Evidence of partial melting in metabasic and meta-andesitic rocks from the Highland Complex, Sri Lanka

Abstract

Partial melting of metamorphic rocks is a common phenomenon in high-temperature zones of mountain belts and its products have been observed in many Precambrian basement areas. While granulites of the Highland Complex of Sri Lanka have been widely studied, remelting of metabasites in the Highland Complex has not yet been documented. Building on the UHT conditions reported in the literature, the aim of this study is to demonstrate whether a set of metabasites and meta-andesites from the Highland Complex, Sri Lanka, record evidence for partial melting. These metabasites and meta-andesites comprise garnet-pyroxene-feldspar-amphibole/biotite granulites with leucosomes parallel to the gneissic layering. Detailed petrographic analysis combined with microprobe and trace element data shows dehydration reactions to have occurred. These reactions induced the formation of a tonalitic melt (now leucosome), together with the peritectic minerals garnet, clinopyroxene, orthopyroxene and K-feldspar. Formation of melt is also supported by the loss of albite at triple points and grain boundaries of plagioclase. Relative enrichment of Rb and Ba into leucosomal K-feldspar and Ba and La into leucosomal quartz provide further evidence for melting. Peak metamorphism was reached at 810 – 925 °C and pressures of 8 – 9.8 kbar (dependent on sample), based on geothermobarometry and Perple_X modelling. Zircon analysis by both CL (cathodoluminescence) and LA-ICP-MS shows the onset of partial melting induced partial dissolution of xenocrystic zircons. Furthermore, the crystallization of melt caused the formation of new zircons and the overgrowth of older cores with low trace element concentrations. Additionally, euhedral biotite grains with a relatively low Ti concentration are formed during melt crystallization. Analyses of Th/U values combined with textural observations in zircons infer a single long metamorphic event, with 585 ± 10 Ma as the timing just after peak metamorphism and 536 ± 6 Ma as the timing of final crystallization of melt.