EXTREME CRUSTAL METAMORPHISM DURING A NEOPROTEROZOIC EVENT IN SRI LANKA: A STUDY OF DRY MAFIC GRANULITES

Abstract

Garnet-clinopyroxene-quartz granulites of the central Highland Complex of Sri Lanka preserve textural and com- positional features indicative of high-pressure, ultrahigh-temperature (HP-UHT) crustal metamorphism and multistage retrogression. Grains of the peak metamorphic assemblage, garnet-clinopyroxene-quartz, are commonly separated and embayed by late orthopyroxene-plagioclase symplectites; however, in some domains, rare grain-to-grain associations of the peak assemblages are still preserved. Thermodynamic modeling in the CaO-Na2O-K2O-FeO-MgO-Al2O3-SiO2 system indicates peak metamorphic conditions of 12.5 kbar at 925C. The temperature estimates using garnet and clinopyroxene core compositions are in the range 844-982C, in agreement with the thermodynamic modeling. In conclusion, the textural, geochemical, and thermodynamic modeling and thermobarometric data indicate a multistage decompression after HP-UHT metamorphism. U-Pb zircon (laser ablation-inductively coupled plasma mass spec- trometry) ages represent the timing of the peak metamorphism at ca. 580 Ma. A Sm-Nd internal isochron from mineral phases (garnet, clinopyroxene, orthopyroxene, and felsic fraction) and from a whole rock yields an age of Ma interpreted as the time of isothermal decompression (retrogression). Our results from the central Highland 534 12 Complex of Sri Lanka provide important constraints on the Neoproterozoic orogeny associated with the assembly of Gondwana.