CHEMISTRY OF TERTIARY PRECORDILLERA MAGMATIC ROCKS AS A GUIDE TO PROCESSES ASSOCIATED WITH THE SHALLOWING OF THE CHILEAN FLATSLAB IN THE CENTRAL ANDES

The Central Chilean-Pampean flat-slab in the central Andes (~ 28-33°S) is the classic example of a currently subducting slab with a flat portion behind an inactive volcanic arc. Many questions remain as to how the flat part of this slab developed and the effects that flattening has had on the overlying crustal lithosphere and mantle wedge. These questions can be addressed by examining the chemistry of the retroarc Albarracín-Ullum region volcanic rocks (31°30’S; 68°50’W) that erupted in the Argentina Precordillera just prior to and as the flatslab segment formed. These andesitic to rhyodacitic volcanic rocks fall in two age groups: ~18-11 Ma and ~10-7 Ma. The older group, which largely consists of ignimbrites, erupted from the Ullum caldera complex. Geochemically, they are characterized by higher heavy REE concentrations (as best seen by Yb) and Ba/Th ratios than the dome and block & ash deposits that were emplaced in the younger group. The change in REE characteristics can be interpreted to reflect an episode of late Miocene granulite/eclogite metamorphism of the deep part of a thickening crust in conjunction with retroarc crustal shortening by contractional deformation over the flattening slab. The higher Ba/Th ratios in the younger magmas can be interpreted as being due to an influx of slab-related fluids into the mantle wedge as the subducting slab arrived below and flattened beneath the region. The isotopic ratios in the volcanic rocks (¹⁴³Nd/¹⁴⁴Nd = 0.51262-0.51275; ²⁰⁷Pb/²⁰⁴Pb = 15.48-15.55, ⁸⁷Sr⁸⁶Sr ratios = 0.7033-0.7038) signal an overall more depleted magmatic source than for the Precambrian Grenville-age xenoliths from the unexposed crystalline basement in the volcanic rocks and reflect a mixture of local and transported lower crustal/lithospheric and mantle wedge components. The evolving chemistry of the volcanic rocks is consistent with crustal thickening and changes in mantle and lithospheric components under the Precordillera linked to the marked flattening of the subducting slab that began with the near normal subduction of the Juan Fernandez ridge at 10-11 Ma and shut off volcanism under the Precordillera region of the retroarc by 7 Ma.