MAGMATIC CONSTRAINTS ON THE TIMING OF THE THICKENING AND ECLOGITIZATION OF THE CRUST UNDER THE ARGENTINE ANDEAN CENTRAL PRECORDILLERA OVER THE CHILEAN-PAMPEAN FLATSLAB

Seismic evidence from precursor and receiver function data show that the Central Andean foreland crust under the Argentine Central Precordillera over the 80 km deep Pampean-Chilean flatslab near 32°S is ~ 60 km thick and includes transitions at ~ 20 and 40 km that are attributed to the top of crystalline basement and a transition to the eclogitic facies. New age and chemical constraints suggest the eclogitization of the lower crust occurred at ~8-7 Ma as the Nazca plate slab flattened and the crust thickened under the region. Evidence for the presence and timing of eclogitization comes from chemical contrasts in the dacitic Albarracín ignimbrites that erupted from the Ullum Complex, whose 14.7±0.2 Ma age (MSWD=1.5) is constrained by near 100 U/Pb laser ICP-MS zircon analyses, nearby dacitic volcanic domes yielding Ar/Ar ages of 7.5±0.2 Ma (Co de la Sal, biotite) and 7.0±0.1 Ma (Co de los Baños; whole rock), and a block and ash deposit with an amphibole Ar/Ar age of 7.5±0.4 Ma. Local field relations show the major thrusting and associated crustal thickening under the region occurred after 15 Ma and likely near the time of the emplacement of the 8-7 Ma deposits. The older ignimbrites have higher HREE concentrations than the younger deposits suggesting the appearance of a crustal eclogitic residue by 8-7 Ma. Regional Andean relations are consistent with the older ignimbrite magmas erupting in response to a general spread of volcanism into the backarc as South America drifted westward over the Nazca plate after ~ 18 Ma. In contrast, the 8-7 Ma volcanic rocks mark the end of backarc volcanism associated with the extreme flattening of the Nazca plate into the underlying flat segment after the subduction of the Juan Fernandez ridge began to the west at ~10 Ma. Fluids from the mantle wedge over the flattening slab are argued to have facilitated eclogitization of the lower crust. Evidence for the presence of mantle wedge melts participating in the magmatism until 7 Ma comes from the volcanic rocks having more primitive isotopic ratios (ε_Nd = 0 to ⁺1.7; ⁸⁷Sr/⁸⁶Sr = 0.70325-0.70345; zircon ε_Hf = ⁺0.5 based on 26 analyses) than the 1246 to 1088 Ma zircons in the ignimbrite and ~ 1100 Ma xenoliths (ε_Nd = 0 to ^-3; ⁸⁷Sr/⁸⁶Sr = 0.704-0.710), which are considered to reflect the composition of the lower crust under the region.