DATING OF NEOPROTEROZOIC AND CAMBRIAN OROGENIES IN TASMANIA

A coherent set of timing constraints is produced for Tasmania's Proter ozoic and Cambrian geology when only mineral ages are considered and w hole-rock ages excluded. The oldest recognised event is the formation of sedimentary deposits which contain detrital zircons that indirectly indicate a depositional age younger than 1180 Ma. Partial melts of th ese sedimentary rocks were incorporated in Neoproterozoic, Devonian an d probably Cambrian felsic magmas. Neoproterozoic granite on King Isla nd has an age of 760 +/- 12 Ma and is pari of a high-level intrusive e pisode that accompanied the Wickham Orogeny, an event with regionally varied strain that is represented in northwestern Tasmania by a low-an gle unconformity, by altered granitoid with a magmatic age of 777 +/- 7 Ma, and by the thick turbidite pile of the Burnie and Oonah Formatio ns with its syndepositional intrusions of Cooee Dolerite, The late Neo proterozoic was relatively quiet tectonically but by early in the Midd le Cambrian a crustal collision which marked the early phase of the Ty ennan Orogeny brought about high-level emplacement of ultramafic-beari ng allochthons and deep-seated metamorphism of quartzose sedimentary a nd basaltic rocks. The ultramafic allochthons carried tonalite that ha d crystallised only shortly before at 510 +/- 6 Ma, while the deep-sea ted metamorphism produced eclogite at 502 +/- 8 Ma. By middle Middle C ambrian times the metamorphic rocks had been uplifted and they experie nced repeated uplift during the period of Mt Read volcanism and onward to the close of the Tyennan Orogeny in the Early Ordovician, an overa ll period of some 20 million years from the early Middle Cambrian. Reg ionally varied strain was again a feature during the Tyennan Orogeny, with the Smithton area in northwestern Tasmania and King Island occupy ing relatively undeformed cratonic positions.