sign in sign up
<<
Home , Carbonatite

American Mineralogist, Volume 72, pages5946, 1987

Mineral chemistryand petrogenesisof carbonatiteintrusions, Perry and Conway Counties,Arkansas Groncn R. McConMrcK,Rrcn,qno C. Hurrrcorr* Departmentof Geology,University of Iowa, Iowa City, Iowa 52242,U.S.A.

AssrRAcr Xenocrystic alvikite (Cr) containing fragmental xenoliths of - , phlogopitite, and argillite togetherwith xenoliths of , syenite,and two varieties of sovite (C,) all exhibiting reaction rims are described from Morrilton, Oppelo dump, Perryville, and Branch, Arkansas. The texture indicates that the bodies were explo- sively emplaced, and anorthoclaseand in syenite xenoliths indicate that feniti- zation precededone or more ofthe carbonatite intrusive phases. The chemistry indicates that green phlogopite representsremnants of the earliest (C,) sovite intrusive phase,and goldenyellow, reversedpleochroic, phlogopite represents the metasomatizedor "dolomitized" phaseof the C, sovite.The metasomatismintroduced Si, Mg, and Fe3*and removed Fe2*,Al, and Ti from the . Zoned and amphibole phenocrystsand xenocrystsfrom the C, sovite indicate that the C, magma was being enriched in Mg and losing Fe; there is also a slight increase in Ti and Al. All ferromagnesianxenocrysts show magnetite-bearingreaction rims where they are in contact with the C, alvikite, which suggestsoxidation of the alvikite magma. Eastonite,which is presentas overgrowths around reaction rims on a number of phlog- opite xenocrysts,contains Ba and also the highestpercentage of Al and Mg of all the . This eastonite may represent a Co alvikite phase, but in any case crystallized after the formation of the Fe-Ti reaction rim.

INrnolucrroN xenoliths, sovite xenoliths, and xenocrysts of fer- Four of six small carbonatite breccia pipes and sills in romagnesian , magnetite, and apatite; over- the vicinity of Oppelo and Perryville in Conway and Per- growths on phlogopite xenocrysts may represent a more ry Counties in west-central Arkansas are still exposed. evolved carbonatite phase. The present mineralogical They trend in a north-south line from Morrilton to Brazil study has been undertaken to document the chemical Branch (Fig. l). Four sills at one locality, known as the changes that occurred during successive phases of car- ArkansasRiver sills, were first recognizedin 1888 (Wil- bonatite emplacement. liams, 1890),but not studiedunlil 1927during the map- ping of the Arkoma basin. Croneis found additional brec- Gnor,ocrc sETTING cia pipes at Oppelo, Perryville, and Brazil Branch (now The carbonatite bodies in this study are the northernmost Tyndal Slough) and described them as known in the Arkansas igneousprovince. They lie on the north whose groundmasshad been altered to secondarycalcite side of the Ouachita fold belt, 47 to 7 | km north of the Magnet (Croneisand Billings, 1930).Steele and Wagner (1979) Cove alkaline carbonatite complex that crops out in the interior describedsills from Morrilton Dam and from a shale pit ofthe Ouachita structural province (Fig. 2). The frontal Ouachita on Highway I 13,called the Oppelodump locality.A nat- Mountains comprise an east-trendingstructural belt character- ural gaswell drilled westof Oppeloin sec.8, T5N, RlTW ized by thrust faults and steep,narrow folds that are locally over- province passedthrough 4l carbonatites,some as much as 30 ft turned. The Arkoma basin,a contiguousstructural north of the Ouachita belt, contains less faulting and broad, low-am- (9.1 m) thick, to a total depth of 11850 ft (3612 m) plitude folds. The country rock in the vicinity ofthe carbonatite (Mitchell,1979). bodies is a siliciclastic sequenceof the Ouachita flysch of Steeleand Wagner(1979) published several analyses of Atokan Age. the carbonatite matrix of the various intrusions but not Southwestofthe Brazil Branch locality, high-altitude photog- of the xenoliths. Mitchell (1979) describedthe igneous raphy shows a tear fault trending Nl5-20"8 and cutting across xenoliths in the Oppelo dump and Brazil Branch bodies the Ouachita structural grain; this fault is on strike with the and speculatedon their origin and geologichistory. Brazil Branch, Perryville, and Morrilton carbonatite outcrops; The brecciapipes are composedof alvikite containing the Oppelo dump exposureis only slightly to the eastof this line. The alignment of the carbonatite brecciasand sills parallel with + Presentaddress: Core Laboratories, Inc., 10703E. Bethany regional lineaments of the Ouachita Mountains and northem Drive,Aurora, Colorado 80014, U.S.A. Arkansas suggeststhat their emplacement was controlled by 0003-o04x/87/0102-o059$02.00 59 60 McCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE

R16W f ARKANSAS tr

/-- )

T,ACKSON CONWAYCO. PERRYCO.

Fig.2. The Arkansasalkalic province. Study area shown by arrow;map modified from Ericksonand Blade (1963). ! : Cre- taceousalkalic intrusive and volcanic rocks; V : Cretaceousbasic volcanicrocks; O: Cretaceousultrabasic intrusive rocks; A : pre-Cretaceousdiabase and . Vertical-line pattern indicates structurallyand topographically high areas; axes ofanticlines and synclinesare shown by appropriatearrows.

