SILICIC MAGMA FORMATION IN OVERTHICKENED CRUST: MELTING OF CHARNOCKITE AND LEUCOGRANITE AT 15, 20 AND 25 KBAR

Abstract

Two models of silicic magma formation have been experimentally tested: (1) generation of A-type granite magma by partial melting of crustal source rocks at depths >50 km; (2) production of syenite magma by partial melting of quartzofeldspathic rocks at pressures >15kbar Melting experiments at 15, 20 and 25 kbar were performed on Archaean biotite-bearing charnockite of opx-bearing granodiorite composition, and on leucogranite. Most experiments were conducted with oxalic acid (OA) to provide 2 wt % H2O and similar to 3 wt % CO2, but several fluid-absent runs with charnockite were also made, The temperature interval of the OA experiments was similar to 925-1100 degrees C;for the dry runs it was similar to 1100-1250 degrees C. In all cases, melting reactions produced garnet (Grt), clinopyroxene (Cpx), ternary feldspar (Tfsp) and 5-85 vol. % melt. The composition of partial melts produced from charnockite for degrees of melting up to similar to 60 vol. % is always granitic and is controlled by residual Tfsp and Qtz. Ternary feldspar acts as a sink for alkalis until totally consumed at similar to 50 vol. % melting. The granitic composition of the initial melt results from incongruent melting of natural silicic rocks, and contrasts with the haplogranitic system Ab-Or-Qtz in which the initial melt is generally considered to Be of syenitic composition at these high pressures, nle presence of Ca, Mg- and Fe-bearing-phases in many natural rocks defines peritectic melting reactions that result in the formation of the low-silica phases Grt and Cpx together with partial melt. The resulting SiO2, enrichment in the partial melt prevents the formation of syenitic liquids by partial melting of quartzofeldspathic rocks, even at very great depth. The predominance of Grt and Cpx in the residue at similar to 50 vol. % partial melting may promote separation of granite magma from restite by settling of these denser minerals. High degrees of melting of similar to 50-60 vol. %, accompanied by settling of residual crystals, would lead to a magma of restricted compositional range with a heavy rare earth element depleted pattern. Overall, the granitic melt composition obtained in the experiments differs from that of A-type granite. However, at degrees of melting below similar to 30%, the composition of partial melt resembles A-type granite although its Al2O3 content is more than 1 wt % higher.