Abstract

Vapor-absent melting experiments on a biotite- and amphibole-bearing, Archean tonalitic gneiss (AGC150) at 10 kbar and 875 to 1050 °C show that amphibole breaks down from 900 to 950 °C, producing garnet, orthopyroxene, and granitic melt. Biotite-dehydration melting produces <10 wt% melt up to 950 °C via incongruent melting reactions that produce garnet, orthopyroxene, and titanomagnetite. Widespread biotite-dehydration melting occurs between 950 and 975 °C and produces orthopyroxene, magnetite, titanomagnetite, and ∼20 wt% fluorine-rich melt (up to 0.31 wt% F). Minor F-rich (2.7 wt%) biotite is present even at 1000 °C. Our experiments show that, under vapor-absent conditions, intrusion of hot, mantle-derived magmas into the lower crust is necessary to initiate widespread biotite-dehydration melting in rocks with compositions like AGC150. We propose that the high thermal stability of biotite in AGC150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat F-enriched biotite. We show that dehydration melting of such F-enriched biotite produces F-rich granitic liquids, with compositions within the range of A-type granite, and leaves behind a granulitic residue consisting of orthopyroxene, plagioclase, quartz, titanomagnetite, and magnetite.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.