Internal textures and U-Pb geochronology of zircons in metamorphic rocks from the Southwestern Highland Complex, Sri Lanka

摘要

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 Pro-terozoic 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.