A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites

Details A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites

Nov 1978

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978gecoa..42.1735g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 42, Issue 11, pp.1735-1747

Mathematics

Logic

6

Scientific paper

Rubidium-strontium isotopic study of intermediate-pressure granulites at Mt. Aloysius, central Australia reveals total rock isochrons that either record the metamorphism or predate it. The gneisses involved, typically quartz + feldspar + orthopyroxene + garnet granulites, occur in five lithological units which outline a simple fold structure. The distribution of isotopic ages in a 25 km 2 area is tested using 74 samples collected in groups of 2 to 4 both along and across strike in each of the units. Two total rock isochron ages of 1200 and 1550 Myr occur, and both are found at different sites in one unit. Mineral ages are younger and independent of location, with feldspars giving 800 Myr and biotites 730 Myr. The 1200 Myr isochrons show the features of outcrop-scale Sr isotopic homogenisation and are taken to record the time of metamorphism. Contemporaneous regional depletion of U, commonly associated with granulite facies metamorphism, confirms the interpretation. The 1550 Myr isochrons describe entire lithological units and are best assigned to the supracrustal genesis of the rocks. The preservation of two ages indicates that isotopic equilibration of anhydrous total rocks is incomplete even within the granulite facies. Careful interpretation is required to assign geological meaning to granulite isochrons.

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