GEOCHRONOLOGY AND RADIOGENIC ISOTOPE GEOLOGY OF MESOZOIC ROCKS FROM EASTERN PALMER LAND, ANTARCTIC PENINSULA - CRUSTAL ANATEXIS IN ARC-RELATED GRANITOID GENESIS

Rb-Sr and Sm-Nd analyses, as well as a few new K-Ar dates, are present ed for plutonic igneous, meta-igneous and metasedimentary rocks from t he central Black Coast area of eastern Palmer Land (southeastern Antar ctic Peninsula). Gneissic and foliated leucogranitoids, previously asc ribed to a ?Precambrian/Paleozoic basement, yield Late Triassic to ear liest Jurassic Rb-Sr isochron ages, with initial Sr-87/Sr-86 ratios of up to 0.721 and epsilonNd(t) values as low as -9. Isotopic comparison with local metasedimentary rocks suggests that the granitoids represe nt granite magmas that were generated, at least in part, by anatexis o f similar metasediments at depth. Contemporaneous I-type granitoids ha ve initial Sr-87/Sr-86 ratios of 0.705-0.707 and epsilonNd(t) values o f -1 to -4. These cannot be distinguished from those of the subsequent , mid-Cretaceous, undeformed granitoids, which represent the climax of Andean subduction-related magmatism in this area. K-Ar ages on basalt ic dikes record emplacement associated with late-stage block-faulting and uplift of the Cretaceous batholith at about 80 Ma. Triassic/Jurass ic crustal anatexis Sic to a period of reduced subduction within a pla te margin regime that had been active during Paleozoic times. It corre sponds to the earliest stages of lithospheric extension in the proto-P acific margin of Gondwana, prior to mid-Jurassic formation of back-arc basins in the western Weddell Sea, and to Late Jurassic continental s eparation.