Mrclscoprc rExruRES Carbonatite samples from all four localities are light gray when freshly broken and reddish brown when Fig. l. Location map of study areaand carbonatiteintrusions: weathered.All are xenocrystic alvikite brecciasthat con- (4) (l) Morrilton Dam, (2) Oppelo dump, (3) Perryville, Brazil tain xenoliths of granite, syenite, magnetite-apatiterock, Branch. sovite, phlogopitite, and argillite. The granite,syenite, and sovite xenoliths range in size from less than 2 mm up to 5 cm, are well rounded, and exhibit reaction rims with basementfaulting, as is common in many carbonatiteprovinces the groundmass.The sovite xenoliths are armored by me- (Heinrich, 1980). dium to finely crystalline . Magnetite-apatite rock and phlogopitite xenoliths are fragmental and generally Mnrrron oF TNvESTIGATToN less than I cm in diameter. The argillite xenoliths are Approximately 500 analyseswere obtained on the enr,-ervrx-su generallywedge shaped and from less than 2 mm up to electron microprobes at the University of Chicagoand the Uni- 9 cm, with an averagelength of 2-3 cm. Pale reaction versity of Iowa using the following standards:Asbestos micro- rims on the black argillite xenoliths are visible megascop- cline (K); Amelia albite (Na, Al, Si); Di$Jd,, glass (Mg); An,o ically in the Brazil Branch and Morrilton Dam samples; glass(Ca); A-128 (Ti); hortonolite(Mn, Fe);and P-140 this type of xenolith was not found in the Perryville or (Si, Mg, Fe). Kakanui hornblende was used as a working Oppelo dump samples. standard for all these elements. Analytic conditions were spot is far greaterin the Brazil size 4 to l8 pm, counting time 60 s, acceleratingvoltage 15 kV The abundanceofxenoliths with 1000nA, and detectionlevel for all elementsapproximately Branch and Morrilton samplesthan in the Perryville or 0.2 wto/o(1o). Background and peak counts were determined by Oppelo dump samples.The majority of xenoliths in the the proceduresof Reed and Ware (1975); correctionswere made Brazil Branch samplesare argillite, and those from Mor- with Bence-Albeefactors (1968). rilton Dam are syenitesand . Phlogopitite xeno- Selectedrepresentative analyses are presentedin Tables l-4; liths are found only in the Morrilton Dam samples.Mag- all analyseswere used as the data base for Figures 3-7. Addi- netite-apatite rock xenoliths and sovite xenoliths are found tionally, reconnaissanceanalyses with the wavelength-dispersive at all four localities. no among severalgen- method disclosed discernible differences Xenocrysts of phlogopite, clinopyroxene, amphibole, erations of apatite in La, Ce, Sr, Y, Cl, or F amounts. Ti values and magnetiteare common in all samples.Xeno- for phlogopite rims obtained by Eos (Table 2) are considered apatite, phlogopite, l0 mm; clinopyrox- suspectbecause reconnaissance wos analysesofa few Perrywille cryst maximum sizesare samplesindicate that Ba (whoseeos peak overlaps that of Ti) is ene, 8 mm; amphibole, 6 mm; and magnetiteand apatite, alwayspresent in eastoniticovergrowths in amounts greaterthan l0 mm. Phlogopite is more common than clinopyroxene 1 u4o/o. and amphibole; amphibole is less abundant than clino- MCCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE 6l

Table 1. Chemicalcomposition of magnetite(EDS microprobe data)

