Petrology and zircon U–Pb geochronology of metagabbro from the Highland Complex, Sri Lanka: Implications for the correlation of Gondwana suture zones

Abstract

The basement in the Sri Lankan fragment of Gondwana is composed of upper amphibolite- to granulite-facies rocks that underwent latest Neoproterozoic – Cambrian high-grade metamorphism. A recent tectonic model suggests that the Highland Complex in the central part of the island corresponds to a suture zone formed by the closure of a Neoproterozoic ocean between the Wanni and Vijayan Complexes. In order to test the model and further unravel the complex tectonic evolution of this region, we present new petrological, pressure–temperature, and zircon U–Pb geochronological data on garnet-bearing metagabbros from the Highland Complex. The metagabbros, which occur as blocks or lenses within metasediments of the Highland Complex, contain granulite-facies mineral assemblages of garnet+clinopyroxene+orthopyroxene+ilmenite+hornblende+plagioclase+quartz (sample HC7) and garnet+plagioclase+clinopyroxene+orthopyroxene+pargasite+quartz+rutile+ilmenite (sample HC8). Application of phase equilibria modeling in the system NCFMASHTO indicates peak P–T conditions of 900–950°C/10.5–11kbar and 800–830°C/8.5–9kbar, respectively, and clockwise P–T paths as inferred from decompression textures and mineral compositional isopleths. These conditions are significantly lower than the high-pressure (>17kbar) and ultrahigh-temperature (>1000°C) conditions obtained previously for similar metagabbros from the Highland Complex, whereas our results are consistent with those of similar rocks from the Palghat-Cauvery Suture Zone in India and the Lützow-Holm Complex in East Antarctica. The latest Neoproterozoic to Cambrian metamorphic ages (574±28Ma and 534±14Ma) as well as the available petrological data suggest that the metagabbros in the Highland Complex could be correlated with those in the Lützow-Holm Complex in East Antarctica. The results of this study confirm that the Highland Complex is an extension of the Lützow-Holm Complex, and that both these terranes involve a package of accreted trench sediments and remnant oceanic lithosphere metamorphosed during the final collision related to the Gondwana assembly.