SPATIAL ISOTOPIC TRENDS IN MAGMATISM ASSOCIATED WITH CORDILLERAN OROGENS

We propose that an intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of magmatism. Magmatism is the most juvenile near the trench and becomes increasingly evolved towards the foreland. We present new and compiled isotopic data from the Central Andes, the U.S. Cordillera, and Tibet to demonstrate that this spatial trend is long-lived and persists throughout the life of a continental subduction margin. The consistency of the isotopic trend through time is surprising considering the plethora of orogenic processes that might be expected to alter the isotopic composition of magmatism. In addition to its longevity, the spatial isotopic trend encompasses a broad spectrum of geochemical compositions that represent diverse petrogenetic processes. We suggest that the isotopic trend is ultimately related to a mantle source and that juvenile asthenospheric mantle and evolved continental lithospheric mantle comprise the two end-members of the trend. Mantle lithosphere thins towards the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes, like delamination, operate to thin or remove the continental mantle lithosphere. In time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust. Our results have implications for interpreting temporal and spatial shifts in isotopic composition within Cordilleran orogens and suggest that the continental mantle lithosphere may be a significant source of magmatism in orogenic interiors.