GSA Connects 2021 in Portland, Oregon

Paper No. 203-1
Presentation Time: 8:05 AM


GRUNDER, Anita, CEOAS, Oregon State University, CEOAS Admin 104, Corvallis, OR 97331

Comparison of arc volcanism and plutonism is confounded because major arc volcanoes have lifetimes of a few hundred thousand to a million years where plutonic complexes have lifetimes on the order of 1-10+ m.y. Owing to the arid climate of northern Chile, about 11 million years of arc volcanism are preserved in the Aucanquilcha Volcanic Cluster (AVC) allowing for comparison to the well-studied 95-85 Ma Tuolumne Intrusive Complex (TIC) . The two suites have remarkable similarities. Both are products of about 10 m.y. of subduction magmatism on the margin of overthickened continental crust. They have similar footprints of about ~1000 (AVC) to ~1500 (TIC) km2, with older pyroxene-bearing more mafic members at the margins and more felsic members with more prominent amphibole and biotite generally inward (broadly normal zoning). They also share a history of sharply increasing magma emplacement rate of about ½ of the volume emplaced after about 7 m.y., that is 2/3 of the lifespan. Silica ranges from 55 in to 70 wt.% in the AVC and, while largely overlapping with the TIC, leucosomes and late granite in the TIC range to as high as 76 wt.%. Sr and Nd isotopic ranges are also largely overlapping with systematic increase in crustal signatures in time; 87Sr/86Sri range is 0.705-0.707 for AVC versus 0.7055-0.7076 for TIC and ╬ÁNd ranges are -1 to -7.5 versus -2 to -9, respectively. Nd versus Sr isotopic ratio plots are slightly concave upward for the AVC, as is common in crustally contaminated (AFC or mix + frac) volcanic suites, and slightly convex downward in the TIC, suggesting that plagioclase residua from earlier fractionation are entrained in subsequent magma batches. Inherited zircons, substantially older than the host magmatic complex, are rare, and modest age dispersion is attributable to recycling of antecrysts. Amphibole barometry for the AVC yields magma storage depths from 1-5 kb, similar to 1-4 kb reported for the TIC. These striking similarities indicate that the AVC and TIC are records of the same history of traverse, differentiation, and storage of magma through and in the arc crust. Leveraging the known volume of the AVC (~300 km3), footprint, and magma storage depth, yields an estimate of an intrusive to extrusive ratio of at least 40, disregarding the large mafic complex that must have underlain both of these systems.