An Ophiolitic complex in the northern Shackleton Range, Antarctica

Three lithotectonic and metamorphic units are exposed in the Bernhardi Heights-Hogbom Outcrops area of the eastern Herberts Mountains (northern Shackleton Range). From N to S, and structurally from top to bottom, they include: i) a high-grade gneiss-migmatite complex containing garnet-bearing meta-intrusives of dioritic to tonalitic composition ('Upper High Grade Gneiss Complex'); ii) a metamorphic 'Ophiolitic Complex' consisting of medium-grade mafic-ultramafic rocks and metasedimentary cover rocks, and iii) a high-grade gneiss complex made up of migmatitic gneisses and rare amphibolites ('Lower High Grade Gneiss Complex'). Prominent rock types in the Ophiolitic Complex are amphibolites of N-type MORB to OIB geochemical and Nd isotope affinity, metagabbros interpreted as metacumulate rocks having been formed in a MORB-type setting and metaperidotites. Single-stage Nd-model ages for the OIB-type amphibolites of around 1.0 Ga constrain a maximum formation age for the igneous protoliths of the ophiolitic unit. The polyphase metamorphic evolution of this complex included an early Barrovian-type metamorphic stage (550°C, 7-8 kbar), followed by decompression with concomitant T increase (up to 650°C, 5-6 kbar) and a late retrogression under greenschist facies conditions. K-Ar amphibole dates for amphibolites which are in the range of 500-490 Ma provide evidence for a late-Pan-African (Ross-age) age of metamorphism. Relict felsic granulites from the Upper High Grade Gneiss Complex record an early high-P granulite facies event (675-750°C, 8.4-11.3 kbar), followed by migmatization under low-P upper amphibolite facies conditions (730-850°C, 5-7 kbar). This early metamorphic event possibly reflects a Proterozoic tectonometamorphic cycle. The subsequent metamorphic stages span from intermediate-P amphibolite facies to greenschist-grade conditions. They document the tectonometamorphic reactivation of the High Grade Gneiss Complex as a consequence of the Ross-age thickening and exhumation history of the northern Shackleton Range crystalline rocks.