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Petrogenetic evolution of arc magmatism associated with Late Oligocene to Late Miocene porphyry-related ore deposits in Ecuador

Abstract:

We present a data set on the geochemical and isotopic composition of mineralized porphyry intrusions, seriate intrusions, and arc volcanic formations spatially associated with late Oligocene to late Miocene, porphyryrelated ore deposits in Ecuador. Our aim is to constrain the petrogenetic evolution of these arc magmas, to evaluate geodynamic and tectonic impact factors on arc magmatism, and to explore the metallogenic significance of these parameters. We distinguish four petrogenetic groups associated with the investigated magmatic centers, indicating different paths of low- and high-pressure crustal magma evolution. Tertiary Ecuadorian arc magmas, which differentiated at higher crustal pressures, are commonly characterized by adakite-like compositions related to fractionation and/or restite equilibration effects of amphibole ± garnet ± titanite, particularly in hydrous silicic melts without concomitant or subsequent major plagioclase fractionation. Trace element distribution patterns of arc magmas through time are broadly systematic and indicate that crustal magma differentiation progressively extends to higher pressures from the late Oligocene to the late Miocene, possibly related to crustal thickening. Locally, adakite-like features-possibly related to high melt H2O contents or accessory phase fractionation - are already observed in the early Miocene in a regional setting otherwise dominated by low-pressure magma evolution. Compositional differences between mineralized porphyry intrusions and spatially associated, seriate precursor intrusions in Ecuador mostly reflect regional-scale changes in magma composition through time and are not observed if the relative emplacement age difference is small. Where porphyry intrusions are associated with shallow, crustal batholith systems in northern-central Ecuador, their emplacement is commonly late with respect to batholith magmatism. Intrusive compositions indicate that porphyry parental melts mostly evolved at higher crustal pressures where metallogenically favorable melt preconditioning might possibly occur. © 2010 Society of Economic Geologists, Inc.