mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and tempo... more mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and temporal association of these rocks with adakites suggests a petrogenetic link between them. Trace element characteristics of the NEB imply that amphibole and ilmenite might be present in their source. We suggest that these minerals could be added metasomatically to the mantle through hybridization by percolating slab melts, during which Nb and Ti are preferentially extracted from the adakitic melts. In an early stage (4-3 Ma) of the subduction of the young and hot Sulu Sea basin crust beneath the ^jimboanga peninsula, adakitic liquids formed at depths of 75-85 km. A few of them were emplaced at the surface but most were consumed through slab melt-mantle metasomatic reactions. Adakite production and emplacement continued later (<2 Ma), while the Nb-enriched mantle was brought by convection to depths that allowed its melting and the subsequent emplacement of NEB behind the adakitic front of the ^pmboanga arc PH 93-39 (HG) PH 93-38 (UG) P 90-35 (MG) P 90-36 (MG)
Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchat... more 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=l.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. 1Now at Paper number 95JB02404. 0148-0227/96/95JB-02404505.00 ratios and incompatible element abundances than those erupted behind the volcanic front [Dickinson, 1975; Whitford et al., 1979; Carr et al., 1990; Tatsumi et al., 1992]. This observation raises the question of whether the relatively depleted lavas at the volcanic front are due to greater degrees of melting of homogeneous mantle, melting of a residue remaining from previous melting events, or melting of completely unrelated source compositions. Experimental results indicate that behind-the-front magmas derive from deep levels [ Tatsumi et al., 1983], but the results say nothing about the degree of melting at various distances from the trench. Evaluations of global major element systematics suggest that degree of melting correlates with crustal thickness [Plank and Langmuir, 1988] but do not address variation within specific arcs. Answers to this type of question will shed light on such geodynamic issues as asthenospheric flow in the mantle wedge. The Kamchatka arc provides important constraints on mantle-wedge processes because several of its volcanoes have erupted a variety of lavas, some of which are relatively 697 698 HOCHSTAEDTER ET AL.: MAGNESIAN LAVAS FROM THE KAMCHATKA ARC
mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and tempo... more mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and temporal association of these rocks with adakites suggests a petrogenetic link between them. Trace element characteristics of the NEB imply that amphibole and ilmenite might be present in their source. We suggest that these minerals could be added metasomatically to the mantle through hybridization by percolating slab melts, during which Nb and Ti are preferentially extracted from the adakitic melts. In an early stage (4-3 Ma) of the subduction of the young and hot Sulu Sea basin crust beneath the ^jimboanga peninsula, adakitic liquids formed at depths of 75-85 km. A few of them were emplaced at the surface but most were consumed through slab melt-mantle metasomatic reactions. Adakite production and emplacement continued later (<2 Ma), while the Nb-enriched mantle was brought by convection to depths that allowed its melting and the subsequent emplacement of NEB behind the adakitic front of the ^pmboanga arc PH 93-39 (HG) PH 93-38 (UG) P 90-35 (MG) P 90-36 (MG)
Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchat... more 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=l.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. 1Now at Paper number 95JB02404. 0148-0227/96/95JB-02404505.00 ratios and incompatible element abundances than those erupted behind the volcanic front [Dickinson, 1975; Whitford et al., 1979; Carr et al., 1990; Tatsumi et al., 1992]. This observation raises the question of whether the relatively depleted lavas at the volcanic front are due to greater degrees of melting of homogeneous mantle, melting of a residue remaining from previous melting events, or melting of completely unrelated source compositions. Experimental results indicate that behind-the-front magmas derive from deep levels [ Tatsumi et al., 1983], but the results say nothing about the degree of melting at various distances from the trench. Evaluations of global major element systematics suggest that degree of melting correlates with crustal thickness [Plank and Langmuir, 1988] but do not address variation within specific arcs. Answers to this type of question will shed light on such geodynamic issues as asthenospheric flow in the mantle wedge. The Kamchatka arc provides important constraints on mantle-wedge processes because several of its volcanoes have erupted a variety of lavas, some of which are relatively 697 698 HOCHSTAEDTER ET AL.: MAGNESIAN LAVAS FROM THE KAMCHATKA ARC
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