Insights into the volcanic arc mantle wedge from magnesian lavas from the Kamchatka arc

Abstract

Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchatka peninsula south of its junction with the Aleutian arc. Most volcanism occurs within the Central Kamchatka Depression (CKD), a large graben oriented parallel to the trench, and along the Eastern Volcanic Front (EVF), located south and east of the CKD and closer to the trench. Differentiation trends range from calc‐alkaline to tholeiitic. Fractionation of a mineral assemblage, including olivine, clinopyroxene, and orthopyroxene, produces the tholeiitic trend, whereas separation of amphibole and magnetite, along with possible crustal assimilation, produces the calc‐alkaline trend. A suite of near‐primitive high‐Mg basalts provides geochemical records of mantle sources and processes unobscured by differentiation. Rare earth element (REE) patterns range from slightly depleted ((Ce/Yb)n = 0.8–1.5) to slightly enriched ((Ce/Yb)n = 1.5–3.5). Rocks with the depleted REE patterns occur at the volcanic front in regions where a volcano or volcanic chain exists behind the volcanic front. Lavas with relatively enriched REE patterns occur behind the volcanic front and along portions of the volcanic front where behind‐the‐front volcanism is absent. Modeling of trace element abundances normalized to 10% MgO indicates that the rocks with the depleted REE patterns are derived from a more depleted source, inferred to represent refractory source material remaining after a previous generation of melt extraction within the arc. Mantle source material apparently convects into the mantle wedge from the rear, producing relatively enriched magmas when it melts for the first time. Relatively depleted magmas are produced if a second period of melting ensues as the mantle reaches the volcanic front.