Eclogites of the Great Caucasus (nature of protolite and it geodynamical typification)

Abstract

Relevance. In structural-formation zone of Front ridge of the Great Caucasus in rock sections of “balkanskoy” and “labardanskoy” suites bodies of eclogites of omphacite-garnet composition with a admixture of amphibole, epidote, zoisite and clinopyroxene are occur. Aim of the work was to study the petrochemical and geochemical features of eclogites in sections of the Greater Caucasus rocks. Methods. Calculation carried out on the basis of garnet-clinopyroxene thermometer, are assess interval temperature of stability of observed association garnet + omphacite within 580-650°С, and carried out assessments of pressure according solubiliy of jadeites in the clinopyroxene, give maximum pressure of observed paragenesis is 13,5 kbar and minimum – 8,5 Kbar. Results. Geochemical investigation of eclogites and garnet amphibolites was carried out and results of it RFA, ICP-MS analysis and also briefly petrographyc discription of investigated rocks have been done. Petrogeochemical characteristics of eclogites have been treated and it premetamorphic nature was determined and the most likely geodynamical typification of initial protolite was deciphed. It was shown, that eclogites correspond on composition to magmatic rocks of basaltic type with a relation of isotopes of 87Sr/86Sr equal to 0.7035. Eclogites were formed from moderate-titanoferous, moderate-aluminiferous and moderate-magnesioferous, low-potassic volcanites with a sodium type of alkalinity. It is suggested, that initial melt of basic composition, was formed at a 8-15% melting of spinelian peridotite. Ni/Со ratio ∑/n equal 2,9 correspond to index of mantle smelted, varying within 2,5-5,0. Low values Mg# =0,55 denotes on possible phenomenon of differentiation of initial melt. Positive europium Eu/Eu*=0,95-2,75 and strontium anomalies allows of primary accumulation of plagioclase in volcanites. Analysis of petrogenic diagrams are show, that dots of eclogites are located in field of basalt Е – MORB type, and also – tholeite of arc island, basalt of backarc basin (margin sea) and medial ocean ridge. Geochemical specialization of the initial melts is siderophile. Incompatible elements and REE are normalized to N-MORB and chondrite, are formed spectra of lines, close to MORB type basalt; Laн/Ybн =0,6-1,7 when ∑/n REE=34 ppm. At the same time obtained spectrums are distinguish from standart of N-MORB by clear negative anomalies of highcharged elements (Nb, Ta, Zr, Hf), that indicate of their oversubduction nature. According to tofality of obtained results and accounting of paleotectonic reconstruction of the Caucasus region, have supposedly, that forming of initial volcanites was occurred in Early Paleozoic in condition of back-arc basin which located on the northern margin of Paleo-Tethys.