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

Rocks of semi-pelitic composition are common in high-grade terranes. The first appearance of orthopyroxene in these rocks marks the transition from amphibolite- to granulite-facies conditions, and is commonly attributed to the process of fluid-absent partial melting. We have conducted fluid-absent melting experiments on two natural semi-pelitic rocks (quartz, plagioclase, alkali feldspar, biotite and garnet) with the specific objective of determining the pressure–temperature conditions necessary to produce orthopyroxene. In contrast to previous experimental studies, our starting materials were obtained from a transitional amphibolite–granulite terrane. Importantly, the high TiO2 (>5 wt %) and F (>1 wt %) contents of biotite in 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 of orthopyroxene in two-stage experiments at 7 and 10 kbar. Fluid-absent melting of biotite began at ∼875°C with the production of a garnet–alkali-feldspar-bearing assemblage, and progressed with rising temperature to the generation of a garnet–orthopyroxene–alkali-feldspar-bearing assemblage. The initiation of the orthopyroxene-forming reaction, biotite + plagioclase + quartz = orthopyroxene + garnet + K-feldspar + melt, was bracketed between 925 and 950°C at 7 kbar, and at 1025 and 1050°C at 10 kbar. At pressures >10 kbar, orthopyroxene appeared at temperatures below 1035°C, suggesting a steepening and possibly a back-bend in the slope of the reaction. Our results indicate that temperatures of at least 875–1025°C are required to stabilize orthopyroxene under fluid-absent conditions at mid- to lower-crustal depths (5–15 kbar). This estimate is 40–120°C higher than reported in previous experimental studies on rocks of similar bulk composition. We attribute the difference to the higher Ti and F content of biotite in our starting materials, which stabilizes it to higher temperatures. The temperatures of fluid-absent orthopyroxene formation indicated by our experiments are also much higher than the 700–800°C temperatures reported for many orthopyroxene-bearing assemblages in amphibolite–granulite transitional terranes. One explanation for this discrepancy is that the geothermometers used to calculate temperatures for these transitional terranes grossly underestimated peak metamorphic temperatures. Alternatively, granulite formation in some of those terranes may not have been fluid absent, but involved the influx of low water activity fluids.

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