The Chemical U-Th-total Pb isochron method (CHIME) dating was performed for internal domains and zones within monazites in garnet-biotite gneiss and garnet-biotite-cordierite gneiss from the Precambrian Southwestern Highland Complex (SWHC), Sri Lanka, to evaluate evolution of the metamorphic rocks which have been subjected to multiple thermal events during the Gondwana amalgamation. Monazites are abundant in garnetbiotite gneisses. The monazites have core-rim zoned, inherited core-bearing, complexly zoned, and oscillatory zoned type internal textures. The core domains of the core-rim zoned, inherited core-bearing, and complexly zoned type monazites show ages of 533-503, 1788-512, and 1686-678 Ma, respectively, and the rim domains show younger ages of 500-434 Ma. Even though its repeated zonings, oscillatory zoned type monazites show the only young age of 470 ± 45 Ma. The determined isochron ages are grouped into four clusters: group I of 1766 ± 140 and 1788 ± 30 Ma (at present 1686 ± 186 Ma age may be grouped into group I); group II of 679 ± 99 Ma; group III of ages in a range between 533 ± 22 and 481 ± 42 Ma; and group IV of ages in a range between 472 ± 17 and 433 ± 14 Ma. The ages of the group I may imply magma emplacement ages. The ages of the group II correspond to the stage of the most prominent thermal event recorded in the region. The groups III and IV can be identified as post-peak thermal events. The age data given for the monazites in the SWHC are consistent with the published data for the Central Highland Complex, and indicate that the SWHC has been subjected to the same thermal events as the Central Highland Complex.

Internal textures and U-Pb ages of zircons in garnet-biotite gneiss of the Southwestern Highland Complex (SWHC), Sri Lanka, were studied to clarify the repeated thermal events in the study area. The zircons from garnet-biotite gneiss consist of the detrital cores and overgrowths with two to five growth stages. The detrital zircon cores are rounded or euhedral to subhedral in shape, and show transgressive internal textures or oscillatory zoning.The rounded or subhedral to euhedral detrital core morphology suggests several major Archaean to Proterozoic sources of detritus zircons in the SWHC. Ages of the detrital cores are in a range of 3.3-1.7 Ga, and can be categorized into five ranges of 3380-3220, 2730-2660, 2550-2490, 2220-2170, and 1900-1700 Ma, implying the source rock ages. The detrital cores in the 3380-2170 Ma have Th/U-ratios of more than 0.3, suggesting igneous origin. The detrital cores in the range 1900-1700 Ma have Th/U of >0.3 or 0.3-0.1, suggesting an igneous or metamorphic origin. Some overgrowths with the ages in the ranges of 1900-1700 and 630-500 Ma have Th/U-ratios less than 0.1 implying that zircon growth corresponds to the thermal events. Zircons lacking detrital cores and having growth zones with characteristic metamorphic internal textures gave ages in a range of 630-500 Ma, implying the generation at the final granulite grade metamorphic event.