Compositional variations of syntectonic white-mica in low-grade ignimbritic mylonite

The Silla del Gobernador Shear Zone (SGSZ), Coastal Range of central Chile (32°S), is a low-grade mylonite generated from a Triassic acidic ignimbritic protolith. Different microstructures (e.g. quarter mats and pressure shadow structures) associated with pressure solution and solution transfer processes and well-preserved porphyroclasts can be observed. White-mica generation appears to be favoured by reaction-softening from dissolved quartz and feldspar porphyroclasts. According to the mylonite fabrics three different types of white-mica can be established (A, B and C) representing mylonites with evidences of relatively low, intermediate and high strain conditions. Deformation mechanisms suggest low-grade (350–400 °C) conditions for mylonitic deformation in the presence of fluids, consistent with the temperature estimations based on chlorite chemistry. The geological setting of the SGSZ also suggests low-pressure (<3 kbar) conditions. No major element mobility with respect to protolith composition was observed in low-strained mylonites, whereas for the highly strained mylonites, a marked K2O secondary input and higher fO2 conditions during deformation are suggested. The chemical composition of the neoformed white-mica correlates with strain intensity: the high-strained mylonites contain phengitic–celadonitic micas, whereas low-strained mylonites have paragonitic and muscovitic micas. White-mica geobarometry in highly deformed rocks yields pressure values with no geological significance, because the metasomatic K-input favour their high K content, independent of the confining pressure. Thus, the white-mica composition in the mylonites of the SGSZ is controlled by the whole-rock geochemistry and is related to strain conditions.