40Ar‐39Ar incremental heating studies on the Tudor Gabbro, Grenville Province, Ontario: its bearing on the North American apparent polar wander path in late Proterozoic times

Summary. Ar‐Ar incremental heating studies have been carried out on samples taken from the Tudor Gabbro, Grenville Province, Ontario. In an earlier K‐Ar study, these rocks have yielded an isochron age of ∼700 Ma together with very high initial argon ratios. Age spectrum plots on whole‐rock samples, in general, display a saddle‐shaped character, with two of them exhibiting minima close to 700 Ma. No clear plateaus are observed for these rocks. A hornblende separate records the time ∼1110 Ma at which the stock finally cooled through the ∼590°C isotherm. The Tudor Gabbro was probably intruded into an area undergoing middle‐amphibolite facies meta‐morphism about 1180 Ma ago. The age spectra of two whole‐rock samples together with that of their plagioclase separates, suggest that the stock cooled to ∼200–250°C at about 720 Ma. Slow cooling, averaging about 1°C Ma is indicated for this section of the Grenville Province for the period 1100–700 Ma. If the age of the Tudor Gabbro's palaeomagnetic pole position is taken to be 720 Ma, the Hadrynian Track Hypothesis leads to very high polar wander rates of > 20cm a for the period 820–720 Ma. If this hypothesis is rejected, the average drift rate for this period would be ∼4 cm a, in much better agreement with published values of ∼5 cm a for the period 1400–820 Ma

Widespread 18O depletion in some precambrian granulites of Australia

In the Strangways Range a broad tract of lower Proterozoic mafic and silicic granulites with δ 18O = 0.1 to 7.3% is depleted in 18O on average by 2-47% compared with high-grade gneisses and granitoids of Canada and other shields. The Fraser Range mafic granulites (δ18O ~ 7.2%) are enriched with respect to unaltered sea-floor basalts (~5.7%). In some rocks depletion in 18O could be related to dehydration during granulite-facies metamorphism and removal of the resultant products of partial melting. In other rocks pre-granulite reaction between heated seawater and hot basic intrusives seems to be a plausible mechanism of depletion in 18O. A direct correlation between depletion in 18O and the abundance of brown granulite hornblende suggests that 18O-depleted water was present in certain mafic rocks before the onset of granulite metamorphism, whereas in others brown hornblende was introduced during a phase of the granulite facies metamorphism itself

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 700°C. 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 K 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 δ 18O∼3% for some quartzose granulites, and δ 18 O∼2% for some interlayered mafic granulites, are common and may reflect climatic or depositional conditions of the Precambrian sedimentation