Ultrapotassic Magma from the Deep Mantle, Leucite Hills Lamproite, Wyoming USA

地学雑誌 Journal of Geography（Chigaku Zasshi） 124（3）515⊖523 2015 doi:10.5026/jgeography.124.515

The 100s: Signiﬁcant Exposures of the World（ No. 11）

Akira ISHIKAWA＊ and Kenji KAWAI＊

［Received 24 March, 2015; Accepted 30 April, 2015］

Abstract The Leucite Hills province is an isolated volcanic ﬁeld of 14 lava-capped mesas and buttes, creating an impressive landscape in southwestern Wyoming. The 3.0-0.89 Ma lavas form exotic rocks

described as lamproites, which are regarded as the most potassic（＜ 13 wt％ K2O） and incompatible trace element-rich primitive magmas generated in the deep Earth. The radiogenic isotope signatures of Leucite Hills lamproites are well beyond the range of any known oceanic basalts, suggesting that the magma originated from ancient, volatile-rich anomalous mantle, recently reactivated by the convective removal of metasomatised lithosphere present at the margin of the Archean Wyoming Craton.

Key words： Leucite Hills, Wyoming Craton, lamproite, isotope geochemistry

I．Introduction

Lamproite, a highly alkaline primitive volcanic rock with distinctive mineralogy and extreme compositions, has attracted significant attention since the host rocks of diamonds in West Kimber- ley, Australia（ Argyle and Ellandale, famous for producing valuable pink and yellow diamonds）, GMT and Arkansas, USA（ Prairie Creek） were recogn ized as olivine-rich lamproite（ Scott-Smith Fig. 1 Locality index of the Leucite Hills Lamproite Province located in southwestern Wyoming, USA. and Skinner, 1984）. Until then, kimberlite had long been regarded as the only primary source of diamonds, and lamproites were considered to and lamprophyre. The growing recognition of bear little scientific and economic importance. the significance of these low-volume, volatile- The Leucite Hills, Wyoming USA（ Fig. 1） is now rich, mantle-derived magmas as probing deepest known as one of three major lamproite localities into the Earth, particularly for assessing the （besides West Kimberley, Australia, and Murcia- geochemical cycles of volatile elements such as Almeria, South Spain）, although the occurrence carbon, water, and halogens, has now resulted of diamonds remains unconfirmed. They were in an increase in the number of science-oriented not referred to as lamproites until Scott-Smith research projects. and Skinner（ 1984） and Mitchell（ 1985） revised Lamproites are unique. They are currently and organized the terminology of small-volume defined using a combination of geochemical and alkaline rocks such as kimberlite, lamproite, mineralogical criteria because of their extremely

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＊ Department of Earth Science and Astronomy, The University of Tokyo, Tokyo, 153-8902, Japan wide range in modal mineralogy. Lamproites rep- 3B）. They are sporadically preserved in an area resent a clan of rocks characterized by being ultra- of 50 km（ Fig. 4; from the easternmost Black Rock

potassic（ MgO ＞ 3％ and K2O ＞ 3％, and having to the westernmost Pilot Butte） × 40 km（ from

K2O/Na2O ratios ＞2）, and have distinctively lower the northernmost Steamboat mountain to the

