Alkaline igneous rocks of Magnet Cove, Arkansas: Metasomatized

19 Pages·2007·2.31 MB·English

American Mineralogist, Volume 74 , pages I I3-13 I, 1989
Alkaline igneousr ocks of Magnet Cove, Arkansas: Metasomatizedi jolite
xenoliths from Diamond Jo quarry
M.cnrA,J . K. Fr,onn, Mlr,cor,vr Ross
959 National Center,U .S. GeologicaSl urvey,R eston,V iryinia 22092,U.5.4.
Ansrucr
Ijolite xenoliths occur in garnet-pseudoleucites yenite that forms part of the outer ring
of the Magnet Cove alkaline ring-dike complex. Xenoliths were collected from the Dia-
mond Jo quarry located in the southern part of the complex. The xenoliths contain abun-
dant diopside and Ti-rich andradite garnets and have been extensively metasomatized.
Garnet-pseudoleucites yenite and nepheline syenite are identified as possible sourceso f
the volatile-rich fluids responsiblef or the metasomatismo f the ijolite xenoliths. Residual,
volatile-rich fluids from the ijolites may have also produced autometasomatica lteration
before the ijolites were entrained as xenoliths in the garnet-pseudoleucites yenite. Non-
isotropic hydrogarnet,f ormed during metasomatism,c ontains up to 3.5 wto/oF . Aegirine,
biotite, and melanite garnet, also formed during metasomatism, contain relatively high
amounts of V andlor Ti, indicating that these elementsw ere transported by the metaso-
matic fluids. Mineral compositions and geochemicald ata indicate that the ijolite xenoliths
share many characteristicsw ith the intrusive ijolites that form the inner rings of the
Magnet Cove complex.
INrnolucrroN then consider the petrogeneticr elationships ofthe ijolite
The Magnet Cove complex is an alkaline ring-dike xenoliths to the garnet ijolite and biotite-garnet ijolite
complex that crops out in an area of approximately 12 that form the inner rings of the complex.
km2 in Hot Spring County, Arkansas. The igneousr ocks
of the complex intrude deformed Paleozoic sedimentary Gnor,ocv
rocks of the Ouachita Mountains. Biotite K-Ar and Rb- The rocks of Magnet Cove are host to numerous min-
Sr ageso btained by Zartman (1977) indicate a Cretaceous eral species,a nd much of the early literature about the
age of 95-102 Ma for the complex. Fission-track ages complex dealsw ith the minerals. The first detailed petro-
(Eby, 1987) indicate two periods of igneous activity at graphic study and geologic map of Magnet Cove were
Magnet Cove. The mean sphenea gef or the emplacement made by Williams (1891).E ricksona nd Blade( 1963)r e-
of the silicater ock units is 101.4 + 1.0 Ma, whereast he mapped the Magnet Cove complex and presenteda de-
carbonatite,w hich occursi n the core of the complex, was tailed lithologic and geochemical study of the igneous
emplaceda bout 5 m.y. later, as indicated by a mean apa- rocks and the surrounding country rocks. A generalized
tite ageo f 95.9 + 0.4 Ma. MagnetC ove is one of several geologicm ap of the complex basedo n their detailed map
alkaline igneous intrusions in central Arkansas. Morris is given in Figure l. Erickson and Blade (1963) concluded
(1987)d escribedth is diverses uiteo frocks and discussed that phonolites and trachytes were intruded first, fol-
possible petrologic relationships among them. One pur- lowed by jacupirangite and the syeniteso fthe outer ring,
pose of the current study of the igneousr ocks from Mag- then the inner-ring ijolites, and finally carbonatite,w hich
net Cove is to elucidate the relationships among the var- is found in the core of the complex. Erickson and Blade
ious rock types and to compare the petrologic evolution (1963) proposedt hat the igneousr ocks of the Magnet
of Magnet Cove to that of the other intrusions of the Cove complex are related by differentiation and fraction-
Arkansas alkaline province. al crystallization of a residual, volatile-enriched phono-
Metasomatism has played an important role in the pet- lite magma and that this magma was derived by fraction-
rogenetich istory of these rocks. All of the samplesf rom al crystallization of an undersaturated olivine basalt
the Diamond Jo quarry examinedt hus far show extensive magma.
replacemento f primary magmatic minerals by secondary Exposureso f fresh rocks are scarcew ithin the complex.
phases.I n this paper, which is the first ofa serieso n the However, the opening of the Diamond Jo quarry in the
Magnet Cove complex, we describeb oth the primary and latter part ofthe l9th century to supply stone ballast for
secondarym ineral assemblagesfo und in ijolite xenoliths the Hot Springs Railroad exposedm uch fresh unweath-
from the garnet-pseudoleucites yenite that forms part of ered rock. This quarry, which is located in the southern
the outer syenite ring of the Magnet Cove complex. We part of the complex and within the outer syeniter ing (Fig.
