Home – Home – Jornals – Russian Geology and Geophysics 2016 number 6

We present nomenclature and geochemical classification of Paleoproterozoic LILE-enriched high-Mg low-Ti mafic-granitoid rocks of the eastern margin of the Sarmatia paleocontinent and substantiate their tectonic position. Two differentiated rock series are recognized: 1) biotite-orthopyroxene melanorite-quartz-meladiorite-melagranodiorite and 2) hornblende-biotite quartz-diorite-tonalite-granodiorite. Both series correspond in chemical composition to calc-alkalic gabbro-diorites, diorites, tonalites, and granodiorites. As follows fr om their mineralogical and geochemical compositions, these are norite-diorite rocks (intrusive analogs of boninites) (SiO2 = 52-65 wt.%, MgO = 5-20 wt.%, TiO2 = 0.2-0.8 wt.%) and high-Mg granitoids (SiO2 = 60-70 wt.%, Na2O/K2O = 0.65-1.33, MgO = 3.23-7.4 wt.%, K2O = 1.9-4.0 wt.%), respectively. Their high Mg# values (67-87) and Cr contents (>100 ppm), on the one hand, and their isotope-geochemical characteristics similar to those of the host metaterrigenous rocks, the magma enrichment in LILE, and the presence of Ni sulfide ores with a predominance of light sulfur isotopes, on the other, testify to crustal contamination of mantle magmas. The rock series are nearly of the same age and belong to the same magmatic system, wh ere high-Mg granitoids are differentiates of parental high-Mg (boninite-like) norite-dioritic magma. This is confirmed by a gradual increase in SiO2 and K2O contents and a decrease in Mg# and Ni, Co, V, and Cr contents in the sequence from norites to granodiorites and by the facies and phase relationships between the series. Intrusion of rocks took place at shallow depths after low-temperature metamorphism and folding under postcollisional collapse of the East Sarmatian orogen.