University of Tasmania
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The Cambrian metamorphic history of Tasmania

posted on 2023-05-26, 05:37 authored by Chmielowski, RM
The Tyennan Orogeny produced low to medium-grade metamorphic rocks distributed across the western third of Tasmania. Chemical U-Th-Pb monazite dating reveals that the peak episode of metamorphism took place in the Cambrian, with a weighted mean age for all units analysed of 505 ± 1 Ma. However, variations in the results by region range from ~ 511 to ~ 497 Ma. The pelitic schists of the Franklin Metamorphic Complex contain garnet porphyroblasts which record a rapid, nearly isothermal, pressure increase; the garnet cores formed at ~ 600o C, 6,000 bars and the rims at ~ 700o C, 14,000 bars at 511 ± 3 Ma. Likewise, the eclogite from the same region records a change from ~ 550o C, 6,250 bars to ~ 650o C, 19,000 bars. The whiteschist, which was obtained from the opposite side of a major local fault, formed garnet cores at ~ 545o C, 19,600 bars; its garnet rims and matrix minerals formed at 506 ± 5 Ma after an increase in temperature of at least 30-90o C. All of these units show evidence of very rapid isothermal exhumation. Other Franklin Metamorphic Complex fault blocks record P/T for peak conditions at ~ 570o C, 8,600 bars (Mt. Mary), and ~ 700o C, 11,400 bars (Raglan Range). The Forth Metamorphic Complex achieved peak metamorphism at 509 ± 7 Ma, at conditions of 670o C, 16,900 bars, and the nearby Settlers Schist gives results of 513 ± 8 Ma. The garnet porphyroblasts of the Port Davey Metamorphic Complex record a single episode of metamorphism which took place at 505 ± 2 Ma at ~ 550 to 570o C and ~ 6,000 bars during which a dehydration event resulted in both a change of garnet composition and texture. The regional geology indicates metamorphism predated post-collisional extension and associated eruption of the Mount Read volcanics at 506 to 500 Ma. Most of the monazite dating is consistent with this observation. However, the Mersey River Metamorphic Complex gives very consistent results of 497 ± 3 Ma. This is problematical, as it would have been at depth undergoing metamorphism after that extension took place. This could be the result of an unknown analytical problem, but the Mersey River monazite grains are indistinguishable chemically from monazite in the other units, and this sample has undergone repeated analysis. Alternatively, this sample reflects a different metamorphic event than that recorded in all other samples studied.


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