0003-004x/8/9o l024r I 3$02.00 I t3
rt4 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS
ffilacupirangite
E '":;:;;i,'p he Ii ne
f,l carnet-paeudo-
leucite syeniie
ffiPnonolirE, Trachyte
Ifl carnet-biotite
melteigita
El Garnet ijotite
Q eiorite-sarnet
iiolite
N rine-grainea
ijolite
I
n Carbonatite
Metagediments
Fig. 1. Generalizedg eologicm ap of the Magnet Cove alkaline igneous complex, Hot Spring County, Arkansas, after Erickson
and Blade (1963). The Diamond Jo quarry (solid triangle) is the sampling site of the ijolite xenoliths describedi n this paper. The
open and solid circles mark, respectively,t he approximate sampling locations ofgarnet ijolites99423/7 and 85-13B-RSS;t fe open
and solid squaresm ark, respectively,t he approximate sampling locations of biotite-garnet ijolites 99423/6 and 85- I 6-RSS.
l), was mapped in detail by Owens and Howard (1989), Wrror,B-nocK coMPosrrroNs oF
and a simplified version of part of their map is shown in M,LcnBr CovB rror,rrps
Figure 2. The two rock types exposed on the walls of the Whole-rock major-, minor-, and trace-element com-
quarry are nepheline syenite (originally mapped by Erick- positionso f ijolite xenoliths 5-DJ7, l-166, and l-143
son and Blade, 1963, as nepheline syenite pegmatite) and (Tables l, 2) reflect the modal diversity of the ijolite xe-
garnet-pseudoleucite syenite. The contact between these nolith suite (Table 3). Included in Tables I and 2 are
two rock types is relatively sharp. The garnet-pseudoleu- analyseso fa garneti jolite and a biotite-garnet ijolite from
cite syenite contains abundant xenoliths ofijolite, ranging the inner-ring ijolite for preliminary comparison with the
from a few millimeters to 3 m in diameter. Xenoliths of
ijolite xenoliths. Major- and minor-element composi-
Stanley shale, now metamorphosed to hornfels, are also tions ofseveral additional ijolites from the inner ring are
found within the garnet-pseudoleucite syenite. Prelimi- reportedb y Erickson and Blade (1963, rheir Table l7).
nary petrographic and electron-microprobe data from the
Descriptions by Erickson and Blade indicate that these
Diamond Jo syenites and ijolite xenoliths were given by samplesa re similar to the inner-ring ijolites included in
Ross (1984) and Flohr and Ross (1985a, 1985b).
this study. Overall, the inner-ring ijolites are slightly more
magnesiana nd contain significantly less F and CO, than
Mrrrrors oF ANALYSTs
the ijolite xenoliths.
Mineral compositions were obtained by using an automated Of the three analyzedi jolite xenoliths, l-143 contains
AnL-sruqr electron microprobe operating at 15 kV with a beam
the highest abundanceo f clinopyroxene plus garnet and
current of 0. I pA using either an 8- or 9-spectrometerc onfigu-
is enriched in Ti, Fe, and Ca and depleted in Si, Al, and
ration. Correction procedures of Bence and Albee (1968) and
Na comparedw ith samplesl -166 and 5-DJ7. Xenolith
alpha-factor modifications of Albee and Ray (1970) were used.
