Experimental Studies of Igneous Rock Series Central Sierra Nevada Batholith, California

Abstract

The phase relationships of four analyzed granites, granodiorites, and tonalites from the Central Sierra Nevada Batholith, California, have been determined in the presence of 15 wt. per cent water at pressures to 3 kb. Pressure-temperature curves were located for the solidus, the disappearance of K-feldspar, quartz, biotite, plagioclase, and hornblende. Compositions and structural states of feldspars in equilibrium with granitic melts at successive temperatures throughout the melting interval at 2 kb. pressure were determined for the three Sierran rock types. Various equilibrium tests made on the Sierran granite and grano-diorite indicate that runs of 4-6 months in duration are necessary to approach a sTable equilibrium condition at temperatures of 725° C, or lower in "granitic" charges oversaturated with water. Reversals which produce the same assemblage of minerals as normal runs on these rock types do not prove that equilibrium was established. The experimental conditions prohibit direct comparison with the natural occurrences, but the persistence of calcic plagioclase, hornblende, and magnetite in the Sierran granodiorites and tonalites at 900° C. and 2 kb. pressure does indicate that if these rocks were formed by anatexis, then either high temperatures are required at the base of the crust or the magmas consisted of a eutectic-like granite liquid with suspended crystals.