METAMORPHIC EVOLUTION OF SPESSARTINE QUARTZITES (COTICULES) IN THE HIGH-PRESSURE, LOW-TEMPERATURE COMPLEX AT BAHIA MANSA, COASTAL CORDILLERA OF SOUTH-CENTRAL CHILE

Abstract

Lenses of spessartine quartzites (coticules) are associated with greenschist intercalations in coastal exposures at Bahia Mansa within the mainly metapsammopelitic Western Series, which forms most of the basement in the Coastal Cordillera of central to southern Chile. The chemical compositions of the coticules can be explained by protoliths formed from ferriferous and manganiferous hydrothermal precipitates mixed with aluminous alteration-derived material on top of oceanic crust. The peak conditions of metamorphism were calculated with multivariant reactions: 270–370 °C, 6–8 kbar. A retrograde P–T evolution was marked by decompression to 2 kbar during cooling below 300°C, influx of an external fluid, and strain-free crystallization of retrograde phases. The appearance of “ferriwinchite”, “ferribarroisite” and phengite in coticules and surrounding metabasic roc ks marks the transition from greenschist to epidote blueschist facies. Oscillatory zoning in garnet within the coticules as well a s in epidote and tourmaline in the metabasic rocks seems to be governed by the local chemical environment during prograde growth of minerals. Although the garnet in the coticules formed by a continuous prograde dehydration–decarbonation reaction, the primary fluid inclusions are predominantly hydrous owing to massive influx of fluid during the early stage of decompression. Th e late stage of decompression is bracketed by isochores of primary fluid inclusions that are clearly below peak conditions of metamorphism. This is attributed to a volume increase of the fluid inclusions and stretching of the host minerals during decompression. Higher densities of fluid inclusions in garnet compared to those in quartz are due to differences in yield stren gth between the two minerals. The transient metamorphic gradient of approximately 12 °C/km revealed by the coticules and adjacent metabasic rocks of oceanic origin suggest slow subduction. Penetrative ductile deformation is mainly restricted to the late pro grade and early retrograde P–T path within an accretionary prism.