Diversity of granitic rocks constrained by disequilibrium melting and subsequent incremental emplacement and differentiation

Abstract

Compositional diversity is a fundamental feature of Earth's granitoids and it has been prevailing on Earth since the early Archean. Although the exotic mechanisms, like magma mixing, crustal assimilation and contamination, could place constraints on the diversity, the compositional diversity of common huge batholiths in the field require intrinsic mechanisms running in the magma itself. This paper highlights the detailed processes from melting of a given source to subsequent incremental emplacement and differentiation control the diversity of granitic rocks as we observed on the Earth's surface. The diversity is firstly introduced from the significant heterogeneity of crustal rocks (in minerals and geochemistry) and the melting behaviors (congruent or incongruent melting), by which disequilibrium melting of crustal rocks could generate heterogeneous radiogenic isotopes. At the second stage, such heterogeneity inherited from source and melting behaviors in different melt batches can be reconciled to some extents during the incremental emplacement of melt batches, self-mixing and differentiation in magma chambers, which represent a homogenization process in magma system. The homogenization extent depends on temperature, residence time and other parameters of granitic melts. A large amount of published Hf isotopic data in zircon from granitic rocks provide us a way to assess the roles of disequilibrium melting and homogenization process. On the basis of the difference in zircon Hf isotope compositions and the diffusivity of Hf in granitic melts, we propose a new thermometry for granitic magmatism. Given a certain melt residence time, the compiled global Hf isotopic ratios in zircon from granitic rocks indicate that the magma temperatures of intermediate–felsic rocks decreased steadily throughout Earth's history, corresponding to the secular cooling of Earth's mantle.