Deciphering crust, mantle, and slab controls on arc magma compositions: A case study from Central Chile, SVZ (33–46˚S)
Tuesday, 15 December 2020: 15:00
Penny Wieser, University of Cambridge, Earth Sciences, Cambridge, United Kingdom, Stephen Turner, University of Massachusetts, Amherst, United States, Tamsin Mather, Univerity of Oxford, Department of Earth Sciences, Oxford, United Kingdom, David M Pyle, University of Oxford, Department of Earth Sciences, Oxford, United Kingdom, Ivan Savov, University of Leeds, Leeds, United Kingdom and Gabriel Orozco, SERNAGEOMIN National Geology and Mining Service, Santiago, Chile
Abstract:
Volcanic products from thick-crusted continental arcs have elevated incompatible element concentrations and enriched radiogenic isotope ratios relative to those from thinner-crusted arcs. This enrichment may derive from the interaction between primary magmas and the continental crust, or from processes operating in the sub-arc mantle (e.g., variable melt extents, variable material transfer from the slab, or heterogeneity in the ambient mantle). In the Andean Southern Volcanic Zone (SVZ; 33–46˚S) in Central Chile, crustal thickness and geochemical enrichment increase northwards, providing a natural laboratory to investigate the relationship between these parameters. While previous arc-scale comparisons have been hindered by the scarcity of mafic samples from the northern SVZ, our analyses of unusually primitive northern lavas (Mg#~70) allows deconvolution of crustal and mantle controls on arc-scale trends.
The enriched compositions of northern lavas cannot result from crustal processing, because the trace element abundances and 87Sr/86Sr and 143Nd/144Nd ratios required of the hypothetical assimilant violate empirical and theoretical constraints on crustal lithologies (both regionally and globally). Models incorporating subduction erosion of the continental crust and/or varying slab additions also fail. Rear-arc lavas, which exhibit the enriched northern signal, implicate the presence of enriched ambient mantle with a composition similar to EM1 ocean-island basalts. Models incorporating along-strike variation in an EM1 mantle component, combined with a decline in melt extent and ubiquitous slab melting, reproduce regional geochemical trends. One possible origin for this EM1-like component is metasomatized subcontinental lithospheric mantle. Our recognition that mantle enrichment prior to the addition of slab inputs is key to the production of enriched continental compositions in the SVZ has important implications for our understanding of continental crust formation.
