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Age of Metamorphism in the High-Grade Rocks of Sri Lanka
A. Kröner and I. S. Williams
The Journal of Geology
Vol. 101, No. 4 (Jul., 1993), pp. 513-521
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/30068803
Page Count: 9
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Controversy concerning the age of regional granulite-grade metamorphism in Sri Lanka stems from the fact that different isotopic systems apparently record different events. Published isotopic data show that zircons from igneous and sedimentary protoliths experienced considerable Pb-loss at 550 Ma and that new growth of zircon, monazite, rutile and garnet between $539 \pm 6 and 608 \pm 3 Ma$ probably dates near-peak metamorphic conditions. Upper concordia intercept ages between ~670 and ~1950 Ma for magmatic zircons from high-grade orthogneisses and charnockites date igneous emplacement and also support the occurrence of granulite metamorphism after ~670 Ma. A granulite of igneous origin from the Kataragama klippe in southern Sri Lanka yielded discordant ion microprobe data that suggest emplacement of the protolith at 1770-1880 Ma. Thus a Rb-Sr whole-rock age of $1930 \pm 130 Ma$ from the same suite is unlikely to reflect isotopic homogenization during granulite formation, and we interpret this and other 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.
The Journal of Geology © 1993 The University of Chicago Press