Some aspects of the stratigraphy, structure and petrology of the Precambrian rocks of Tasmania

The Precambrian rocks of Tasmania may be divided into two main groups, the unmetamorphosed sediments (?younger Precambrian) and the metamorphic rocks (? older Precambrian). The metamorphic rocks from Frenchmans Cap, the headwaters of the Mersey and Forth Rivers, the lower Pieman River, Ulverstone and Port Davey, show that the regional metamorphism of the Frenchman Orogeny contained at least two phases of deformation, F\\(_1\\) and F\\(_2\\). The highest grade of metamorphism was reached during F\\(_1\\) at Frenchmans Cap but the isoclinal folds were produced during F\\(_2\\). It is postulated that the metamorphic zones formed during F\\(_1\\) were folded into large recumbent folds during F\\(_2\\). The fabric of the metamorphic rocks is shown to be the result of repeated deformation. Quartz, mica etc. have a preferred orientation and a symmetry related to macroscopic features such as lineation. The metamorphic rocks belong to several subfacies of the Greenschist Facies. A thick succession of unmetamorphosed quartz-sandstones, subgreywackes, siltstones, slates and dolomite occurs widely throughout the north-west of Tasmania. The relations between the metamorphosed (\older\") and unmetamorphosed (\"younger\") groups are obscure mainly because both were refolded during the Devonian. A comparison of their tectonic styles suggests that the two are separated by a tectonic and metamorphic episode. The mapped major structures in the metamorphic rocks are of Devonian age and the Palaeozoic Holds may be unfolded to leave bedding foliations and the axial surfaces of isoclinal folds in a sub - horizontal position. Basic and ultrabasic igneous rocks which intrude the meta-morphosed sediments have been metamorphosed to amphibolites greenschists and eclogites. The Cooee Dolerites form a very extensive dyke swarm which intruded the unmetamorphosed sediments during the Penguin Movement of the late Precambrian."