Petrofabric and Raman microspectroscopy study of the Mina Afortunada gneiss dome: Mapping its thermal gradient

Abstract

Black quartzites and migmatitic orthogneisses from the Mina Afortunada dome have been analysed through quartz and graphite petrofabric analysis and Raman microspectrometry on graphitized carbonaceous material. The results permit us to recognize a deformation temperature increase from the dome envelope towards its core marking the transition from basal-<a> to rhomb-<a> quartz intracrystalline slip systems at maximum T of 475 °C, and from rhomb-<a> to prism-<a> at maximum T of 515 °C. The complementary petrofabric and Raman study on graphite discloses a change from basal to prismatic <a> slip systems at maximum T of ca. 470 °C, accompanied by a strengthening of its structural order.

The Raman study of large graphite grains revealed a non-random crystallinity organization in them, with domains of weaker lattice structural order (reflecting lower formation T) in core areas and higher crystallinity sectors (higher formation T) at the rims. This finding might reflect a shielding effect during prograde metamorphism, the rim material preventing grain core material from reorganization and increase in its crystallinity. This study shows also that graphite can be a good candidate for pressure-temperature-time path reconstruction in metamorphosed organic-rich rocks.