AGE AND GEODYNAMIC SETTING OF THE MAZHALYK PERIDOTITE-PYROXENITE- ANORTHOSITE-GABBRO COMPLEX, EASTERN TUVA

Abstract

Rocks of the peridotite-pyroxenite-anorthosite-gabbro association of the Mazhalyk complex are widespread in the Early Caledonian domain of the mosaic structure of Central Asia. This rock association has previously been regarded as part of the Early Cambrian Aktovrakskii complex or was assigned to the early phase of the Tannuola gabbro-diorite-plagiogranite complex. The Mazhalyk massif in eastern Tuva is a representative example of this association. The most common rocks of the massif are wehrlite, olivine gabbro, gabbro, clinopyroxenite, gabbronorite, and gabbro-anorthosite. A U-Pb geochronological study was carried out with zircons from the gabbro-anorthosites. A concordant age of 478 ± 1.4 Ma was obtained for the crystallization of melts of the Mazhalyk complex. The rocks of the Mazhalyk massif show fractionated trace element distribution patterns with an enrichment in the LIL elements, distinct negative Nb-Ta, Zr-Hf, and Ti anomalies, an enrichment in the light REE, and a depletion in the heavy REE. These features are typical of island-arc basic-ultrabasic complexes. The rocks of the massif show positive (though lower than in the depleted mantle) εNd(T) values between +4.8 and +5.7, which is also characteristic of island-arc complexes. On the other hand, the geologic setting and age of the rocks of the Mazhalyk massif and the entire complex suggest that their formation could not be related to the development of Vendian ophiolitic and island-arc complexes. The geochemical resemblance to island-arc complexes could originate from the entrainment of a metasomatized suprasubduction mantle wedge into melting processes. Our geochronological, geochemical, and isotopic data suggest a postcollisional intraplate geodynamic setting of the Mazhalyk complex, whose formation was related to the activity of a mantle plume source.