PLUTON EMPLACEMENT IN THE ANDEAN CONTRACTIONAL BELT, CHILE: THE ROLE OF TECTONIC SHORTENING IN MAGMA SEGREGATION, ACCUMULATION, AND ESCAPE

We conducted a detailed petrographic and fabric-based study of the ~7-6 Ma Risco Bayo-Huemul pluton in the Chilean Andes. This body was emplaced into an active fold-and-thrust belt, and is now coincident with the frontal magmatic arc of the Andes. Detailed geochronology has documented the timing of the intrusions. The 7.2-6.4 Ma Risco Bayo pluton comprises gabbros, granodiorites, and diorites. In contrast, the 6.4–6.2 Ma Huemul pluton consists of high-silica granite, granite, and a quartz monzonite. Geochemical evidence suggests a genetic relation among the constituents of the Huemul pluton: The high-silica granite (the extracted rhyolite) was extracted from a granite-like parent, leaving behind a quartz monzonite (the residual silicic cumulate). Fabrics in all three units of the Huemul are solely hypersolidus. Anisotropy of Magnetic Susceptibility fabric analyses in the Huemul pluton reveal oblate magnetic fabrics characterized by NNW-striking, subvertical magnetic foliations. Magnetic lineations vary from sub-vertical to sub-horizontal. These fabrics cross-cut contacts between compositional domains, indicating ENE-directed shortening while the pluton was partially molten. This orientation is consistent with adjacent Miocene regional structures (NNW-striking folds and reverse faults). From petrographic analysis, the fabrics recorded by the early feldspars in the cumulates are interpreted to have developed in the presence of melt, indicating interstitial rhyolitic melt flow from the quartz monzonites toward the high-silica granites (e.g., to the top of the magma reservoir). In general, the fabrics are consistent with shortening-assisted melt segregation, suggesting that tectonically induced filter pressing has occurred. This process may occur despite the view that subvolcanic rhyolite production is independent of tectonic deformation, because the timescales of magma extraction/ volcanism (seconds to kyrs) in rhyolitic systems are significantly shorter than contractional tectonism (Myrs). Rather, we interpret that both plutonic emplacement processes and melt extraction are likely intimately linked to tectonic deformation in fold-and-thrust belt settings.