PETROGRAPHIC AND GEOCHRONOLOGICAL INVESTIGATIONS ON A GARNET-TOURMALINE PEGMATITE FROM RINGGOLD KNOLL, OATES COAST, ANTARCTICA

The Ringgold Knoll pegmatite, a late-stage member of the Granite Harbo ur Intrusives, crosscuts high-grade Wilson gneisses of the Oates Coast , which forms the westernmost part of the Wilson Terrane at the Pacifi c end of the Cambro-Ordovician Ross orogenic belt in West Antarctica. The pegmatite mineral assemblage consists of K-feldspar, plagioclase, quartz, garnet (almandine-spessartine-pyrope), dark tourmaline (schorl -dravite), muscovite, apatite, monazite, zircon, blue Al-rich tourmali ne and dumortierite in order of decreasing abundances. Major, minor an d rare earth elements are reported for the greater part of the mineral assemblage. The time of pegmatite emplacement is constrained by Rb-Sr and Sm-Nd isochron ages of 492 +/- 8 (2sigma) Ma and 500 +/- 40 (2sig ma) Ma, respectively. High initial Sr-87/Sr-86 of 0.7315 +/- 0.0003 an d low is-an-element-of Nd,t of -8.7 +/- 1.2 strongly support an origin of the magma from highly evolved crustal source rocks. K-Ar and Ar-40 -Ar-39 model ages of about 470 to 475 Ma for igneous muscovite indicat e that the pegmatite together with its wall rocks spent a prolonged pe riod at elevated temperatures before final cooling below about 350-deg rees-C. The muscovite dates may give an estimate for the time of exhum ation of the Oates Coast crystalline basement along two major late Ros s orogenic detachment zones within the Wilson Terrane i.e. the Wilson and the Exiles thrusts (c.f. Flottmann and Kleinschmidt, 1991).