Al2O3 and CaO concentrations compared to other southernmost Zirkel Mesa）, and are situated alkaline rocks such as kimberlite and lampro- on an almost flat-lying sedimentary sequence phyre（ Foley et al., 1987）. Lamproites are also of late Tertiary to early Cretaceous shales and recognized as the most incompatible element- sandstones（ Bridger, Green River, Wasatch, and enriched silicate magma to have erupted on the Laramie Formations in descending order）. Earth. For example, some Spanish lamproites Although the present-day geography of the contain large ion lithophile elements（ LILE） entire volcanic field is largely influenced by up to 10000 × primitive mantle values. These continuous erosion, the total volume of erupted compositional features are reflected in their magma has been estimated to be less than～0.66 petrography and mineralogy with the presence of km3, after corrections for the vesicularity of lava rare minerals, such as phlogopite（ mica group）, K- flows（ 15-40％） and maximum extent of erosion richterite（ amphibole group）, sanidine（ feldspar （20-50％） applied to the present-day volume of～ 3 group）, and leucite（ KAlSi2O6）, and are now used 0.58 km（ Lange et al., 2000）. This suggests that for the nomenclature of lamproite（ Mitchell and the individual localities represent isolated centers Bergman, 1991）. Before the term lamproite was of relatively low volcanic activity, contrary to the revised, the old type locality nomenclature was idea that a continuous lava shield once covered applied for classifying members of the lamproite the entire field. Based on the 40Ar/ 39Ar age varia- clan. Wyomingite（ diopside-leucite-phlogopite tion, volcanic activity in the Leucite Hills spanned lamproite）, orendite（ diopside-sanidine-phlogop- 3.0 to 0.89 Ma, and approximately 84％ of the ite lamproite）, and madupite（ diopside madupitic lavas erupted between 0.94 and 0.89 Ma with an lamproite: the term madupitic means that a lam- average eruption rate of～5 m3/km2/year（ Lange proite contains poikilitic groundmass phlogopite） et al., 2000）. This extremely low eruption rate were first recognized in the Leucite Hills by Closs （for example, ～4 orders of magnitude lower than （1897）, significantly after the initial discovery of that of slow-spreading mid-ocean ridge）, combined this volcanic field by S.F. Emmons in 1871. The with the facts that（ 1） contemporaneous volca- name Leucite Hills was given by Emmons（ 1877） nism leading to eruption of normal lavas occurs in connection with the first occurrence of leucite- more than 300 km from this region（ e.g. rhyolite bearing rocks on the North American continent. eruption at Yellowstone）, and（ 2） mantle-derived xenoliths and xenocrysts occur in some of Leucite II．Geology and age of the Leucite Hills Hills lavas（ Fig. 5; Black Rock and Hatcher Mesa）, The Leucite Hills lamproite province occurs precludes the long-term storage of magma at the at the southern edge of the Archean Wyoming crustal level, and suggests that the timing of the Craton（ Fig. 2A）, which has been largely stable eruption is closely related to the timing of melt since～2.6-2.7 Ga（ e.g. Mueller and Frost, 2006）. generation in the underlying mantle. The southern limit of the Wyoming Craton lies III．Significance of lamproite magmatism ～100 km south of the province and is defined by the Cheyenne Belt, which separates Archean One of the main questions regarding lamproite Wyoming Craton from the surrounding Pro- magmatism in the Leucite Hills is the source of terozoic rocks of the Colorado Province（ Fig. partial melt having strange compositions. It has 2B; Central Plains Orogen）. The Leucite Hills been considered that lamproites and kimberlites lamproites consist of 14 lava-capped mesas and are among the most extreme products of mantle buttes（ Fig. 3A）, along with nine additional enrichment processes（ Hawkesworth et al., 1985）, exposures of volcanic necks, dykes, and plugs（ Fig. which can be acquired in response to:（ 1） the

— —516 Fig. 2 （A） The North American continent showing the location of the Archean Wyoming Craton and the Leucite Hills（ from Hausel, 1998）.（ B） Generalized tectonic map of the Archean Wyoming Craton showing the location of the Leucite Hills Lamproite Province（ from Carlson et al., 2004）.（ C） Seismic velocity structure beneath the western United States （at 200 km depth） based on the joint inversion of body- and surface-waves（ Obrebski et al., 2011）. Dashed lines show the position of vertical cross-sections（ D） -（ E）. Pink line and star represent the locations of Cheyenne belt（ CB） and Leucite Hills, respectively. Green line encompasses the Colorado Plateau（ CP）. introduction of ﬂuid/melt into fairly normal mantle, of lamproites, which has been invoked to be a or（ 2） the remobilization of ancient, volatile- lherzolitic-harzburgitic lithosphere veined with rich anomalous mantle. Based on radio genic hydrous minerals（ e.g. phlogopite and amphibole）. isotope systematics such as Sr-Nd-Pb-Hf, the Thus, lamproite magmatism is thought to be latter would seem to be a more likely source unrelated to active subduction zones, but rather

— —517 Fig. 3 （ A） North Table Mountain forming a volcanic mesa consisting of a single lava ﬂow, and South Table Mountain behind（. B） Boar＇s Tusk forming a prominent volcanic neck composed of diopside-leucite-phlogopite lamproite（ wyomingite） in the northwestern area.

