American Mineralogist, Volume 74, pages 132-140, 1989

Major-cation substitution in phlogopite and evolution of carbonatite in thePotash Sulphur Springs complex, Garland Countyo Arkansas

Rrcn,cno C. HnnrHcorE.* Gnoncr R. McConnnrcxDepartment of Geology, The University of Iowa, Iowa City, lowa 52242, U.S.A.

Arsrru.cr

The petrogenetic relationships among three carbonatite units in the Potash SulphurSprings alkaline complex are interpreted using petrographically determined relative agesand mineral chemistry of phlogopite contained in the carbonatites. The oldest sovite (C,)is interpreted to have magmatically differentiated to the youngest sovite (Cr). The inter-mediate-age alvikite (Cr) is interpreted to have formed by magma mixing of the C, soviteand residual ijolite magma. Composition of the hybrid C, alvikite was probably controlledin part by the degree of sovite magma evolution at the time of the mixing. The compo-sitional trend from C, to C3 sovite is Si and Mg enrichment and Al, Ti, and Fe depletion.Oxidation of Fe occurred concurrently with depletion of Al, causing the later phlogopitegenerations to incorporate Fe3* in tetrahedral sites. The compositional evolution of car-bonatitic phlogopites is generalized as the replacement of a hypothetical Ti analogue ofsiderophyllite [ArFe]+TiRr*Si4R;+Orrl by ferriphlogopite [ArMgSidFq+Orr]. The phlogo-pite of C, alvikite differs from that in C, sovite by replacement of end-member phlogopite[ArMguSiuR]+Orrl with end-member annite [ArFeU+Si6R3+Orr]. Similar compositions ofphlogopite in alvikite, feldspathic fenite, and phlogopitized melteigitic and ijolitic rockssuggests that separate K-Al- and Mg-Fe-bearing fenitizations occurred when alvikite mag-ma was present. Oscillatory zoning in phlogopites of C, and C. carbonatites suggest thatquiescent conditions existed in alvikite and sovite magma chambers late in the evolutionof these magmas.

INrnonucrroN sorption or by incipient crystallization of other minerals.

r nrs rnvesrgauon was designed to determine what ma- llt-1t-:::litnuous variation is superimposed on gradual

. ,"-".:: compositional variation caused by magmatic differentia-JOr-eremenl vanauons exrst rn mlcas ln the carbonatltesuite of the poorly exposed Potash Sulphur Springs intru-

tto*t"^,^- ^^.,sive comprex, west-centrar Arkansas (Fig. r), "l?.1,.:- ,"# 5fiH"Jffi:3:H::iHlbjHL'::,Hfi1:#Hltermine whether any observed variations can be used to ;:,:,;';:

reconsrrucr the perrogenesis of rhe carbonatir"";il i; 9"1"j:-1If"sh,1972), occulrences in the So'iet union

rhe absence of diagnostic field relarions. Mica -.;; T-9 uf{i (Rass and Boronikhin' 1976)' southern west

vide a record of evilving magma composition r".rirJ" "r

9::,1!t!techer and Larsen' 1980)' Jacupiranga' Brazil

the wide variety of cations rhar readily ubrtit;;;;;;; !o^Tl^1 "l-o wvllie' 1982)' and central Arkansas (Mc-

various srrucrural sites (Nockholds, lg47; r'.rt"., iieo; 9:Tt:l^:lu Heathcote, 1987)' core-to-rim composi-

N.mec, re72; Speer, re84). Nockoro'."po.t"ou *iJJ :f,i:l;iK:':fi,;''rfit:,1iiH#lii;8:1,3"H"$T;of Al,o,, Mgo, and Feo proporrions in -i.ur_t::: ;ffi;;iil (1982). Eckerman (1948) and vartiainenvariety of rock types and concluded that AlrO, content, ;;;^. 1

.'

relative to Mgo una r"o, is dependent on -m"J^putul ( t 989) presented optical proof of carbonatitic phlogopite

genesis. Nockolds also concluded that the relatiie'Jri- zonation and discussed the implications of this for mag-

portion of Mgo and Feo is dependent on tne oegrel'Jr 1a dltrelntiation' In this report' petrographic evidence

magma differentiation and is independent "rp"t"gZr*ir.

