Geochemistry of the 1100Ma intrusive rocks from the Ahlmannryggen region, Dronning Maud Land, Antarctica

摘要

The recognition of a Mesoproterozoic large igneous province (LIP) across large parts of southern Africa has been strengthened by recent geochronology, geochemistry and petrology. The c. 1100 Ma Umkondo province has been recognized across parts of Botswana, Zimbabwe, South Africa and Mozambique where tholeiitic sills, dykes and rare lava flows have been correlated into a single magmatic province emplaced in the interval 1108–1112 Ma. The extension of the province into the Dronning Maud Land region of Antarctica has been suggested by several workers, but detailed analyses of geochemistry and petrogenesis are lacking, as are comparative studies. This study investigates 25 dykes and sills of the Borgmassivet intrusions which include several of the major diorite sills of the province, up to 300m in thickness. The dykes and sills are also considered to be c. 1100 Ma and they were emplaced, in part, synchronously with the Ritscherflya Supergroup sedimentary sequence. The Borgmassivet intrusions are characterized by geochemical signatures that suggest the magmas were either extensively contaminated by continental crust or derived from an enriched lithospheric mantle source, where the enrichment was related to earlier subduction. The limited geochemical range of the Borgmassivet and Umkondo intrusions are probably not consistent with significant levels of crustal contamination. Furthermore, the trace element ratios indicate a source in the sub-lithospheric mantle, followed by gabbroic fractionation and interaction with lithospheric wall rocks.