Mesoproterozoic subduction under the eastern edge of the Kalahari-Grunehogna Craton preceding Rodinia assembly: the Ritscherflya detrital zircon record, Ahlmannryggen (Dronning Maud Land, Antarctica)

The ∼2000 m thick clastic and volcaniclastic sedimentary rock pile of the Mesoproterozoic Ritscherflya Supergroup is located near the eastern margin of the Archaean Grunehogna Craton of Dronning Maud Land (East Antarctica). The sedimentary rocks were deposited proximal to an active volcanic arc formed during subduction prior to the assembly of the Rodinia supercontinent. In this study, we investigated internal zonation and U-Pb ages of detrital zircon grains from all formations of the Ahlmannryggen and Jutulstraumen groups of the Ritscherflya Supergroup. Our results show an age distribution with a dominant age peak at ∼ 1130 Ma, close to the sedimentation age of the sedimentary rocks (∼ 1130-1107 Ma), which strongly supports the model of deposition of the sediments in a convergent margin setting. Older peaks in the Ritscherflya sedimentary rock zircon spectrum with ages up to 3445± 7 Ma that were also identified in samples from the Grunehogna Craton basement reflect tectono-magmatic events in the Kalahari Craton. This provides further evidence for the Archaean and Proterozoic connection of the Grunehogna province to the African Kalahari Craton. Parts of the Mesoproterozoic volcanic arc were located on Archaean cratonic basement (∼2800-3450 Ma), whereas other parts tapped late Palaeoproterozoic crust (∼1750 Ma). This is evident from a number of inherited Archaean and Proterozoic cores in zircons with Stenian rims. The Ritscherflya zircon record, therefore, supports models of the eastern margin of the Kalahari-Grunehogna Craton that include inward subduction with an active continental margin prior to collision in Dronning Maud Land. The intercalation of the clastic sedimentary rocks with volcaniclastic materials strongly support the interpretation of a very proximal volcanic source. The sedimentary rocks were affected by regional low-grade metamorphism during the collisional orogeny related to Rodinia assembly and during the Pan-African orogeny related to the assembly of Gondwana. This is evident from metamorphic recrystallisation of zircon at 1086± 4 Ma and from discordancy of many grains pointing to late Neoproterozoic to early Phanerozoic lead loss.

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