GEOCHEMISTRY AND RB-SR AGE OF GRAPHITE-BEARING GRANITOIDS IN THE SOUTHEAST OF THE MAIN SAYAN FAULT

Abstract

The paper deals with the petrography and geochemistry of graphite-bearing granitoids (garnet-biotite granites, leucogranites, pegmatoid granites, pegmatites, plagioclasites, and syenites) composing up to 2 m thick veins among blastomylonites and blastocataclasites in the southeast of the Main Sayan Fault, at the boundary between the Sharyzhalgai uplift of the basement of the Siberian Platform and the Caledonian Slyudyanka crystalline complex. The granitoids are the products of crystallization of low-temperature (760–710 °C) low-alkali melt; crystallization of more water-saturated granites took place at P H 2 O ≈ 2 kbar. The granites have much lower (compared with the clarkes) contents of F, Li, Ba, Sr, Zr, and Zn (also Rb and Y in the leucocratic and pegmatoid granites), low Ba/Rb and Sr/Rb (0.9–3.5 and 0.3–1.7, respectively), high K/Rb (199–375), and medium to low contents of REE, being depleted in LREE; their REE patterns show both negative and positive Eu anomalies. Traces of K, Rb, Be, Nb, Th, Pb, and Ta might be due to their fluid supply into the melt. The residual highly carbonaceous melt shows a tendency for concentration of Au, Pt, Pd, Cu, Ni, and U. The Rb-Sr isochron constructed for garnet-biotite granites yields an age of 334±14 Ma ((87 Sr/ 86 Sr)0 = 0.7058±0.0041, MSWD = 4) and evidences Early Carboniferous granitoid magmatism in the marginal-suture zone. The carbon isotope composition of graphite of the studied granites (δ 13 CPDB = –12.5...–8.7‰) differs from that of graphite of metamorphic rocks (δ 13 CPDB = –24.3...–20.2‰) and is similar to the carbon isotope composition of graphite veinlets in Mg-Ca-containing apoaluminosilicate metasomatites (δ 13 CPDB = –11.2...–7.4‰).