INITIAL DATA ON THE GEOCHEMISTRY OF THE TORRES DEL PAINE LACCOLITH

The Torres del Paine Laccolith (TPL) in Patagonia/Chile is part of a chain of isolated Miocene intrusions which intruded into the eastern foothills of the southernmost Andes of Chile and Argentina. The TPL has been dated by Halpern (1973) at 12±2 Ma (Rb/Sr model) and13±1 Ma (K-Ar biotite), respectively. It intruded at a shallow level (2-4 km) into mudstones, sandstones and conglomerates of the Cretaceous Punta Barrosa and Cerro Torre Formation. The basal part of this composite laccolith consists mainly of layered monzonites and minor olivine-bearing gabbros (Paine-Mafic-Complex, PMC). The main I-type granite can be subdivided into alkali-feldspar porphyritic medium grained granite and a fluid-saturated miarolitic granophyric alkali-granite, which is found towards the host-rock. Miaroles contain mainly quartz and feldspar, with some biotite, tourmaline, sphene, pyrite and fayalite. Field observations indicate that both gabbros and monzonites predate the granite intrusion. Late dykes of basaltic composition cross-cut the granite and the underlying mafic intrusion. Major and trace element analyses show a typical calc-alkaline trend. The structurally oldest rocks are monzonites, whose major, trace and REE characteristics are more evolved than those of the later ol-bearing gabbros. Monzonites are more LREE-enriched with respect to HREE than ol-gabbros ((La/Lu)N = 6.8 and 4.7, respectively). Compatible elements (Cr,Co,Zn) show values comparable to primitive mantle or below. A weak negative Eu-anomaly is visible in the monzonitic rocks, pointing to plagioclase fractionation. The evolved alkali-granites (over 76wt.% SiO2) show enrichment in REE compared to the main porphyritic granite (70-73wt.% SiO2). Eu anomalies are strongest in these rocks. They are considered to be the last phase to crystallize from a magmatic fluid with eutectic composition. The overall major, trace and REE data is compatible with the evolution of a single magmatic suite.