Ushuaia pluton: petrographic facies and geochemical signature. Tierra del Fuego Andes.

CERREDO, MARÍA ELENA; REMESAL, MARCELA B.; TASSONE, ALEJANDRO; MENICHETTI, MARCO; PERONI, JAVIER I.

19-20 de noviembre. Santiago. Chile

Simposio; International Geological Congress on the Southern Hemisfere (Geosur 2).; 2007

GEOSUR

The Ushuaia Pluton (UP) is a poorly exposed epizonal intrusive body hosted in the Early Cretaceous Yahgan Formation (YF) on the northern margin of Beagle Channel (BC) within the area enclosed by a major aeromagnetic anomaly of around 70 km2. In spite of the very restricted UP outcrops (< 3 km along BC shoreline), an extended metamorphic contact aureole ( > 1km) is recognized within the turbidites of YF reaching up to the K-feldspar zone where local pockets of partial melts were produced. UP is compositional varied and includes several facies: a cumular basic to ultrabasic facies, a heterogenous facies where magma mixing processes are evident and marginal roof facies of volcanic/hypabyssal character. The cumular facies is mainly composed of coarse-grained hornblendites often cut across by syenitic veins and dykes. The magma mixing domain is characterized by the presence of mafic microgranular enclaves (MME) ranging from cm to m, with typical crenate outlines hosted in mesocratic to leucocratic rocks of mozonitic to syenitic composition. The roof facies are distinguished by its typical subvolcanic textures and range in composition from andesite to dacite/trachyte. Generally small (< 10cm) MME are unevenly distributed within this facies. Rocks of the three facies are metaluminous and span from the medium-, through the high- K up to the shoshonite series. Basic and ultrabasic rocks show relatively low Mg# (33-45), and very low Ni, Cr and Co contents, which suggest some Fe-Mg silicate fractionation at the source area. Sr spykes on multielement plots along with smooth to null Eu/Eu* indicate that no significant plagioclase fractionation controlled liquid evolution. Some trace element contents and interelemental ratios clearly indicate the subduction-related nature of UP: typical Nb-Ta-Ti troughs on spidergrams; Ba/La ratios within arc magmatism values; Ba contents are variable but high which suggests a metasomatized mantle source; Ba/Nb >30 are typical of melts produced in a metasomatized mantle wedge above a dehydrating subducting slab. Furthermore, high primitive mantle normalized Srn/Pn (>2, 5) may indicate slab derived fluids Information gained from microstructure recognition indicates that magmatic intrusion took place in a tectonically active setting. Different magma batches intervening in pluton assembly display deformation features which span from the early synmagmatic to the waning stages of liquid crystallization. Moreover, widespread brittle fractures with epidote infillings (likely a posthumous magmatic product) which are in turn affected by brittle shearing suggest that deformation outlasted pluton emplacement. In this connection it is important to recall UP location along the strike of the major transtensive structure of BC fault system which could have assisted and controlled magmatic channelways and final pluton emplacement.