Were Phanerozoic (micro)continent-continent collisions frontal or oblique? Possible criteria and the Devonian collision of Chilenia with SW Gondwana as an example

Abstract

The understanding of ancient continent-continent collisions requires information on the type of accretion (frontal or oblique) of the colliding plates. The shape of the P-T path of a rock, which was metamorphosed during ongoing continent-continent collision, has so far been ignored to obtain this information. However, the consideration of corresponding P-T paths related to Phanerozoic orogens suggests that the shape of the burial path has the potential to distinguish between frontal (60-90° angle between collisional front and plate convergence direction) and oblique (<45°) collisions. In case of an oblique collision, the P-T path is nearly isothermal towards peak-pressure conditions. These conditions should be in the range of 12-20 kbar although somewhat higher pressures cannot be excluded. In this contribution, this finding is applied exemplary to the collision of SW Gondwana with microcontinent Chilenia being part of the Palaeozoic collage of microcontinents in the southern part of South America. However, only a single P-T path of a metasediment (Guarguaraz Complex) in the literature provides the requested information. This path is characterized by nearly isothermal burial reaching peak pressures of ~14 kbar at about 480 °C. Thus, it is suggested that the approach of Chilenia to Gondwana was oblique. The abundance of serpentinite bodies in the Guarguaraz Complex is not in conflict with this suggestion.