Abstract
The article focuses on the issues related to the primary nature and metamorphism of gneisses from the Larba block of the Dzhugdzhur-Stanovoy fold area, which are based on the study of the mineralogical composition of supracrustal rocks. PT-parameters for the highest temperature and pressure metamorphic transformations of rocks of the Larba block are estimated as P = 6.5–8.5 kbar and T > 800°C. Relatively low-temperature garnet-biotite, garnet-biotite-chlorite mineral parageneses were formed at the regressive stage of metamorphism at ~4–6 kbar and temperatures up to 500–600°C. Wide variations in paragneiss compositions indicate that protoliths of the studied gneisses differed in composition. Overall, Larba metasedimentary rocks had weakly and moderately weathered source areas. Most of the compositions of paragneisses are localized in the field of chlorite-montmorillonite, montmorillonite-kaolinite-hydromica and chlorite clays formed from the mafic substrate. Low alkalinity in combination with a high MgO content indicates the presence of rock erosion products with a significant admixture of basic volcanic material in the detrital material. Migmatization of rocks is the most significant factor distorting the reconstruction of the original rock composition.
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