The first zircon U-Pb dating (SHRIMP II) for the silurian ophiolites in the Urals

Abstract

The rocks of the ophiolite association are wide� spread within the Ural mobile belt. For this reason dating of their formation is of great interest due to its tectonic significance, allowing interpretation of the history of geological development of the region. For a long time, the geochronological dating of ophiolite rocks has been limited to the search for faunal remains in interlayers of sedimentary rocks among the volca� nites of the upper part of the ophiolite section. Over the last 20 years, in connection with enhancement of the analytical possibilities, a set of isotope data for rocks of the ophiolite association has been obtained (mainly from the Polar segment of the Urals) [1–5 etc.]. The age data available indicate that the forma� tion of the Uralian ophiolites is connected mainly with two separate time intervals on the geochronological age scale. Older rocks (604–490 Ma) formed during the Late Vendian–Cambrian; the younger rocks (410–370 Ma) formed during the Devonian. Single intermediate dating (Ordovician and Silurian), dated by the Ar–Ar method [6, 7], is not regarded as conclu� sive evidence, so it lies in a very wide time interval (70–90 Ma for the same geological object, dated by the Ar–Ar method). At the same time, according to the present geological evidence, the predominant part of ophiolites on the eastern slope of the Urals formed between the Late Ordovician and the Silurian. Based on the zircon U–Pb age for ophiolite gabbro from the eastern zone of the Middle Urals using the SHRIMP II ion microprobe (Center for Isotope Research, VSEGEI), good evidence of the occurrence of Sil� urian ophiolites in the Urals was obtained for the first time. The Eastern zone of the Urals is known as a zone made of essentially Middle Paleozoic volcanogenic formations and comagmatic intrusive bodies, which extends along the eastern border of the Trans Ural region, eastward of the zone of granite batholiths (the main granite axis of the Urals) [8 etc.]. The ophiolite rocks, which are widespread within the zone, are rep� resented by dunite–harzburgite massifs, layered ultra� basite–gabbro complexes, and parallel dolerite dykes