STABLE ISOTOPE SYSTEMATIC OF THE TORRES DEL PAINE INTRUSIVE COMPLEX (PATAGONIA, CHILE)

The 12 Ma old Torres del Paine laccolith is located in the southernmost Andes of Chile. The Torres del Paine Intrusives form a calc-alcaline suite, ranging from gabbros through monzonites to leucogranites. Gabbroic and monzonitic rocks are exposed at the lower basal levels and in the root zone. The main body is composed of a medium-grained porhyritic granite. Field observations strongly suggest that the emplacement of the mafic rocks pre-date the granitic intrusive phases. Oxygen isotope data for granites range from 8.0-10.5 per mil for quartz and 5.1-6.0 per mil for biotite. The miaroles show a similar range in their quartz values of 8.8-11.1per mil. The preservation of original igneous oxygen isotope values indicates that the laccolith was not strongly affected by meteoric water nor low-T hydrothermal fluids. This is supported by the pristine D/H ratios of igneous biotites and high temperature fractionations between mineral like (e.g., quartz-fayalite = 750±30°C). Isotopically, we can distinguish two types of gabbros. Fine-grained monzonitic gabbros, which are chemically evolved , show primitive (mantle-like) oxygen isotope compositions with hornblende and pyroxene values of 5.3 to 5.6 per mil and biotite values of 4.1 to 4.6 per mil. In contrast, the chemically more primitive, olivine-gabbros show enriched oxygen isotopic composition with hornblende and pyroxene values of 6.0 to 6.7 per mil and biotite values of 5.8 to 6.4 per mil. Preliminary analyses of mafic dykes yield similar results – an evolved basaltic dyke shows the most primitive oxygen isotope composition (whole rock = 5.3 per mil) whereas olivine-bearing dykes show high whole-rocks values of 7.6-8.4 per mil. In addition, field observations cleary suggest that the monzonites pre-date the primitive olivine-gabbros in the intrusive cycle. This is consist with the evolved oxygen isotope signature and would suggest contamination in the source region or a lower seated magma chamber by crustal material or fluids. However, more data are needed to test this hypothesis.