Tectonic driving of Neoproterozoic glaciations: Evidence from extreme oxygen isotope signature of meteoric water in granite

摘要

The global context of glaciation in the Neoproterozoic has been hypothesized to result from the massive reorganization of the Earth's land and ocean systems due to breakup and dispersal of the supercontinent Rodinia at about 750 Ma. This hypothesis gains support from unusually light O isotope records of hydrothermally altered rocks in a rift tectonic zone at that time, which is indicative of interaction with meteoric water of low mean annual temperature, and thus tectonic driving of cold palcoclimate at the time of hydrothermal alteration. Very negative delta O-18 values of -14.4 to -10 parts per thousand are found for gamet from Neoproterozoic granite in South China, which are the lightest O isotope record so far reported for minerals from igneous rocks. Negative delta D values of -129 to -109%o are obtained for gamet, magnetite and zircon. Thus high-T meteoric-hydrothermal alteration occurred during magma emplacement. SHRIMP U-Pb dating for magmatic and hydrothermal zircons yields two groups of ages at 782 +/- 3 Ma and 748 +/- 3 Ma, respectively, responsible for granite crystallization and hydrothermal alteration. The garnet is in O isotope disequilibrium with zircon, indicating differential effects of subsolidus hydrothermal alteration on the minerals of different O diffiusion rates. Occurrence of the unusually negative delta O-18 granite at mid-low paleolatitudes provides a geochemical proxy for a cold paleoclimate at 748 +/- 3 Ma, possibly a continental glaciation corresponding to the Kaigas iceage. It suggests a tectonic link to the climatic effect of uplifted rift flanks due to the Rodinia breakup at about 750 Ma, and thus the ice-fire interaction by syn-rift magmatism of low delta O-18 imprints in association with the mantle event of asthenospheric upwelling. Hence the tectonic driving is evident for regional glaciations in supercontinental rift basins. (c) 2007 Elsevier B.V. All rights reserved.