Mineral chemistry and pressure-temperature evolution of two contrasting high-pressure-low-temperature belts in the Chonos Archipelago, Southern Chile

The Chonos Metamorphic complex forms part of a belt of low-grade metamorphi c rocks in the Chilean coastal Cordillera that are interpreted as Palaeozoi c-Mesozoic accretionary complexes. It comprises metapsammopelitic schists, metabasites and metaironstones occurring in two contrasting units. Special attention during microprobe study of key samples was given to the chemical zonation of minerals. Subsequently, conventional geothermobarometry and tha t using thermodynamic calculations were applied. The Eastern belt comprises rocks that are metamorphosed to pumpellyite-actinolite facies conditions a nd show a low degree of deformation with well-preserved sedimentary and ign eous structures. Maximum P-T conditions were around 5.5 kbar and 250-280 de grees C. The rocks of the Western belt are characterized by a transition be tween greenschist and albite-epidote-amphibolite facies metamorphism and sh ow a penetrative tectonic transposition foliation S-2 formed close to the p ressure maximum. Maximum P-T conditions vary around 8-10 kbar and 380-500 d egrees C overstepping the stilpnomelane + phengite stability. High pressure s in this belt are confirmed by regionally distributed phengites with high Si contents up to 3.5 Si per formula unit. Regional distribution of maximum temperatures is reflected by the composition of actinolitic hornblends wit hin the metabasites. In a garnet-bearing metabasite of the Western belt, os cillatory growth zoning of garnet was observed. The composition of correspo nding mineral inclusions suggests that a prograde P-T path during garnet gr owth evolved from 7.5 kbar and 375 degrees C to about 9.4 kbar and 500 degr ees C. Late garnet grew synkinematically with penetrative deformation. The retrograde P-T path in the rocks of the Western belt is constrained by the composition of mainly late strain-free minerals and involves slight cooling during decompression. Both belts are part of a subduction system. The appa rent P-T gap between the belts is due to their juxtaposition during exhumat ion. The Eastern belt constitutes the transition towards the backstop syste m of the accretionary prism that is represented by the Western belt whereas the absence of very low grade rocks west of the Western belt is attributed to tectonic erosion, which was possibly caused by subduction of a ridge.