Late Mesoproterozoic Arc and Back-arc Volcanism in the Heimefrontfjella (East Antarctica) and Implications for the Palaeogeography at the Southeastern Margin of the Kaapvaal-Grunehogna Craton

Abstract

The nunataks of Heimefrontfjella in Dronning Maud Land (Antarctica) are dominantly composed of juvenile late Mesoproterozoic rocks that were metamorphosed and deformed at about 1080 and 500 Ma. Three discontinuity-bound terranes have been identified, namely the Kottas, Sivorg and Vardeklettane terranes. In the Sivorg terrane the basement is mainly made up of fine-grained banded felsic and mafic gneisses, representing a metamorphosed bimodal volcanic sequence. In contrast, the basement in the Kottas terrane is composed of banded gneisses of tonalitic to dioritic composition and calc-alkaline metaplutonic rocks. The metamorphic rocks of the Vardeklettane terrane are granulites of supra- to infracrustal origin. Geochemical data characterize the protoliths of the bimodal sequence in the Sivorg terrane as tholeiitic basalts with MORB signature, alternating with high-k rhyolites derived from a highly fractionated granitic magma. Four U-Pb zircon SHRIMP ages prove a magmatic acrivity in the Sivorg terrane at least from ∼1160 to 1090 Ma. The orthogneisses from the Kottas terrane plot in the field of volcanic arc granites. Their protoliths may be derived from a calc-alkaline magmatic suite. According to recent reconstructions of the Kalahari continent the basement of the Kottas terrane is an Antarctic counterpart of the Mzumbe terrane in Natal. Our data allow a reconstruction of a subduction-related volcanic arc (Kottas Arc) along a southerly directed subduction zone which existed train ∼1200 to 1100 Ma, finally colliding with the southeastern margin of the Kaapvaal-Grunehogna craton. South of the volcanic arc, a back-arc basin (Sivorg terrane) developed during latest Mesoproterozoic times. Undeformed calc-alkaline granites in the northernmost Heimefrontfjella point to a final northward directed subduction of the oceanic lithosphere of the back-arc basin which led to the collision of the Coats Land block with the Kaapvaal-Grunehogna craton.