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Título

PETROGRAPHY AND MINERAL CHEMISTRY OF A KYANITE-BEARING CHLORITOID-GARNET-MUSCOVITE-QUARTZ SCHIST FROM THE RASPAS COMPLEX, SW ECUADOR

Texto do resumo

Metapelites are rocks usually sensitive to Pressure-Temperature (P-T) variations, and therefore important for the study of metamorphism, since they allow us to determine metamorphic conditions, which can thus be linked to ages to reconstruct its metamorphic history. The Raspas Complex is one of the best preserved exposed oceanic crust sections and constitutes the southernmost high-pressure occurrence in the Andes. The Complex hosts eclogites, blueschists, metapelites, serpentinised peridotites, greenschists and garnet amphibolites. Petrographic and mineral chemistry studies were carried out in the metapelitic schist part of this sequence in order to investigate its formation and evolution. The rock displays garnet porphyroblasts embedded in a fine to medium-grained anastomosed lepidogranoblastic matrix and is composed of quartz (54 vol%), white mica (14%), garnet (11%), chloritoid (6%), graphite (6%), stilpnomelane (3%), kyanite (2%), rutile (2%), chlorite (1%), and apatite (<1%). The foliation is marked by the orientation of white mica, graphite and chloritoid, while the granoblastic portion is defined by quartz. The garnet porphyroblasts (< 4mm) are fractured and contain occasional inclusions of rutile and quartz. These porphyroblasts present three main zones based on their compositional variation: core (Alm71-73Sps0-3Prp<8Grs>17-19); mantle (Alm72-74Sps2-3Prp<8Grs17-15); and rim (Alm69-7Sps1-2Prp>1.2Grs15-12). White mica in the matrix displays variable grain size (from 0.1 mm to 3.2 mm), oriented in the main foliation involving the garnet porphyroblasts. It presents Si content of 3.14-3.4 a.p.f.u., with XMg [Mg/(Mg + Fe)] of 0.7-0.8. Graphite occurs in the matrix, associated with white mica. Kyanite does not present a preferential orientation, occurring in association with quartz and mica. Chlorite exhibits pleochroism in shades of light green and is associated mainly with mica and chloritoid. Accessory minerals include rutile and apatite. Stilpnomelane occurs at the rims of the garnet and in the rock matrix with radial distribution. Based on these observations, we identified that the rock underwent through three stages of evolution: stage 1 growth of chloritoid, white mica and rutile in a quartz-rich environment, in which core garnet began its development; during the stage 2, foliation developed around garnet growth (rim) and together with the formation of kyanite; and the stage 3, with the formation of chlorite. Zr-in-rutile thermometry data for 25-point analyses (Zr = 52-192 ppm) indicate temperature conditions of 504-612 ºC, for the estimated pressure of 15 kbar, in accordance to previous data for similar rocks (T = 573-646 ºC for P = 15 kbar, using the Grt-Phe and Grt-Chl thermometers. Given the lack of studies on high-pressure conditions in pelitic systems in this region, the analysed rock is a strong candidate to help reconstruct the metamorphic history of the area. Further characterisation, especially in terms of metamorphic modelling, will allow the establishment of a better-constrained P-T trajectory.

Palavras Chave

Garnet; Metapelite; Zr-in-rutile; High-pressure metamorphism; Metasediment