this paper, petrological and geochemical analyses (whole-rock geochemistry, Nd–Sr and zircon in-situ Hf isotopic compositions) are used to address this controversial issue. Mafic enclaves in the Taihang felsic plutons (north China craton) are fine-grained and show typical magmatic textures. They occur as rounded to ellipsoidal globules, and are sometimes highly elongated (without solid state deformation) due to stretching within the partially crystallized, convective felsic magma. Such occurrence suggests that the mafic enclaves and host rocks represent two co-existing, compositionally distinct magmas. The mafic enclaves contain abundant hydrous phases (hornblende and biotite) and show the reaction relationship of clinopyroxene being rimmed by hornblende. They also contain plagioclase xenocrysts showing compositional and textural disequilibrium. In terms of Nd–Hf isotopes, mafic enclaves have ε Nd( t) values of − 10 to − 15, and host rocks have lower values as a whole (− 13 to − 18). Zircons from a single enclave show a large variation in Hf isotopic compositions ( ε Hf( t) = − 5 to − 20). A model is proposed whereby an evolved basaltic magma first mixed with a granitic magma of crustal origin at depth; and then the mixed magma broke up into discrete globules upon entering the felsic magma chamber, forming enclaves by convective motion, or forceful injection in the host felsic magma. This was followed by a double mechanical transfer of plagioclase crystals at the contact of the two magmas, and by the diffusion of mobile elements (e.g., Si, alkalis, P, Y, Yb, Pb) from felsic magma to enclave-forming magma.