P1-6-A P1-X-11 P1-10-C M15-9-M M18-6-A M18-3-C M4-5-H OD6-16-13 0D6-5-A SX SXRMASXR RSXR Mgo 2.71 z.Ia 4.11 2.77 2.87 2.39 2.74 3.14 3.48 Alro3 053 0.48 4.40 0.89 0.62 0.72 0.77 0.96 3.87 sio, 026 o.26 0 0 0.16 0 0.16 0 0.38 o.24 0.43 0.29 o22 0.13 0 0 0 0.18 Tio, 8.00 8.17 11.74 10.34 8.85 10.83 10.63 9.44 13.88 MnO 1.21 0.94 0.52 1.07 1.19 1.06 0.95 101 0.39 FeO 81.86 82.59 75.26 79.33 80.97 78.37 78.72 81.94 77.12 VrO" 0.39 0.36 0.37 0.33 Sum 94.82 95.62 96.32 95.03 95.13 93.72 94.29 96.49 99.29 Numbers ions 24 of oxygen '| of on the basis of atoms Mg 11 1.12 1.55 1.11 1.16 0.97 1.11 1.25 1.26 AI o.17 0.15 1.31 0.28 0.20 0.23 0.24 0.30 1.11 si 0.07 0.07 0 0 0.04 0 0.04 0 0.09 0.07 0.12 0.08 0.07 0.04 0 0 0 0.05 Ti 165 1.67 2.23 2.09 1.81 2.22 2.16 1.89 2.54 Mn o.28 0.22 0.11 o.25 0.27 o25 0.22 0.23 0.08 FE 18.83 18.82 15.86 17.84 18.41 17.88 17.80 18.28 15.69 0.01 0.08 0.08 0.07 Nofe; Locations are shown by the letters in the sample numbers: M : Morrilton, OD : Oppelo dump, P : Perryville.Other abbre- viations: SX : sovite xenolith, R : reaction rim, MA : magnetite-apatitexenolith. pyroxene in all but the Brazil Branch samples(in which Phlogopite. Two types of phlogopite xenocrysts are it is subequal). present:(1) a pale golden-yellow variety with weakly re- verse to normal pleochroism and (2) a green to colorless CovrposrrroN oF THE GROUNDMASS or pale green strongly normal pleochroic variety. The green The groundmassof all but the Brazil Branch samples phlogopite contains more Al in the tetrahedral site, a wider is a finely crystalline mosaic of calcite, apatite, and an range of Fe/(Fe + Mg) in the octahedral site, and higher unidentified green, highly birefringent (possibly Ti content than the golden-yellow variety (Figs. 3, 4). a ferromagnesianmineral that has altered to clay). Mod- Green phlogopitesare occasionallyzoned, the outer zone erate amounts of magnetite, phlogopite, , and being more Fe rich; golden-yellow phlogopites are rarely accessoryperovskite, , and pyrite are also usually zoned with no discerniblecompositional gradients.- present.The phlogopite of the groundmassis eastonitic, en-yellow phlogopite contains insufficient Al and Si to fill similar in composition to overgrowths found on phlog- tetrahedral sites, and we suspectthat Fe3+fills the site. opite xenocrysts.The Morrilton Dam alvikite contains Edgesof xenocrystic phlogopite in all samplesexhibit thin veins (<2 mm thick) of calcite and vugs of ferroan rounding and formation of magnetite-bearing reaction calcite and dolomite. rims. Xenocrysts often have overgrowths around the re- The texture of the groundmass in the Brazil Branch action rims of very pale greenish-white eastonitic mica samplesfeatures rhomboid to tabular crystals of calcite containing more Al and Mg (Table 2; Figs. 3, 4). Recon- in a microcrystalline apatite-bearingmatrix. The ground- naissancewos analysesofthese overgrowths indicate Ba mass of the Brazil Branch alvikite ditrers in having an in amounts estimated to be as high as 5 wto/o,and TiO, abundanceof acicular aegirine crystalsgrown together in generally in trace amounts only. All analyses of green clots adjacent to argillite xenoliths. rims (Table 2) exhibit low totals that may also reflect the problem with Ti vs. Ba in the analyses. MrNBnar- CHEMISTRyoF xENocRysrs Clinopyroxene. Xenocrystic clinopyroxene is euhedral Textural evidencesuch as fragmental shapes,corroded to idiomorphic fragmental with scalloped and embayed edges,and reaction rims on the ferromagnesianminerals margins. It is weakly zoned and has microcrystalline re- in all of the alvikites indicate that theseminerals (phlog- action rims containing magnetite. The compositions of opite, clinopyroxene,and amphibole) are xenocrysts.The clinopyroxene xenocrystsfrom all four localities are sal- larger magnetite and apatite grains are also consideredto itic and nearly identical. Xenocrysts from Brazil Branch, be xenocrysts, a deduction based on the similarity of however, exhibit a greatervariation in Na and Fe content composition (Table l) and intergrowth textures to the than those from other localities (Table 3, Fig. 5). Analyses magnetite-apatiterock xenoliths. A separategeneration for Mg, Fe, and Ca (Fig. 6) of cores of clinopyroxene of magnetitegrains-which contain no apatite inclusions xenocrystsexhibit moderate scatter, whereasanalyses of and are commonly attached to ferromagnesiangrains or rirns are clustered.The rims are all slightly more calcic. are presentas isolated grains in the groundmass-formed Amphibole. Amphibole xenocrystshave scallopedand as a reaction product between the ferromagnesianxeno- embayed margins indicating reaction with the ground- crysts and the alvikitic liquid. mass;the outer zones(clearly overgrowths on some grains) 62 MCCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE

Table2. Chemicalcomposition of phlogopiteand biotite(EDS microprobe data)

