Metamorphic rocks with different pressure–temperature–time paths bounded by a ductile shear zone at Oyayubi ridge, Brattnipene, Sør Rondane Mountains, East Antarctica

Abstract

The Sør Rondane Mountains, East Antarctica have been thought to be situated in the collision zone between East and West Gondwana during the final stage of amalgamation of the Gondwana supercontinent. They are therefore recognized as a key region for understanding the geological phenomena during the collisions and for testing the proposed tectonic models. We identified metamorphic rocks with different pressure–temperature–time paths that are bounded by a ductile shear zone at Oyayubi ridge, Brattnipene, Sør Rondane Mountains. Based on field and microscopic observations, chemical analyses of minerals, and zircon U–Pb dating, the sillimanite–garnet–biotite gneisses (i.e., pelitic gneisses) from higher structural level show a peak metamorphism at ca. 590 Ma that took place under conditions of ca. 830–840 °C and 0.8–0.9 GPa, and these high-temperature conditions lasted until ca. 550 Ma. These rocks underwent isothermal decompression and then retrograde hydration under lower pressure–temperature conditions than 530 °C and 0.4 GPa at ca. 530 Ma. In contrast, the orthogneisses that consist of hornblende–biotite gneiss and garnet–clinopyroxene gneiss from lower structural levels did not undergo metamorphism at ca. 600 Ma, but underwent metamorphism at ca. 570 Ma and reached peak conditions of 700–760 °C and 0.6–0.9 GPa at ca. 560–550 Ma. These observations suggest thrusting of the pelitic gneiss over the orthogneiss at ca. 570–550 Ma, causing a prograde metamorphism of the orthogneisses and a decompression of the pelitic gneisses as well as uplift and subsequent rapid denudation. The results indicate two stages of collision in the Sør Rondane Mountains, and that the ductile shear zone that bounds the pelitic gneiss and orthogneiss units may have been part of the continental plate collision boundary at ca. 570–550 Ma.