INSIGHTS INTO THE VOLCANIC ARC MANTLE WEDGE FROM MAGNESIAN LAVAS FROMTHE KAMCHATKA ARC

Active volcanism in the Kamchatka are occurs where the Pacific Plate s ubducts beneath the Kamchatka peninsula south of its junction with the Aleutian are. Most volcanism occurs within the Central Kamchatka Depr ession (CKD), a large graben oriented parallel to the trench, and alon g the Eastern Volcanic Front (EVF), located south and east of the CKD and closer to the trench. Differentiation trends range from calc-alkal ine to tholeiitic. Fractionation of a mineral assemblage including oli vine, clinopyroxene, and orthopyroxene, produces the tholeiitic trend, whereas separation of amphibole and magnetite, along with possible cr ustal assimilation, produces the calc-alkaline trend. A suite of near- primitive high-Mg basalts provides geochemical records of mantle sourc es and processes unobscured by differentiation. Rare earth element (RE E) patterns range from slightly depleted ((Ce/Yb) n=0.8-1.5) to slight ly enriched ((Ce/Yb)n=1.5-3.5). Rocks with the depleted REE patterns o ccur at the volcanic front in regions where a volcano or volcanic chai n exists behind the volcanic front. Lavas with relatively enriched REE patterns occur behind the volcanic front and along portions of the vo lcanic front where behind-the-front volcanism is absent. Modeling of t race element abundances normalized to 10% MgO indicates that the rocks with the depleted REE patterns are derived from a more depleted sourc e, inferred to represent refractory source material remaining after a previous generation of melt extraction within the are. Mantle source m aterial apparently convects into the mantle wedge from the rear, produ cing relatively enriched magmas when it melts for the first time. Rela tively depleted magmas are produced if a second period of melting ensu es as the mantle reaches the volcanic front.