Late-Neoproterozoic ultrahigh-temperature metamorphism in the Highland Complex, Sri Lanka

Abstract

The Sri Lankan crustal blocks preserve the records of Neoproterozoic tectonothermal events associated with the final assembly of the Gondwana supercontinent. Here we present petrological and U–Pb zircon geochronological data on a suite of ultrahigh-temperature (UHT) metapelites and mafic granulite from the Highland Complex in central Sri Lanka which provide convincing evidence for Late Neoproterozoic UHT metamorphism. Three sapphirine granulites, two corundum bearing metapelites and one mafic granulite were studied for petrology and zircon U–Pb geochronology. The petrographic observations coupled with conventional thermobarometric calculations indicate that the pelitic granulites attained P=11–12kbar and T=850°C during prograde evolution. Subsequently the rocks have undergone a prograde decompression stage with increasing temperature up to UHT conditions. The peak metamorphism was attained at P–T conditions around 900–975°C and 9–10kbar, followed by near isobaric cooling and rapid decompression.LA-ICPMS zircon U–Pb analyses of magmatic cores in detrital zircons from the sapphirine granulites show 206Pb/238U ages ranging from 834±12 to 722±14Ma, reflecting the timing of crystallization of these grains in their protoliths. Metamorphic zircons in these samples yield ages ranging 535±5 and 567±4Ma representing the age of the Neoproterozoic UHT metamorphism. The metamorphic zircons from corundum-bearing metapelites yield ages from 530±4 to 578±4Ma. An older age population between 1676Ma and 2365Ma from these metasediments suggests the timing of emplacement of the Paleoproterozoic sources. Our data suggest variable degree of input from multiple sources into the protolith sediments of the Highland Complex prior to the latest Neoproterozoic UHT metamorphism at ∼530–580Ma which is broadly coeval with the collisional assembly of the Gondwana supercontinent.