Plt*t::: *e is described for three carbonatites of the

Rimsaite (1969, p.359) concluded that micas i" t"h"'iil; fotasf Sulphur Springs complex' and the nature of ma-

carbonatite "represent rhe entire crystallization ;;;d; J-:,t-*:ll-l,t"bstitution is interpreted for phlogopite that

of that unit. periographic evidence from oka."r;;,;- clslallizej as these magmas evolved'

cording to Rimsaite, rhat disconrirruo.r, "o-po1]iioiir

r419Ti! phlogopite crystals selected for analvsis pos-

shifts between zones in micas were caused "id;;;-

sess sharp maryins and euhedral to subhedral shapes; theyare not altered. Phlogopite crystals that display anhedralshape and altered margins or cleavage traces are inter-

* Present address: 2825 Harmony Drive, Bakersfield, Califor- preted to be xenocrysts from elsewhere in the complex.nia 93306, U.S.A. The sources of the most common xenocrysts and xeno-

0003-004x/89/0 102-0 I 32S02.00 r32

HEATHCOTE AND McCORMICK: PHLOGOPITE AND EVOLUTION OF CARBONATITE I J J

liths found in the Potash Sulphur Springs carbonatites arethe ijolitic rocks and the fenites.

Mnrrrons oF IIwESTIGATIoN

Mineral compositions were obtained from polished thin sec-tions by energy-dispersive X-ray methods on ml-rux-su mi-croprobes at the University of Chicago and the University ofIowa. Microprobe standards for the major elements were naturalminerals and synthetic silicate glasses; Kakanui hornblende wasused as a working standard. Analytical precision estimated fromrepeated analyses of major oxides in Kakanui hornblende was+6.2 relative percent or better at one standard deviation; theminimum detectable level for the elements of interest was 0.2wt0/0. Reconnaissance analyses by wavelength-dispersive spec-trometry indicated that F is present in all phlogopite populationsof this study. Ba concentrations are below eos detectability inall samples; thus interference of Ba Z-series X-ray emission peakswith Ti K-series peaks is not significant in the analyses of thisstudy. Analytical conditions were l5-pm spot size, l5-kV accel-erating voltage at 100 nA, and 60-s counting time. Backgroundand peak counts for the elements were determined with the pro-cedures of Reed and Ware (1975); matrix corrections were madewith the factors ofBence and Albee (1968),

Representative phlogopite and clinopyroxene analyses are giv-en in Tables I and2; all phlogopite analyses obtained were usedin constructing Figures 2-6. Fe is reported as Fe2*. Mineral for-mulas were not recalculated for Fe3* although it is known to bepresent in some minerals. Vanadium ore occurs in the PotashSulphur Springs complex, and V was detected in certain clino-pyroxenes (Table 2), but standards for this element were notemployed in the ros routine so the significance ofthe recordedV value is unknown.

Gnor,ocrc sETTTNG

The Potash Sulphur Springs complex is a subvolcanicstructure, roughly circular in plan, with approximately2.6-km2 surface exposure (Fig. l). Unpublished geophys-ical information indicates that the igneous body plunges

SOUTHWESTERN GARLANDC O U N T Y

Fig. l. Location of the Potash Sulphur Springs complex. Cur-rent exposure of the alkalic complex is within a folded and fault-ed sedimentary country-rock section that is dominated by mas-sively bedded chert.

steeply toward the Magnet Cove complex, 8 km to theeast (D. R. Owens, pers. comm.). Syenites are the dom-inant outcropping rock types of the complex; small ex-posures exist of pyroxenite, melteigite, ijolite, malignite,nephelinite, phonolite, and carbonatite. Numerous lam-prophyre dikes are also present (Hollingsworth, 1967;Howard, 1974), and sodic and potassic fenites are devel-oped at the periphery of the complex. Natural exposureis limited owing to deep weathering and dense vegetation;however, large exposures were excavated on the east andnorth sides of the complex after Union Carbide Corpo-

TABLE 1. Chemical composition of phlogopite (eos microprobe data)

ARKANSAS

R l g w R r E W

Sovite (la)M439-2-1

Sovite (l)M439-3-6

Sovite (lb) Sovite (lll)

M307-1-8 M339-2-2 M488-2 M159-8-1

Sovitepegmatite

(tv)DG8-10-4

Alvikite (ll)

M173-10-1 BC-1-1 M15-5-4

Melteigite(ll) Fenite (ll)

