High Mg basalts and basaltic andesites from pyroxenite melts(?) in a continental arc: Tatara-San Pedro, Chilean Andes
Abstract
A primary challenge for understanding magma generation at continental arc volcanoes is to distinguish subducted slab and upper mantle contributions from crustal contamination. High-density sampling at the Quaternary frontal arc Tatara-San Pedro complex (TSPC) of the Southern Volcanic Zone (SVZ) in Chile has yielded one of the most complete eruptive chrono-stratigraphies of any arc volcano on Earth, providing a unique opportunity to elucidate magma source heterogeneity and the effects of slab, upper mantle, and crustal inputs. While most TSPC lavas are impacted by crustal contamination, these effects can be distinguished. Beyond them, we have recognized three distinct mantle-derived magma types: (1) the "background SVZ" mantle, which is the subduction-modified depleted mantle wedge, (2) a fluid-rich component derived from fluxing of the "background SVZ" mantle wedge by slab-derived hydrous fluids, and (3) an "enriched mantle" component, with much higher incompatible element and HFSE abundances, LREE/HREE, and Sr isotopes, but lower Nd and Hf isotopes, than the other two endmembers. Lavas containing the "fluid-rich" and "enriched" components are generated in separate regions of the subduction regime, as both lava types show mixing with the "background SVZ" but never with each other. While the loss of fluids from subducted ocean crust and melting of the subduction-modified mantle wedge reflect classic processes associated with convergent plate boundaries, the origin of the "enriched mantle" component is not part of the subduction paradigm. In the SVZ, the presence of the "enriched mantle" was observed in basaltic cones behind the arc front near Villarica by Hickey et al. (1989) and interpreted as either derived from continued input of subduction components into the mantle wedge behind the main frontal arc, or from melting of the subcontinental lithospheric mantle. At TSPC, the inferred parent magmas of basalts and basaltic andesites of the enriched component have elevated MgO, FeO/MnO, and Ni concentrations as well as low CaO contents at a given SiO2. These data are consistent with recent model calculations for partial melting of pyroxenite sources (e.g. Herzberg 2011) as well as the compositions of olivine phenocrysts in lavas interpreted as pyroxenite melts (e.g. Sobolev et al. 2005). The "enriched mantle" endmember also has chemical affinities with nearby voluminous behind the arc basalts in Argentina. We propose that the extensive melting in the back arc produced pyroxenite veins in the upper limb of the convecting mantle wedge that migrated toward the arc front and have melted to generate the "enriched mantle" endmember lavas observed at the TSPC.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V32C..08J
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 8413 VOLCANOLOGY / Subduction zone processes