Abstract

During the late Miocene, the decreasing angle of subduction below the Andes of central Chile caused a reduction in the influx of new magma and heat into the base of magmatic systems that had been active throughout the Miocene. This led to their cooling and solidification, triggering the release of large volumes of metal-rich magmatic fluids from middle- and upper-crustal magma chambers and generating a group of economically significant Cu-rich breccia pipes. The Sr- and Nd-isotopic compositions of the magmatic fluids that formed these brecccas, determined from breccia-matrix minerals, were variable, implying that these fluids were not exsolved from a specific Cu-porphyry magma, as is often invoked to explain Andean Cu deposits. Instead, the formation of the late Miocene deposits in central Chile was tectonically triggered by cooling of a variety of magma types during the last stages of existence of long-lived Andean magmatic systems as both subduction angle and, as a result, subarc magma supply decreased.

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