Physics
Scientific paper
Sep 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.2593b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 9, pp.2593-2602
Physics
3
Scientific paper
Ilmenites occurring in amphibolite and granulite-facies orthogneisses in Sri Lanka yield precise U-Pb ages. Coexisting ilmenite-magnetite in the granulite-facies rocks indicate temperature conditions of 680 ± 60°C and log f O 2 = -17.0 ± 1.2. The U-Pb age of 498.2 ± 3.3 Ma obtained for ilmenite is indistinguishable from the Pb-Pb age of 501 ±5 Ma derived for coexisting silicate minerals, and suggests that oxide-silicate equilibration has been preserved. In contrast, Fe-Ti oxides in adjacent amphibolitefacies rocks indicate a temperature of 830 ± 60° C and log f O 2 = -12.7 ± 0.8 and record U-Pb ages of 1093.7 ± 7.6 Ma and 1095.4 ± 4.2 Ma. These ages are some 600 Ma older than those obtained for coexisting silicate phases in the same sample, or for both oxide and silicate minerals in the nearby charnockite and relate either to the time of emplacement of the granitic precursor or to some earlier metamorphic episode. These results suggest that temperature, f O 2 and fluid conditions estimated from Fe-Ti oxides should be interpreted with extreme caution.
Burton Kevin W.
Keith O'Nions R.
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