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

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.

► Four periods of bulk horizontal shortening during the long-lasted Isan Orogeny
► Low-P/high-T metamorphism accompanied NE–SW shortening aged at 1649 ± 12 Ma.
► Medium-P/high-T metamorphism accompanied periods of N–S shortening aged at 1645 ± 7.
► High-P/high-T metamorphism accompanied W–E shortening aged at 1591 ± 10 Ma.
► Introduction of mafic dykes and sills was the major source of heat.