Counterclockwise P-T path and isobaric cooling of metapelites from Brattnipene, Sør Rondane Mountains, East Antarctica: Implications for a tectonothermal event at the proto-Gondwana margin

In this paper we provide evidence for a counterclockwise P-T path and isobaric cooling for metapelitic rocks from the Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica. The counterclockwise path was determined using the following mineral textures and relationships: (1) garnet coronae on sillimanite where the sillimanite is associated with spinel in orthopyroxene-bearing granulite; (2) garnets that contain inclusions of spinel, sillimanite, and corundum, and where the sillimanite and corundum have sparse tiny spinel inclusions; (3) the garnet-orthopyroxene-plagioclase-quartz equilibria relationships using the contrasting compositions of high-Mg garnet - plagioclase inclusions and high-Ca garnet coronae - matrix plagioclase point to an increase in pressure. The peak metamorphic conditions were determined by thermobarometry and pseudosection analyses, and are consistently within the ranges 850-900°C and 8-9kbar. Orthopyroxene porphyroblasts close to garnet coronae have relatively high Al₂O₃ contents (~6.1wt.%), consistent with the highest temperature conditions reported so far from the Sør Rondane Mountains. Prismatic subidioblastic staurolite formed as a secondary mineral along the margins of the garnets, and this retrograde staurolite indicates a back-reaction during isobaric cooling. The conditions for staurolite formation are taken to be 680-700°C and 8.5kbar using isopleths, and this is consistent with the appearance of secondary kyanite in the matrix. On the basis of the metamorphic P-T path and the tectonic setting of the precursor rocks, the following tentative scenarios can be inferred: (1) the heat source for the early sequence of metamorphism was derived from advective heat flow at an active continental margin, and (2) the subsequent increase in pressure was caused by overthrusting or obduction of either oceanic crust or oceanic island arc onto the Brattnipene region. These tectonothermal events took place along a possible proto-Gondwana margin at ca. 650-600Ma.