Neoproterozoic rifting in the Upper Yangtze Continental Block: Constraints from granites in the Well W117 borehole, South China.

Well W117 in the Sichuan Basin reveals a suite of ~814 Ma quartz monzonites, unconformably overlain by Sinian clastic and carbonate sediments. The quartz monzonites contain no muscovite and amphibole, and are characterized by high SiO2 (72.26-77.93%), total alkali, and TFe2O3/MgO content, and low P2O5 and CaO abundance, with variable A/CNK ratio (0.93-1.19), classified as metaluminous to weakly aluminous highly fractionated I-type granites. They are preserved in the Neoproterozoic rift and exhibit restricted negative εNd(t) values (-7.0 to -5.2) and variable zircon εHf(t) values (-13.9 to 2.3), suggesting their generation via melting of both ancient and juvenile crustal materials in an extensional setting. Their parent magmas were formed in a low-temperature condition (831-650 °C) and finally emplaced at ca. 9-10 km below the surface, indicating that the intrusion underwent exhumation before the deposition of Sinian sag basin. Such geological processes, together with evidence for Neoproterozoic structures in the surrounding area, support that the Upper Yangtze craton experienced two main phases of rifting from 830-635 Ma. The Well W117 granites and its overlying sediments record a geodynamic evolution from orogenic collapse to continental rifting, and to thermal subsidence, probably related to the Rodinia supercontinent breakup.