BB1- BB1. BB1- BB2- oD6- oD6- oD1- oD1- P2- P2- DI P1- Pr- 18-2 18-1 4-1 16-2 o-z 13-1 13-2 3-1 3-2 4-6 4-7 uu un YC GP GR na .GR YC YR GC GR sio, 3s.73 3590 3765 3753 37.02 34 12 39.28 34.08 42 01 32 55 40.69 37.66 33.86 Tio, 3.81 3 33 2.61 215 378 2j5 2.'t5 2.86 0.72 3.70 1.12 4.60 3.13 '12.10 Al,o3 1711 1786 13.57 1580 20.61 13 70 20 44 11 .30 21.90 11 86 16.76 21.70 FeO 13 25 12.37 22.74 17.17 13.92 4 88 10.72 5 63 8.74 5 98 I 16 9.36 5.48 MnO 0 0 0.39 030 0 0.33 0 027 0 0 0 0 038 Mgo 3 70 1697 11.97 14.86 16.56 21.75 20 54 21 27 22.95 20.41 22.75 19.18 21.46 CaO 0 37 0 22 0.32 0.31 0 0.40 0.39 0.s3 0.63 o.47 0.53 0.37 0 47 Naro 040 0 041 0.39 0 0 044 0 0.44 000.550 KrO 9.36 939 886 8.93 9.54 8,92 9.33 8.59 7 59 7.9 9.20 9.50 8.85 Sum 96.99 96.04 9705 9522 9663 93.17 96.56 93.67 94 39 92.91 95.31 97.99 95.33 Numbersof ionson thebasis of 22 atomsof oxygen Si 5.22 5.25 5 74 5 68 5 43 4.98 5.67 4 96 6.06 4.76 5.89 5.34 4 84 Ti 0.42 0.37 0.30 0 24 0 42 0.24 0 23 0 31 0.08 0.41 0j2 0 49 0.34 A(rv) 278 2.95 2.18 2.32 2 57 3.02 2 33 3 04 1.92 3.24 2 02 2.66 3.16 A(vr) 017 0.33 0 0.10 0.16 052 0.01 0.46 0 0.54 0 0.14 0 50 Fe(lV) 0 0 0.08 000000.02 0 0.09 0 0 Fe(Vl) 162 152 282 2.17 1.71 0.60 1.29 0.69 1 04 0.73 1.O2 111 066 Mn 0 0 005 0.04 0 004 0 0.03 0 0 0 0 005 Mg 3.70 3.70 272 3 35 3.62 4.73 4.42 4-61 4 93 4.45 4.91 4.05 4.57 0 06 0.03 0 05 005 0 0.06 0.06 0.08 0 10 0.07 0.08 0.06 0.07 NA 0.11 0 0.12 012 0 0j2 0,12 0 0.12 0 0 0.15 0 K 1.74 175 172 1.72 1.78 1 72 1.72 1.59 1.40 148 1.70 1.71 1.62 P2- P2- P2- P2- M17- M30- MX- MX- M18- M18- M7- M7- 17-2 17-3 I 3-1 13-2 3-A 7-E M16-1 M16-2 4-3 4-1 4-A 4-B 7-D 10-G uu un uh uu YC YR GC GR YP YPSS sio, 37.12 3251 37.25 3200 37.27 38.06 39.30 31.05 37.10 32.00 4080 40.59 38.24 38 63 Tio, 3.81 3.75 3.97 414 3.25 4.74 1 14 3.37 4.18 2.07 1 53 1.47 2.91 2 62 Al,o3 1681 2275 16.67 22.53 1558 16.40 10.67 21.37 1595 1951 12.56 12.39 1356 14.13 FeO 14.01 5 85 1329 s.70 1340 7.98 7.49 5 68 11.56 4.79 10.03 9 59 16.58 17.00 MnO 0 04.1 0 0.30 0 0 0.24 0 13 0 0.25 00 0 021 lr/gO 16.80 2108 1744 20.85 16.73 19.29 22 66 20 11 16.99 20 04 21.29 21.52 14.t6 13.93 CaO 0.31 0.87 0 27 0.49 0 0 0 31 0.64 0.57 2 46 00 00 Naro 0.42 0 054 0 0.26 0.53 0 43 0 40 0.61 0.19 0 63 0.50 0.22 0.30 K.O 9.44 I24 I 40 8.38 9.90 9.92 9 84 I 01 8.62 8.70 9 88 9.84 1026 10.51 Sum 98.72 95.44 98.83 9447 96.39 96.92 92 09 90.76 95.58 90.02 9668 9s.84 9593 9732 Numberof ionson thebasis of 22 atomsof oxygen Si 5 33 4 65 5.33 4 63 5.48 5.41 5.90 4.68 5.42 4.89 5 86 5.87 5.74 5 73 Ti 0 41 0.40 0.43 0 45 0 36 0.51 0.13 0.38 0 46 0 24 0.17 0.16 0.33 0.29 Ar(rv) 2.67 3 35 2.67 3.36 2 52 2.59 1.89 3 32 2.58 3.11 2j3 211 2.26 2.27 Ar(vr) 0.18 0 49 o 14 0 48 0.18 0.16 0 0 48 0.17 0.40 0 0 0.14 0 20 Fe(lV) 0 0 0 0 0 0 0.22 0 0 0 0.01 002 00 Fe(Vl) 1 68 0.70 1.59 0.69 1.65 0.95 072 0.72 1.41 0 61 1.20 1.14 2.08 2.11 Mn 0 005 0 0.50 0 0 0.03 0.o2 0 0.03 0 0 0 0.03 Mg 3 60 4.50 3.72 4.s0 3 67 4.09 5.07 4.52 3 70 4.56 4.s5 4.64 3.17 3.08 0.05 013 0.04 0.08 0 0 0.0s 0.10 0.09 0.40 0 0 00 Na 0 12 0 0.15 0 0.08 0.s3 0.13 0 12 0.17 0.0s 0 18 0 14 0.06 0 09 K 1.73 150 1.71 155 1.86 1.80 1.88 1.54 1.61 1.70 1.81 1.82 1o7 100 Notei Locationsare shown by the lettersin the samplenumbers: BB : BrazilBranch, M : Morrilton,OD : Oppelodump, P : Perryville.Other abbreviations:GC : core of green phlogopitexenocryst, GR : rim of green phlogopitexenocryst, GP : phenocrystof green phlogopitein sovite xenolith,S : silicatexenolith, YC : cole of yellow phlogopitexenocryst, YR : rim of yellow phlogopitexenocryst, YP : phenocrystof yellow phlogopitein sovitexenolith; ( ) connectscore and rim of samegrain.

