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Compositional Diversity of Late Cenozoic Basalts in a Transect Across the Southern Washington Cascades: Implications for Subduction Zone Magmatism

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Compositional Diversity of Late Cenozoic Basalts in a Transect Across the Southern Washington Cascades: Implications for Subduction Zone Magmatism Trinity University Digital Commons @ Trinity Geosciences Faculty Research Geosciences Department 11-10-1990 Compositional Diversity of Late Cenozoic Basalts in a Transect Across the Southern Washington Cascades: Implications for Subduction Zone Magmatism W. P. Leeman Diane R. Smith Trinity University, [email protected] W. Hildreth Z. Palacz N. Rogers Follow this and additional works at: https://digitalcommons.trinity.edu/geo_faculty Part of the Earth Sciences Commons Repository Citation Leeman, W. P., Smith, D. R., Hildreth, W., Palacz, Z., & Rogers, N. (1990). Compositional diversity of late Cenozoic basalts in a transect across the southern Washington Cascades: Implications for subduction zone magmatism. Journal of Geophysical Research: Solid Earth, 95(B12), 19561-19582. http://doi.org/ 10.1029/JB095iB12p19561 This Article is brought to you for free and open access by the Geosciences Department at Digital Commons @ Trinity. It has been accepted for inclusion in Geosciences Faculty Research by an authorized administrator of Digital Commons @ Trinity. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH,VOL. 95, NO. BI2, PAGES19,561-19,582, NOVEMBER 10, 1990 Compositional Diversity of Late Cenozoic Basalts in a Transect Across the SouthernWashington Cascades' Implicationsfor SubductionZone Magmatism WILLIAM P. LEEMAN,• DIANE R. SMITH,2 WESHILDRETH, 3 ZEN PALACZ,4.5 AND NICK ROGERS4 Major volcanoesof the SouthernWashington Cascades (SWC) includethe large Quaternary stratovolcanoesof Mount St. Helens(MSH) andMount Adams(MA} and the IndianHeaven (IH) and SimcoeMountain (SiM) volcanicfields. There are significant differences among these volcanic centers in termsof theircomposition and evolutionary history. The stratovolcanoesconsist largely of andesitic to daciticlavas and pyroelastics with minorbasalt flows. IH consistsdominantly of basalticwith minor andesitclavas, all eruptedfrom monogeneticrift andcinder cone vents. SIM hasa poorlyexposed andesitcto rhyolitecore but mainly consists of basalticlavas erupted from numerous widely dispersed vents; it has the morphologyof a shield volcano. Distribution of mafic lavas acrossthe SWC is related to north-northwesttrending faults and fissure zones that indicate a significantcomponent of east-west extensionwithin the area.There is overlapin eruptivehistory for theareas studied, but it appearsthat peakactivity was progressivelyolder (MSH (<40 Ka), IH (mostly<0.5 Ma), MA (<0.5 Ma), SIM (I.-4 Ma)) and morealkalic towardthe east. A varietyof compositionallydistinct mafic magma types has been identifiedin the SWC, includinglow large ion lithophileelement (LILE) tholeiitic basalts, moderateLILE calcalkalicbasalts, basalts transitional between these two, LILE-enriched mildly alkalic basalts,and basalticandesites. Compositional diversity among basaltic lavas, both within individual centersas well as acrossthe arc, is an importantcharacteristic of the SWC traverse. The fact that the basaltic magmaseither show no correlation between isotopicand trace element componentsor show trends quite distinctfrom those of the associatedevolved lavas, suggeststhat their compositionalvariability is attributableto subcrustalprocesses. Both the primitivenature of the erupted basaltsand the fact that they are relatively commonin the SWC sectoralso imply that such magmashad little residencetime in the crust. A majority of the SWC basalticsamples studies are indistinguishablefrom oceanicisland basalts(OIB) in terms of trace elementand isotopiccomposi- tions, and more importantly, most do not display the typical high field strengthelement (HFSE) depletion seenin subduction-relatedmagmas in volcanicarcs elsewhere.LILE enrichmentand HRSE depletioncharacteristics of most arc magmasare generallyattributed to the role of fluids releasedby dehydrationof subductedoceanic lithosphere and to the effectsof sedimentsubduction. Because most SWC basalts lack these compositionalfeatures, we concludethat subductedfluids and sedimentsdo not play an essentialrole in producingthese magmas.Rather, we infer that they formed by variable degree melting of a mixed mantle source consistingmainly of heterogeneouslydistributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrencesof HFSE-depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatizedsubarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequenceof (1) tectonic mixing associatedwith accretionof oceanicand islandarc terranesalong the Pacific margin of North America prior to Neogenetime, and possibly(2) a