CONTACT-REACTIONARY PROCESSES IN THE NYURUNDUKAN ULTRAMAFIC-MAFIC MASSIF (NORTHERN PREBAIKALIA)

Abstract

We present results of study of contact-reactionary rocks from the Nyurundukan ultramafic-mafic massif in northern Prebaikalia. The host metabasites metamorphosed in the conditions of amphibolite facies are shown to have undergone high-temperature (T ≈880 °C) metamorphism as a result of the intrusion of the Nyurundukan ultramafic-mafic pluton. The contact-reactionary processes produced endomorphic gabbros, pyroxene-plagioclase-bearing hornfels-like rocks, and beerbachites. We have established that the interaction of basic magma with the host metamorphic basic rocks was allochemical and was accompanied by the redistribution of macrocomponents and trace elements within the zone of endocontact gabbros. This was probably due to the high-pressure conditions. Detailed mineralogo-petrographic studies using amphibole and amphibole-plagioclase thermobarometers permitted determination of PT-conditions at several stages of metamorphism. Based on the generalized age data and results of the performed research, a scheme of the sequence of magmatic and metamorphic events in the study region is proposed. According to the scheme, the ultramafic-mafic intrusion formed within one or two stages of regional metamorphism after the formation of the protolith of the Nyurundukan series. Hence, these objects cannot be genetically related as was suggested from the ophiolite model.