have ferroan pargasite compositions that are similar display unreactedmargins; the larger grains are fragmen- among all xenocrysts,whereas the coresof the xenocrysts tal, the smallerones euhedral. rangein composition from ferroan pargasitichornblende sovITE xENoLITHS to ferroan pargasite(Table 4, Fig. 7). Somexenocrysts are MrNBn.ll CHEMISTRyoF intergrowthsof amphiboleand clinopyroxene. Sovite xenoliths contain abundant phenocrystsof gold- Magnetite and apatite. Magnetite and apatite are pres- en-yellow phlogopite and a few pale greenclinopyroxene ent in two distinct size ranges:0.5-l mm and lessthan phenocrystsin a groundmassof very coarse-grainedcal- 0.2 mm. Magnetitegrains commonly contain inclusions cite, apatite, and magnetite. One xenolith from Brazil of carbonategroundmass and apatite(Table 1). Most of Branch contained a green pleochroic phlogopite pheno- the larger magnetite grains are embayed and rounded, cryst similar in composition to the green phlogopite indicating reaction with the groundmass.Apatite grains xenocrysts.Xenoliths always display armored margins of MCCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE 63

Table3. Chemicalcomposition of clinopyroxene(Eos microprobe data) B81-1-1 BB1-1-2 BB1-11-2 BB2-3-1 BB2-3-2 BB2-3-3 0D1-13-3 oD1-13-4 0D1-16-1 R C C .... .M...... R C ..R C.

sio, 51.40 5003 49.92 44.79 44.12 48.13 55.14 44.25 5Z.Vb Tio, 0.29 0.64 0.43 1.90 2.66 1.32 0 2.49 0 Alro3 2.5'l 2.63 J.Ub 7.64 7.94 4.48 0 8.39 0.48 FeO 14.59 7.91 11.12 Y.CO 11.47 8.44 4.14 8.84 5.71 MnO 0.41 0 0 0 0 0 0 0 0 Mgo 879 13.44 10.87 1040 12.02 16.12 11.19 1515 CaO 1765 23.85 20.87 22.41 21.91 22.92 24.13 23.49 23.80 Naro 3.52 0.41 1.40 0.71 1.01 0.61 0.44 0 0.34 Sum 99.18 98.92 97.66 97.40 98.67 97.93 99.98 98.65 98.42 Numbersof ions on the basis of 6 atoms of oxygen Si 1.98 1.90 1.93 1.74 1.71 185 2.07 1.68 1.99 Ti 0.01 0.01 0.01 0.06 0.08 0.04 0 0.07 0 A(rv) 0.o2 0.09 0.07 0.26 0.29 0.15 0 0.32 0.01 A(vr) 0.10 0.03 007 0.09 0.04 0.05 0 0.06 0.01 Fe 0.47 o.25 0.36 0.31 nea 0.27 0.13 0.28 0.18 Mn 0.01 0 0 0 0 0 0 0 0 Mg 0.51 076 063 0.60 u.cc 0.69 0.88 0.63 0.85 Ca o.73 0.97 0.87 0.94 091 0.94 0.95 U.YO noA Na 0.26 003 0.11 0.05 0.08 0.05 003 0 0.03 oD1-t6-3 M4-3-B Ml8-3-A P't-12-1 P2-22-1 P2-23-1 P2-21-1 M30-5-G M2-1-A R R R SS sio, 48.24 51.18 54.66 5378 52.78 50.16 46.58 54.31 54.28 Tio, 103 0.66 0 0 0.26 114 2.07 1.13 0.90 Alros 436 2.61 0.38 u.t5 1.39 348 7.24 1.02 0.82 tr6n 7.49 7.34 5.27 6.23 7.39 8.30 9.69 17.62 20.20 MnO 0 0 0 0 0 0 0 1.02 o.82 Mgo 13.27 13.09 15.18 15.08 13.74 13.09 11.10 5.84 4.62 CaO 23.46 23.84 24.83 24.02 23.66 23.71 23.57 11.23 9.61 Naro 0 0.71 0.51 0.42 0 0 0.66 8.16 9.19 Sum 97.85 99.44 100.82 100.28 99.22 99.88 100.93 100.31 100.43 Numbersof ions on the basis of 6 atoms of oxygen Si 1.85 1.92 2.00 1.98 107 1.88 1.75 2.08 2.01 Ti 0.03 0.02 0 0 001 0.03 0.06 0.03 0.02 A(rv) 0.16 0.08 0 0.02 0.02 0.'12 0.25 0 0 A(vr) 0.04 0.04 0.02 0.02 0.04 0.03 0.07 0.05 0.04 Fe 0.24 0.23 0.16 0.19 o.23 0.27 0.30 u.c/ 0.63 Mn 0 0 0 0 0 0 0 0.03 203 Mg 0.76 0.73 0.83 083 077 0.73 0.62 0.33 0.26 Ca noA 0.96 0.97 uv5 0.95 0.95 0.95 0.46 0.38 Na 0 0.05 0.04 003 0 0 U.UC 0.61 0.66 Nofe: Locations are shown by the letters in the sample numbers: BB : Brazil Branch, M : Morrilton, OD : Oppelo dump, P : Perryville.Other abbreviations:C: coreof xenocryst,M: middleofxenocryst, R: rim of xenocryst,S: silicatexenolith;( )connectscoreand rimof samegrain.

