Was the Devonian collision of Chilenia with south-western Gondwana frontal or oblique?

Abstract

The understanding of ancient continent-continent collisions requires the information of the type of approach (frontal or oblique) of colliding continental plates. To obtain this information, the shape of a P-T path for a rock that was metamorphosed during ongoing continent-continent collision, was not taken into account so far. However, a consideration of P-T paths of metamorphic rocks of Phanerozoic orogens suggests that the shape of the burial path has the potential to distinguish between frontal or oblique collision. In case of oblique collision, this part of the P-T path should be characterized by nearly isothermal burial to peak-pressure conditions at elevated temperatures. These conditions should be in the range of 12-20 kbar although somewhat higher pressures cannot be excluded. The application of this finding to the Palaeozoic collage of microcontinents in the southern part of South America is difficult because of the rarity of P-T paths with constrained burial path. Fortunately, a single P-T path of a metasediment related to the collision of the microcontinent Chilenia with south-western Gondwana provides the requested information. According to the nearly isothermal burial path reaching peak pressures of 14 kbar at about 480 °C, it is suggested that the approach of Chilenia to Gondwana was oblique. The abundance of serpentinite bodies in the corresponding outcrop area supports this suggestion.