P-T paths and tectonic evolution of shear zones separating high-grade terrains from cratons: examples from Kola Peninsula (Russia) and Limpopo Region(South Africa)

The petrology and P-T evolution of mica schists from two regional scale tec tonic (shear) zones that separate high grade terrains ("mobile belts") from cratons are described. These are the 2.4-1.9 Ga Tanaelv Belt, a suture zon e that separates the Lapland granulite complex from the Karelian craton (Ko la Peninsula-Fennoscandia), and the 2.69 Ga Hout River Shear Zone that sepa rates the > 2.9 Ga Kaapvaal craton from the 2.69 Ga South Marginal Zone of the Limpopo high-grade terrain (South Africa). Two metamorphic zones are identified in strongly deformed mica schists from the 1.9 Ga Korva Tundra Group of the Tanaelv belt: (1) a chlorite-stauroli te zone tectonically overlaying gneisses of the Karelian craton, and (2) a kyanite-biotite zone tectonically underlying garnet amphibolites of the Tan aelv Belt, which are in tectonic contact with the Lapland granulite complex . The prograde reaction Chl + St + Ms double right arrow Ky + Bt + Qtz + H2 O clearly defines a boundary between zones (1) and (2). Rotated garnet porp hyroblasts from zone (1) contain numerous inclusions (Otz, Chi, Ms), and sh ow clear Mg/Fe chemical zoning, suggesting garnet growth during prograde me tamorphism. The metamorphic peak, T = 650 degrees C and P = 7.5 kbar, is re corded in the kyanite-biotite zone and characterized by unzoned snowball ga rnet. In many samples of mica schists euhedral garnet porphyroblasts of the retrograde stage are completely devoid of mineral inclusions while N-Mg de creases from core to rim, indicating a decrease in P-T from 650 degrees C, 7.5 kbar to 530 degrees C, 5 kbar. The Hout River Shear Zone (South Africa) shows metamorphic zonation from gr eenschists through epidote amphibolites to garnet amphibolites. Rare strong ly deformed mica schists (Chl + Grt + Pl + Ms + Bt + Qtz) occur as thin lay ers among epidoteamphibolites only. Garnet porphyroblasts in the schists ar e similar to that of the Tanaelv Belt recording a prograde P-T path with pe ak conditions of T = 600 degrees C and P similar to 5.5 kbar. The retrograd e stage is documented by the continuous reaction Prp + 2Ms + Phl double rig ht arrow 6Qtz + 3East recording a minimum T = 520 degrees C and P similar t o 3.3 kbar: Similar narrow clockwise P-T loops recorded in mica schists fro m both studied shear zones suggest similarities in the geodynamic history o f both sheer zones under consideration.