American Mineralogist, Volume 76, pages I26I-1278, 1991 Eruptive pegmatite magma: Rhyolite of the Honeycomb Hillso Utah Rocnn D. CoNcooN* Department of Earth and Planetary Sciences,Johns Hopkins University, Baltimore, Maryland 21218, U.S.A. W. P. Nlsn Departmentof Geologyand Geophysics,University of Utah, Salt Lake City, Utah 84112-l183, U.S.A. Ansrn-lcr The Honeycomb Hills rhyolite representsdifferentiation in a highly evolved magma. A pyroclastic sequence12.5 m thick and a dome of -0.2 km3 occur in western Utah in a region populated with severalTertiary topaz rhyolites. Phenocrystsconsist of quartz, san- idine, and albite (10-500/ototal) in a glassyor fine-grained groundmass.Primary pheno- crysts and megacrystsof topaz and fluorsiderophyllite (- lolototal) and accessoryphases usually associatedwith rare-elementpegmatites occur: fergusonite,ishikawaite, columbite, fluocerite, thorite, monazite, and zircon. Whole-rock composition (SiO, : 73.3o/o,TiO, : 0.01,AlrO3 : 14.0,Fe,O, : 0.28,FeO : 0.55,MnO : 0.07,MgO < 0.01, CaO:0.42, NarO : 4.59,&O : 4.44,PrO, < 0.01,F: 0.61,Cl : 0.10,and maximum valuesRb : 1960ppm, Cs : 78,Li: 344, Sn : 33, Be : 80, and Y : 156)is peraluminous,highly evolved, and comparable to rare element pegmatites.Elevated F contents of up to 2.30/o in glass account for low silica and high alumina contents becausethe granite minimum shifts toward the Ab apex of the Q-Ab-Or ternary with increasing F. Mineralogy and distribution of trace elementswith order of eruption indicate evacuation of a cool (570- 'C), 610 chemically stratified magma chamber. Chemical variation within the erupted volume can be modeled by Rayleigh fractionation of 7 5o/oof the phenocrystphases. Spatial variation of some elements,notably Li, Be, B, F, and Cs, may be due to volatile transfer. Enrichment of HrO and F in interstitial melt during crystallization reduced viscosity enough to allow eruption of the highly crystalline lava of the dome. IurnooucrroN of the Honeycomb Hills. Previously, the rhyolite body possibly The Honeycomb Hills in west-central Utah are the was consideredto be the remains of two or three product of a single eruptive cycle of unusual lavas. The lava domes (McAnulty and kvinson,1964; Christiansen lavas are silicic, contain high concentrations of F, are et al., 1986). However, flow banding orientations reveal crystal rich, were erupted at low temperature,and contain that the Hills are the eroded remnants of a single dome a suite of unusual accessoryminerals. The parent magma that emanatedfrom a central conduit that can be distin- was the result of extensive differentiation and was com- guished by concentric vertical flow banding in coarsely positionally zoned. High silica concentration and crystal crystalline rhyolite (Congdon and Nash, 1988).There are content together with low temperatureswould normally several other volcanic units in the immediate vicinity, mitigate against eruption. The lavas of the Honeycomb but all are older and unrelated to the genesisofthe Hon- Hills provide an opportunity to examine these unusual eycomb Hills magma body. An underlying tuff (Ttrt) is features,to assesstheir effectson eruption of the lavas, similar in composition to tuffs erupted from the Thomas and to place constraintson differentiation mechanismsin Range 50 km to the east and dated at 6.1 Ma (Turley and highly evolved silicic magmas. In addition, these lavas Nash, 1980). Latites and dacites of Oligoceneage (Hogg, are a rare example of an eruptive pegmatite magma, and 1972) outcrop adjacent to the Honeycomb Hills and are they provide specific information on crystal-liquid rela- underlain by the Kalamazoo Tuff dated at 34 Ma (Hag- tions in such magmas that are not obtainable from peg- strum and Gans, 1989). The Kalamazoo Tuff(Tjt) lies matites themselves. unconformably on the Devonian Guilmette Formation, a thick carbonate sequencethat overlies about 5 km of Gnor,ocrc sETTTNG lower Paleozoicsediments (Hintze, 1988). The lavas of the Honeycomb Hills were erupted at 4.7 The rhyolites of the Honeycomb Hills can be conve- Ma in the eastern Basin and Range province (Lindsey, niently divided into two units, a pyroclastic sequenceand 1977;Turley and Nash, 1980).Figure I is a geologicmap the overlying lavas that form a dome. The proximal py- roclastic sequenceconsists of 12.5 m of pumice-bearing t Presentaddress: U.S. GeologicalSurvey, M.S. 956,Reston, air-fall tuffs with rare interbedded surge deposits. The Viryinia 22092,U.S.A. tephra deposit is capped by a vitrophyric breccia that 0003-{04x/9l /0708-l26 I $02.00 t26l r262 CONGDON AND NASH: ERUPTIVE PEGMATITE MAGMA representsa vitric bomb bed marking the end of the ex- plosive phase of the eruption. The upper unit is a flow- banded felsite that forms the single dome; based upon a presentday extent and height (240 m), it had an original volume of approximalely 0.2 km3. Any original pumi- ceouscarapace has been lost to erosion that has exposed the internal structure of the dome including the central conduit. AN.c.LyrrcAL METHoDS Whole-rock SiO2, Al2O3, CaO, Fe,o,,TiOr, MnO, Cl, Rb, Y, Zr, and Nb were measuredby X-ray fluorescence spectroscopy.MgO and Fe2* were determined volumet- rically, PrO, by colorimetry, NarO and KrO by flame photometry, Li by atomic absorption, and F by selective ion electrode. Be, B, As, Cs, Th, U, and rare earth ele- ments (REE) were determined by instrumental neutron ----\_ contact activation analysis (INAA) by Neutron -------- Activation Ser- o--l]-T-n.5-1.