The monte Kranck pluton: a new evidence of magmatic activity in the Fuegian Andes of Argentina.

GONZÁLEZ GUILLOT, M.; ACEVEDO, R.; ESCAYOLA, M.

Santiago de Chile

Congreso; GeoSur; 2007

Monte Kranck is located in the Beauvoir range, in the Argentinian Fuegian Andes(AFA). At this locality a small pluton (3 km2) crops out, intruding the BeauvoirFormation (Early Cretaceous). It is the northernmost plutonic body known in the AFA.Its presence was indirectly infened previously by the presence of a positive magneticanomaly, a different colour pattern compared to the sunounding rocks in satellitalimages, and by the presence of a hornfels some hundreds of meters to the east, but nodirect observation had been made until date.Lithologically, it shows similarities with the other plutons of the AFA (HornblenditaUshuaia, Plutón Diorítico Moat and Cerro Jeu-Jepén, introduced from south to north):ultramafic bodies (hornblendites and pyroxenites) crystallized early, followed bygabbros, diorites and monzodiorites. Latter pulses are represented by monzonites andsyenites, and finally syenitic to alkali-feldspar syenitic veins cut the assemblage. Thelatest manifestations consist of lamprophyric dykes.Unlike the other aforementioned plutons, ultramafic rocks are scarcely represented,and the most evolved rocks are richer in K-feldspar than equivalent lithologies from theother localities. Thus, large bodies of monzonites classify locally as syenites, and thedifferentiated veins are of K-feldspar syenite composition. These characteristics areshared with the Cerro Jeu-Jepén pluton.Monzonites and a melagabbro-meladiorite (no plagioclase composition wasdetermined) form the dominant rock types. The latter is characterized by a lamprophyrictexture, represented by 50-70% of mafic phenocrysts (hornblende and clinopyroxene)set in a phaneritic groundmass of the same minerals plus plagioclase ± biotite, alkalifeldspar and accessory phases (titanite, apatite and opaques). The monzonites are poorin mafic minerals, and K-feldspar is occasionally arranged in a trachytic texture. Theyform a main body in the eastern part of the pluton, and randomly distributed dykes.The lithological and petrographical characteristics of the Monte Kranck plutonsuggest that it can be correlated to the other plutons in the AFA, and so it can be statedthat there is a volumetrical increase in the abundance of differentiated rocks (syenites),along with a decrease in uItramafic terms, from south to north. This is accompaniedgeochemical1y by an increase in K2O content in whole rock and in some mafic minerals.It was recently proposed that these plutons belong to a Lower Cretaceous shoshoniticsuite of a back-arc enviromnent. In this model, the magmas that gave rise to each plutonwere generated at progressively greater depths in the mantle as the slab sank(northward), acquiring higher K2O contents, probably in response to decreasing degreeof pmiial melting. Although we lack geochemical data for the Monte Kranck pluton, itslithology could represent the farthest-from-the-trench (considering the Cretaceous arcaxis on the Chilean archipelago) batches of magma generated, in agreement with the K-hmodel of Dickinson (1975).