Experimental Studies of Igneous Rock Series: Felsic Body Suite from the Needle Point Pluton, Wallowa Batholith, Oregon

Abstract

The phase relationships of four analyzed granitic rocks which comprise a late-stage felsic body sequence intruding the Mesozoic granodiorites and tonalites of the Needle Point pluton, Wallowa Batholith, Oregon, have been determined in the presence of 15 wt percent water at pressures to 3 kbar. Pressure-temperature curves were located for the beginning of melting, and for the disappearance of K-feldspar, quartz, plagio-clase, biotite, and hornblende in granodiorite no. 678, quartz monzonites nos. 685 and 774, and granite no. 705. Compositions and structural states of plagioclases from the granitic rocks, basic masses intruding the batholith, and mafic inclusions, as well as the compositions of coexisting feldspars, were determined using the electron microprobe analyzer. Variations in each of these parameters throughout the melting interval of each granitic rock were measured at 2 kbar pressure. Although experimental conditions (excess $$H_{2}O$$ present; $$fO_{2}$$ regulated but not controlled) forbid detailed comparison with natural occurrences, the results indicate that, at 2 kbar pressure, temperatures of at least 730° C and 780° C are required to generate liquids of granite and quartz monzonite composition. Several factors suggest that the normal product of partial fusion of many crustal rock types is a $$H_{2}O$$-undersaturated granite liquid; crystal mushes with this type of liquid are probably involved if intermediate magmas are derived from crustal sources.