Geochronology and radiogenic isotope geology of Mesozoic rocks from eastern Palmer Land, Antarctic Peninsula: crustal anatexis in arc-related granitoid genesis

Abstract

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 87Sr/ 86Sr ratios of up to 0.721 and εNd 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 87Sr/ 86Sr ratios of 0.705–0.707 and εNd 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 is related 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.