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Evolution of Mauna Kea Volcano, Hawaii' Petrologicand Geochemical Constraints on Postshieldvolcanism

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Evolution of Mauna Kea Volcano, Hawaii' Petrologicand Geochemical Constraints on Postshieldvolcanism JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. B2, PAGES 1271-1300, FEBRUARY 10, 1990 Evolution of Mauna Kea Volcano, Hawaii' Petrologicand Geochemical Constraints on PostshieldVolcanism F. A.FREY, 1W. S.WISE, 2 M. O.GARCIA, 3 H. WEST, 3 S.-T. KWON, 2 AND A. KENNEDY,1 All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages:basaltic (-240-70 ka) and hawaiitic (-66-4 ka). The basaltic substage(Hamakua Voltanits) contains a diverse array of lava types including pierites, ankaramites,alkalic and tholeiitic basalt, and high Fe-Ti basalt. In contrast, the hawaiitic substage(Laupahoehoe Voltanits) contains only evolved alkalic lavas, hawaiite, and mugearite;basalts are absent. Sr and Nd isotopicratios for lavas from the two substagesare similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substagecan not be related to the older basalts by shallow pressurefractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene-dominatedassemblage. We conclude that the petrogeneticprocesses forming the postshieldlavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulatethe following sequenceof events for postshieldvolcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased,major changesin volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fracturesenabling magma ascentto the magma chamberclosed. (2) Therefore subsequentbasaltic magma ascendingfrom the mantle stagnatedwithin the lower crust, or perhapsat the crust-mantleboundary. Eruptionsof basalticmagma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low-density,residual hawaiitic melts. Minor assimilationof MORB-relatedwall rocks, reflected by a trendtoward lower 206pb/204/pb in evolved postshieldlavas, may have occurredat this time. A compositionalgap developedbecause magma ascent was not possible until a low-density hawaiitic melt could escape from a largely crystallinemush. Eruption of this melt created aphyric hawaiite and mugearitelavas which incorporatedcumulate gabbro, whetlite, and dunitc xenolithsduring ascent. 1. INTRODUCTION [Macdonaldet al., 1983,pp. 145-154;Clague and Dalrymple, 1987]. The most voluminous is the shield stage (the The HawaiianRidge is the type exampleof a hot spot terminologyused here is that of Clague and Dalrymple trace [Wilson,1963; Christofferson,1968; Morgan, 1972]. [1987]), which is dominatedby tholeiitic basalt and is Geochemicalstudies of lavaserupted along this trace provide characterizedby a high magma supply rate (Figure 1). importantinformation on the mantlesources of hot spot Toward the end of this stage,eruption rates diminish, rift volcanism[e.g., Tatsumoto,1978; Feigenson,1986; Stille et zones become less active, and eruptionsare more widely al., 1986; Frey and Roden, 1987]. In order to evaluate distributed. The subsequentpostshield stage forms a geochemicaldifferences between Hawaiian volcanoes,it is relatively thin veneerof lavas (<10 m to 1 km), including necessaryto understandhow individualHawaiian volcanoes tholeiitic, transitional, and alkalic basalts; hawaiite; evolve and the processescontrolling their evolution. The mugearite; and rarely, trachyte. The final stage in the growth stages of a Hawaiian volcano are well defined eruptivehistory of someHawaiian volcanoes (the rejuvenated or posterosionalstage) is the eruptionof alkalicbasalts after a periodof volcanicquiescence of up to 2 m.y. [Clagueand Dalrymple, 1987]. 1DepartmentofEarth, Atmospheric, andPlanetary Sciences, The transitionbetween the shieldstage and the postshield MassachusettsInstitute of Technology, stagevaries from volcanoto volcano. Recentg•chemical Cambridge. studiesof postshieldlavas using stratigraphic sections from 2DepartmentofGeological Sciences, Haleakala [Chen and Frey, 1983, 1985; Chen et al., 1985; Universityof California, Santa Barbara. West and Leemah, 1987a,b; Kurz et al., 1987] and Kohala 3HawaiiInstitute ofGeophysics, volcanoes[Feigenson et al., 1983; Hofmann et al., 1987; University of Hawaii Lanphereand Frey, 1987; Spenglerand Garcia, 1988] have Honolulu. shownthat the shieldand postshieldlavas are derivedfrom distinctparental magmas formed by mixing of components Copyright1990 by the American GeophysicalUnion. fromcornpositionally different manfie sources. At MaunaKea thepostshield stage can be dividedinto two Papernumber 89JB- 1326. substages:the oldestand most voluminousconsists largely 0148-0227/90/89JB-01326505.00 of basalticlavas (basaltic substage) and