BT-8-2 E73-1-4

sio,Tio,Alro3FeO'MnOMgoCaOKrONa,O

Sum

34.s9 36.112.68 3.88

15.46 16.4018.85 12.030.46 0

13.82 1 8.400.24 0.319.91 10.280.46 0

96.47 97.41

5.27 5.250.31 0.422.78 2.812.40 1.460.06 03.14 3.980.04 0.051.93 1 .900 1 4 0

39.97 38.440.95 1.35

13.61 12.589.68 13.710.30 0.32

22.38 19.050 09.93 9.530.32 0

97.14 94.98

5.72 5.740.10 0 .152.29 2.211 . 1 6 1 . 7 10.04 0.044.77 4.240 01.81 1 .810.09 0

41 44 44.66 35.24 37.090.97 0.36 3.13 2.84

11.37 9.32 1 1 .83 12.028.36 6.06 25.65 20.890.29 0 0 0.71

21.21 25.05 10.36 14.440.30 0.95 0.60 0

10.38 11 .19 10 .01 10 .500 0 0 0

94.32 97.59 96.82 98.49

Numbers of ions on the basis of 22 atoms of oxygen

40.36 38.35 4',t.621.17 1 .83 2 .03

11 .14 11.96 1't .7415.03 15 .87 11 .810.95 0.39 0.35

17.60 17.30 1 8.950.34 0.20 09.99 10.35 10.210 0 0.36

96.58 96.2s 97.07

38.952.26

11.2317.920.59

16.240.41

10.290

97.89

5.800.25t C l 7

2.230.073.600.061.950

SiTiAIFe'MnMgCaKNa

6.07 6.26 5.54 5.590.11 0.04 0.37 0.321.96 1 .54 2 j9 2 .131.02 0.71 3.37 2.630.04 0 0 0.094.63 5.23 2.43 3.240.05 0 .14 0 .10 01.94 2.00 2.01 2.020 0 0 0

5.98 5.750.13 0.211.94 2.1' l1.86 1 .990.12 0.053.89 3.860.05 0.031.89 1 .980 0

6.010.222.O01.430.044.0801.880.10

- Fe is reported as Fe2*

134 HEATHCOTE AND McCORMICK: PHLOGOPITE AND EVOLUTION OF CARBONATITE

sio, 48.21Tio, 2.2OAlro3 5.32FeO. 6.31MnO 0MgO 13.69CaO 23.91Na,O 0.51

Sum 100.15

TABLE 2. Chemical composition of clinopyroxene (EDS micro-probe data)

Mel- liolite Alvi-teigite M185- Syenite Fenite- Alvikite kite--BT-1-3 cpx KSS30a5 DK-4-4 BC-3-1 M15-4-4

51.45 52.60 53.32 51.61 43.700 0 1.06 0 4.301 .37 0.9s 0.47 0.34 7.64

12.50 17.12 20.88 14.78 7.320.48 1 .38 1.56 0.91 0

10.37 7 .64 4.50 8.81 12.2122.29 18.33 9.10 20.62 24.231.47 3.60 9.33 3.03 0

99.93 101 .62 100.22 1 00.10 99.40Numbers ot ions on the basis ol 6 atoms of oxygen

si 1 .80 1.97 2.01 2.07 1 .99 1 .66Ti 0.06 0 0 0.03 0 0.12Al o.23 0.06 0.04 0.02 0.01 0.34Fe. 0.20 0.40 0.5s 0.68 0.48 0.23Mn 0 0.01 0.04 0.05 0.03 0Mg 0 76 0.59 0.43 0.26 0.s1 0.69Ca 0.9s 0.91 0.75 0.38 0.85 0.99Na 0.04 0.11 O.27 0.70 0.23 0

. Contains -0 92 wt% V,O"; Fe is reported as Fer*** Salite xenocryst in alvikite matrix.

ration commenced mining for vanadium in 1966 (Hol-lingsworth, 1967).

An Early Cretaceous (Albian) age of 100 + 2 Ma wasobtained for the Potash Sulphur Springs complex byU-Th-Pb dating of zircon (Zartman and Howard, 1985);Zartman et al. (1967) reported contemporaneous ages of95 to 97 + 5 Ma (K-Ar biotite) and 99 + 8 Ma (Rb-Srbiotite) for syenitic rocks of the Magnet Cove complex.The ages of both complexes have been confirmed by fis-sion-track dating ofapatite and titanite (Eby, 1987). Bothcomplexes were emplaced into Paleozoic sedimentaryrocks that had been folded and faulted during the com-pressional, Pennsylvanian-Permian Ouachita orogeny.Very low grade (below greenschist facies) regional meta-morphism in the Ouachita orogenic belt (Jackson,1977;Guthrie et al., 1986) indicates current exposure is of avery shallow level in the subvolcanic structures.

Cretaceous alkaline igneous activity that included for-mation of the Potash Sulphur Springs complex spatiallyoverlapped the Late Triassic tholeiitic province associ-ated with Gulf of Mexico rifting (Moody, 1949; Brockand Heyl, 196l; Pilger, 1980; Edick and Byerly, l98l)and occurred adjacent to the Permian Wauboukigou al-noite province of southern Illinois and vicinity (I-ewisand Mitchell, 1987). Cretaceous alkaline activity is closelyassociated with a belt of middle to Late Cretaceous upliftthat extends from northwestern Louisiana to central Mis-sissippi (Kidwell, l95l) including the Sabine and Monroeuplifts and the Jackson dome. The uplift belt crosses theaxis of the Mississippi embayment, a regional downwarpalso active in the Cretaceous, that developed over the latePrecambrian Reelfoot aulacogen (Ervin and McGinnis,1975). The Potash Sulphur Springs complex is located onthe western margin of the Mississippi embayment, 160km north of the uplift belt.

