Analogous diagenetic conditions of dark enclave and its host granite derived by magma mixing: Evidence for a post-mixing magmatic process

Abstract

Mafic microgranular enclaves (MMEs) are commonly formed during magma mixing and usually record contrasting physicochemical diagenetic characteristics relative to their host rocks. We present a finding that some MMEs from the Cretaceous Qianjia pluton, which is believed to be the product of magma mixing, exhibit diagenetic conditions analogous to those of their host granites. Integrated calculations based on in situ compositions of minerals (e.g., zircon, titanite, amphibole, feldspar, and apatite) from the Qianjia monzonitic MMEs and their host granites indicate that they exhibit similar diagenetic temperatures (700–800 °C) and pressures (0.20–0.28 GPa), but discrepant oxygen fugacity and water activity. The MMEs have higher oxygen fugacity and water activity, as well as lower Th/U ratios, compared to the host granites, suggesting more oxidized and H2O-rich circumstances. Besides, high F concentrations and F/Cl ratios in apatites from both the MMEs and host granites and their parent magma indicate a relatively F-rich magma source that was probably derived by F-bearing mineral dehydration during plate subduction. Similar physicochemical diagenetic conditions between the MMEs and their host granites seemingly reveal their cognate features; however, slight differences in the oxygen fugacity, water activity, and chemistry of certain minerals (e.g., apatite and titanite) provide clues on magma mixing. We therefore propose that the MMEs involved might have been derived from local heterogeneous hybrid melts or by early crystallization during a post-mixing process. In this case, information about the magma mixing has been almost eliminated due to the geochemical re-equilibration. Therefore, in the case of magma-mixing derived intrusions, the MMEs could be formed not only during magma mixing but also during a post-mixing stage, in which certain minerals most likely preserved a few geochemical characteristics of magma mixing.