the youngerconsists 1271 1272 FREY ET AL.: POSTSHIELDVOLCANISM ON MAUNA KEA VOLCANO,HAWAII only of hawaiiteand mugearite(hawaiitic substage, Figure substage[West et al., 1988] andbasalt substage (F.A. Frey et 1). Withinthe basaltic substage the oldest subaerial lavas are al., manuscriptin preparation,1989). This paperfocuses on intercalatedtholeiitic and alkalicbasalt. This substageis the transitionbetween the basalticand hawaiitic substages. characterizedby a largedecrease in eruptionrates. Basedon MaunaKea volcano,the highestof the Hawaiianvolcanoes datafor the currentlyactive shields, Kilauea and MaunaLoa, andthe youngestvolcano with a fully developedpostshield eruption rates during shield formation are about 30 x stage,is ideal for studyingthis transition.Postshield lavas 106 m3/yrwith magma production rates of about100 x are well exposedon the dry west side of the volcanoand 106m3/yr [Dzurisin etal., 1984; Lockwood and Lipman, excellent cross sections of theselavas axe available in gulches 1987]. In contrast,during the basaltic substage of postshield on the wet east flank. We conclude that the marked decrease volcanismatMauna Kea, about 850 km 3 oflava erupted over in eruptionrate (Figure 1), presumedto reflect drift of the -170x 103years with eruption rates decreasing from10 to 2 volancofrom the hot spot,caused shallow magma chambers x 106m3/yr (Figure 1). A similarlylow eruption rate, 2 x to solidify; subsequentascending alkalic basalt magmas stagnatedat depth,perhaps at the crust-mantleboundary (15- 106 m3/yr,has been estimated for recenteruptions at 20 km). Crystallizationand segregationof densecrystalline Hualalaivolcano [Moore et al., 1987],which appears to be in phasesin thesedeeper magma chambers formed a low-density the basaltic substageof postshieldgrowth. During the hawaiitemagma with sufficientbuoyancy to eruptand form hawaiite substage at Mauna Kea, eruption rates have thehawaiite substage. continuedto decrease, averaging only -0.4 x 106m3/yr (Figure1 andWest et al. [1988]). 2. STRATIGRAPHY AND AGE CONTROL Previouspetrologic and geochemical studies of MaunaKea werereconnaissance in nature [Macdonald and Katsura, 1964; Macdonald,1968; Budahn and Schmitt, 1985] or focussedon Stearnsand Macdonald[1946] dividedthe volcanicrocks of onlyone substage (e.g., the hawaiite substage [West et al., Mauna Kea into two units, the Hamakuaand Laupahoehoe 1988])or onegeochemical parameter (e.g., Pb isotopesVolcanic Series. These have been renamed Hamakua [Tatsumoto,1978]). Ourobjective is to userelative age, Volcanics and LaupahoehoeVolcanics [Langenheimand eruptionrates and geochemical data for 149Mauna Kea lavas Clague, 1987]. Althoughin the type sectionan erosional tounderstand the origin and evolution of thepostshield stage unconformityseparates the two units,Stearns and Macdonald at MaunaKea volcano. In otherpapers we discussand [1946]found no widespreaduncomformity between the units. interpret compositionalvariations within the hawaiite Nevertheless,lavas from the two groups are distinctive. 1 O0 - ß Postshieldstage basaltic - hawaiitic • Pre- Shieldstage substage I substage _c 50 shield 32000km3 850 km3 ', 25 km3 stage • 500km3 ._o Hamakua Laupahoehoe > 10 'Volcanics'•, •:..•.• o0 • • i • I vo,n,sI 700 500 300 0 Age,xl 0 3 ¾rs. Fig.1. Growthrate as a functionof agefor Mauna Kea [Swanson,1972]).In order to allow for a gradualincrease at volcano,illustrating the development of the three volcanic the beginning of the shield stage and a decreasetoward the stages(modified from Wise [1982]). Rectangles bounded by end,the duration isestimated tobe -400,000 years. The end lightlines are proportional to estimated rock volumes for of theshield stage is assumedtohave occurred when lava eachstage. Heavy line shows variation in supplyrate with flowswere no longer produced ata ratesufficient tomaintain time.The diagram was constructed byestimating thevolume the sharp break in slopeat sealevel, i.e., when subsidence of magma(reduced todense rock equivalent) added to the ratesexceeded growth rates (Moore and Fiske, [1969], and edificeduring each stage. The preshield stage isassumed to Figure3). Forthe preshield and shield stages the volcano havea volumesimilar to Loihi. The shield volume was growthrate equals the magma supply rate. However, the estimatedfrom its present shape adjusted for overlap with volumeof thepostshield stage is based only on extruded neighboringvolcanoes andsubsidence [Moore, 1987]. The lavas;atthis stage a significant fraction ofthe magma supply estimatedtime span for the development of the shield stage solidifiedto formdeep crustal cumulates. Details of the assumesa maximum magma supply rate equal
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