Experimental Studies of Igneous Rock Series: A Zoned Pluton in the Wallowa Batholith, Oregon

Abstract

The phase relationships of five analyzed granodiorites and tonalites from the zoned Needle Point Pluton of the Wallowa Batholith, Oregon, supplied by W. H. Taubeneck, have been determined in the presence of 15-25 weight per cent H_2O at pressures to 3 kb. Pressure-temperature curves were located for the beginning of melting and for the disappearance of K-feldspar, quartz, plagioclase, biotite, and hornblende. For two specimens, a granodiorite and a tonalite, the melting behavior at 2 kb. pressure was determined in more detail with approximate point counts of crushed fragments. Representative results are: granodiorite (Differentiation Index [D.I.] 76) melting begins at 705° C., K-feldspar disappears at 710° C., quartz disappears at 735° C.; tonalite (D.I. 55) melting begins at 725° C., the trace of K-feldspar disappears at the same temperature, quartz disappears at 755° C. In both rocks, calcic plagioclase, amphibole, and opaque minerals remain at 900° C. The hornblende becomes unstable at a slightly higher temperature and passes into the melt phase. Forty-seven per cent of the granodiorite melts between 705° and 730° C., and 55 per cent melts at 900° C. Twenty-nine per cent of the tonalite melts between 730° and 800° C., and 32 per cent melts at 900° C. At 900° C., the approximate plagioclase content of the granodiorite is 27 per cent, and of the tonalite 40 per cent; the glass in each rock has a refractive index of 1.510 ± 0.002. The experimental conditions (excess H_2O; f_(O_2) not controlled) prohibit detailed comparison with the natural occurrences. However, the persistence of calcic plagioclase and hornblende in abundance at 900° C. at 2 kb. pressure indicates 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-type granite liquid with suspended crystals. The high liquidus temperatures suggest that gabbroic magma may be involved in the development of magmas of intermediate composition in batholiths.