TECTONIC TRIGGER FOR THE FORMATION OF LATE MIOCENE CU-RICH BRECCIA PIPES IN THE ANDES OF CENTRAL CHILE

During the late Miocene, the decreasing angle of subduction below the Andes of central Chile caused a reduction in the influx of new magma a nd heat into the base of magmatic systems that had been active through out the Miocene. This led to their cooling and solidification, trigger ing the release of large volumes of metal-rich magmatic fluids from mi ddle- and upper-crustal magma chambers and generating a group of econo mically significant Cu-rich breccia pipes. The Sr- and Nd-isotopic com positions of the magmatic fluids that formed these breccias, determine d from breccia-matrix minerals, were variable, implying that these flu ids were not exsolved from a specific Cu-porphyry magma, as is often i nvoked to explain Andean Cu deposits. Instead, the formation of the la te Miocene deposits in central Chile was tectonically triggered by coo ling of a variety of magma types during the last stages of existence o f long-lived Andean magmatic systems as both subduction angle and, as a result, subarc magma supply decreased.