During analysis of cancrinite and natrolite, the electron beam l-143 is alsoe nrichedi n V, Y, andZr, comparedt o sam-
was rasteredo ver an area of 4 pm'?i n order to minimize loss of ples 5-DJ7 and l-166. These are trace elementsc om-
Na due to mobilization. Data collection proceduresw ere out- monly found in notable concentrations in garnets from
lined by McGee (1983, 1985),a nd backgroundsw ere calculated alkaline igneous rocks (e.g., Deer et al., 1982 and this
using an interpolation routine describedb y M. Mangan and J. study). Schnetzlera nd Philpotts (1970) also reported that
J. McGee (written comm., 1985).A variety of natural and syn- garnet has high distribution coefrcients for the heavy rare-
thetic materials were used as standards.T raverses of clinopy- earth elements( HREEs)c omparedt o other common rock-
roxenesw ere obtained using an ARL-EMx3 -channele lectron mi-
forming minerals. In the ijolite xenoliths, as the abun-
croprobe. X-ray single-crystala nd powder-diffraction data were
dance of garnet increasesf rom 100/oin 5-DJ7 to an av-
obtained on selectedc rystals. Whole-rock analyses were ob-
tainedo n threei jolite xenoliths( l-166B, 1-143,a nd 5-DJ7)a nd erageo f 37o/oin I - I 4 3, the HREE concentrationi ncreases
two inner-ringi jolites (85-13B-RSSa nd 85-16-RSS)u singa na- (Fig. 3). Garnet ijolite 85-I3B-RSS from the inner-ring
lytical methodsd escribedb y Baedecke(r1 987). ijolite has REE concentrations( Table 2) and a REE pat-
tern similar to those of the ijolite xenoliths, whereasb io-
t Any use of trade names in this report is for descriptive pur- tite-garneti jolite 85-16-RSSh as a much more fraction-
poseso nly and does not imply endorsementb y the U.S. Geo- ated REE pattern. Ijolites from the Fen complex, Norway
logical Survey. (Mitchell and Brunfelt, 1975), Seabrook Lake, Ontario
FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS ll5
mssr '2-D J 12
^-e.99,11 J:1-116
^'{ffi3^}-5t7+s
6-DJ 10
n\
:r"-"?if#ii "t. 4-DJ 10
;>9'$'
TALUS ii;:ii:ili::\-G ,.
2-Dr7 a?,_ 01-DJ7 \_ri_!-
Is-DJlO
, l;':".o]-',
DIAilOND JO OUARRY
O CORE SAMPLE
I BULK SAMPLE
./ coNTAcr
Fig. 2. Geologic map of the Diamond Jo quarry, Magnet Ms, Stanley shale; f, fenite; ns, nepheline syenite xenoliths (?);
Cove complex, after Owens and Howard (1989) indicating lo- gi, garnet ijolite xenoliths; ms, metamorphosed Stanley shale
cations of all core and bulk sampleso f ijolite xenoliths and sy- xenoliths; MCZ, mafic clot zone; CH and TH, crest and toe
enitesc ollected.S amples8 4-1-RSS-Aa nd B are grab samples respectively,o f highwall. The solid triangle marks the location
collected from the talus slope. Contour interval is 10 ft (3.048 of surveys tationD J8 atlat3426'17"N and long9 251'45'W.
m). NS, nepheline syenite; GPS, garnet-pseudoleucites yenite;
(Cullersa nd Medaris, 1977) and Oka, Quebec( Eby, 1975), all three xenoliths (Table 2) are notable exceptions.I jo-
have relatively fractionated REE patterns that are similar lites from the inner ring contain significantly lessP b than
to the pattern of the biotite-garnet ijolite from Magnet the ijolite xenoliths. Garnet ijolite 85-I3B-RSS shares
Cove. Mitchell and Brunfelt (1975) noted rhar with in- some characteristicsw ith the ijolite xenoliths, but is de-
creasing abundance of garnet, the REE patterns of the pleted in Ba and Rb relative to them, unlike garnet-bio-
ijolites from the Fen complex becamep rogressivelyf lat- tite ijolite 85-16-RSS,w hich containss ignificantlym ore
ter, i.e., the light REEs (LREEs) are lesse nriched relative Ba. and also Sr. but has a similar amount of Rb. Addi-
to the HREEs. Abundance of garnet in the ijolites from tional data are neededt o fully compare the geochemistry
Magnet Cove thus exercisess ignificant control over the of the ijolite xenoliths and inner-ring ijolites.
REE patterns of these rocks.
