Abstract:
The origin and significance of microgranular enclaves as indicators of the occurrence of magma mixing and/or mingling episodes between basic magmas and anatectic acid magmas, either I- or S-type, are re-evaluated. A model for studying microgranular enclave and host rock associations has been developed, based on the geochemical characteristics and the outcrop relationships observed in microgranular enclave-bearing granitoid suites. The model consists of three main stages, concerning the injection (stage 1), the evolution (stage 2), and the mixing (stage 3) processes that basic magmas experience when injected into anatectic crustal environments. In stage 1, an acid magma is intruded by one or more injections of almost completely liquid basic magma which is hotter and less viscous than the acid magma. The two systems do not mix easily, but remain as discrete entities until thermal equilibrium and comparable viscosities are reached, and freezing of the basic magma and superheating of the acid magma are operative along their boundaries. In stage 2, the basic magma experiences both the physical processes of stretching, convective stirring, and mingling with the acid magma, and the chemical processes of crystal fractionation, and contamination with the acid magma (CFC process). Repeated cycles of these physical and chemical processes result in the formation of both microgranular enclaves and an evolved liquid which is thermally equilibrated with the acid magma. Accordingly, microgranular enclaves record the steps of evolution of the basic magma. In stage 3, the evolved products of the basic magma (tonalitic to granodioritic in composition) and the acid magma participate in a two-endmember mixing process which accounts for the geochemical evolution of granitoid plutons bearing microgranular enclaves. The model sheds new light on the magmatic processes occurring in plutonic environments during the formation of composite batholiths, and also suggests some ideas on the petrogenesis of tonalitic plutons. Finally, the observed scale-independent property of microgranular enclaves suggests that fractal geometry, a relatively new topic of mathematics, can play a determinant role in the understanding of the chaotic flow mechanics of viscous fluids, i.e., the kinematics of the mingling and stretching of basic magmas.