Mesoproterozoic HT-UHT granulites from the central Bushmanland Domain, Namaqua Metamorphic Province, South Africa: Metamorphic P-T evolution and geochronological constraints

Abstract

The Bushmanland Domain of the western Namaqua-Natal Metamorphic Province encompasses Paleoproterozoic to Mesoproterozoic intrusives as well as metamorphosed volcano-sedimentary supracrustal rocks. In this largest domain of the Namaqua Sector of the Namaqua-Natal Province, high-grade metamorphism prevails, with granulite-facies rocks occupying the central and southern parts. Previous studies of the granulites have revealed evidence for peak metamorphism reaching conditions that are at least transitional to ultrahigh temperatures (UHT). We present first evidence for coexisting sapphirine and quartz, and related peak conditions of 1000℃ – 1050℃ and 7 – 8 kbar. The contrast to previous P-T estimates and a clockwise P-T path possibly point to an older M1 metamorphic cycle that predates the well-established 1040 – 1020 Ma Namaquan (M2) cycle, characterised by lower-T and lower-P granulite-facies metamorphism. This M2 metamorphism shows a general anticlockwise P-T path as recorded in mafic rocks of the study area, with peak conditions of 770 – 830℃ and 4.9 – 6.4 kbar. Zircon and monazite geochronology yields two distinct age groups, with a Late Mesoproterozoic group confirming the ~ 1040 to 1020 Ma metamorphic age of the Namaquan orogeny related to M2, and a Paleoproterozoic group with a relatively narrow age range of ~1787 – 1773 Ma. This latter group encompasses the zircon cores and relates to magmatic crystallisation of felsic host rocks. The magmatic zircons had been through a stage of clastic transport before obtaining their metamorphic overgrowth within the Bushmanland Group supracrustal sequence. In the compiled zircon age spectrum for the Bushmanland Domain, this particular age group had not shown up prominently before. It points to a significant source area which is either not exposed, or could not be localized yet