Chemical U – Th – Pb monazite dating of the Cambrian Tyennan Orogeny, Tasmania*

Abstract

Chemical U – Th – Pb dating of monazite from 12 schists throughout western and central Tasmania define a peak metamorphic age of ca 510 Ma. This age is very close to the age of arc – continent collision and ophiolite emplacement, implying very rapid uplift and cooling. To the south, along the western margin of the South Tasman Rise, metamorphism occurred later at 495 Ma, which correlates with a late stage of the Ross Orogeny, Antarctica. The Tyennan Orogeny in Tasmania has a three-stage history similar in age to the Delamerian Orogeny in South Australia. However, the Tyennan Orogeny only produced metamorphic rocks during the early stage associated with ophiolite obduction at 515 – 505 Ma. The intense compressional event recognised in the Delamerian and Ross Orogeny at 500 – 495 Ma is correlated with a mild basin inversion in Tasmania, and no metamorphism on mainland Tasmania has been recognised associated with this event. The western margin of the South Tasman Rise is a fragment of the Ross Orogen and does not correlate directly with Tasmania.