Polychron zircons from rocks of the Kalbakdag mafic-ultramafic massif and questions of its genesis (Central Tuva)

The article presents the results of 30 determinations of isotopic age in 18 zircon grains performed by the LA-SF-ICP-MS method from four bulk samples of olivine gabbronorite (K-1, K-2), olivine gabbro (K-5) and plagioverlite (K-7) selected in the central part of the Kalbakdag mafic-ultramafic massif, which was previously considered as a typical layered intrusion. This massif is located in the central part of the territory of Tuva within the Ondum island-arc subzone of the Early Caledonids and breaks through the metavulcanogenic- terrigenous strata of the Late Neoproterozoic - Lower Cambrian. The rocks of the massif are characterized by moderate magnesium content, low alkali content, very low titanium and phosphorus content. The content of rare earth elements in them is lower than NMORB at (La/Yb)n = 0.52-1.71, positive anomalies of K, Sr, Zr, Hf, Eu of low intensity and negative anomalies of Nb and Ta are noted on spiderograms. The dated zircons, in addition to differences in morphological and optical properties, are characterized by significant variations in isotopic age, that is, they are polychronous. The entire collection of dated zircons is conditionally divided into four “populations” (clusters): Neoarchean-Paleoproterozoic (2747-1862 Ma, zircons from samples K-2 and K-5), Neoproterozoic (799-648 Ma, zircons from samples K-1 and K-5); Cambrian-Devonian (525-383 Ma, zircons from samples K-2 and K-7) and Carbonian-Permian (325-279 Ma, zircons from sample K-5). Zircons from the first and second “populations” are proposed to be considered as a xenogenic phase introduced into the parent melts of the gabbroids of the Upper Mantle protolith massif. They are to varying degrees “rejuvenated” very ancient juvenile zircons that were previously present in this protolith and preserved in ultramafic restites formed on it. In turn, zircons from the Cambrian-Devonian “population” are proposed to be considered as a syngenetic phase, the age of which corresponds to the time of formation of gabbroids of this array. The concordant age of syngenetic zircons was 477 ± 5 Ma. Zircons from the Permian “population” have been identified as an epigenetic phase formed during infiltration into rocks of an array of fluids separated from late granitoid melts. According to the totality of available data, the Kalbakdag massif is considered by us as a polygenic association of spatially converged fragments of earlier protrusion of hybridized restitogenic ultramafites (plagioverlites and pyroxenites) and mesoabyssal gabbroid intrusive (olivine and non-olivine gabbro and gabbronorites) introduced in the Early Paleozoic.

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