Cn nnoN,q,rrrn PETRocRAPHY

Carbonatite samples were collected from a drillcore lo-cated near the center ofthe Potash Sulphur Springs com-plex (Heathcote, 1987) and from exposures in the north-east quadrant of the complex. Four carbonatite units werefound to contain phlogopite. Rock names have been as-signed following the guidelines of Streckeisen (1979); asingle mineral prefix is used to distinguish the two sovitesin this study.

Phlogopite sovite

Phlogopite sovite contains fine- to medium-grainedapatite, magnetite, and phlogopite phenocrysts (1.0-> 10.0mm) in a mosaic-textured matrix of medium-grained (l.G-6.0 mm), inclusion-free calcite and minor dolomite;curved contacts between calcite crystals predominate. Rarexenocrysts of salite and melanite are present. Concentra-tions of magnetite, apatite, and phlogopite in some sam-ples are suggestive of flow accumulations. Phlogopite isstrongly pleochroic (X: pale yellow, y : light green, Z: light green; Z > Y > X), and inclusion-free except forrare apatite.

Alvikite

Alvikite contains very fine grained phenocrysts (0.02-0.40 mm) of phlogopite, t itanite, aegirine-augite. apatite,and nosean (nosean identified by optical properties andqualititative ros analysis), with accessory magnetite andiron sulfide in a mosaic-textured matrix of fine-grained(0.3-1.5 mm), inclusion-rich calcite and minor dolomite.Apatite (0.02-0.07 mm diameter) has a distinctive ovaloutline in this carbonatite and occurs abundantly as in-clusions in matrix carbonate. Planar contacts between ad-jacent carbonate crystals predominate, and flow bandingis preserved in some specimens. Phlogopite phenocrystscommonly exhibit numerous concentric, optical zones andcontain abundant inclusions of titanite, apatite, nosean,magnetite, iron sulfide, and melanite. Pleochroism ofphlogopite is strong (X: pale yellow to orange, Y: Z:green to red-brown).

Xenoliths of melteigite, ijolite, malignite, melanephe-linite, and orthoclase fenite are common. Xenocrysts ofgreen phlogopite, nepheline, perovskite, salitic to ae-girine-augitic clinopyroxenes, melanite, orthoclase, andapatite are abundant in some specimens. Nepheline xeno-crysts have been replaced by microcrystalline aggregatesof cancrinite that have been subsequently mantled by no-sean; perovskite is mantled by titanite. Melanite is typi-cally replaced to some degree by calcite; salitic clinopy-roxenes commonly exhibit margins altered tomicrocrystalline intergrowths of titanite, calcite, andarfvedsonite (arfvedsonite identified by qualitative eosanalysis). Green phlogopite xenocrysts (0.2-8.0 mm) fromphlogopite sovite are invariably mantled by red-brown,magmatic phlogopite in the alvikite. Partial dissolutionof xenocrystic phlogopite beneath alvikitic phlogopitemantles yields distinctive atoll-shaped aggregates of mag-matic phlogopite in this unit. Atoll-phlogopite tends to

occur together with highly altered salite xenocrysts. Frag-ments of orthoclase and apatite appear unaltered in al-vikite matrix.

Ferriphlogopite sovite

Ferriphlogopite sovite contains fine- to medium-grainedphlogopite phenocrysts (0.1-3.0 mm), with accessoryapatite, titanite, and pyrochlore, in a fine-grained (0.1-1.5 mm), mosaic-textured matrix of calcite and minordolomite. Planar contacts between calcite crystals pre-dominate in some samples; other samples exhibit exten-sive recrystallization with stylolites and strongly inter-penetrative matrix calcite crystals. Dysanalyte (identifiedby optical character and qualitative EDs analysis), mas-sive pyrrhotite, spherulitic clumps of zoned, limpid toinclusion-rich apatite, and an anastomosing network ofunidentified green f,bers are associated with stylolites.Xenoliths of alvikite and orthoclase fenite occur in thisunit.

Ferriphlogopite commonly exhibits faintly visible os-cillatory zoning and rarely contains apatite inclusions.Pleochroism ranges from weakly normal (X: pale yel-low, : Z : pale orange) to weakly reverse (X: orange,Y: Z: pale orange). This mica is very similar in opticalproperties and composition to tetraferriphlogopite de-scribed by Puustinen (1973). M6ssbauer spectra obtainedfrom samples Ml59 (Table l) and M2l3 (not shown)confirm the presence of tetrahedral Fe3* in amounts of360/o + 30/o and 29o/o + 30/o of the Fe present in these twosamples, respectively. The remaining Fe is octahedrallycoordinated Fe'z* (M. D. Dyar, written comm., 1988).

