Primitive Magmas at Five Cascade Volcanic Fields 413
397 The Canadian M ine ralo g i st Vol. 35, pp. 397423 (1997) PRIMITIVEMAGMAS AT FIVECASCADE VOLCANIC FIELDS: MELTSFROM HOT, HETEROGENEOUS SUB.ARC MANTLE CHARLES R. BACONI, PEGGY E. BRUGGMAN, ROBERT L. CHRISTIANSEN, MICHAEL A. CLYNNE" JULIE M. DONNELLY-NOLAN ANDWES HILDRETH U.S.Geological Survey, 345 Mitdl.efield Road" Mmlo Parh Califurnia94025-3591,U.SA. ABSTRACT Major and trace elementconcenftations, including REE by isotopedilution, and Sr, Nd, Pb, and O isotoperatios have been determinedfor 38 mafic lavasfrom the Mount Adams,Crater Lake, Mount ShastaMedicine Lake, and Lassenvolcanic flelds, in the Cascadearc, northwestempart of the United States.Many of the sampleshave a high Mg# tl@Mg/(Mg + FeD > 601and Ni content(>140 ppm) suchthat we considerthem to be primitive. We rccognlzerhree end-member p:,jmillrve magma groups in the Cascades,characterized mainly by their trace-elementand alkali-metal abundances:(1) High-alumina olivine tholeiite (HAOT) hastrace element abundaaces similarto N-MORB, exceptfor slightly elevatedLllE, andhas Eu/Eu* > 1. (2) Arc basalt andbasaltic andesite have notably higher L/lE contents,generally have higher SiO2contents, are more oxidized andhave higher Cr for a given Ni abundancethan HAOT. Theselavas show relative depletioninl/F.i4 havelowerl/ftEE andhigherl,ftEEthan HAOT, andhave smallerEulEu* (0.94-1.06).(3) Alkali basaltfrom the Simcoevolcanic field eastof Mount Adamsreprcsents the third end-membr, which contributesan intraplate geochemicalsigpature to magna compositions.Notable geochemical featuresamong
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