Garnet Lu-Hf geochronology and P-T path of the Gridino-type eclogite in the Belomorian Province, Russia

Eclogites are commonly seen as markers of subduction and thus their presence in Proterozoic and Archean orogenic provinces is crucial information for determining the initiation of modern plate tectonic. The Belomorian province hosts some of the oldest known eclogite-facies rocks. Here we present new garnet Lu-Hf geochronology that constrains the prograde stage of Gridino-type eclogite to ca. 1.96-1.92 Ga. Inherited magmatic zircon cores have Hf-176/Hf-177 ratios that indicate isotopic disequilibrium between the inherited zircon cores and the metamorphic mineral assemblages, while the metamorphic rims have variable Hf-176/Hf-177 ratios that reflect Paleoproterozoic eclogite-facies metamorphism. Iterative thermodynamic models, involving the optimization of pressure (P), temperature (T) and reactive bulk composition (X) was used to reconstruct the P-T conditions recorded by garnet, and define the prograde trajectory. Zr-in-ruffle thermometry, combined with equilibrium phase diagrams, constrains the peak P-T conditions at 725-750 degrees C and above 18 kbar, corresponding to an average apparent thermal gradient of below 41 degrees C/kbar. The P-T conditions of the high-pressure granulite facies overprint are 680-730 degrees C at 9-10 kbar. Thus, the metamorphic evolution of Gridino-type eclogite followed a clockwise P-T path with a cooling decompression stage, recording the assembly of the Columbia supercontinent during the Paleoproterozoic. (C) 2018 Published by Elsevier B.V.