Significance of oxygen isotope studies on granulite facies rocks

Oxygen isotope geothermometry, which has been used to estimate the temperature of metamorphism of low-grade and hydrous metamorphic rocks, may also be used for rocks of granulite metamorphic facies, provided they contain suitable pairs of minerals which still retain the oxygen isotopic fractionation developed at the time of initial metamorphism. Coarse-grained orthopyroxene and quartz appear to be very suitable and yield 'reasonable' temperatures of about 7000C. Magnetite and ilmenite, however, are generally unsuitable for they show oxygen isotope exchange with intergranular fluids during the slow cooling of the granulites. There is also evidence that under some conditions oxygen isotopic temperatures may be more meaningful than temperatures deduced from Ksub(D.Fe-Mg) in coexisting pyroxenes. Whole-rock oxygen values of some large bodies of mafic granulites show little change during granulite facies metamorphism. However, some narrow mafic layers set within a thick silicic metasedimentary sequence appear to have become equilibrated with the pore fluids of the sedimentary rocks. In one extensive metasedimentary terrane in central Australia abnormally light oxygen values of delta180approximately3% for some quartzose granulites, and delta 18O approximately 2% for some interlayered mafic granulites, are common and may reflect climatic or depositional conditions of the Precambrian sedimentation. (Auth.)