Other carbonatites

Sovite pegmatite is the fourth mica-bearing carbonatitein the Potash Sulphur Springs complex. Biotite in thisunit displays ragged crystal margins and is interpreted tobe xenocrystic although it has not been found elsewherein the complex. It is strongly pleochroic (X : red,, Y : Z: black). The sovite pegmatite, present in the East Wil-son pit, cuts syenite and is cut by pyritiferous manganoancalcite veins. A fenite breccia with siderite matrix, pres-ent in the East Wilson pit, has been identified as ferro-carbonatite. This unit postdates orthoclase fenite, but isofuncertain age relative to other carbonatites.

MrNrn.q,L CHEMISTRY oF PHLocoPITE

Compositional trends

Representative phlogopite and biotite analyses are pre-sented in Table 1. Four separate types ofphlogopite aredetermined by their optical characteristics and AI vs. Fe/(Fe + Mg) proportions (Fig. 2). The green phlogopite ofphlogopite sovite is designated type I and is subdividedinto subtypes I, Ia, and Ib on the basis ofclusters evidentin Figure 2. The red-brown to green phlogopite of alvikiteis designated type II. Analyses from xenolith-poor alvi-kite samples plot within the dashed oval of Figure 2; mi-cas with similar optical character occurring in silicate rocksscatter across the larger parl ofthe type II field. Orange,

t 3 5

t 4 1 6 1 8 2 0 ^ l '

2 4 2 6 2 8

Fig.2. Plot of Al (p.. zZ o*vgln) vs. Fe/(Fe + Mg) for phlog-opite; alvikite and sovite trends are designated as described intext. Enclosed fields correspond to phlogopite types described intext. Trend lines connect multiple analyses of single grains; ar-rows point rimward. Dashed oval in field II contains all analysesfrom xenolith-poor alvikites. Stars represent analyses from phlo-gopitized melteigite.

normal- to reverse-pleochroic phlogopite of ferriphlogo-pite sovite is designated type III. The field in Figure 2labeled "M488" represents an aluminous ferriphlogopitepopulation that is identified as type III by optical char-acter. Biotite analyses from sovite pegmatite, designatedtype IV, cluster tightly at an Fe-enriched composition.

Relative ages of phlogopite compositional types are es-tablished by the consistent zoning sequences in individ-ual crystals and by xenocrystic and crosscutting criteria.Examples of zoning and overgrowth sequences from in-dividual grains are shown in Figures 2 ar.d 4 to 6 by trendlines with arrows indicating the rimward direction.Phlogopite of subtype I occurs in sovite as unzoned crys-tals and in alvikite as cores of cryptically zoned crystalsthat bear rims of subtype Ia. Alvikite veins containingtype II crystals cut sovite veins that contain subtype Ibcrystals. Phlogopite subtypes Ib and I have not been foundtogether, but occur in sovite samples that are similar toone another in grain size and mineral assemblage. Fer-riphlogopite sovite contains xenoliths of alvikite thatcontain type II phlogopite. Based on these relationships,type III is determined to be the youngest ofthe phlogo-pites, subtype I is the oldest, and the intermediate-agetype II is younger than subtypes Ia and Ib.

Inclusion and xenocryst relationships and phlogopitezoning stratigraphy are used, together with similarities ofrock texture and mineral assemblage, to define the sovitetrend (I - Ib - M488 - III) and the alvikite trend (Ia- II) shown in Figure 2. It is clear from the establishedage relationships that crystallization of type II phlogopiteon the alvikite trend began later than subtype I crystal-

HEATHCOTE AND MCCORMICK: PHLOGOPITE AND EVOLUTION OF CARBONATITE

o l

OLL

@LL

488

136 HEATHCOTE AND McCORMICK: PHLOGOPITE AND EVOLUTION OF CARBONATITE

3 0Aln

Fig. 3. Plot of tetrahedral Al vs. Si; symbols as in Fig. 2.Intrasite substitution line, Si + Al : 8, represents the diocta-hedral-trioctahedral substitution. Subtype I phlogopite is the onlypopulation that consistently contains octahedrally coordinatedAl. Tetrahedral Fe3* is assumed for other types.

lization on the sovite trend, but ceased prior to crystal-lization oftype III phlogopite on the sovite trend.

