Geology, Petrology and O and H isotope geochemistry of remarkably 18O depleted Paleoproterozoic rocks of the Belomorian Belt, Karelia, Russia, attributed to global glaciation 2.4 Ga

This paper deals with strongly O-18-depleted (down to -27.3 parts per thousand VSMOW) 1.9Ga Paleoproterozoic mid-grade metamorphic rocks found in the Belomorian Belt of Karelia (E. Baltic Shield). The protolith of these rocks is attributed to have been altered by glacial meltwaters during the world's first 2.4-2.3 Ga Paleoproterozoic "Slushball" glaciation, when Karelia was located near equatorial latitudes. We describe in detail three and report seven new localities with unusually depleted O-18 signatures that now span 220 km across the Belomorian Belt. Hydrogen isotope ratios measured in amphibole, biotite and staurolite also display remarkably low values of -212 to -235 parts per thousand. Isotope mapping in the three best exposed localities has allowed us to identify the world's most O-18 depleted rock, located at Khitostrov with a delta O-18 value 27 parts per thousand. In Khitostrov samples, zircons have normal delta O-18 detrital cores and low-delta O-18 metamorphic rims. Mapping demonstrates that zones of delta O-18 depletion occur in a concentric pattern 100-400 m in dimension, and each locality displays significant delta O-18 and ED heterogeneity on a meter to centimeter scale, characteristic of meteoric-hydrothermal systems worldwide. The zone of maximum delta O-18 depletions usually has the highest concentration of metamorphic corundum, rutile, and zircon and also display doubled concentrations of insoluble trace elements (Zr, Ti, Cr, HREE). These results are explained by elemental enrichment upon mass loss during hydrothermal dissolution in pH-neutral meteoric fluid. Remarkably low-delta O-18 and ED values suggest that alteration could have only happened by glacial meltwaters in a subglacial rift zone. Many localities with delta O-18 depletions occur inside metamorphozed 2.4 Ga gabbro-noritic intrusions, or near their contact with Belomorian gneisses, implying that the intrusions were driving meteoric hydrothermal systems during the known 2.4 Ga episode of Belomorian rifting. Given that the isotopically-depleted localities now spread over 200 km, the extent of the Karelian ice cap is estimated to be at least that large. Svecofennian 1.9 Ga metamorphism is seen to cause metamorphic recrystallization of hydrothermally-altered rocks into coarse-grained assemblages, and causing local metasomatism through devolatilization of the underlying hydrous low-delta O-18 protolith, further depleting ED via volatilization. This process led to gem-quality rubies and kyanites that preserve these remarkable delta O-18 values in the geologic record. (C) 2011 Elsevier B.V. All rights reserved.