— —518 signatures obviously require sources that have ex- perienced long-term enrichment of Rb and LREE and depletion of Lu over Hf. Extreme 87Sr/ 86Sr variations seen in samples from West Australia, Spain, and other Mediterranean region are likely to have originated from isotopic ingrowth in a mica-rich vein developed within the lithosphere or mantle-wedge contaminated by sediment-derived fluid/melt（ Prelevic et al., 2008）, whereas the smaller 87Sr/ 86Sr ratios of the Leucite Hills may reflect derivation from ancient metasomes domi- nated more by amphibole than mica（ Nowell et al., 2008）. The metasomatic event that generated the source of Leucite Hills lamproites has been envisioned to take place during the stabilization of the Wyoming Craton 3.2 to 2.5 Ga or subsequent Fig. 4 Simplified geological map of the Leucite Hills Lam- subduction along the margin of the Wyoming proite Province（ after Kemp and Knight, 1903）. Craton, but not later than 1.2 Ga（ Vollmer et al., 1984）. The other question is what triggered the recent is related to paleo-Benioff zones stranded in the remobilization of the ancient veined mantle. sub-continental lithosphere（ e.g. Mitchell and Global lamproites typically occur along craton Bergman, 1991）. margins or in accreted mobile belts with a thick Figure 6A and B shows Sr-Nd and Hf-Nd isotope crust（＞40-50 km） and lithosphere（＞150-250 systematics of global lamproite rocks, respectively, km）. This occurrence contrasts with that of both of which clearly demonstrate that the isotopic kimberlites, which are commonly restricted to values of lamproites lie well beyond the range within craton interiors, suggesting that lamproites of any known oceanic basalts. Because isotopic and kimberlites magmatism are distinct in terms variations in oceanic basalts probably represent of not only source material inferred from isotope the extents to which heterogeneity persisted in data（ Fig. 6）, but also tectonic setting and melting the convective mantle, continental lithosphere conditions. These distinctions are an important isolated from the convective mantle is an excellent key to a complete understanding of their origins candidate as the source of lamproite. Two trends and geodynamic contexts. In the case of the can be identified in Sr-Nd diagrams: one shows Leucite Hills, seismic studies reveal an abrupt extreme 87Sr/ 86Sr variations up to 0.722, which is change in the lithospheric structure across defined by samples from West Australia, Spain, the border of the Wyoming Craton（ Fig. 2C-E, and other Mediterranean regions; the other forms Obrebski et al., 2011）. A high-velocity keel is a steeper trend having larger variations of initial identified beneath the Archean Wyoming province, εNd（－10 to －25） with moderate 87Sr/ 86Sr ratios whereas a low-velocity zone is widely developed ranging from 0.705 to 0.707, which are defined by beneath Yellowstone, ～300 km northwest from samples from Leucite Hills and other lamproites the Leucite Hills（ Fig. 2B）, implying potential in Wyoming Craton（ Montana）. Whereas in the effects of the Yellowstone hotspot（ e.g. Mitchell Hf-Nd diagram, there is a broad correlation along and Bergman, 1991）. Alternatively, Lange et al., the MORB-OIB array, which extends down to （2000） emphasized the close proximity of the extremely low Hf isotope ratios（ initial εHf as low Leucite Hills to the uplifted Colorado Plateau as －35）, although the majority of samples tend （5.3-2.6 Ma）, now present further south over the to deviate below the array. These extreme isotope Cheyenne belt（ Fig. 2C）. Levander et al.（ 2011）

— —519 Fig. 5 （A） Black Rock mesa located in the easternmost area.（ B） Hatcher mesa located in the central area.（ C） Enlarged view of Black Rock mesa comprising a basal pyroclastic olivine bearing diopside–sanidine–phlogopite lamproite（ orendite） overlain with a lava flow.（ D） A lava flow of Hatcher Mesa containing a number of crustal xenoliths.（ E） A small mantle xenolith in brownish vesicular lava（ left） and olivine xenocrysts（ right） collected in Black rock mesa.（ F） felsic（ left） and mafic（ right） crustal xenoliths in Hatcher Mesa lamproite.

