Extreme crustal metamorphism during a Neoproterozoic event in Sri Lanka; a study of dry mafic granulites

Garnet-clinopyroxene-quartz granulites of the central Highland Complex of Sri Lanka preserve textural and compositional 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-Na (sub 2) O-K (sub 2) O-FeO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) system indicates peak metamorphic conditions of 12.5 kbar at 925 degrees C. The temperature estimates using garnet and clinopyroxene core compositions are in the range 844 degrees -982 degrees C, 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 spectrometry) 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 534+ or -12 Ma interpreted as the time of isothermal decompression (retrogression). Our results from the central Highland Complex of Sri Lanka provide important constraints on the Neoproterozoic orogeny associated with the assembly of Gondwana.