Geochemical variations in the <5 Ma Wrangell Volcanic Field, Alaska: implications for the magmatic and tectonic development of a complex continental arc system
We report geochemical data for nearly 400 new volcanic samples from the <5 Ma Wrangell Volcanic Field (WVF) of southern Alaska that, in combination with existing data, provide the first comprehensive regional insights into the magmatic development of this continental arc system. Complex spatial-chemical relationships are documented and provide important perspectives on the interplay between magma generation and evolution processes and the tectonic elements of the arc. Three volcanic rock suites are identified: a dominant, arc-wide low TiO 2, calc-alkaline suite (trend 2a); a second low TiO 2, calc-alkaline suite (trend 2b) that is restricted to the northwestern- and southeastern-most volcanoes of the arc system and that is characterized by adakitic, amphibole dacite magmas; and a high TiO 2 transitional tholeiitic suite (trend 1) that provides evidence for localized intra-arc extension over at least the last 1 Ma. All three suites contain mafic parental compositions consistent with derivation from a MORB-like mantle wedge that has been heterogeneously enriched via the addition of subduction components. The tholeiitic parental magmas were produced by relatively low degrees of partial melting in response to the development of an intra-arc extensional regime. The calc-alkaline mafic magmas were generated by somewhat higher degrees of partial melting catalyzed by active fluid fluxing into the mantle wedge. The calc-alkaline adakitic dacites were produced under highly oxidizing conditions via partial melting of the downgoing slab induced by unique plate margin-like tectonic configurations. Explosive eruptions of these adakitic magmas produced the only known widespread tephra deposits from the WVF. Post magma generation open system processes including magma mixing, magma-crust interaction and magma-mantle interaction, along with fractional crystallization of anhydrous (trend 1) and hydrous (trend 2) mineral assemblages, are responsible for within-suite differentiation to more evolved compositions and for diversity within the adakitic suite. The relative degree to which these open system processes affected magma evolution exhibits a geographic control. This further highlights the importance of local and regional tectonic complexities on the production and evolution of magmas along the southern Alaskan continental margin over the past 5 million years.