Late-Tonian to early-Cryogenian apparent depositional ages for metacarbonate rocks from the Sør Rondane Mountains, East Antarctica

The Sør Rondane Mountains (SRM), located in the Neoproterozoic to Early Cambrian East African-Antarctic collisional orogen is composed of medium- to high-grade metasedimentary, metaigneous and intrusive rocks of diverse composition. Within the metasedimentary rocks, the metacarbonate rocks are considered to have deposited chemically in the so-called the "Mozambique Ocean" that separated the continental blocks that amalgamated to form Gondwana and possibly record geochemical signatures of contemporaneous seawater. Here we attempt to constrain the apparent age of sedimentation of metasedimentary sequences using strontium isotope chemostratigraphy of the least altered metacarbonate rocks. Pure metacarbonate samples, collected during the 51st Japanese Antarctic Research Expedition from different regions throughout the SRM, were selected based on careful screening using multiple geochemical parameters, such as carbon and oxygen isotopic composition and trace and rare earth element contents. We could successfully test the extent of alteration for the Sør Rondane metacarbonate samples, using Mn/Sr ratios, which show a positive correlation with ⁸⁷Sr/⁸⁶Sr ratios. After a rigorous geochemical screening in terms of post-depositional alterations, 18 samples were identified as least altered. These samples collectively gave regional initial ⁸⁷Sr/⁸⁶Sr ratios between 0.70566 and 0.70630, from Balchen, Brattnipene, Menipa and Tanngarden regions of the SRM, with the exception of Perlebandet region. These Sr isotopic ratios reflect seawater compositions of late-Tonian and early-Cryogenian age (880-850Ma and 820-790Ma), when compared with the evolution of Sr isotopes in the Neoproterozoic Oceans. Furthermore, these estimates are consistent with the carbon isotope chemostratigraphic curves of Neoproterozoic. The estimated apparent depositional ages of carbonate rocks in the SRM are also conformable with the reported detrital and metamorphic ages for this region. Carbonate rocks in the Perlebandet region shows low initial Sr isotope ratio (0.70482), suggesting that these rocks may have deposited earlier than other carbonate rocks in the SRM. Our results can be correlated with the chemostratigraphic depositional ages reported for carbonates from the Montepuetz Complex, Mozambique, suggesting the presence of contemporaneous platform environment on both sides of the possible suture. The finding of late-Tonian and early-Cryogenian carbonate deposition, potentially points toward a platform environment surrounding the tonalitic continental arc in the SW region of the SRM, prior to the amalgamation of Gondwana. The results obtained need to be tested with similar studies on metacarbonate sequences from the surrounding regions, which would help to resolve the processes and sequence of collision events that finally amalgamated Gondwana.