GEOCHEMICAL FEATURES OF GABBROIDS AND GRANITOIDS FROM THE NYATYGRANSKY COMPLEX (BUREYA MASSIF)

Аннотация

Gneissic gabbros, gabbrodiorites, granodiorites, and granites of the Late Proterozoic Nyatygransky Complex, exposed by erosion in the eastern part of the Bureya massif, form small intrusive bodies within the boundaries of the Melgin metalliferous trough, as well as around it. Analysis of chemical and trace-element compositions of rocks from the Nyatygransky Complex revealed their geochemical features. The calculated agpaitic index values for gabbroids are very low and range from 0.11 to 0.33. Granodiorites have slightly higher index values ranging from 0.5 to 0.38, and only gneiss-granites are plumasitic with agpaitic index values between 0.45 and 0.75. Both gabbroids and granitoids are assigned to calc-alkaline rocks by their agpaitic index. Gneissic gabbros, dominated by dark-colored minerals (labradorite, hornblende), show enhanced alkalinity not common to gabbros (4%); in them, the magnesium content prevails over the iron content. They are characterized by high alumina content (15–18%), reaching 20.38%. Geochemically, they are interpreted as I-type igneous rocks. Gneissic granitoids with a slight predominance of the iron content over the magnesium content are less aluminous (Al₂O₃ 12–15%). They are interpreted as S-type (sedimentary) and I-type (igneous) granitoids, formed in sedimentary and granitic layers of the earth’s crust. According to petrochemical diagrams, the gabbroids and granitoids of the Nyatigransky Complex belong to the volcanic arcs of the active continental margin.