Assessment of the age and conditions of metamorphism of high-pressure blastolites of the Gonzhinsky block of the Argun superterrane of the eastern part of the Central Asian folded belt

Abstract

As a result of petrographic and thermobaric studies, 4 mineral associations were identified in the blastolite: relict, main, association corresponding to biotitisation and association of retrograde changes. The relict association probably represented eclogites The main mineral association corresponds to apoeclogite zoisite amphibolites. Determination of the P-T parameters of the formation of this association showed an interval of P = 13.0–15.0 kbar and T = 580-670 °C. At the same time, if we take the most ferruginous outer rim of amphibole-1 for calculations, then the P-T parameters shift to a higher temperature region of P = 13.0–17.0 kbar and T = 660-780 °C. The association corresponding to the imposed biotitisation is well structurally expressed. The lines of monomineral equilibria of the biotite association constructed from various combinations of garnet, biotite and plagioclase compositions showed good convergence in the local region corresponding to the interval P = 12.5-13.2 kbar and T = 810-830 °C. The last association of retrograde changes includes low-alumina amphibole-2.The association corresponding to the imposed biotitisation.ше is well structurally expressed. The last association of retrograde changes includes low-alumina amphibole-2. Based on the obtained isotopic data, a mineral Sm-Nd isochron was constructed for the rock, garnet and two amphiboles. The age of the isochron is 171 ± 3 million years, the latter is interpreted as the time of formation of the main mineral association and corresponds to its crystallization at the peak of metamorphism.