Zircon inheritance from long-lived sources of Late Triassic post-orogenic plutons, High Andes, Central Chile (~30°S): Magmatic feedbacks and petrogenetic implications

Abstract A comprehensive understanding of the magmatic system evolution and petrogenetic environment is crucial for the correct interpretation of magmatic zircon age spectra. In the Central Andes, a protracted Late Paleozoic period of collisional to post-collisional magmatism was followed by Late Triassic emplacement of post-orogenic plutons and associated mantle precursors. Some of these magma products are crowded with inherited zircons with a large age span at the sample scale (>25 Myr). This work deals with two contrasting inheritance-rich plutons: the peraluminous cordierite-bearing Los Tilos pluton (LTp) and the peralkaline hypersolvus Ferro-edenite/hedenbergite-bearing Monte Grande pluton (MGp). Both plutons were derived from melting of the crust: the peraluminous S-type granite represents derivation from mixed materials, whereas the peralkaline A-type leucogranite embodies nearly eutectic melts enriched in incompatible elements. Rapid cooling of entrained assemblages suggests that segregation and subsequent ascent was rapid, following emplacement-site isobaric cooling at ca. 3.5 and 2 kbar, for LTp and MGp respectively. Most zircons were inherited; their disparate chemistry originates from contrasting sources, unveiling an open-system behavior. The effects of variations in magma temperature, emplacement pressure, and water content were evaluated in light of zircon entrainment and dissolution potential, within a thermodynamically consistent framework. Relatively dry magmas ( We suggest that the large age span observed in zircon samples of both plutons reflects a long-lived source that fed arc-related granites in the Early to Middle Triassic, culminating in the Late Triassic with the final extraction of highly enriched crustal melts, in a post-orogenic context. Dry high-silica (>75 SiO2 wt%) magmatic flare-ups, in extensional settings, fulfill the inheritance requisites presented here. Contrastingly, a broad zircon age span in large calc-alkaline batholiths emplaced at similar depths, reflect not source processes but rather magmatic erosion, crystal armoring and recycling of previous magma batches, and a complex crystallization history, spanning the whole duration of batholith construction.