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- W226041293 abstract "Voluminous dacitic ignimbrites have erupted in a subduction zone setting along the compressional Central Andean margin in the last 11 Ma. A compilation of some 600 published and new analyses allows a view of the spatialtemporal crustal evolution of the ignimbrite complexes that erupted through the thickened crust of the Puna plateau between 22°S and 27°`S latitude. Their variable chemistry reflects the conditions that generate large crustal magma systems. AFC modeling largely based on Sr/Sr ratios and δO/O shows that the large volume ignimbrites (>500 km) are best modeled as approximately 50-50 mixtures of mantlewedge derived basalt and variable composition crustal contaminants. Erupted mafic magmas show the mantle basalt is isotopically enriched (Sr/Sr ~ 0.7055; eNd ~ -2), most likely by crustal recycling due to forearc subduction erosion and delamination. An estimated volume of 8200 km for the erupted ignimbrites with a 90 km/km/Ma arc magma production rate produces a plutonic/volcanic ratio of 4:1 and a 5.4 km thickness of new crust distributed across the plateau. Crustal contaminants in the ignimbrites are spatially and temporally variable with Sr/Sr ratios from about 0.715 to 0.735 at 300 to 125 ppm Sr and δO/O from about 11 to 14. Variable Al/(K+Na+Ca), Na/K and isotopic ratios require a more sedimentary-like and radiogenic contaminant in the north than in the south. Temporal trends to lower Sr/Sr ratios and more metaluminous compositions in the north indicate an evolving crustal contaminant as new magmas enter the crust. Trends from higher to lower La/Yb (40 to 10) and Sm/Yb (7.5 to 2) ratios in the same regions indicate a changing role for garnet as a controlling residual phase. Trace element evidence for residual garnet and a requirement for high Sr contents in crustal contaminants indicate that crustal contamination occurred in the deep crust and that the contaminated melts accumulated in magma chambers like those seen near 20 km depth on seismic images. Negative Eu anomalies superimposed on high pressure REE patterns and calculated bulk distribution coefficients are best explained by feldspar fractionation in mid-crustal magma chambers. CL characterization of plagioclase in fault rocks M. KAYAMA*, K. OKABAYASHI, K. KAWAMOTO AND H. NISHIDO Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama, 700-005 Japan (*correspondence: kayama0127@gmail.com) Oshika Geological Museum of Japan Median Tectonic Line, 988 Ohgawara, Oshika, Nagano, 399-3502 Japan" @default.
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- W226041293 date "2008-07-01" @default.
- W226041293 modified "2023-09-24" @default.
- W226041293 title "Implications of chemical and isotopic variations in Neogene Puna Plateau Ignimbrites for Central Andean crustal evolution" @default.
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