— —520 identiﬁed a vertical high-seismic-velocity anomaly beneath the Colorado Plateau extending from the lower crust to more than 200 km in depth, and suggested that delamination of the lower crust and continental lithosphere induce the uplift and magmatism observed in the Colorado Plateau. As illustrated in Fig. 7, the invasion of the asthenospheric mantle into the shallow region is caused by the removal of the Colorado Plateau lithosphere, and has driven passive upwelling of the deep mantle. It is therefore possible that the trigger for Leucite Hills magmatism is related to

Fig. 6 （ A） Sr-Nd isotope and（ B） Hf-Nd isotope systematics of global lamproite rocks including Leucite Hills. Data are from Mirnejad and Bell（ 2006）, Tappe et al.（ 2007）, Prelevic et al.（ 2008） and Nowell et al. （2008）.

Fig. 7 Drawings（ 4:1 vertical exaggeration, VE） illustrating the inferred progression of the convective removal of Colorado Plateau lithosphere（ after Levander et al., 2011）, leading to the eruption of Leucite Hills lamproites.（ A） The lithosphere and asthenosphere as they existed after the formation of the modern lithosphere beneath Basin and Range（ TBL, thermal boundary layer）. The Colorado Plateau lithosphere has been hydrated possibly due to preceding Farallon subduction.（ B） The viscosity reduction from hydration and increase in density from freezing small-volume melts （solid black lines） destabilized the lithosphere, and localized downwelling was initiated.（ C） The Colorado Plateau lithosphere has been removed with delaminating of the lowermost crust, and forms the drip inferred from seismic data. Asthenosphere is invading the region from beneath the drip（ red arrows） and around the peripheries of the drip（ dashed red arrow）. This passive upwelling of asthenosphere reactivated the ancient veined mantle present in the basal section of the Wyoming Craton lithosphere, leading to the Leucite Hills magmatism（ pink lines）.（ D） Cross-sections of mantle S-wave velocity variations along the southern United States（ Grand et al., 1997）.

