EVOLUTION OF THE CHILEAN FLATSLAB: A MIOCENE MAGMATIC PERSPECTIVE FROM THE ULLUM-ALBARRACÍN VOLCANIC REGION OF THE ARGENTINE PRECORDILLERA

The modern central Andean Chilean-Pampean flatslab (~28-33°S) is a classic example of a subducting slab with a flat segment developed inland of an inactive volcanic arc. Clues to the processes of slab shallowing on the backarc lithosphere and mantle wedge come from the distribution, petrology and geochemistry of the ~18-6 Ma eastern Central Precordillera Albarracín-Ullum region (~31.5°S; 68.8°W) andesitic to rhyodacitic volcanic rocks, whose evolution reflects the history of crustal deformation and thickening over the evolving mantle wedge at ~12-11 to ~ 7-6 Ma as the slab flattened in response to the subduction of the Juan Fernandez ridge. Existing K/Ar, Ar/Ar and zircon ages show that these volcanic rocks can be grouped into the ~18-14 Ma pre-flatslab Ullum caldera related ignimbrites and domes that predate local thrusting, a minor group of 13-9 Ma ashes and domes that erupted as the thrust front advanced from the west and relatively voluminous ~8-5 Ma “adakitic” dacitic block & ash deposits and domes that intruded as the thrust front arrived and merged with a developing westward verging basement wedge from the east. The evolving chemistry of these volcanic rocks progressively reflects the shallowing slab as the overlying mantle dehydrated and geochemical indicators like Ba/Th ratios increased as eastward directed slab-derived fluids exited the dehydrating wedge. At the same time, the overlying crust progressively underwent thickening, partial melting and eclogitization as mantle-derived magmas intruded and interacted with the evolving granulitic to eclogitic-bearing crust to produce the magmas with “adakitic-like” REE and trace element signatures that formed the < 7.5 Ma domes. The relatively depleted isotopic ratios in the domes (¹⁴³Nd/¹⁴⁴Nd = 0.51262-0.51275; ²⁰⁷Pb/²⁰⁴Pb = 15.48-15.55; ⁸⁷Sr⁸⁶Sr = 0.7033-0.7038) compared to those of their enclosed Grenville basement xenoliths support melting of in situ and tectonically transported crust by mantle derived magmas. The age, evolution and chemistry of the Precordillera Albarracín region magmas have some surprising parallels with the timing and evolution of some Cordillera Blanca region magmas over the Peruvian flatslab making comparisons instructive in understanding volcanism and shallowing processes of flat subduction zones.