medium to finely crystalline calcite. A few ferromagne- magnetite reaction rims on ferromagnesian xenocrysts sian phenocrystsin sovite xenoliths protrude into the host (Table l). groundmass;the protrusion invariably has developed a reaction margin. MrNpru.r- cHEMrsrRy oF sILTcATExENoLrrHS Phlogopite. Golden-yellow phlogopites are identical in Xenoliths of argillite, granite, and syenite are present composition to golden-yellow xenocrysts described pre- in all samples.Representative mineral analyseswere ob- viously (Table 2; Figs. 3, 4). tained for granite and syenite xenoliths. Analyses for Clinopyroxene.Pale greenclinopyroxenes are nonpleo- mineral grains in the microcrystalline argillite were not chroic to weakly pleochroic and salitic; they have com- obtained. positions similar to cores of xenocrystic clinopyroxenes Granite. Xenoliths of granite are fine grained and equi- (Table 3; Figs. 5, 6). granular; they contain quartz, K-feldspar [&ruNaooo- Magnetite and apatite. Magnetite in sovite xenoliths (A1304sis e6)O rol, zoned plagioclasethat varies from core usually appearsas vermiform blebs or regularporous net- [Na, onCa,55(Al4 52Si, os)Oro] to rim [Nar rrCaour(Al3 68Sis 32)- works often having inclusions ofcarbonate and euhedral Oro],biotite [K, nrNaoou(Mg, ,rTio ,rFe, o8Al0 3e)(Al2 26Si5 74)- apatite. The composition of sovitic magnetite is similar O,0(OH)41@igs 3, 4), and amphibole [(NaoouIG,Mgrur- to that of larger magnetite grains in the alvikite matrix Tio,rMnoorFe,erAlo 2r)(Al,,rsi6 88)Or0(OH)r] (Fig. 7). and to xenoliths of magnetite-apatiterock. The magnetite Syenite.Xenoliths ofsyenite are fine grained and equi- has slightly lower Ti, Mg, Si, and Mn contents than the granular;they contain anorthoclase[Na, r,K, re(Al2eesie.o3)- 64 MCCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE

34

32

EASTONITE 3 O

f Z zs l J 4 za

E. 6 24 U I ,, UE PHLOGOPITE2 O

oo o1 t**ri*-'-**,t o6 1o oa

06 50 52 54 56 56 60 Fig. 3. Compositional varieties of phlogopite-biotite exhib- si+4 ited in terms of atomic percenttetrahedral Al vs. ratio of atomic Fig. 4. Compositionalvarieties of phlogopiteexhibited in percentFe'?+/(Fe'z+ + Mgt*). Phlogopitein silicate xenoliths also atomicpercent octahedral Fe vs. atomicpercent Si4*. plots in the "green core" area. termsof Phlogopitein silicatexenoliths also plots in the "greencore" alea. Oro] and aegirine [(Na, o.Mg, ..Fe, ,uMno ,rTio ,.Ca, ,rAlo rr)- (sis3o)or4l. Textural evidenceindicates that thesefour igneousbod- DrscussroN AND INTERPRETATIoN ies were explosively emplaced.The sourceof the magma The carbonatite pipes and sills are composed of C, al- cannot be determined; however, the presenceof xenoliths vikite breccia (nomenclature after Le Bas, 1977) contain- ofgranite, syenite,and argillite indicates that the carbon- ing xenoliths of granite, syenite, argillite, magnetite-apa- atite originated below or within the crystalline basement. tite rock, phlogopitite, and two varieties of C' sovite The syenite is composed of anorthoclase and aegirine' together with xenocrysts of ferromagnesian minerals, which indicate that fenitization precededone or more of magnetite, and apatite. Eastonitic overgrowths containing the carbonatite intrusive phases.Angular argillite xenol- Ba on some phlogopite xenocrysts may represent a late iths are probably metamorphosedshales of the Ouachita Co alvikite phase. The C. ferrocarbonatite phase of Le Bas Formation. (1977) is not seen. The history ofcarbonatite differentiation and intrusive

Table4. Chemicalcomposition of amphibole(EDS microprobe data)

