Sources of granitoid magmas at convergent plate boundaries

Abstract

Source rocks for magmas in subduction zones include subducted oceanic crust, mantle peridotite, and metamorphic rocks of continental crust. Phase relationships for these materials, including the compositions of liquids developed at various pressures, temperatures, and with H_2O and CO_2, provide a starting point for the petrogenesis of granitoid intrusions. Four thermal structures are obtained by combinations of warm or cool subducted oceanic crust, and cool or warm overlying mantle. These encompass the range of dehydration and melting sites and processes likely to occur in subduction. Petrological structures derived by combining major rock structures with the assumed thermal structures and the phase relationships of materials indicate that initial magma generation in any of the three major source rocks may be followed by a sequence of events leading to the intrusion of granitoid magmas into continental crust. Magmas of different compositions may rise independently from different sources at different depths, introducing the prospect of hybridization and magma mixing in the mantle and continental crust. Many granitoid intrusions are initiated by the transfer of heat and magma from below the continental crust.