MnNCKFMASH models of subduction and exhumation in HP/LT metabasites in Elephant Island (S. Shetland Islands, Antarctica).

RECHE ESTRADA, J. Y ACEVEDO, R.D.

Santiago de Chile

Congreso; GeoSur 2007; 2007

Pontificia Universidad Católica de Chile

Petrologic models (P-T pseudosections) are used here to show the metamorphic assemblages that can appear along hypotetical P-T paths in two metabasite samples collected and studied by Acevedo, (1999) from Stinker Point (E71: 61º13´29´´S, 55º21´57´´W) and Mesa Bay (E24: 61º08´07´´S, 55º25´49´´W),W of Elephant Island. The Elephant Island Group is a fragment with Cretaceous (120 - 80 My.; Hervé et. al., 1990, Trouw et.al., 1990) age metamorphism of the HP/LT Sambagawa type Scotia metamorphic complex (Tanner et. al., 1982; Dalziel, 1984). According to Trouw et.al., (1998), the fragment followed an earlier prograde, increasing pressure, sin-D1 (subduction towards the SW) P-T trajectory, followed by heating with minor decompression related to D2 backthrusting and by decompression and cooling, related to D3 deformation and to exhumation and retrogradation of the metamorphic assemblages. Models have been constructed with the software THERMOCALC v. tc326 (Powell et.al., 1998), using the latest (tcds55) thermodynamic dataset (Holland and Powell, 1998 with 5fth, 2003 upgrade). The mineral phases considered in the models are: blue and green amphiboles (glaucophane, actinolite and hornblende), omphacitic piroxene, chlorite, white mica, plagioclase, lawsonite, clinozoisite and albite in the chemical systems NCKFMASH and MnNCKFMASH. The metamorphic assemblage present in the Stinker Point (E71: garnet + hornblende + plagioclase ± clinozoisite + quartz + white mica) appears in a 4-variant field centered around 480ºC and 4.5 kbar in the P-T pseudosection. This is a notable coincidence with the geothermobarometric results of Trouw et. al., (1998). Other mineralogical and textural features predicted by the model of E7 are: garnet present since the subduction stage, late plagioclase with albite-oligoclase-albite zoning from core to rim related to destruction of Na-bearing phases (glaucophane) during second stage heating, chlorite as inclusions (prograde, destabilized during heating) and as matrix phase (retrograde, formed during the late cooling-decompressional stage) and clinozoisite also present as abundant inclusions in plagioclase and in the matrix, consistent with its first destabilization during heating to give Ca-component of oligoclase ± grossular ± green amphibole and with its re-growth related to late retrograde albite. By contrast, the assemblage present in Mesa Bay (E24: glaucophane + chlorite + epidote ± actinolite depicts conditions consistent with the 350º C and 7.5 kbar given by Trouw et. al., (1998) and must have followed a heating and late cooling-decompression trajectory inside the same wide 3-variant (act gl chl cz) field, given that almost no hornblende and/or albite-oligoclase growth is detected. Some features suggested the possible presence of early lawsonite. The P-T pseudosection predicts also the presence of lawsonite during the earliest compressional stage and its early destabilization in E24 during the heating stage.