Cation substitutions

Phlogopite compositions provide a basis from whichelemental changes in the evolving carbonatite magma canbe determined. Specific cation substitutions are inferredfrom bivariate scattergrams where elongate data fieldsparallel permissible octahedral and tetrahedral substitu-tion lines (Figs. 3-6). Because the ros analyses in thisstudy are limited to five major cations, the substitutionsdiscussed below are the simplest among many choices(e.g., Hewitt and Abrecht, 1986) and are not claimed tobe unique characteizations of phlogopite crystal chem-istry in the Potash Sulphur Springs carbonatites. IJncer-tainties in A-site and anionic substitutions preclude a de-tailed mica-component exchange analysis. Intrasitesubstitutions (substitution restricted to either octahedralor tetrahedral sites) that fit the data are dioctahedral-trioctahedral (161R3+l4lR3+ : I61R2+l4lSi' Fig. 3), annite-phlogopite (Fsz* : I61Mg; Fig. 2), and Ti vacancy (Ti:2t6rR2+' Forbes and Flower, 1974l-Fig.4). Alignmentsof data fields along intersite substitution trends (pairedsubstitutions in octahedral and tetrahedral sites), corre-sponding to the given intrasite substitutions, are shownin Figures 5 and 6. Elongation ofthe type II and III fieldsperpendicular to the Si:R2* trend (Fig. 5) is an artifactcaused by the assumption that total Fe is octahedrallycoordinated Fe2* when a portion is actually tetrahedrallycoordinated Fe3*. Tetrahedral Fe3* is assumed, in the ab-sence of Mdssbauer analysis, for phlogopite types II, [I,and some of subtype Ib because Si + Al sums are lessthan 8.00.

Fig. 4. Plot of Ti vs. R'z*; symbols as in Fig. 2. Intrasitesubstitution line, -2R3*:Ti, represents the Ti-vacancy substitu-tion. Subtype Ia phlogopite plots at the low-Ti end ofthe subtypeI field; subtype Ib phlogopite and type III phlotopite of sampleM488 are indistinguishable on this scrttergram.

Alignments parallel to the trends -2R2*:Ti (Fig. 4) and2Si:R'z* (Fig. 5) suggest operation of the Ti-Tschermaksubstitution (6rTi2t4rR3+ : t6lR2+2t4lsi, e.g., Czamanske andWones, 1973; Robert,1976), however, the 2Si:R'zt partof this substitution requires a much steeper slope for Mgvs. Si (ideally l:2) than is evident for the broad 4Mg:Sialignment in Figure 6. No simple combination of Ti-Tschermak and other substitutions can produce the ob-served alignment. Simultaneous exchange of 2Mg for Ti,of Mg for Fe2* and of Mg + $i fq1 t0l(:+ + t41R3+ in thepreviously noted substitutions does yield the 4Mg:Si ra-tro.

A general charucteization of the cation substitutionsgiven above may be made in terms of ideal, anhydrousmica end-members. The depletion of Fe2+, Ti, and Al,enrichment of Mg and Si, and Fe oxidation of the sovitetrend correspond to replacement ofa hypothetical Ti an-alogue of siderophyllite [Argery.u +1'p'tXSio,o,R'*rOrr, Otferriphlogopite end-member [Ar(tei14tu)(SiutatFe ]+)O,,(Annersten et al., 1971)1. Derivation of the alvikite trendfrom the sovite trend corresponds to replacement ofphlogopite end-member [Ar(qMguxS'utorRl*)O,,] by an-nite end-member [Ar(teipsu +XSiutotp'*rorr]. Substitutionsalong the alvikite trend are similar to those along thesovite trend, but Mg enrichment at the expense of Fe isless evident.

DrscussrouDetails of carbonatite magma genesis are obscure in

the Potash Sulphur Springs complex. Gross relationshipsamong fenites and the sovite, alvikite, and silicate mag-mas can be inferred from available inclusion relations.

o.4. } ?o 2

HEATHCOTE AND MCCORMICK: PHLOGOPITE AND EVOLUTION OF CARBONATITE 137

on l

44

5.0 ^2+ 60 70K

Fig. 5. Plot of R'?* vs. Si; symbols as in Fig. 2. Intersite sub-stitution line, Si:R'?*, represents dioctahedral-trioctahedral sub-stitution; 2Si:R'?* represents Ti-Tschermak substitution. Sub-types I and Ia are indistinguishable on this scattergram, as arethe two fenite fields. Field II and III elongations along the -Si:R'?* trend are caused by assumption that all Fe is octahedrallycoordinated Fe2* when tetrahedrally coordinated Fe3* is actuallyalso present.

