Home – Home – Jornals – Russian Geology and Geophysics 2015 number 9

We summarize results of geological, geochronological, petrogeochemical, and isotope-geochemical (Sr-Nd) studies of Late Vendian-Early Paleozoic granitoid batholiths in Eastern Tuva (Kaa-Khem, East Tannu-Ola, Khamsara, etc.). Analysis of geochronological (U-Pb, Ar-Ar) data has shown that the Late Vendian-Early Paleozoic granitoids in Eastern Tuva formed in several stages in the time interval 562-450 Ma and at different geodynamic stages of the regional evolution: island-arc (562-518 Ma) and accretion-collision (500-450 Ma), with the latter stage characterized by more intense granitoid magmatism. Diorite-tonalite-plagiogranite associations with different petrogeochemical parameters are the most widespread in the region. Petrogeochemical studies of the Late Vendian-Early Paleozoic plagiogranitoid associations have revealed high- and low-alumina varieties reflecting different conditions of formation of parental melts. At the island-arc stage of the regional evolution, only low-alumina plagiogranites of tholeiitic ( M -type) and calc-alkalic ( I -type) series formed. Their parental melts were generated at 3-8 kbar through the partial melting of N-MORB-type metabasalts in equilibrium with amphibole restite. Isotope-geochemical studies have shown positive e _Nd values (6.9-6.3) and low Sr isotope ratios (( ⁸⁷Sr/ ⁸⁶Sr) ₀ = 0.7034-0.7046). The lower (as compared with the depleted mantle) e _Nd values and specific petrogeochemical composition (positive Nb-Ta and negative Ti anomalies) of the plagiogranites reflect the subduction nature of metabasic substratum and the subordinate role of ancient crustal material. At the accretion-collision stage of the regional evolution, high- and low-alumina plagiogranitoids of calc-alkalic series (I-type) formed. The high-alumina plagiogranitoids are products of melting of N-MORB-type metabasalts in equilibrium with garnet restite at ³15 kbar in the lower part of the collisional structures, and the low-alumina ones formed through the melting of metabasites in equilibrium with amphibole restite at ≤8 kbar in the upper part of the same structures. The Sr-Nd isotope data for the high- and low-alumina plagiogranitoids generated at the accretion-collision stage show that the rejuvenation of rocks is accompanied by the decrease in e _Nd (from 6.2 to 3.4) and the increase in their model Nd age T _Nd(DM) (from 0.73 to 0.92 Ga) and ( ⁸⁷Sr/ ⁸⁶Sr)0 (0.7036-0.7048). This points to the essentially metabasic composition of the parental substratum, as in the case of the island-arc plagiogranitoids, and the progressive supply of ancient crustal material to the magma chamber.