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 presented for plutonic igneous, meta-igneous and metasedimentary rocks from the central Black Coast area of eastern Palmer Land (southeastern Antarctic Peninsula). Gneissic and foliated leucogranitoids, previously ascribed to a ?Precambrian/Paleozoic basement, yield Late Triassic to earliest 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 represent granite magmas that were generated, at least in part, by anatexis of similar metasediments at depth. Contemporaneous I-type granitoids have initial Sr-87/Sr-86 ratios of 0.705-0.707 and epsilonNd(t) values of -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 basaltic dikes record emplacement associated with late-stage block-faulting and uplift of the Cretaceous batholith at about 80 Ma. Triassic/Jurassic crustal anatexis Sic to a period of reduced subduction within a plate margin regime that had been active during Paleozoic times. It corresponds to the earliest stages of lithospheric extension in the proto-Pacific margin of Gondwana, prior to mid-Jurassic formation of back-arc basins in the western Weddell Sea, and to Late Jurassic continental separation.