Fluid-absent Melting of High-grade Semi-pelites: P-T Constraints on Orthopyroxene Formation and Implications for Granulite Genesis

Abstract

experimental studies on rocks of similar bulk composition. We Rocks of semi-pelitic composition are common in high-grade terranes. attribute the difference to the higher Ti and F content of biotite in The first appearance of orthopyroxene in these rocks marks the our starting materials, which stabilizes it to higher temperatures. transition from amphiboliteto granulite-facies conditions, and is The temperatures of fluid-absent orthopyroxene formation indicated commonly attributed to the process of fluid-absent partial melting. by our experiments are also much higher than the 700–800°C We have conducted fluid-absent melting experiments on two natural temperatures reported for many orthopyroxene-bearing assemblages semi-pelitic rocks (quartz, plagioclase, alkali feldspar, biotite and in amphibolite–granulite transitional terranes. One explanation for garnet) with the specific objective of determining the pressure– this discrepancy is that the geothermometers used to calculate temperature conditions necessary to produce orthopyroxene. In contrast temperatures for these transitional terranes grossly underestimated to previous experimental studies, our starting materials were obtained peak metamorphic temperatures. Alternatively, granulite formation from a transitional amphibolite–granulite terrane. Importantly, the in some of those terranes may not have been fluid absent, but high TiO2 (>5 wt %) and F (>1 wt %) contents of biotite in involved the influx of low water activity fluids. our experiments are more representative of biotite found in rocks on the verge of granulite-facies conditions than those used in earlier studies. Experiments were conducted in a piston-cylinder apparatus at 800–1050°C and 7–15 kbar. We reversed the first appearance