Isotopic and Geochemical Signals of the Neogene Los Frailes Volcanic Complex as Recorders of Delamination and Lower Crustal Flow under the Southern Altiplano of the Central Andes.

COIRA, B.L.; KAY, S.M.; JIMÉNEZ, N.; KATO, J.

Cochabamba

Simposio; 11th SSAGI Simposio SudAmericano de Geología Isotópica.; 2018

Colegio de Geólogos de Bolivia

The eruption history, O-Sr-Nd isotopes, major and trace elements and mineralogy of the Los Frailes Volcanic Complex record the crustal thickening and delamination events under the southern Altiplano after a slab steepening event at 25-22 Ma. Temporal changes in the chemistry in these andesitic/-rhyodacitic magmas refine a crustal shortening-uplift history inferred from balanced cross-sections, surface uplift indicators and seismic studies by others. The strongly peraluminous character(Al/Ca+K+Na =1.05-1.35), high δ18O (+9.73-11.09? in quartz),changing initial 87Sr/86Sr (0.707-0.7125)and 144Nd/143Nd (0.51231-0.51215)ratios and variably steep REE patterns (La/Yb up to 100) in the Los Frailes rocks implicate mid to lower crustal metapelitic crustal contaminants.O-Sr-Nd AFC modeling suggests the erupted magmas are ~50:50 mixtures of enriched mantle basalt (87Sr/86Sr=0.7055, 500-950 ppm; 144Nd/143Nd =0.51260, 25-30 ppm; δ18O=+5.8?) and variable peraluminous crustal melts (δ18O=+13-16?, 87Sr/86Sr =0.730-0.750, 95-160 ppm;144Nd/143Nd =0.51190, 25-40 ppm).Support for a deep crustal origin for the silicic components comes from steep to extremely steep HREE patterns (Sm/Yb=3-14),high Sr (400-650 ppm)and seismic data revealing a relatively silicic middle to lower crust that requires a ductile structural mechanism to transfer peraluminous upper crust into the deep crust. Negative Eu/Eu* (0.6-0.9) anomalies implicate feldspar fractionationat higher levels,and a low pressure crystal cargo (±cordierite)signals shallow pre-eruption crystallization. In short, the temporal trends in the REE patterns and isotopes in the intermittently erupted Frailes complex record crustal thickening in line with uplift and thrusting in the Eastern Cordillera as Sm/Yb ratios requiring increasingly higher pressure garnet-bearing residue,increase from ~4 at ~19 Ma to ~11 at ~12 Ma. At the same time, decreasing 144Nd/143Nd (0.5123-0.5122) atnear constant 87Sr/86Sr (~0.710) ratios support incorporation of Brazilian lithosphere into the site of magma evolution. Subsequently, amarked drop in Sm/Yb ratios to~5-6 inthe 8-7 Ma Livicucho/Condor Nasa ignimbrites as brittle deformation shifted into the Subandean belt and after a major surface uplift event,signals an important crustal delamination event after 12 and before 8 Ma. Apost-12 Ma shift to higher 87Sr/86Sr ratios supports exposure of mantle melts to more isotopically evolved silicic crust in the wedge and overlying crust. Lower crustal flow of the hot crust can explain a return to higher Sm/Yb at~4-2 Ma. Finally, the voluminous ~1.5 Ma Los Frailes Ignimbrites erupted as the crust thickened above a thinned lithosphere.