Extensional collapse of the late Neoproterozoic-early Palaeozoic East African-Antarctic Orogen in central Dronning Maud Land, East Antarctica

Abstract

Abstract The East African-Antarctica Orogen resulted from the continent-continent collision of East and West Gondwana, or parts thereof, during the Pan-African event at c . 650–510 Ma. The collision overprinted large areas of older, mainly Mesoproterozoic, crust up to granulite facies grade in East Antarctica. The collision history is well documented by folding and thrusting, isothermal decompression and metamorphic zircon growth at c. 580–560 Ma (Pan-African I). The convergence was succeeded by an extensional phase, probably representing orogenic collapse. This Pan-African II event at c . 530–510 Ma is characterized by large-scale extensional structures, finally resulting in the post-tectonic intrusion of voluminous A2-type granitoids. In central Dronning Maud Land the Pan-African II event started with the intrusion of syntectonic igneous rocks within an overall extensional setting. Two new SHRIMP data from gabbro zircons of the Zwiesel Gabbro give ages of 521±5.6 and 527±5.1 Ma. These ages are interpreted as crystallization ages and confirm the interpretation that the gabbro was emplaced early during the Pan-African II event. The gabbro was intruded by a network of leucogranite dykes and veins. Whereas the gabbro appears entirely undeformed, the leucogranite dykes are strongly mylonitized along extensional shear zones, indicating pronounced strain partitioning of the gabbro complex. Within the leucogranite mylonites, large tension gashes developed during mylonitization, indicating very high strain rates. Quartz c-axis orientations from quartz of the tension gashes show a distinct cross-girdle that formed during pure shear deformation. Fluid inclusion data from the leucogranite mylonites and the associated tension gashes mainly reveal recrystallization-related intracrystalline CO 2 -dominant inclusions with relatively low densities of < 1 g cm −3 . The fluid inclusion data are interpreted to represent the last stages of a retrograde P-T path that is characterized by simultaneous cooling and decompression during extensional exhumation, probably succeeding the collapse of overthickened crust. A comparable orogenic collapse of the East African-Antarctic Orogen is reported from other parts of the orogen, such as from western Madagascar and the northern Arabian-Nubian Shield.