Determination of crustal anatexis conditions from thermodynamic modelling and zircon data: the case of the late triassic a And S-Type granites in the high Andes of Central Chile (~30°S)

Abstract

This thesis is focalized in the thermodynamic conditions that favor the generation of anatectic melts in different scenarios, and also as a study case, about the origin of crust-related plutons of the Late Triassic, in the High Andes of Central Chile. The magmatic environment in which zircon is crystallized and preserved, inherited from source, is also disentangled in light of the anatectic conditions achieved. In a subduction setting, the thermal scenario that explains crust anatexis could correspond to combined effects due to an increased production of radioactive heat, and/or heat advection by upwelling of mantle magmas, among many others. To partially melt the crust, within the melt extraction efficiency window (crystallinities between 50 to 70 vol.%), an addition of 0.2 kJ/g would be required. Magmas generated in the Lower Crust reach these crystallinities in a wide temperature interval, in contrast with those generated in the Middle and Upper Crust, in which crystallinities <50 vol.% are set to be held at temperatures no higher than 50° from their respective solidii. Peraluminous melts are reproduced in the whole modeled range, for the Lower Crust, whereas anatectic melts of peralkaline composition are mostly common in the Middle and Upper crust, at temperatures above 700°C. This study case corresponds to rocks generated in the igneous activity of the Late Triassic, in Central Chile. This activity is characterized by a phase of arc magmatism, in which some singular plutons share geochemical signatures related to post-orogenic A- and S-type granites. The igneous bodies formed in this period pierce the root of the Upper Paleozoic arc, reaching an epizonal level of emplacement. The A-type granites show agpaitic and hypersolvus mineral associations, whereas S-type granites highlight abundant cordieritic nodules. Both of them show clear evidence of rapid cooling of metastable peritectic minerals (cordierite, β-quartz paramorphs, and hedenbergite). This work reveals U-Pb in zircon ages within the 214±2 to 241±2(1σ) Ma and 209±3 to 236±2 (1σ) Ma, for samples corresponding to the peraluminous cordierite-bearing granite of the Los Tilos pluton, and for the peralkaline hedenbergite and Fe-edenite-bearing leucogranite of the Monte Grande pluton, respectively. The ample age range, in each of the selected samples, suggests the presence of an important fraction of foraneous zircons as xenocrysts and/or crystals inherited from the source. The thermodynamic and kinetic models developed in this study, reveal that zircons dragged in anatectic melts could correspond to inherited crystals formed at depth, and carried in rapidly ascending melts (reaching the granite solidus in less than 12 kyr after extraction). It is inferred that zircon grains were crystallized in the feeding magma s source, by zirconium enrichment of the primary anatectic melts, not by temperature variations; this is consistent with lattice stress models calculations, and with the remarkably low Ti-in-zircon concentration reported here. In this case, the adiabatic ascent of water-undersaturated melts might have been facilitated by structural discontinuities inherited from previous tectonic cycles, remnants of the final amalgamation of Gondwana. These melts reached an epizonal level of emplacement, rapidly enough to prevent melt stagnation en route to the surface, by which efficient crystal inheritance from their crustal source was achieved.