seawardjump in the locus of subduction at about 40 Ma. The Cascadesarc is unusual in that the subductingoceanic plate is very youngand hot. We suggestthat slabdehydration outboard of the volcanicfront resultedin a diminishedrole of aqueousfluids in generatingor subsequentlymodifying SWC magmascompared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generationis presumablytriggered by other processesthat increasethe temperatureof the mantle wedge (e.g., convective mantle flow, shear heating, etc.). INTRODUCTION Range has been consideredto be a more or less typical subduction-relatedarc [e.g., McBirney, 1978;McBirney and The CascadeRange is a continentalvolcanic arc situated White, 1982], although petrogenetic studies have largely alongthe PacificNorthwest margin of North America.It has focusedon the more topographicallyimposing young strato- beena site of vigorousmagmatic activity in responseto volcanoesof the High Cascades. We have undertaken pet- obliquesubduction of the Farallon and Juande Fuca plates rologicstudies of major central volcanoesand nearbybasal- duringapproximately the last 38 m.y. [cf. Lux, 1982;Vet- tic lava fields within an east-west transect across the planckand Duncan, 1987].In manyrespects, the Cascade southern Washington Cascades to document the spatial distributionof magmatypes in this portion of the arc. This lKeith-Wiess Geological Laboratories, Rice University, Hous- regionis an area of transition[Weave• and Malone, 19871 ton, Texas. "GeologyDepartment, Trinity University, San Antonio, Texas. which separatesthe isolatedcentral volcanoesto the north '*U.S.Geological Survey, Menlo Park, California. (Mount Rainier, Mount Baker, and Glacier Peak) from the 4Departmentof Earth Sciences, Open University, Milton Key- broadvolcanic plateau of the OregonCascades to the south nes, England. which has a widespreadand nearly continuousvolcanic SNowat VGIsotopes Limited, Winsford, Cheshire, England. cover from both large stratovolcanoes(e.g., Mount Hood, Copyright1990 by theAmerican Geophysical Union. Mount Jefferson,Mount Washington,and the Three Sisters) Pa•r Number89JB03490. and numerousmonogenetic vents [cf. Luedke and Smith, 0148-0227/90189JB-03490505.00 !982]. Although magmaticactivity in the vicinity of the 19,561 19,562 LEEMAN ET AL.: CHEMICAL DIVERSITY OF CASCADES ARC BASALTS SouthernWashington Cascades (SWC) transectdates back volcanicand sedimentary rocks. Each of thesevolcanoes to at least 36 Ma [cf. Evarts et al., 1987; Korosec, 1987a, b], comprisesa variety of magmatypes ranging from basalt to this paper concentrateson magmaticactivity within this part rhyodacite[Hoblitt etal., 1980; Hildreth and Fierstein, 1985• of the Cascadesduring the past few million years (mainly Ortet al., 1983].Distributed among them are the Indian late Plioceneto Recent) and principallyon petrogenesisof Heaven(iH) andnumerous smaller lava fields, small shiekt the basaltic lavas and implications for subcrustalmagmatic volcanoes,and monogenetic vents comprising mainly ba•. processes related to Cascades subduction. ticlavas [e.g., Hammond and Korosec, 1983]. Many of the lattervents are distributed in roughlyNNE to NNWtrend. TECTONIC SETTING OF THE SWC ingalignments orfissure systems that presumably areper. pendiculartothe direction of leastprinciple horizontal stress The Cascadessubduction zone is somewhatatypical com- [e.g.,Nakamura, 1977]. Loci of the largestratovolcanoes pared to other convergent margins [Duncan and Kultn, straddlesuch structures, and most notably Mount St. Helens 1989]. There are no deep (> 100 km) earthquakesassociated [C.S. Weaveret al., 1987]and Mount Hood [Williams etal., with the present Juan de Fuca subduction [Weaver and 1982]appear to be situatedabove zones of localcrustal Baker, 1988], althougha subductingslab has been imagedto extensionalong active NW trendingstrike-slip zones. greater depths by seismic tomography [Rasmussen and MountSt. Helens has been intermittently active during the Humphreys, 1988]. Because the Juan de Fuca spreading last 36,000years [Mullineaux, 1986], during which time it ridge is exceptionally close to the subductionzone, the producedpredominantly dacitic magmas[Smith and Lee. subductingplate is relatively thin and warm; this factor may man, 1987].in the last •-2000years, its eruptiveproducts explain the paucity of deep earthquakesand may influence havebecome more diverse, ranging from basalt to rhyo- the dip angle of the subductingplate as well as the rate of dacite.However, true basaltic lavas were produced only subduction.There is no topographicallyexpressed trench duringthe CastleCreek eruptive period (2200-1700 yea.rs along the convergent margin, possibly because the trench B.P. [Hoblitt et al., 1980]).Somewhat older (about 690,• has been filled with terrigenoussediments owing to high to 8,000years B.P. [Hammondand Korosec,1983])
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