P-T conditions of metapelites from metamorphic complexes in Aysen, Chile

Abstract

Pressure and temperature metamorphic conditions were determined for K-white mica and chlorite in metapelites from the eastern Andean metamorphic complex (EAMC) and from the Chonos metamorphic complex (CMC) of southern Chile. The Kubler index (KI) of K-white micas was measured on a < 2 mu m fraction of 60 metapelites from both complexes. The KI-values obtained from the EAMC vary between 0.26 and 0.16 Delta degrees 2 theta; in the CMC, their range is 0.27-0.17 Delta degrees 2 theta, which indicates that the metamorphic temperatures reached by the metapelites are higher than 300 degrees C for both complexes. As confirmed by chlorite geothermometer, the temperatures are between 310 and 380 degrees C. Determination of the metamorphic pressure was carried out through the b-parameter of K-white micas and a phengite geobarometer. The b-parameter for the EAMC (9.033 + 0.10 angstrom) corresponds to a pressure of 4.0 + 1.2 kbar, and the phengite geobarometer yields pressures of approximately 2.7 kbar. For the CMC, 5.2 + 0.7 kbar (9.035 + 0.007 angstrom) is obtained by b-parameter and 3.8 kbar by phengite geobarometer. Although they differ in the pressure values, they are consistent, in that those of the EAMC are relatively lower than those of the CMC. The P-T metamorphic conditions are consistent with a subduction complex along the Gondwandan margin, of which the CMC and EAMC were parts.