CONTACT VS REGIONAL METAMORPHISM IN THE AREA OF TORRES DEL PAINE (SOUTHERN CHILE) – COMBINING RAMAN SPECTROSCOPY AND CLAYINVESTIGATIONS

The transition from a low-temperature metapelitic contact aureole to a pre-intrusive, low-temperature regional metamorphism is rarely recognized in outcrop scale. This study reports results from the transition zone from contact to regional metamorphism by using Raman spectroscopy on organic matter, illite crystallinity and chlorite chemistry.

The studied contact aureole of the Torres del Paine intrusive complex (TPIC) is located in the Magallanes Basin at 51°S. The thermal evolution in the Magallanes Basin is characterized by a complex interaction of static burial, deformation during fold-and-thrust belt formation, and contact metamorphism. The deformation process has been dated at around 60 Ma under regional anchizonal metamorphic conditions. The TPIC was emplaced into the Cretaceous Cerro Toro and Punta Barrosa formations between 12.59 and 12.43 Ma and post-dates deformation [1,2,3]. Three profiles as well as individual samples were collected starting from the cordierite-in isograde of the contact aureole towards only regionally affected units of the Cerro Toro Formation and were subjected to XRD, SEM and EMP analysis. The initial regional metamorphic conditions, prior to the emplacement of the TPIC, are anchizonal, with temperatures between 240 and 270°C based on Raman spectroscopy of organic matter. The regional metamorphic shale paragenesis consists of albite, chlorite and illite (dominant clay phases). The influence of the TPIC contact metamorphic zone (CMZ) is documented by elevated Raman temperatures, the appearance of epidote and retrograde Fe-rich chlorite. The CMZ can be identified at a distance of 1.2 km and thus, spreads ca. 800 m further out than the hornfels aureole. Temperatures gradually increase towards the cordierite-in isograde. However, illite crystallinity values remain unchanged and record the regionally observed anchizonal values.

In this study, organic matter responses to rapid temperature increase, whereas illite crystallinity seems to react less sensitively to thermal changes. Possible explanations for the slow reaction kinetics of illite include the short heating time of a few thousand years, and a lack of induced stress and probably a low fluid activity.