A.N. SIROTKIN (PMGRE), A.V. BEREZIN (IPGG RAS, St. Petersburg State University), S.G. SKUBLOV (IPGG RAS, St. Petersburg Mining University)

The granulites of North­Western Spitsbergen: geological position, PT parameters and age of metamorphism

The blocks of granulites discovered in the north­west of the Spitsbergen archipelago were described in detail for the first time. Their geological position, chemical and mineral composition, age, and conditions of formation were analyzed. Granulites are represented by ortho­rocks whose composition corresponds to the gabbroids of normal alkalinity. Petrographic study of these rocks allows to distinguish an early granulite paragenesis Opx + Cpx + Bt + Pl + Qz and a later superimposed association Amp + Chl + Ms + Ab. The PT parameters of granulite metamorphism (T = 860 ± 20°C and P = 6,5 ± 1 kbar) and of the regressive stage (T = 500 ± 40°C and P = 1,5 ± 1 kbar) are calculated by a set of methods. The age of the granulites was determined by the ion microprobe SHRIMP II (CIR VSEGEI) using the U­Pb method for zircon. The resulting discordia is characterized by the following parameters: the upper intersection corresponds to the age of 1766 ± 89, whereas the lower intersection is 597 ± 92 Ma. Geochemical study (REE and trace elements) of zircon showed that grains, according to which the ancient age was calculated, were formed at the stage of granulite metamorphism. Thus, the presented data demonstrate that in the rocks of the crystalline basement of the northwestern part of the Spitsbergen archipelago there are blocks that have undergone granulite metamorphism in the Late Karelian period.
Keywords: granulites, PT parameters, zircon, U-­Pb age, Spitsbergen.