SOUTHERNMOST ANDES AND SOUTH GEORGIA ISLAND, NORTH SCOTIA RIDGE - ZIRCON U-PB AND MUSCOVITE AR-40 AR-39 AGE CONSTRAINTS ON TECTONIC EVOLUTION OF SOUTHWESTERN GONDWANALAND/

Zircon U-Pb and muscovite Ar-40/Ar-39 isotopic ages have been determin ed on rocks from the southernmost Andes and South Georgia Island, Nort h Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from st ratigraphic relations. The U-Pb systematics of four zircon fractions f rom one sample show that proto-marginal basin magmatism in the norther n Scotia are, creating the peraluminous Darwin granite suite and subma rine rhyolite sequences of the Tobifera Formation, had begun by the Mi ddle Jurassic (164.1 +/- 1.7 Ma). Seven zircon fractions from two othe r Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn t hrough these points on concordia diagrams give upper intercept ages of ccc. 1500 Ma, interpreted as a minimum age for the inherited zircon c omponent. This component is believed to have been derived from sedimen tary rocks in the Gondwanaland margin accretionary wedge that forms th e basement of the region, or else directly from the cratonic ''back st op'' of that wedge. Ophiolitic remnants of the Rocas Verdes marginal b asin preserved in the Larsen Harbour complex on South Georgia yield th e first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 +/- 1 Ma). The geographic pattern in the observed age ra nge of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin Boor northwestward from South Georgia Island to the Sarmiento Complex in so uthern Chile. Rocks of the Beagle granite suite, emplaced post-tectoni cally within the uplifted marginal basin Boor have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 +/- 1.0 Ma, but their upper inter cept is not well known. Inasmuch as this age is similar to the Ar-40/A r-39 age of secondary muscovite growing in extensional fractures of pu lled-apart feldspar phenocrysts in a Beagle suite granitic pluton (pla teau age is 68.1 +/- 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the high-grade metamorp hic complex of Cordillera Darwin in Tierra del Fuego. Copyright (C) 19 96 Elsevier Science Ltd & Earth Sciences & Resources Institute