+ os -.---- approximatecontact vices, Toronto, Canada. Electron microprobe analyses { adit were performed with Applied Research Laboratories unconsolidatedalluvium ** topazhorizon EMX-SM and Cameca SX-50 instruments at the Uni- lg| andlake deposits s2. samplesite location versity of Utah, using a combination of natural minerals Flowbanded domal topaz ffi$l Poorlysorted crystal and synthetic lThhrl materials as standards and applying ZAF l-aala'l rhyoliteflows fi]'et$llvitric tuff and Q@z)(PAP) correction procedures.Mineral separates llflll Airfall pyroclastic Latiteand dacite of biotite, plagioclase,sanidine, topaz, and quartz were 1""''ldeposits E analyzedfor trace elementsby INAA. Separationwas by Crossstratified Kalamazootuff use of heavy liquids and a Franz isodynamic magnetic pyroclasticdeposits ffi separator. Separateswere examined under a binocular Fig. 1. Geologicmap of theHoneycomb Hills area.Location microscope, and foreign grains were removed by hand. is approximately113'35'W, 39%3'N. For completekey to the Small accessoryminerals were not always eliminated, as pyroclasticsection, see Figure 2. discussedbelow. Modal analyses were made by point counting of samplesin thin section. The total number of points counted varied between 1000 and 2000, depend- Eureka and Prospect Mountain lie at depths ing on sample size. the Quartzites of about I and 5 km, respectively,below the Honeycomb Hills. Pumice fragments contain from l0 to 250lophe- Prrnocru.prrv nocrysts consisting of plagioclase,sanidine, quartz, and The proximal stratigraphic section is illustrated in Fig- less abundant F-rich annite and topaz set in a glassy ure 2, in which the pyroclastic section is shown to scale. groundmass(Table l). Feldsparphenocrysts ar€ euhedral, Pyroclastic depositsare light colored (white to light gay) whereasquartz is commonly embayed and subhedral. when fresh and oxidized to brown or red-brown when The dome-forming felsites are markedly flow banded, weathered. Pumice fragments up to 50 cm in diameter hght gray in color when fresh, and brown when weath- are abundant, comprising at least 500/oof the deposit. ered. They are holocrystalline, with phenocryst contents Latite clasts averaging 10 cm in diameter are present in ranging from 30 to 40o/othat consist of sanidine, plagro- conformable layers up to 50 cm thick, and these often clase, quartz, biotite, and topaz in decreasingorder of comprise up to 500/oof an individual layer. These lithic- abundance(Table l). The averagegrain size of the crys- rich layers delineate"throat clearing" episodesin the ini- talline groundmassis 0.3 mm; groundmasstopaz occurs tial stagesof the eruptive cycle. Other xenoliths include as needlesup to 0.1 mm in length. Xenoliths of quartzite quartzite and limestone clasts up to 50 cm in diameter. up to 50 cm in diameter tre presentin the dome. Cognate Two distinct varieties of quartzite occur as xenoliths. The (?) xenoliths of pegmatite up to 20 cm in diameter and dome contains red, hematite-stained, slightly conglom- topaz-rich xenoliths are present.The pegmatite xenoliths eratic quartzite derived from the Prospect Mountain contain miarolitic cavities and are composed of potassi- Quartzite of lower Cambrian age. Within the tephra are um feldspar, plagioclase,quartz, biotite, and topaz, typ- xenoliths of white quartzite from either the Eureka ically l-2 cm in diameter. Topaz-rich xenoliths are poi- Quartzite (Ordovician) or sandstonelenses in the Guil- kilitic in texture and contain up to 200/otopaz together mette Formation (Devonian). The Guilmette Formation with quartz, potassium feldspar,plagioclase, and fluorite. outcrops a few kilometers west of the Honeycomb Hills. Megacrystsof topaz, sanidine, and biotite are common Based on stratigraphic sections for the adjacent Fish in a zone donoted "Topaz Horizon" in Figure 1. This is Springsand southern Deep Creek Range (Hintze, 1988), a horizon approximately 10-20 m thick and subparallel CONGDON AND NASH: ERUPTIVE PEGMATITE MAGMA r263 Samolenumbers Lithologydescription Magnetite is more abundant in the pyroclastic se- Unconsolidatedfluvial quence but is still present only in trace amounts; it is .F and lakedeDosits commonly oxidized. Other trace phasesin both units in- I clude zircon, thorite, monazite, fluorite, fluocerite, co- o Flow banded domal o lumbite, fergusonite,and ishikawaite. .*+S++{ ll:lg:l8rhyolite flows o + $??:3i:3? z -.,* Mrxnur,ocv 33:13:1f Quartz Airfall tuffs. Mafic EI!SSITc)4wC) 50 quartz claststhroughout, more Phenocrystsof are smokey throughout but ap- ! pear to be less often embayed in felsite than in pumice. oeu.o@ ug(D 49 abundantin zonesup lo 1mwide.