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Clinopyroxenes from the Keweenawan Lavas of Minnesota

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Clinopyroxenes from the Keweenawan Lavas of Minnesota American Mineralogist, Volume 59, pages 1190-1197, 1974 Clinopyroxenesfrom the KeweenawanLavas of Minnesota T.qolsHI KoNna Department of Geology, Yamagata Uniuersity, Yamagata 990, Iapan eNo JonN C. GnnrN Department of Geology, Uniuersity ol Minnesota, Duluth, Duluth. Minnesota 5 58 I 2 Abstract The Keweenawan(I-ate Precambrian) lava flows in northern Minnesota include the volcanic series: tholeiitic olivine basalt, basaltic andesite,quartz latite, and rhyolite, which is assumed to be derived from a tholeiitic parental magma. These lavas contain calcic clinopyroxene which ranges from CarnMgnuFe,,to CaoMgr"Fenu.The crystallization trend of these clinopyroxenes is characterized by marked iron enrichment and is similar to that of the associated Beaver Bay Gabbro Complex and of the Skaergaard Intrusion. The mineral assemblageof olivine f fer- roaugite + quartz and the crystallization trend of the ferroaugites are characteristic of equi- librium crystallization of pyroxene during the late stage of difterentiation of tholeiitic magma. fntuoduction gabbro far to the northeastand southwestof those The compositionaltrends and the phaserelations consideredhere, all of the rocks (including the of pyroxenesin intermediateand felsic volcanic BeaverBay Complex)which intrude the North Shore -+15 rocks of tholeiitic derivationhave beenrcported by Volcanic Group are within m.y. of the same many investigators(e.g., Carmichael,1960; Kuno, ageas the lavas (Silverand Green, 1972), showthe 1969; Emeleus,Dunham, and Thompson,l97l). same magneticpole position (Beck and Lindsley, One suchrather complete series of lavasof generally 1969; Books, 1972; Books and Green, 1.972) and tholeiitic affinitiesis the North ShoreVolcanic Group areassumed to be cogeneticwith thelavas. of Late Precambrianage, about 1130 million years The basaltsof the North ShoreVolcanic Group old, which underlies(Fig. 1) most of the shore of are nearly alLnon-porphyritic, the pyroxenesoccur- Lake Superior in northeasternMinnesota, U.S.A. ring generallyeither as poikilitic crystals in ophitic (Green, 1972). This series,which includesapproxi- or subophiticintergrowth with plagioclaseor as small mately7500 meters(25,000 feet) of lavas,contains equantcrystals in an intergranulartexture. The inter- sometypes which are more potassicthan is typical mediate and felsic lavas, however, are nearly all in pure tholeiiticsuites, and it rangesin composition porphyritic, with small plagioclase,clinopyroxene, from olivine tholeiites through q\artz tholeiites, and commonly also magnetiteand olivine pheno- basaltic andesites, trachybasalts,andesites, and crysts.The olivinehas in all casesbeen destroyed by quartz latites to rhyolites. The olivine tholeiites are secondaryalteration, but the clinopyroxeneis fresh the most abundantsingle rock type. The flows are in manyrocks. intruded by the immenseDuluth Gabbro Complex To understandthe completetrend of compositions and by locally abundant shallow intrusive rocks, of the clinopyroxenesduring fractionationof tholei- principally olivine diabase.The Beaver Bay Com- itic magma,partial analysesof many crystalsfrom plex, describedby Grout and Schwartz(1939) and this volcanicseries were obtainedusing an electron Green (1972) and in part by Gehman(1957) is microprobe (Green), and the clinopyroxenesfrom one such group of intrusions.Muir (1954) and four large sampleswere separatedand analyzedby Konda (1970) have studiedthe clinopyroxenesof conventionalwet chemical methods (Konda). the Beaver Bay Complex. Except for lavas and The samplesstudied for this report are from lava I 190 CLINOPYROXENES FROM KEWEENAWAN LAVAS 11,91 flows of the uppermost 2000 meters of the North laths; subhedral olivine now completely altered to Shore Volcanic Group, plus a dike that crosscutsthe serpentine or chlorite; and both ilmenite and Ti- flows roughly 2600 meters from the top of the ex- magnetite. This basalt is the most "primitive" posed section. The sample localities are shown on (highest Mg/Fe ratio, lowest K2O) of the olivine Figure 1. This paper presents the results of these tholeiites analyzed in the North Shore Volcanic analyses and discussesthe petrologic significance of Group, and, according to calculations by Phinney the pyroxene crystallization trend thus outlined. (I970), it may be close to the parental magma com- position for the Duluth Gabbro Complex as well as Petrography for the North Shore Volcanic Group. Chemical analyses of most of the rocks sampled (b) Olivine basalt (T-22); on old road, NW 1/4 are given in Green (1972); they are plotted on an sec. 2, T.58N., R.5W. Medium-grained, subophitic; AFM diagram in Figure 2. Brief petrographic de- the upper-central part of a