TESTING FORMATION OF THE TORRES DEL PAINE GRANITE, SOUTHERN CHILE BY MELT-ROCK REACTION USING SR AND PB ISOTOPES AND MINERAL MAPPING

Granite formation processes have seen renewed debate, as our most-commonly-taught explanations for magma differentiation (fractional crystallization and partial melting) fail to explain geochronology and thermobarometry of many intrusive bodies. Here we assess whether the granite of the Torres del Paine Intrusive Complex (TDPIC) could have formed via reactions between low temperature (500 C) melt (LTM) and gabbro.

While the beautifully exposed contact between the TDPIC gabbro and overlying granite is sharp at macro scale, mineral compositions smoothly transition between the two bodies, age dating contradicts cross cutting relations that suggest intrusion of granite into gabbro, and element analyses show that the mafic and felsic minerals are consistent with reaction toward chemical equilibrium.

We hypothesize that the overlying granite formed through a process of melt-rock reaction with pre-existing gabbro within the mafic suite via heat input by underplating sills and migration of buoyant, alkalic, LTM upward.

We have begun to tackle this broad hypothesis via isotopic measurement and major element analysis of potassium feldspar and plagioclase crystals within samples collected along the granitic-to-mafic transition zone of the TDPIC. Previous whole-rock studies show ⁸⁷Sr/⁸⁶Sr of the gabbro consistent with those expected for a mantle magma, higher ⁸⁷Sr/⁸⁶Sr in the granite due to crustal input, and homogeneous lead ratios between the gabbro and granite.

Preliminary study of micro drilled plagioclase shows ⁸⁷Sr/⁸⁶Sr vary by 0.0002, with cores of those in the granite equal to the gabbro whole rock values. EDS analyses show plagioclase with more anorthitic cores relative to rims. Future work includes laser ablation of potassium felspars for lead isotope values.

While more work is required to firmly suggest melt-rock reaction as the mechanism of granite formation in the TDPIC, these studies show agreement with what might be expected if granite formed from a pre-existing gabbro via interaction with a more felsic LTM.

If it is true that such a process occurs at temperatures below those that we typically considered to be igneous, we must not only reassess current ideas about granite formation and the distinction between igneous and metamorphic processes, but pedagogical approaches for igneous petrology as a whole.