Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: example from the Eastern Fold Belt, Mount Isa Inlier, Australia

Abu Sharib, A.S.A.A.;Sanislav, I.V.

Abstract

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group, the southeastern corner of Mount Isa Inlier, record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (~ 1650–1500 Ma). Low-P/high-T metamorphism (M₁) prevailed during a period of NE–SW bulk horizontal shortening as indicated by the early growth of cordierite porphyroblasts, which entrap monazite grains that gave an average age of 1649 ± 12 Ma. This was followed by medium-P/high-T (M₂) and high-P/high-T (M₃) metamorphisms that accompanied periods of N–S and W–E bulk horizontal shortening, respectively. Growth of first generation garnet, andalusite and staurolite porphyroblasts having an average age of 1645 ± 7 Ma identify the former, whereas growth of the 1591 ± 10 Ma second generation garnet, staurolite and andalusite porphyroblasts together with fewer kyanite porphyroblasts characterizes the latter. The sediments of the Soldiers Cap Group were deposited in a tectonic setting that has the characteristics of an intra-continental rift basin. The upper limit of the age of sedimentation is constrained by detrital zircons at 1654 ± 4 Ma suggesting that tectonism and metamorphism were either active during the final stage of the basin filling or immediate after deposition. Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was the major source of heat.

Journal

Tectonophysics

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587

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0040-1951

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Elsevier

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DOI

10.1016/j.tecto.2012.06.051