The last Neoproterozoic rift magmatism on the western margin of Yangtze block, South China: New insights of Marinoan onset from low-δ18O magmatic events

Abstract

The geological record preserves evidence of “Snowball Earth” glaciations during the Cryogenian Period, the Sturtian onset at ca. 720 Ma and the Marinoan onset at ca. 650 Ma. Compared to the Sturtian, the trigger for the Marinoan is still unclear due to the absence of large igneous provinces (LIPs). This study first discovered low-δ18O Luoluobuzi (LLBZ) granite which emplaced on the eve of the Marinoan at the Yangtze Block in South China. The granite with U-Pb age of 651.2 ± 2.1 Ma and has εHf (t) values between + 6.8 to + 13.3, and low-δ18O values between + 2.74 to + 5.34 ‰. Granite samples show Ba, Nb, Ta, Sr, Eu, and Ti depletion and enrichment in Rb, U, Th, and Pb, modeling calculations to suggest they originated from partial melting of nearby Neoproterozoic mafic rocks and formed in a continental rift. We systematically compiled data and found an interglacial period between the Sturtian-Marinoan glaciations is absent of magmatic activities. Extensional magmatic activity similar to the LLBZ granite started at ca. 651 Ma, which within errorsoverlapped the onset of the Marinoan. Such temporal ruling also existed in other worldwide rift-related magmatic activities. After combining chemical index of alteration (CIA) and palaeomagnetic results, we found the last episode of Rodinia breakup played an important role in the onset of the Marinoan.