Differentiation of the parental Potash Sulphur Springs sil-icate magma may reasonably be assumed to have pro-duced a petrogenetic sequen@ such as that documentedfor other complexes (e.g., King, 1965; Sorensen, 1974;LeBas, 1977; Robins, 1980), wherein early pyroxenite ormelteigite is succeeded by ijolite or malignite, with sy-enites forming latest. Clinopyroxenes crystallizing duringdifferentiation ofthis sequence grade from salitic towardacmitic compositions (e.g., Nash, 1972). The presence ofsalite xenocrysts and the absence of aegirine-augite inphlogopite sovite of the Potash Sulphur Springs complexindicate that sovite formed later than salite-bearing py-roxenite and melteigite and possibly prior to the forma-tion of aegerine-augite-bearing ijolite and malignite. Fur-thermore, subtype I phlogopite contains the highestproportion of Ti siderophyllite component in the com-plex, which indicates that it formed at higher temperatureor pressnre (e.g., Robert, 1976) than the other phlogopitetypes.

The presence in alvikite of abundant sovitic, type Iphlogopite xenocrysts and unaltered, ijolitic, aegirine-au-gite xenocrysts indicates that alvikite is intimately linkedwith sovite and ijolite; salite xenocrysts from melteigiteexhibit reacted margins in alvikite. Alvikite containsmagmatic (euhedral) titanite, aegirine-augite, nosean, andmelanite; it is mineralogically similar to ijolite. For thesereasons, and the established relative ages of phlogopite

2 0 3 0 4 0 5 0 6 0

M g

Fig. 6. Plot of Mg vs. Si; symbols as in Fig. 2. Intersite sub-stitution ratio, 4Mg:Si, along which the data broadly trends, canresult from combined dioctahedral-trioctahedral, annite-phlog-opite, and Ti-vacancy substitutions acting in unison toward Mgenrichment.

types, alvikite is interpreted to have formed as a hybridmagma when ascending sovite magma encountered apocket of residual ijolitic magma (Fig. 7). The abundancein alvikite of ijolitic xenoliths and xenocrysts is inter-preted to indicate that the ijolite magma pocket wasmostly crystalline at the time of the encounter with sovitemagma, and only a residual, interstitial ijolitic liquid waspresent. An additional indication ofthe interpreted pet-rogenetic relationship is the similar cation ratios foundbetween salitic clinopyroxene and type I phlogopite andbetween aegirine-augite and type II phlogopite. The par-allel variations of Si, Al, Ti, Mg, Fe, and Fe oxidationevident between clinopyroxenes contained in melteigiteand ijolite and between phlogopites contained in phlog-opite sovite and alvikite also suggest the interpreted pet-rogenesis (Tables I and 2).

Subtype Ia phlogopite is found only in zoned xeno-crysts in alvikite, indicating that it is not a crystallizationproduct of sovite magma. It is suggested that subtype Iarepresents a transient stage of Fe uptake by phlogopitethat occurred between the initial encounter of sovite withthe more Fe-rich ijolite and the subsequent formation ofalvikite magma. After subtype Ia crystallized, the alvikitemagma abruptly began to crystallize type II phlogopite,which involved an increase in available Si and depletionof Al over what had been available during crystallizationofsubtype Ia.

The petrogenetic relationship between fenitized rocksand carbonatites of the Potash Sulphur Springs complexremains obscure, but the following relationships have beenfound in this study. Some orthoclase fenite samples ("F,"Figs. 2-6) contain type II phlogopite, whereas others("Fenite," Figs. 2-6) contain phlogopite on an edge ofthetype II field, with tetrahedral Fe3* similar to the type III

138 HEATHCOTE AND MCCORMICK: PHLOGOPITE AND EVOLUTION OF CARBONATITE

Fig. 7. Schematic representation of magmatic activity in the Potash Sulphur Springs complex between the time of phlogopitesovite (Cr) formation and later solidification of ferriphlogopite sovite (Cr). This period is interpreted to have preceded the formationof syenite. Relative ages of magmatic units are indicated by crosscutting. Dots noted as sample points represent sample coverageof the geologic units and crosscutting relationships discussed in text. Mica types are noted adjacent to the units in which they arefound.

composition of sovite specimen M488 (Fig. 2). The pres-ence oforthoclase fenite xenoliths (fenitized chert) in al-vikite indicates that the alvikite magma formed afterK-Al-bearing fenitization, but the presence of type IIphlogopite in phlogopitized silicate rocks, and also in somesamples of orthoclase fenite, suggests that a Mg-Fe-bear-ing fenitization event followed K-Al-bearing fenitizationand coincided with the presence of alvikite magma. Thedrop in Al content between subtype Ia and type II phlog-opite on the alvikite trend may relate to feldspathic (K-Al-bearing) fenitization ofthe chert country rock.

The presence of alvikite and fenite xenoliths in ferri-phlogopite sovite indicates that evolution of sovite mag-ma, from crystallization of subtype I to crystallization oftype III phlogopite, bracketed the formation and solidi-fication of alvikite magma. Oxidation of Fe in sovite toproduce ferriphlogopite must have occurred during or af-ter the suggested Mg-Fe metasomatism. The sovite mag-ma apparently solidified prior to the formation of syeniteand sovite pegmatite.

