Fragments of the Oceanic and Continental Mantle in Ophiolites of the Northwestern Pacific Margins: Composition, age and Genesis of Sakhalin Peridotites
Results are presented of the study of original and published data on the chemical composition and age of mantle peridotites from the Sakhalin Island ophiolites. Material and genetic proximity had been established of peridotites from Berezovsky and Shelting plutons, on the one hand, and the melange zone serpentinites, on the other. By compositions and Fe<sub>2</sub>O<sub>3</sub> и MgO variations, the Sakhalin peridotites radically differ from those of the North-East Asia ophiolite complexes (Krasnogorsky Massif, Karaginsky Island Massif, etc.) that are the fragments of the Pacific Plate mantle. On the contrary, Sakhalin peridotites have the sub-continental genesis, and are compositionally close to xenoliths of lherzolites from the Hankai Micro-continent''s mantle (southern Sikhote-Alin). The rythmics had been considered of the alternation in compression and expansion at the edge of Asian continent in the last 180 million years, caused by cyclic changes in the Pacific spreading velocity. Formation of Berezovsky Massif, according to the data obtained by U-Pb method over zircons, took place 169-154 million years ago under the Jura expansion of the continent edge. Concerted data on the age and composition demonstrate that Sakhalin ophiolites had been formed within the limits of marginal sea basin in the process of rift induced destruction of the Khankai craton periphery. The assumed tectonic situation was close to that being reconstructed for the Josephine Jura ophiolites of the Californian edge of North American continent. Continental genesis of the Sakhalin ophiolites is in conformity with the age and tectonic mode of the ophiolite formation at the Island of Sakhalin.