— —521 transient heating of the basal part of lithosphere downwelling. Nature, 472, 461-465. beneath the Archean Wyoming Craton. Mirnejad, H. and Bell, K.（ 2006）: Origin and source evolution of the Leucite Hills Lamproites: Evidence from Sr-Nd-Pb-O isotopic compositions. Journal of Acknowledgement Petrology, 47, 2463-2489. We are grateful to Professor Y. Ogasawara of Waseda Mitchell, R.H.（ 1985）: A review of the mineralogy of University for his valuable suggestions and advice on lamproite rocks. Transactions of the Geological Society planning our ﬁeld trip. of South Africa, 88, 411-437. Mitchell, R.H. and Bergman, S.C.（ 1991）: Petrology of References Lamproite Rocks. Prenum Press, New York, 447p. Carlson, R.W., Irving, A.J., Schulze, D.J. and Hearn, B.C. Mueller, P.A. and Frost, C.D.（ 2006）: The Wyoming （2004）: Timing of Precambrian melt depletion and Province: A distinctive Archean craton in Laurentian Phanerozoic refertilization events in the lithospheric North America. Canadian Journal of Earth Sciences, mantle of the Wyoming Craton and adjacent Central 43, 1391-1397. Plains Orogen. Lithos, 77, 453-472. Nowell, G.M., Pearson, D.G. and Irving, A.J.（ 2008）: Lu- Closs, W.（ 1897）: Igneous rocks of the Leucite Hils and Hf and Re-Os isotopic studies of lamproite genesis. Pilot Butte, Wyoming. American Journal of Science, 4, Extended Abstracts 9th International Kimberlite 115-141. Conference. Emmons, S.F.（ 1877）: Descriptive Geology; in Report of Obrebski, M., Allen, R.M., Pollitz, F. and Hung, S.- the Geological Exploration of the 40th Parallel. U.S. H.（ 2011）: Lithosphere–asthenosphere interaction Army Engineer Department; Professional Papers. beneath the western United States from the joint Foley, S.F., Venturelli, G., Green, D.H. and Toscani, L. inversion of body-wave traveltimes and surface-wave （1987）: The ultrapotassic rocks: Characteristics, phase velocities. Geophysical Journal International, classiﬁcation, and constraints for petrogenetic models. 185, 1003-1021. Earth-Science Reviews, 24, 81-134. Prelevic, D., Foley, S.F., Romer, R. and Conticelli, S. Grand, S.P., van der Hilst, R.D. and Widiyantoro, S. （2008）: Mediterranean tertiary lamproites derived （1997）: Global seismic tomography: A snapshot of from multiple source components in postcollisional convection in the Earth. GSA Today, 7, 1-7. geodynamics. Geochimica et Cosmochimica Acta, 72, Hausel, W.D.（ 1998）: Diamonds and Mantle Source 2125-2156. Rocks in the Wyoming Craton, with a Discussion of Scott-Smith, B.H. and Skinner, E.M.W（. 1984）: Diamond- Other US Occurrences. Wyoming State Geological iferous lamproites. Journal of Geology, 92, 433-438. Survey, Report of Investigations, 53, 93p. Tappe, S., Foley, S.F., Stracke, A., Romer, R.L., Kjarsgaard, Hawkesworth, C.J., Fraser, K.J. and Rogers, N.W（. 1985）: B.A., Heaman, L.M. and Joyce, N.（ 2007）: Craton Kimberlites and lamproites : Extreme products of reactivation on the Labrador Sea margins: Ar-40/ mantle enrichment processes. Transactions of the Ar-39 age and Sr-Nd-Hf-Pb isotope constraints Geological Society of South Africa, 88, 439-447. from alkaline and carbonatite intrusives. Earth and Kemp, J.F. and Knight, W.C.（ 1903）: Leucite Hills of Planetary Science Letters, 256, 433-454. Wyoming. Geological Society of America Bulletin, 4 Vollmer, R., Ogden, P., Schilling, J.G., Kingsley, R.H. 305-336. and Waggoner, D.G.（ 1984）: Nd and Sr isotopes in Lange, R.A., Carmichael, I.S.E. and Hall, C.M.（ 2000）: ultrapotassic volcanic rocks from the Leucite Hills, 40Ar/39Ar chronology of the Leucite Hills, Wyoming: Wyoming. Contributions to Mineralogy and Petrology, Eruption rates, erosion rates, and an evolving tem- 87, 359-368. perature structure of the underlying mantle. Earth and Planetary Science Letters, 174, 329-340. Levander, A., Schmandt, B., Miller, M.S., Liu, K., Karlstrom, K.E., Crow, R.S., Lee, C.T.A. and Hum- phreys, E.D.（ 2011）: Continuing Colorado plateau uplift by delamination-style convective lithospheric

— —522 アメリカ，ワイオミング州のリューサイトヒルズ・ランプロアイト

― 深部マントルに由来する異常マグマ ―

石川晃＊河合研志＊

アメリカ，ワイオミング州南西部に広がる平原とからも，そのマグマ源はマントル深部に存在すには，300 ～ 89 万年前の溶岩に覆われたメサ・る。リューサイトヒルズ・ランプロアイトの放射ビュート群が特徴的な景観をつくりだしている。性元素同位体組成は，海洋マントルに認められるリューサイトヒルズと称される一連の溶岩類は，範囲を大きく逸脱していることから，過去に交代その名が示す通りリューサイトを石基鉱物として作用を被った太古代-原生代大陸リソスフェアが含む珍しいマグマ活動により形成されており，ラその起源マントル物質の候補として考えられ，南ンプロアイトと呼ばれる岩石に分類される。ラン方に位置するコロラド台地を形成した「リソスプロアイトは地球上で最も不適合元素に富んだ始フェア落下イベント」により誘発されたマグマ活原的マグマであり，時にダイアモンドを産するこ動であると推測される。

キーワード：リューサイトヒルズ，ワイオミング地塊，ランプロアイト，同位体地球化学

＊東京大学大学院総合文化研究科宇宙地球科学教室

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