Spatial and temporal radiogenic isotopic trends of magmatism in Cordilleran orogens

Title of Publication: 
Spatial and temporal radiogenic isotopic trends of magmatism in Cordilleran orogens
Chapman, J.B., Ducea, M.N., Kapp, P., Gehrels, G.E., DeCelles, P.G.
Publication Info: 1342-937X/© 2017 International Association for Gondwana Research. Published by Elsevier B.V.

An intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of subduction-related magmatism. Magmatism is most isotopically juvenile near the trench and becomes increasingly evolved landward. A compilation of radiogenic isotopic data from the central Andes, U.S. Cordillera, and Tibet (themostwell-studied examples ofmodern and ancient Cordilleran systems) demonstrate such spatial trends are long-lived and persist throughout the life of these continental subduction margins. The consistency of the isotopic trend through time in magmatic products is surprising considering the plethora of orogenic processes that might be expected to alter them. In addition to longevity, spatial isotopic trends encompass a broad spectrum of geochemical compositions that represent diverse petrogenetic and geodynamic processes. The two end-members of the spatial isotopic trends are represented by melts sourcedwithin isotopically juvenile asthenospheric mantle and melts sourced from isotopically evolved continental lithospheric mantle and/or lower crust. Mantle lithosphere generally thins toward the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes such as delamination, subduction erosion, and subduction ablation, operate to thin or remove the continental mantle lithosphere.With time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust, giving rise to an isotopically homogenous deep lithosphere. The results of this analysis have significant 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.

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Fig. 2. Radiogenic isotopic data from Cordilleran orogens with magmatic rocks distinguished by age. A) In the U.S. Cordillera, magmatism b20 Ma is shifted to more juvenile isotopic compositions (blue arrows), potentially related to infiltration of asthenosphere into the continental lithospheric mantle during Miocene extension. Although these periods represent different tectonic scenarios (compression vs. extension) they display a similar spatial isotopic trend. B) In the Andes the spatial isotopic trend appears to have been relatively similar since the Mesozoic. Note that widespread magmatism during the Neogene is not isotopically constant when observed across the width of the orogen. C) In Tibet, magmatism b50 Ma is shifted to more evolved isotopic compositions (blue arrows), that may be related to crustal thickening and increased crustal assimilation following India-Asia collision. D) The exact isotopic value of the spatial isotopic trend in Tibet has varied during the last 100+ Ma, however, the overall shape and magnitude has remained relatively similar. Colored lines are interpreted spatial isotopic trends that correspond to the data points of the same color. εHf and εNd data are plotted together in A-D and are converted using the terrestrial array of Vervoort et al. (1999). Data sources are discussed in the text.