BB1-12-1 BB1-12-2 BB1-11-1 BB1-13-1 BB1-13-2 0D1-15-5 0D1-15-3 M11-3-B M4-3-L rcR CR 45.36 ao co sio, J/.O l 39.28 39.81 41.62 39.16 39.06 39.94 Tio, 3.26 2.84 2.36 2.'t7 2.10 3.36 3.71 1.46 3.10 Al,o3 1442 13.84 1154 10.59 13.77 14.41 1398 /.YC 13.19 FeO 1665 1269 1667 1852 12.26 12.55 1180 tc.oY 1215 MnO 0 0 0 0.31 0 0 0 u.cc 0 Mgo 10.03 12.89 10.62 9.47 13.69 12.39 12.98 11.85 12.56 1167 12.25 10.61 9.29 11.86 11.81 11.46 11.54 12.29 Naro | .co 1.99 2.66 3.81 1.84 157 1.99 171 1.76 KrO 2.95 2.78 1.84 1.68 236 2.52 2.31 113 2.72 Sum 98.14 98 57 YC.6Z 97.46 97 04 97.67 98.16 97 25 v/ bb Numbersof ions on the basisoJ 23 atoms of oxygen 5.99 Si f,./d 3.O/ 6.15 6.39 5.91 5.86 5.92 6.82 Ti 0.38 0.32 0.28 0.25 0.24 0.38 041 0.17 0.3s A(rv) 222 213 185 161 2.09 214 2.O8 1.18 2.01 A(vr) 038 0.31 0.26 0.31 036 040 0.79 0.23 032 Fe 2.13 1.59 2.17 2.38 1.55 1.57 1.46 1.97 1.53 Mn 0 0 0 0.04 0 0 0 0.07 0 ,47 Mg 229 287 2.46 2.17 3 08 2.77 2.66 2.81 191 196 177 r53 1.92 1.90 182 186 1.98 NA 0.46 0.58 0.80 1.14 0.54 0.46 0.57 050 0.51 K 0.58 0.53 o.37 0.33 0.45 0.48 0.44 o.22 0.52 : Note..Locations are shown by the lettersin the samplenumbers: BB : Brazil Branch,M : Morrilton,OD Oppelodump' Other abbreviations: C: coreofxenocryst,R: rimofxenocryst,S: silicatexenolith,l: intergrowthwithclinopyroxene; ( )connectscoreandrimofsamegrain. MCCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE 65

