Geochemical Signals and Magmatic Consequences of Arc Migration in the Central Andes

Cornell University, Ithaca NY

Investigators

Abstract

Kay EAR-0126000 The Central Andean margin, the classic example of an oceanic slab subducting beneath a continent, is a laboratory for studying processes that typify continental crustal formation, modification and destruction. This study focuses on such a process - the migration of an arc magmatic front towards the backarc - that has reconfigured the forearc, arc and backarc of continental margins since plate tectonics began. Lithospheric scale profiles indicate significant mass balance problems that imply recycling of continental crustal and mantle lithosphere into the mantle. We plan a trace element and isotopic study of magmas erupted before, during and after arc migration as probes to changing magmatic source regions and lithospheric configurations beneath the migrating arc. The focus area from 26.5?S to 28?S latitude overlaps the transition between the modern Central Volcanic Zone (CVZ) and the non-volcanic region above the shallowing dipping subduction zone to the south. In this region, the magmatic arc front migrated ~ 50 km eastward from the Maricunga belt to the southern CVZ and Bonete region between ~7 and 2 Ma. Existing geologic maps, K/Ar ages (~300) and major element and Instrumental Neutron Activation trace element data (~ 400 samples at Cornell) from virtually every center provide a basis for an in-depth ICP-MS and Nd, Sr, Pb and O isotopic study of magmatism in this environment. Chemical data show that lavas erupted in the old dying arc and as the front migrated eastward have extremely steep REE patterns (La/Yb > 60) and high field strength (HFS) element depletions (La/Ta to 85) that contrast with lower ratios in both pre-8 Ma and post-4 Ma arc lavas. The magmas with the steepest REE patterns appear better analogues to anomalous Archean (TTG) magmatic suites than do magmas thought to be melts of the subducting slab (adakites). Variable age mafic magmas (<58% SiO2) in the region, which include the most mantle-like Central Andean lavas, are critical to deciphering mantle processes whereas silicic magmas are keys to crustal processes. A crucial question is why very steep REE patterns occur in lavas erupted as the arc terminates and migrates, but not before or after. Residual garnet must play a role, but is that garnet associated with melting in the mantle, in situ thickened crust, or in crust removed by subduction erosion from the base of the arc and forearc and cycled through the subduction zone? Tests are to correlate spatial and temporal isotopic patterns with variations expected across the arc region and to look at interrelations between the isotopes and trace elements. Other questions are the origin of La/Ta ratio variations and the history of fluid interaction as the slab shallows and the arc migrates. Oxygen isotopes and expanded trace element data sets can be used with fluid/melt distribution coefficients to constrain spatial and temporal variations. Expanded trace element data sets including Nb, Zr as well as Ta, Hf, and Ti will allow modeling of residual phases and source areas. Longer range questions are the importance of crustal thickening, arc migration and removal of young continental lithosphere in both the modern Central Andes and the Archean, and the extent to which continental lithospheric is recycled in this setting. As Andean analogies are used to understand processes in both ancient and young mountain belts, results of this project will set up Os-Re and Lu-Hf isotopic studies and define geophysical targets addressing fundamental questions in Earth's continental lithospheric evolution.