AGE OF METAMORPHISM IN THE HIGH-GRADE ROCKS OF SRI-LANKA

Controversy concerning the age of regional granulite-grade metamorphis m in Sri Lanka stems from the fact that different isotopic systems app arently record different events. Published isotopic data show that zir cons from igneous and sedimentary protoliths experienced considerable Pb-loss at 550 Ma and that new growth of zircon, monazite, rutile and garnet between 539 +/- 6 and 608 +/- 3 Ma probably dates near-peak met amorphic conditions. Upper concordia intercept ages between approximat ely 70 and approximately 1950 Ma for magmatic zircons from high-grade orthogneisses and charnockites date igneous emplacement and also suppo rt the occurrence of granulite metamorphism after approximately 670 Ma . A granulite of igneous origin from the Kataragama klippe in southern Sri Lanka yielded discordant ion microprobe data that suggest emplace ment of the protolith at 1770-1880 Ma. Thus a Rb-Sr whole-rock age of 1930 +/- 130 Ma from the same suite is unlikely to reflect isotopic ho mogenization during granulite formation, and we interpret this and oth er best-fit lines as a memory effect in the Rb-Sr isotopic system that provides approximate ages for the gneiss protoliths, not for the high -grade metamorphism.