The Puente Quemado Gabbro, to the West of Ushuaia, Tierra del Fuego, Argentina.

VILLAR, L.; ACEVEDO, R.D.; LAGORIO, S.

Santiago de Chile

Congreso; GeoSur 2007; 2007

Pontificia Universidad Católica de Chile

A small gabbro intrusive with not clear contacts and no magnetic reply is exposed at Parque Nacional de Tierra del Fuego entrance in “Puente Quemado” place. Outcrops show medium plutonic to fine textures, are lodged in the slates of the Yahgan Formation of Lower Cretaceous age and show clear deformation. Rocks display subophitic to intergranular microtextures; primary mineralogy mainly consists of diopside and plagioclase, with subordinate opaque mineralas (ilmenite of pyrophanite type, pyrite), interstitial quarz, orthopyroxene and scarse olivine (both latter phases only optically determined). These gabbros are affected by a low grade metamorphism that partly obliterate its mineralogy and texture. Secondary (metamorphic) mineralogy is composed of chlorite, tremolite-actinolite, titanite, albite, quartz, epidote, clinozoisite, interlayered chlorite/smectite, stilpnomelane and white mica. The plagioclase is partially replaced mainly by chlorite and/or interlayered chlorite/smectite, tremolite-actinolite, titanite, albite and scarce white mica. The clinopyroxene presents partially alteration to chlorite, tremolite-actinolite, titanite, stilpnomelane, epidote and clinozoisite. Secondary mineralogy points out therefore low grade metamorphism in greenschists facies. Geochemical data show that in the TAS diagram rocks fall in the basaltic field, which is in accordance with the classification through an immobile element plot (Nb/Y vs. Zr/TiO2), special for metamorphosed rocks; in the latter diagram the sub-alkaline character is also shown. Normative composition consisting of quartz, diopside and hypersthene agrees with modal mineralogy. In the AFM diagram the gabbros plot in the tholeitic field near the calc-alkaline one. In diagrams as Zr-Ti/100-Y*3, Zr-Zr/Y, Zr/4-Nb*2-Y these gabbros fall in the MORB field. The REE patterns (N-MORB normalized) show LREE enrichment; incompatible multi-elemental diagrams are comparable with those of N-MORB type, showing Ba and Rb enrichments as well as negative anomalies for Sr, Nb, and Ta. The former trough is related to plagioclase fractionation while the others must point out a subduction component. Tholeiitic characteristics show affinities with Monte Olivia diabases and Paso Garibaldi basaltic lenses. Consequently, these rocks also represent part of the remnant oceanic crust that belong to the Rocas Verdes back-arc basin, ophiolite complex studied by diverse authors (e.g. Dalziel, 1981; Stern and de Wit, 2003; Cerredo et al., 2005). Hydrothermal ocean-floor metamorphism has been overprinted over magmatic activity under extensional tectonics, later strongly deformed by Andean compression.