very thick flow. About scriptions of the rocks from which pyroxenes were 3 percent small olivine crystals, completely altered; studied follow. augite up to 3 mm diam., some as skeletal inter- (a) Olivine basalt (T-56) : lakeshore at E. edge growths with plagioclase; both ilm. and Ti-mag; sec. 12, T.59N., R.4W. Slightly porphyritic, fine- to plagioclase laths zoned, An65-An2p.Trace of inter- medium-grained basalt flow between 7-8 -m thick stitial granophyre. Saponite and zeolites fill dikty- (Fig. 3,A.). Major minerals: clots of small, euhedral taxitic cavities. bytownite phenocrysts zoned Ans5-Anool poikilitic (c) Ophitic olivine basalt (DY-3): lakeshore at augite up to 2 mm across enclosing small labradorite north edge of Sec. 15, T.60N., R.2W. The lower Flc. 1. Generalizedgeologic map of Keweenawanrocks in the western Lake Superior area showing location of samplesstudied. Check pattern, intrusive rocks; vertical ruling, Lower Keweenawanrocks (magnetically reversed); stippled, Middle Keweenawan lavas (magnetically normal). After Green (1972, Fi'e. V-3). t192 T, KONDA, AND I. C. GREEN maryinal zoning by changesin extinction angle and birefringence.Some augite shows very thin exsolu- tion lamellae(pigeonite?). Groundmass intergranu- lar in texture, with plagioclase, alkali-feldspar, augite, pigeonite, opaques, and greenish clay min- erals.The pigeonitetends to be oxidized along rims and fractures. il-z .xt-4 (f ) Basalticandesite (MI-4): lakeshore,SE 1'/4 " Fal ' 1-16 Sec. 22, T.62N., R.3E. Fine-grained red-brown aphyric andesiteor trachy-andesiteflow showingthin oxidation banding. Intergranular to intersertal tex- ture with small plagioclaselaths (An 53-45), ffi?8- netite octahedra,and 8 percent blocky, fresh ferro- augitesin a groundmassof oxidized glass,magnetite, alkali-feldspar, quartz, and amphibole (Fig. 3D). Cavities are filled with quartz, chlorite, clay, and Fro. 2. AFM diagram for rocks from which pyroxene zeolites.Flow thicknessunknown. analyses aro given in this paper. Point labelled GH-7a is (g) Porphyriticandesite or trachyandesite(H-4): derived from rock analysis S & G-5-9 (from the same flow Highway61, centerline Sec.12,T.62N, R.4E. Very but a diferent locality), and point DY-3 is derived frorn fine-grained,dark brown dike, which transectsthe rock analysis DY-6b (from a similar flow slightly lower in lavas about 2600 m below the top of the exposed the sestion than DY-3), Rock analyses from Green (1912). section. About 3 percent of small phenocrysts of subhedraloligoclase (Anzu), ferroaugite,and mag- middle part of a very thick (200 m) flow of netite, in a groundmassof plagioclase,alkali-feld- fine- to medium-grainedbasalt. Tabular plagioclase spar, ferroaugite, magnetite, anhedral hornblende, predominates, with fresh ophitic augite, olivine possibledevitrified brown glass,and alteration prod- pseudomorphs,magnetite and ilmenite (Fig. 3B). ucts. Interstitial cavitiescontain zeolites and calcite. (h) Porphyritic intermediate qaartz latite (MI- (d) Porphyriticbasalt (GH-7A): lakeshore500 2): Highway61 at W. edgeSec. 6, T.62N., R.3E. m westof powerplant at GrandMarais; SE 1/4 SW Fifty m-thick lava flow rather similar to rocks termed l/4 Sec.20, T.61N., R.lE. The chilledbase of a "icelandites"by Carmichael(1964). Reddishbrown, thick (110 m) basaltic to andesiticsheet that is fine-grained;about 7 percent of small phenocrysts probablya flow but may be a sill. Abundant 1-2 mm of stout tabular, unzoned andesine(Atuo), ferro- phenocrystsof zonedplagioclase, abott 2 percent of augite, altered olivine, and magrretite.The ferro- fresh prismatic augite crystals (0.5-1.5 mm) and augite phenocrystsare fresh, about 1 mm across; fewer small altered olivine phenocrystsset in a sub- some are aggregatedwith the other phenocrysts spherulitic, fine-grained groundmassof plagioclase (Fig. 3E). The groundmassminerals are plagioclase, tablets,clinopyroxene prisms, and magnetiteblocks, alkali-feldspar,interstitial quartz,opaques, and green with interstitial alkali feldspar in the coarser areas. alteration products, probably chlorite. Although Original small miarolitic cavities filled with quartz there may have been clinopyroxenein the ground- andan oxidizedchloritic mineral. mass,it is not now recognizable. (e) Porphyritic basaltic andesite(F-7a): lake- (i) Porphyritic rhyolite (GM-10): lakeshore, shore,NE l/4 Sec.21, T.57N, R.6W. The basal, centerline Sec. 14, T.61N, R.1E. Basal spherulitic chilled portion of a very thick (at least 100 rr) zone of a rhyolite lava flow that is as much as 155 m flow. Red-brown, fine-grained potassic basaltic thick. Orange-red,aphanitic, about 3 percent small andesitewith 10 percent small phenocrystsof plagi- blocky phenocrystsof plagioclase(An35-26) and rare oclase (Anss), augite, rare pigeonite,and olivine qvartz, ferroaugite, magnetite and altered olivine. (Fig. 3C). The
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