Magma differentiation and contamination: constraints from experimental and field evidences

Abstract

Differentiation and contamination of silicic magmas are common phenomena characterizing the granite batholiths and large igneous provinces that build up most of the continental crust. Although they can be identified by means of geochemical relations of igneous rocks exposed in the continents, the mechanisms allowing magmas to undergo the necessary crystal–liquid separation and digestion of country rocks for differentiation and contamination are poorly constrained. In this chapter, we show two independent approaches that are essential to understand fractionation and contamination of magmas. These are (a) the study and interpretation of field relations in exposed deep sections of batholiths and (b) the results of laboratory experiments carried out at middle– upper crust pressure. Experiments support that fractionation is intrinsic to crystallization of waterbearing magmas in thermal boundary layers created at the sidewalls of ascent conduits and walls of magma chambers. Gravitational collapse and fluid migration are processes identified in experimental capsules. Similarly, reaction experiments in mixed capsules support reactive bulk assimilation as a plausible mechanism that is compatible with field and petrographic observations in contaminated granitic rocks