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Magnesian Andesites and the Subduction Component in a Strongly Calc-Alkaline Series at Piip Volcano, Far Western Aleutians |
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| Authors: | YOGODZINSKI G M; VOLYNETS O N; KOLOSKOV A V; SELIVERSTOV N I; MATVENKOV V V |
| Institution: | 1Department of Geological Sciences & INSTOC, Cornell University Ithaca, New York 14850 2The Institute of Volcanic Geology and Geochemistry Petropavlovsk, Kamchatka 683006, Russia 3The Institute of Volcanology Petropavlovsk, Kamchatka, 683006, Russia 4The Institute of Oceanology Moscow, 117218, Russia |
| Abstract: | Piip Volcano is a hydrothermally active seamount located inthe strike-slip regime immediately north of the far WesternAleutian Ridge. Fractionation of hydrous and oxidized magnesianandesites (MA) produced an igneous rock series at Piip Volcanowith a lower average FeO*/MgO (more strongly calc-alkaline)than any in the Central or Eastern Aleutian arc. Basaltic rocksin the Piip Volcano area are rare, and those that do occur havecharacteristics transitional toward MA (high SiO2 and Na2O;low CaO/Al2O3). The compositions of the MA and their predominanceas parental magmas throughout the Western Aleutians since MiddleMiocene time suggest that transpressional tectonics causes primitivebasaltic melts of the mantle wedge to pool immediately belowthe arc crust, where they interact with warm, ambient peridotiteto produce highly silica-oversaturated lavas of mantle origin.Proposed consequences of a long melting column in the mantlewedge (e.g., high percentage melting and tholeiitic volcanism)are not observed at Piip Volcano, despite that fact that itis built on very thin crust. Arc-related incompatible element signatures in volcanic rocksof the Piip Volcano area (e.g., high Ba/La, La/Sm, and Th/Ta)are broadly transitional between mid-ocean ridge basalt (MORB)and basalts of the Central and Eastern Aleutians. Interelementand isotopic ratios are, however, MORB-like (206Pb/204Pb<18.2, Nd>10, 87Sr/86Sr<0.7028). Mixtures of hypothetical slabmelts (Western Aleutian adakite) and depleted MORB mantle producean enriched peridotite source with incompatible element andisotopic characteristics of the Western Alcutian rocks. Componentsfrom recycled marine sediment (e.g., radiogenic Pb) are absent,possibly because they have been stripped out at shallow levelsby the long, oblique subduction path beneath the forearc. Theincompatible element signatures of the Western Aleutian rocks(including Ta depletion) are largely inherited from small percentagemelts of the subducting slab, which enrich the mantle wedgesource. Fluid-dominated processes of mass transport are notrequired to explain the arc-type incompatible element signatureof the Western Aleutian rocks. |
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