TIMING OF EARLY PROTEROZOIC COLLISIONAL AND EXTENSIONAL EVENTS IN THE GRANULITE-GNEISS-CHARNOCKITE-GRANITE COMPLEX, LAKE BAIKAL, USSR - A U-PB, RB-SR, AND SM-ND ISOTOPIC STUDY

In the granulite-gneiss-charnockite-granite Sharyzhalgay Complex of the Lake Baikal region in eastern Siberia Early Proterozoic collisional and extensional events were separated by ca. 100 m.yr. The earlier collisional event, associated with the development of granulites and gneisses as the result of high-grade dynamothermal metamorphism, took place close to 1965 +/- 4 Ma, the mean of Pb-207/Pb-206 ages for the coarser zircon fractions from a garnetiferous gneiss. A Pb-207/Pb-204 vs. Pb-206/Pb-204 isochron for zircon from five size fractions and a six point Rb-Sr whole-rock errorchron give generally corresponding ages of 1956 +/- 8 and 1963 +/- 163 Ma, respectively. The later extensional event, associated with charnockitization due to the uprise of fluids and heat in a regime corresponding to the middle to upper crustal levels of a Basin and Range-type province, was initiated in the 1880-1860 Ma period. This is shown by a 1873 +/- 4 Ma concordant age for the coarsest size fraction of zircon from a charnockite, a lower concordia intercept of 1871 +/- 10 Ma for three size fractions from the same rock, and a concordant age of 1862 +/- 4 Ma for monazite from the garnetiferous gneiss. The event was continued with magmatic emplacement of granitic masses into the deep levels of caldera-like structures, possibly during the upper time range of lower concordia intercept ages of 1817 +30/ -32 and 1797 +40/ -44 Ma for two distinctly different zircon populations in a pyroxene-bearing granodiorite interpreted as an evolved (and contaminated) product of the mantle-derived magma that was the source of CO2 involved in the charnockitization. Upper intercept ages of 2784 +48/ -45 and 2775 +61/ -55 Ma indicate late Archean crust at depth as the source region of the incorporated zircon. Late Archean (to earliest Proterozoic) rocks also occur in a tectonic slice(s) of basement amongst the granulite-gneiss cover rocks. This is indicated by an upper intercept age of 2568 +95/ -47 Ma for discordant zircon and monazite fractions from a kinzigite, with the mean of Pb-207/Pb-206 10 Ma. However, T(DM) ages from Sm-Nd isotopic data for structurally concordant metamorphosed basic rocks and metasedimentary garnetiferous gneiss show that the protolith of the lithologically layered supracrustal assemblage, subsequently polyphase deformed and polymetamorphosed in Early Proterozoic times, was also formed in Early Proterozoic (not Archean) times. The epsilon Nd (2000) values indicate contamination of mantle-derived material by LREE-enriched crust.