Carbonaceous Material Thermometry of Metapelites From the Onot Greenstone Belt (SW Siberian Craton)

ABSTRACT

In recent decades, a Raman spectroscopy of carbonaceous material (RSCM) geothermometer has been actively developed for studying metamorphic complexes. Some aspects of the method application remain insufficiently addressed. This study focuses on carbonaceous garnet‐staurolite schists from the Onot greenstone belt and presents the results of applying this method in combination with conventional geothermometry and thermodynamic modeling. The textural and structural features of the rocks, petrographic characteristics of mineral assemblages, and chemical heterogeneity of minerals were investigated. It was found that carbonaceous material (CM) is represented by black, opaque, irregularly shaped flakes that occur as inclusions within garnet and staurolite porphyroblasts, as well as within quartz, plagioclase, and biotite from the rock matrix. According to the RSCM geothermometer, the temperatures obtained from the CM inclusions in quartz and plagioclase averaged 580 °C–605 °C, in biotite 560 °C–580 °C, in staurolite 555 °C–580 °C, and in garnet 525 °C–560 °C. Peak metamorphic conditions estimated by using mineral geothermometers and geobarometers were 565 °C–600 °C and 6.9–8.2 kbar. Thermodynamic modeling showed that almandine, grossular, and spessartine isoplets reflecting the garnet rim intersect at temperatures of 575 °C–590 °C and pressures of 7.7–8.0 kbar. The results demonstrate that the degree of carbonaceous material structural order in the studied schists corresponds to the peak metamorphic temperature when it is located in the rock matrix. The CM inclusions in porphyroblasts are capable of preserving less ordered carbonaceous material.