POLYGENESIS OF MAFIC-ULTRAMAFIC COMPLEXES: ISOTOPE-GEOCHRONOLOGICAL AND GEOCHEMICAL EVIDENCE FROM ZIRCONS OF THE BEREZOVKA MASSIF ROCKS (Sakhalin Island)
F.P. Lesnov1, V.V. Khlestov1,2, V.G. Gal’versen3, S.A. Sergeev4,5
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
3Sakhalin Geological-Prospecting Expedition, ul. A. Matrosova 28, Yuzhno-Sakhalinsk, 693005, Russia
4A.P. Karpinsky Russian Geological Research Institute, Srednii prosp. 74, St. Petersburg, 199106, Russia
5St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia
Keywords: Zircons, U-Pb age, SHRIMP II, geochemistry, LA-ICP-MS, gabbroids, ultramafic rocks, ophiolites, Berezovka massif, Sakhalin Island
Abstract
Results of comprehensive isotope-geochronological (U-Pb and SHRIMP II dating) and geochemical (LA-ICP-MS) studies of zircons from different rocks of the Berezovka polygenetic mafic-ultramafic massif of the East Sakhalin ophiolite association are presented. The massif includes three proximal but genetically autonomous structure-lithologic complexes of different ages: protrusion of ultramafic rocks of restite nature, gabbroid intrusion breaking through it, and contact reaction zone located along their boundaries. The isotopic age of zircons in the massif as a whole and in its individual rocks varies over a broad range of values. The zircons belong to several populations according to their age (Ma) and other features: relict and xenogenous (~3100-990, 70-410, and ~395-210) and syngenetic (~200-100, ~90-65, and ~30-20). They differ in grain size and morphology, optical and cathodoluminescence images, and trace-element patterns. By morphology, the grains are divided into short-prismatic crystals with well-developed faces and edges, long-prismatic crystals with well-developed faces and edges, prismatic crystals with slightly resorbed faces and edges, prismatic crystals with strongly resorbed faces and edges, and intensely resorbed grains totally or partly lacking faceting. The ages of zircons depend inversely on the contents of La, Ce, and Yb, total contents of REE, (Ce/Ce*) n , and (Eu/Eu*) n . Some grains are characterized by abnormal REE and trace-element patterns due to their epigenetic redistribution. The wide scatter of intermediate ages of the relict and xenogenous zircon grains, their resorption and disturbed optical and geochemical features are probably due to the nonuniform rejuvenation of their isotope systems and variations in other parameters, caused by the effect of younger mafic melt and its fluids, whose crystallization gave rise to a gabbroid intrusion dated at 170-150 Ma. The obtained data on the isotopic age and other properties of zircons from the Berezovka massif rocks agree with the geological model of its polygenesis.
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