Geochronology of discrete structural-metamorphic events in a multiply deformed precambrian terrain

Abstract

Rb—Sr total-rock and mineral studies are combined with detailed structural-metamorphic work to investigate polymetamorphism within the Georgetown Inlier of north Queensland, Australia. A sequence of five major, temporally discrete, regional structural events is established. The earliest events, which were marked by tight folding and prograde metamorphism, occurred at 1570 ± 20 and 1469 + 20 m.y. Subsequent deformations were less intense, and generally associated with retrogressive metamorphism. The third is documented by Rb—Sr total-rock isochrons at 967 ± 28 m.y., and the fourth by mineral and total-rock studies at about 400 m.y. The last deformation is inferred to have occurred at about 300 m.y., a time of extensive igneous activity in the east of the area. The isotopic data indicate that Sr isotopic homogenisation is approached on the scale of a cubic metre during amphibolite-facies regional metamorphism and deformation. Consequently, the Rb—Sr isochron age of a metamorphic unit is highly dependent on its deformation-metamorphic history, and in particular its last significant metamorphism; a structural-metamorphic event which forms a prominent schistosity appears to be an important requisite. The overall success of the method demonstrates its potential in unravelling the geological history of other polymetamorphic terrains.