The southwestern Dana Mountains of the southern Antarctic Peninsula are underlain by the western part of a composite concentrically zoned Upper Cretaceous batholith consisting largely of granodiorite. The granodiorite (felsic phase) was intrusive into older heterogeneous gabbro-diorite (mafic phase) which makes up the margin of the batholith. Flat-lying pegmatite bodies and abundant inclusions, roof pendants, and septa of metasedimentary rocks and gabbrodiorite suggest that the granodiorite lies near the roof of the batholith. Petrography, chemistry, field relations, and radiometric age dates indicate that the batholith is part of the Andean intrusive suite, a suite of calc-alkaline plutonic rocks that crops out extensively throughout the Antarctic Peninsula and Chilean and Patagonian Andes. Textural features, mineralogy, and chemical data indicate that both phases of the batholith in the southwestern Dana Mountains have been contaminated by assimilation of the Latady Formation, an Upper Jurassic sequence of shale, siltstone, and sandstone that underlies large parts of the southern Antarctic Peninsula. Composition of the granodiorite also has been modified by assimilation of lesser amounts of the gabbro-diorite phase. Such contaminated plutonic rocks occur in several places in the northern Antarctic Peninsula; the batholith in the southwestern Dana Mountains represents the southernmost exposures of contaminated igneous rocks known from the Antarctic Peninsula. Hornblende andesite dikes cut both the mafic and felsic phases of the batholith. Chemical data imply that the dikes represent small amounts of mafic material remobilized by intrusion of the granodiorite. During the austral summers of 1969
