T24 Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

Sunday, 22 September 2024
8:00 AM - 12:00 PM
207C

LINKING THE FOREARC RASPAS COMPLEX ECLOGITE OF EL ORO MASSIF TO JURASSIC ANDEAN BATHOLITH GENERATION IN ECUADOR

Paper number: T24-3
Program: Technical Sessions
Time: 8:30 AM – 8:45 AM

Authors

Abstract

The cordilleras of western North and South America comprise the greatest orogenic system produced by subduction of oceanic crust along a continental margin. A geologic hallmark of this process is the great chain of Mesozoic subduction-related continental margin batholiths extending ~12,500 km from Alaska to Patagonia. In North America the ~1,000 km-long Franciscan Complex records more than 150 Myr of east-dipping subduction beginning in Early Jurassic time that is directly linked to batholith formation to the east. Yet, the situation in South America is dramatically different. Mesozoic subduction assemblages paired to Andean batholith belts are so far unrecognized. This is due to large-scale forearc subduction erosion at the Peru and Chile margins, and further north in Ecuador and Colombia, to the effects of Late Cretaceous oceanic plateau accretion. The Raspas Complex eclogite-blueschist assemblage located in the El Oro massif of SW Ecuador is one of the few forearc subduction records preserved anywhere in South America. We present new U-Pb analyses of zircon from eclogite facies garnet-kyanite-chloritoid metapelites in the Raspas Complex (~P-T conditions 20 kbar, 550-600 C) that yield a broad range of ages (163-943 Ma), however grains <212 Ma have distinctly high U/Th ratios consistent with a metamorphic origin. Older igneous grains suggest a provenance linkage to the Amazon craton and provide a maximum depositional age estimate of ~245 Ma for the shale protolith. Metamorphic zircon ages range between ~ 210 and 165 Ma and are not tightly clustered, most likely recording multiple generations or a protracted zircon growth history. This finding is consistent with the presence of two distinct foliations preserved in the Raspas metapelite. To compare these results to the evolution of the coeval arc, new and previously published detrital zircon results from modern rivers in Ecuador were compiled and show several distinctive flare-ups in the Mesozoic arc. The most prominent of these occurs in the Early to Middle Jurassic (~190-170 Ma) and is synchronous with the eclogite subduction history of the Raspas Complex recorded in the metamorphic zircons. This is the first example of Andean subduction metamorphism being related to Andean batholith generation.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-403630

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