Vapor-absent melting at 10 kbar of a biotite- and amphibole-bearing tonalitic gneiss: Implications for the generation of A-type granites

• Published in: Geology (Boulder) 1992-03, Vol.20 (3), p.263-266
• Main Authors: Skjerlie, Kjell P, Johnston, A Dana
• Format: Article
• Language: English
• ISSN: 0091-7613
• DOI: 10.1130/0091-7613(1992)0202.3.CO;2

Vapor-absent melting experiments on a biotite- and amphibole-bearing, Archean tonalitic gneiss (AGC150) at 10 kbar and 875 to 1050 {degrees}C show that amphibole breaks down from 900 to 950 {degrees}C, producing garnet, orthopyroxene, and granitic melt. Biotite-dehydration melting produces < 10 wt% melt up to 950 {degrees}C via incongruent melting reactions that produce garnet, orthopyroxene, and titanomagnetite. Widespread biotite-dehydration melting occurs between 950 and 975 {degrees}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 {degrees}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.