Na*1 1 Nar

Na+l

Fe*2 +Mn*2

MORRILTON Fe*2 *Mn*2

Fe+2 +Mn+2

Mg+2 Fe*2+Mn*2 Fig. 5. Variationin clinopyroxenecomposition exhibited in termsof atomicpercent Na+, Mg'z*, and Fe2+ + Mn2+. oener-o f;fff sucrre xelourH-+ anlzl analcxff! uonntrOl-coREr phasesis recordedprimarily in the chemistry of the micas Frg.1. Variationin amphibolecomposition exhibited in terms much as was noted at Oka by Rimsaite (1969).Discon- of atomicpercent Sia+ vs. ratioof atomicpercent Mg'?+/(Mg:+ * tinuous shifts of composition similar to those described Fe,*). Tie linesconnect core and rim valuesofthe samegrain. by Rimsaite (1969) occur twice in this study: some Cr Modifiedfrom leake(1978). greenphlogopite was metasomaticallyaltered to C' gold- en-yellowphlogopite and someofboth the C' greenphlog- ble 2; Figs. 3, 4). The phlogopite chemistry (Table 2; Figs. opite and the C, golden-yellowphlogopite were altered to 3, 4) indicates that if the sovite xenoliths that contain eastonite. golden-yellowphlogopite were formed by Green phlogopitexenocrysts and the singlesovite xeno- of the earlier C, sovite, Si, Mg, and Fe3+were introduced lith a green phlogopite xenocryst represent containing and Al, Fe'*, and Ti were removed. earliestC, sovite intrusive phase.Golden- remnantsofthe Pyroxene and amphibole xenoliths and xenocrystsare yellow, pleochroic, unzoned phlogopite is present reverse weakly zoned. The analysesof the cores indicate slight phenocrystsin one variety of Cr sovite xenoliths and as compositional scatter, whereas the analysesof the rims xenocrysts in the C, alvikite (Table 2, Fig. 3). This as areclustered (Tables 3, 4; Figs.6,7).Itappears asthough reversepleochroic phlogopite appearsto be similar to the thesexenocrysts and phenocrystsformed during the crys- phlogopite that Rimskaya-Korsakova and Sokolova tallization of the C, magma, and therefore the zoning (Kapustin, 1980,p. 201).Ka- namedtetraferriphlogopite representschanges in chemicalcomposition ofthe magma pustin describedtetraferriphlogopite as an indicator min- during this phase.The compositions of pyroxenesin the the "dolomitized" stageof the early carbonatite eral of C, sovite in the Oppelo dump, Perryville, and Morrilton phase(C, of Le Bas, 1977)formed by metasomaticre- are identical and exhibit little scatter, whereas placement. zonesare related to zonesof The dolomitized pyroxene in the Brazil Branch breccia contains more Fe brecciation and schistosity; the brecciation is often as- and Na and exhibits g.reaterscatter (Fig' 5). Aeginne is with linear zones,as is the casehere. Analogously, sociated presentin the C, alvikite phaseonly in the Brazil Branch golden-yellow, reverse pleochroic phlogopite xenocrysts specimens,suggesting that the carbonatitic magma at the sovite xenoliths containing phenocrysts of reverse and southern end of this district was more alkaline. Chemical pleochroic phlogopite may representremnants of a meta- changesfrom ferromagnesianmineral coresto their rims (dolomitized?)phase of the early C' sovite (Ta- somatized indicate that the magma was being enriched in Mg and losing Fe; there also is a very slight increasein Ti and Al. Qa*2 This effect could be causedby the precipitation of mag- BRAztLBRANCH(:HE: netite or Fe-rich ,which depletedthe magma in Fe. "="""u,...<:,;lr: All ferromagnesianmineral xenocrystsin the C, alvikite oppElo<:H.^" have reaction rims containing magnetite together with many unidentifiable small mineral grains. that form in thesereaction rims contain higher amounts of Ti, Si, Mg, and Al than xenocrysticmagnetites. Such reaction rims suggestoxidation of the alvikite magma in a silica- poor analogue to the oxidation of annite described by Wonesand Eugster(1959). Fig. 6. Variation in clinopyroxene composition exhibited in Eastonitic overgrowths that contain more Mg and Al termsof atomicpercent Ca2*, Mg2+, and Fe2** Mn2+.Tie lines than the other phlogopitesare presenton the groundmass connect core and rim values of the same grain. side of reaction rims on a number of phlogopite xeno- 66 MCCORMICK AND HEATHCOTE: PETROGENESIS OF CARBONATITE crysts. wDS analysis has shown that these rims contain Ba Potash Sulfur Springs,Arkansas. U.S. GeologicalSurvey Pro- much as that reported by Gasper and Wyllie (1982) in fessionalPaper 425,95 p. Gaspar, Wyllie, P.J. (1982) phlogopite phlogopites from Jacupiranga. This eastonite, which may J.C., and from the Jacupirangacarbonatite, Brazil. American Mineralogist, represent a phase, Co alvikite crystallized at the close of 67,997-1000. crystallization of the carbonate groundmass after the for- Heinrich, E.W. (1980) The geology of carbonatites.Robert E. mation of the Fe-Ti oxide reaction rims that removed Fe Krieger Pub. Co., Huntington, New York. and Ti from the magma. Leake, Bernard A. (1978) Nomenclature of amphiboles. Amer- ican Mineralogist, 63, 1023-1052. Le Bas, M.J. (1977) Carbonatite-nephelinitevolcanism. Wiley, AcxNowlBncMENTs New York. Kapustin, Y.L. (1980)Mineralogy of carbonatites.Amerind Pub. We want to express our gratitude to Norman F. Williams, Co., New Delhi. Director of the ArkansasGeologic Commission, for the generous Mitchell, J.R. (1979) The petrology of metamorphic xenoliths in financial support the Arkansas Geologic Commission furnished carbonatite intrusions of west-centralArkansas. M.Sc. thesis, for instrument and time chargesfor the microprobe analyses. University of Arkansas,Fayetteville. The GraduateCollege of the University of Iowa kindly furnished Reed,S.J.B., and Ware, N.G. (1975) electronmi- partial photographic Quantitative subsidyfor drafting and charges.J. V. Smith, croprobe analysis of using energy-dispersiveX-ray as always, was most helpful in furnishing us with microprobe spectrometry.Journal of Petrology, 16, 499-5 19. standards and allowing us time on the Chicagomicroprobe. Sam- Rimsaite, J. (1969) Evolution of zoned micas and associated ples from Perryville, Oppelo dump, and Brazil Branch were sup- silicatesin the Oka carbonatite. Contributions to Mineralogy plied by K. C. Jacksonofthe University of Arkansas,Fayetteville. and Petrology,23, 340-360. The figureswere ably drafted by Mark Morton. We are indebted Steele,K.F., and Wagner, G.H. (1979) Relationship of the Mur- for the time and effort of an unknown reviewer whose critical freesborokimberlite and other igneous rocks ofArkansas. In and accurate review forced a rethinking of the data that has F.R. Boyd and H.A.O. Meyer, Eds. SecondInternational Kim- resultedin this revised draft. berlite Conference,Proceedings, vol. I, p.393-399. American GeophysicalUnion, Washington, D.C. RrrnnnNcns Williams, J.F. (1890) The igneousrocks of Arkansas.Arkansas GeologicalSurvey Annual Report,2,457 p. Bence,A.8., and Albee, A.L. ( I 968) Empirical correction factors Wones,D.R., andEugster, H.P. (1959)Biotites on thejoin phlog- for the electron microanalysisof silicatesand oxides. Journal opite (KMg3AlSi3O,0(OH)r) - annite (KFerAlSi3Oro(OH),). of Ceology,76, 383-403. CarnegieInstitution of WashingtonAnnual Report 1958-1959, Croneis,C.G., and Billings,M.P. (1930)Igneous rocks in central 127-132. Arkansas.Arkansas Geological Survey Bulletin, 3, 437 p. Erickson,R.L., and Blade,L.V. (1963) Geochemistryand pe- MlNuscnpr REcETVEDNover"rnER. 8, 1985 trology of the alkali igneous complex at Magnet Cove and Mnruscnrpr AccEprEDSrrrrlrssn 2. 1986