Origin and crystallization of some peraluminous (S-type) granitic magmas.

Abstract

Experiments were carried out on four compositions from central Victoria, Australia, S-type volcanic plutonic suites in the ranges 700o-900oC, 1-7 kbar, a H2 = 0-1.0, fO2 = or < QFM, but the results of only one composition, Strathbogie (granite 889), are reported in detail (chemical and modal analyses). Early crystallization of low-F biotite with plagioclase (up to 3 kbar) and quartz (at higher P) requires > 2.5 wt.% H2O in the melt. A wide field of crystallization for orthopyroxene was found, but its rarity in S-type granitic rocks is probably due to peritectic reaction to biotite with decreasing T. Cordierite crystallizes near the liquidus at 1-2 kbar, but does not develop above the solidus at P = or > 4-5 kbar. Thus, cordierite is not likely to be a restite phase. Almandine-rich garnet becomes a near-liquidus phase at P = or > 4-5 kbar. Integration of geothermometric and geobarometric calculations and crystallization sequences indicates that early-crystallizing garnet, biotite, orthopyroxene and plagioclase are present at T = or < 850oC, P = or > 4 kbar, in melts with 3-5 wt.% H2O. Experiments also showed that Al2SiO5-saturation is unnecessary to produce mildly peraluminous melts; thus mildly peraluminous sedimentary rocks may produce peraluminous melts. It is believed that peraluminous melts are undersaturated with respect to H2O, which is derived from hydrous minerals, principally precursor biotite. In less aluminous precursor rocks biotite may persist to higher T. Biotite-orthopyroxene partial melting in granulite-facies rocks in the range 5-8 kbar yields granitic liquids with 3-5 wt.% H2O. Experiments with rhyodacite starting materials produced peraluminous glasses (3 analyses) in quenched runs from 750o-850oC.-S.A.K.