The Monte Kranck pluton: new evidence of magmatic activity in the

GONZALEZ GUILLOT M.; ACEVEDO R. D.; ESCAYOLA M. P.

Santiago de Chile

Congreso; Geosur 2007; 2007

Monte Kranck is located in the Beauvoir range, in the Argentinian Fuegian Andes (AFA). At this locality a small pluton (-3 knl) crops out, intruding the Beauvoir Formation (Early Cretaceous). It is the northernmost plutonic body known in the AFA. Its presence was indirectly infened previously by the presence of a positive magnetic anomaly, a different colour pattern compared to the sunounding rocks in satellital images, and by the presence of a hornfels some hundreds of meters to the east, but no direct observation had been made until date. Lithologically, it shows similarities with the other plutons of the AFA (Hornblendita Ushuaia, Plutón Diorítico Moat and Cerro Jeu-Jepén, introduced from south to north): ultramafic bodies (hornblendites and pyroxenites) crystallized early, followed by gabbros, diorites and monzodiorites. Latter pulses are represented by monzonites and syenites, and finally syenitic to alkali-feldspar syenitic veins cut the assemblage. The latest 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 the other localities. Thus, large bodies of monzonites classify 10cal1y as syenites, and the differentiated veins are of K-feldspar syenite composition. These characteristics are shared with the Cerro Jeu-Jepén pluton. Monzonites and a melagabbro-meladiorite (no plagioclase composltlOn was determined) form the dominant rock types. The latter is characterized by a lamprophyric texture, represented by 50-70% of mafic phenocrysts (hornblende and clinopyroxene) set in a phaneritic groundmass of the same minerals plus plagioclase ± biotite, alkali feldspar and accessory phases (titanite, apatite and opaques). The monzonites are poor in mafic mineral s, and K-feldspar is occasionally ananged in a trachytic texture. They form a main body in the eastern part of the pluton, and randomly distributed dykes. The lithological and petrographical characteristics of the Monte Kranck pluton suggest that it can be correlated to the other plutons in the AFA, and so it can be stated that 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 accompanied geochemical1y by an increase in K20 content in whole rock and in some mafic minerals. It was recently proposed that these plutons belong to a Lower Cretaceous shoshonitic suite of a back-arc enviromnent. In this model, the magmas that gave rise to each pluton were generated at progressively greater depths in the mantle as the slab sank (northward), acquiring higher K20 contents, probably in response to decreasing degree of pmiial melting. Although we lack geochemical data for the Monte Kranck pluton, its lithology could represent the farthest-from-the-trench (considering the Cretaceous arc axis on the Chilean archipelago) batches of magma generated, in agreement with the Kh model ofDickinson (1975).(-3 knl) crops out, intruding the Beauvoir Formation (Early Cretaceous). It is the northernmost plutonic body known in the AFA. Its presence was indirectly infened previously by the presence of a positive magnetic anomaly, a different colour pattern compared to the sunounding rocks in satellital images, and by the presence of a hornfels some hundreds of meters to the east, but no direct observation had been made until date. Lithologically, it shows similarities with the other plutons of the AFA (Hornblendita Ushuaia, Plutón Diorítico Moat and Cerro Jeu-Jepén, introduced from south to north): ultramafic bodies (hornblendites and pyroxenites) crystallized early, followed by gabbros, diorites and monzodiorites. Latter pulses are represented by monzonites and syenites, and finally syenitic to alkali-feldspar syenitic veins cut the assemblage. The latest 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 the other localities. Thus, large bodies of monzonites classify 10cal1y as syenites, and the differentiated veins are of K-feldspar syenite composition. These characteristics are shared with the Cerro Jeu-Jepén pluton. Monzonites and a melagabbro-meladiorite (no plagioclase composltlOn was determined) form the dominant rock types. The latter is characterized by a lamprophyric texture, represented by 50-70% of mafic phenocrysts (hornblende and clinopyroxene) set in a phaneritic groundmass of the same minerals plus plagioclase ± biotite, alkali feldspar and accessory phases (titanite, apatite and opaques). The monzonites are poor in mafic mineral s, and K-feldspar is occasionally ananged in a trachytic texture. They form a main body in the eastern part of the pluton, and randomly distributed dykes. The lithological and petrographical characteristics of the Monte Kranck pluton suggest that it can be correlated to the other plutons in the AFA, and so it can be stated that 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 accompanied geochemical1y by an increase in K20 content in whole rock and in some mafic minerals. It was recently proposed that these plutons belong to a Lower Cretaceous shoshonitic suite of a back-arc enviromnent. In this model, the magmas that gave rise to each pluton were generated at progressively greater depths in the mantle as the slab sank (northward), acquiring higher K20 contents, probably in response to decreasing degree of pmiial melting. Although we lack geochemical data for the Monte Kranck pluton, its lithology could represent the farthest-from-the-trench (considering the Cretaceous arc axis on the Chilean archipelago) batches of magma generated, in agreement with the Kh model ofDickinson (1975).± biotite, alkali feldspar and accessory phases (titanite, apatite and opaques). The monzonites are poor in mafic mineral s, and K-feldspar is occasionally ananged in a trachytic texture. They form a main body in the eastern part of the pluton, and randomly distributed dykes. The lithological and petrographical characteristics of the Monte Kranck pluton suggest that it can be correlated to the other plutons in the AFA, and so it can be stated that 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 accompanied geochemical1y by an increase in K20 content in whole rock and in some mafic minerals. It was recently proposed that these plutons belong to a Lower Cretaceous shoshonitic suite of a back-arc enviromnent. In this model, the magmas that gave rise to each pluton were generated at progressively greater depths in the mantle as the slab sank (northward), acquiring higher K20 contents, probably in response to decreasing degree of pmiial melting. Although we lack geochemical data for the Monte Kranck pluton, its lithology could represent the farthest-from-the-trench (considering the Cretaceous arc axis on the Chilean archipelago) batches of magma generated, in agreement with the Kh model ofDickinson (1975).