Isotope composition of strontium, carbon, and oxygen in magnesian carbonates of the Onot greenstone belt

Abstract

341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan marginal block of the Siberian Platform basement (Fig. 1). The OGP occurs on ancient tonalite–trondhjemite–gran odiorite gneiss containing zircons with a U–Pb age of ≈3.34 Ga [1] and is intruded by Early Proterozoic granite of the Shumikhin Complex (the U–Pb age of zircons is 1.87 Ga [2]). The belt is composed of metavolcanic rocks of the tholeiite–rhyolite series, metaterrigenous and biotite–garnet gneisses, amphib olite, quartzite, and magnesite and dolomite marbles [3, 4]. Rocks are metamorphosed under the condi tions of the amphibolite and epidote–amphibolite facies and have undergone intense ultrametamorphic and metasomatic transformations. The age of the pro tolith of metarhyolite from the OGB is 2.89 Ga (U– Pb age by zircon) [5]. The Rb–Sr age of ultrameta morphic granitoids from the Onot Complex is 2.64 Ga [6]. Thus, the supracrustal complex of the OGB is one of the oldest in Asia, and its age is consistent with that of typical Archean greenstone belts of the Earth.