Oscillatory zoning in late-formed phlogopites of bothalvikite and sovite suggests these magmas were notstrongly stirred prior to solidification, but experiencedinstead a quiescent period that permitted expression ofsubtle crystalJiquid interface processes, similar to whathas been proposed to produce oscillatory zoning in ig-neous plagioclases (e.9., Alldgre et al., l98l). In earlier(hotter) magmatic stages, conditions for oscillatory zon-ing either did not occur or such zoning was obscured byionic diftrsion in the micas as suggested by Loomis ( I 983).Oscillatory zoning in alvikitic phlogopite involves ex-change of Ti siderophyllite and ferriphlogopite compo-nents (Figs. 2 and,4). The zoning in ferriphlogopite soviteis more subdued than in alvikite and is dominated byexchange of Si and Al; Fe3* evidently is not involved (Fig.

2). This difference in phlogopite oscillatory zoning be-tween the two carbonatites may be due to the relativedepletion of Fe and Al in late-stage sovite magma com-pared to alvikite magma.

A modification of the standard designation for carbon-atites (LeBas, 1977, p. 268) is applied to units of thePotash Sulphur Springs complex on the basis of relativeages determined in this study: C, : phlogopite sovite(phlogopite subtypes I and Ib), C, : alvikite (t1pe II), andC, : ferriphlogopite sovite (type III). In the standard des-ignation, ferrocarbonatite is usually designated C.. Latercarbonatites are also present, but their place in the evo-lution of the complex is not clear.

Sovite magma (C,) remained mineralogically simple,crystallizing phlogopite, magnetite, and apatite, until latein its history (Cr) when other minerals (e.g., pyrochlore)formed. The relative abundance of calcite and dolomitedid not change appreciably during this evolution. Com-positional evolution ofsovite, as recorded in phlogopite,involved progressive Mg and Si enrichment and Fe, Ti,and Al depletion concurrent with Fe oxidation.

Alvikite magma (C) is interpreted to have formed whensovite magma (C,) encountered residual ijolite magma.Alvikite magma then evolved rapidly to a relatively uni-form composition richer in Si, Fe, and Ti (Ti as titanite)than sovite magma. The presence of magmatic nosean(with Na:K estimated as 7: l) in alvikite indicates that thismagma also contained sulfate and was enriched in alka-lies relative to sovite. This means that alkalies must havebeen contributed by the ijolite magma when alvikitemagma formed.

The Fe depletion and Si and Mg enrichment of car-bonatite determined in this study mirrors the cationmovement in fenitization of metapelites by carbonatitethat has been suggested by Mian and kBas (1987), but

SEQUENCE OF EVENTSOLDEST AT BOTTOM

. SAMPLE____ - PRESENT EXPOSURE

1n l.iirl rennteHrocoprrE sovrrE - cal : : , r : ' . ' , : l

4 ffif l alvrxrrr- ce, wrrH AUREoLEIo [ff i 'ql loF PHLOGOPITIZATION

rb @l pHloooprrE SOVTTE- crI I ]

RESIDUAL IJOLIT IC MAGMA

MELANEPHELINITE, PHONOLITE.IJOLITE AND FENITE

SEDIMENTARY ROCK CONTACT

Al depletion evident in carbonatite of the Potash SulphurSprings complex contrasts with the Al movement fromwallrock toward carbonatite inferred by those authors.

The petrogenetic relationship between C, and C, car-bonatites is interpreted to be purely accidental. The mix-ing event between silicate and C, carbonatite magmascould easily have occurred at a later stage, ifat all, in theseparate evolutions of the original magmas and then wouldhave yielded different mineral assemblages and a differentcarbonatite suite.

AcxNowr,nocMENTS

We want to express our glatitude to Norman F. Williams, Director ofthe Arkansas Geological Commission, for the generous financial supportfumished by the Commission for instrument and time charges for micro-probe analyses. Sigma Xi and the Graduate College of the University ofIowa kindly fumished partial subsidy for drafting and photographic charges.We wish especially to thank Don R. Owens (Hot Springs, Arkansas) andUnion Carbide Corporation for hospitality, help, and access to the WilsonSprings Vanadium Operation. J. V. Smith, as always, was most helpfulin furnishing us with microprobe standards and allowing us time on theUniversity ofChicago microprobe. The assistance and hospitality oflanSteele and Rick Hervig during microprobe sessions at the University ofChicago are gratefully acknowledged. Reviews of an earlier draft of thispaper by J. A. Speer and R. J. Arculus are much appreciated.

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Mlruscnrrr RECETVED JAN(mv 7, 1988Mmuscnrm Accm SerrslGBR.26. 1988