Despite the range of trace-elementc ompositions in the Pprnocn-q.pHy AND MTNERALc oMposrrroNs oF
three analyzedi jolite xenoliths, these rocks are generally THE IJOLITE XENOLITHS
typical of mafic alkaline rocks (e.g.,G erasimovsky et al., Nine samples of ijolite xenoliths were collected from
1968;G erasimovsky,1 974;S age,1 987).R elativelyh igh the Diamond Jo quarry; six are bulk samples( l-143A,
concentrationso f Pb in xenoliths 5-DJ7 and l-166 and l-143B, l-1668, 5-DJ7, 84-l-RSS-A,a nd 84-l-RSS-B)
low concentrationso f Th. Nb. and Ta. and low Th/U in and three are drill-core samplesh aving diameters of 2.54
l16 FLOHR AND ROSS:M ETASOMATIZED IJOLITE XENOLITHS
TABLE1 , Whole-rockm aior- and minor-elemenct ompositions TABLE2. Whole-rockm inor-a nd trace-elemencto mpositions(i n
(in wt%) of ijolites from the Magnet Cove igneous ppm) of ijolitesf rom the Magnet Cove igneousc om-
complex,A rkansas plex,A rkansas
ljolitex enoliths, ljolitex enoliths,
DiamondJ o quarry Inner-ringij olites DiamondJ o quarry Inner-ringij olites
Sample: 5-DJ7 1-166 1-143 85-13- 85-16 - Sample: 1-166 1-143 85-138- 85-16-
n5b RSS H55 H55
sio, 40.5 37.5 35.4 41.1 34.2 21 8 158 zc.c 24.5 92.2
Tio, t.o 23 5.9 2.40 361 Ce 28.8 20.5 386 93
Alro3 19.1 179 12.O 14.2 lo. I Nd 10.8 11.0 21 6 197 26.0
Fer03 c.J 5.3 8.9 5 56' 8.13 ' Sm 2,74 3.61 10.29 4.6 6.8
FeO 22 39 3.8 3.27 285 Eu 0.94 1.34 469 1.59 2.46
MnO 0.21 0.19 0.28 0.40 030 Tb 0.527 0.98 0.94 1.31
Mgo 3.0 3.8 3.3 5.13 7.16 Yb 3.20 6.51 477 5.59 2.03
CaO 13.1 14.6 19.5 16.2 14.0 Lu 0.478 0.91 6.01 0.80 0.204
Naro ao 7.3 4.3 6.03 3.40
KrO 3.6 2.7 1.7 3.20 2.34 Ba 778 440 617 25 2040
Rb 171 117 149 51 119
DA 064 0.80 0.72 1.08 1.22
H| 2rvO5 1.40 1.78 1.92 0.46 5.99 Sr 874 695 843 678 1120
Nb 32 12 54 21 224
CO, 0.74 2.0 0.98 0.28 0.24
cl 0 055 0.140 0.009 0.02 <0.01 Ta 121 0.80 3 05 17 3 378
Zl 439 572 1720 714 42
F 045 0.30 0.24 0.07 0.04
S 023 0.26 0.39 0.21 0.42 Hf 505 70e el o 13.3 0.69
Y 28 52 394 37 26
Sum 101.1 99.61 100.0 Th 0.97 019 157 1 77 1.06
U 1.09 0.72 2.05 1.33 5.99
Note; Maior oxides,e xcludingF eO,d eterminedb y tcp-AE(Sin ductively
coupledp lasma-atomice missions pectrometryb) y M Kavulak,J . Mari- Sc 031 0.74 6.30 4.71 2.31
980 970 1300 810 700
nenko,a nd R. Rait in the ijolitex enolithsa, nd by X-rayf luorescenceb y J.
Cr 4.4 3.6 34 7.6 3.6
Taggart,A . J. Bartel,a nd D. Siemsi n garneti jolite8 5-138-RSSa ndb iotite-
garneti jolite8 5-16-RSSF. eO,H ,O,C O,,S , F,a ndC ld eterminedb y various Co 4.68 13.8 16.0 18.9 36.2
Ni <50 <50 <70 <70 <80
techniquesbyEB. randt,J.H Christie,J.S harkey,L .Jackson,M Kavulak,
Cu 12 11 5.0 18 26
J. lVarinenkoa, nd N. Rait.
. Zn 52.8 47 153 55 82
Feros calculatedf rom total Fe obtained as Fe,Oaa nd reported FeO.
Sb 25.9 3.20 10.2 0.155 0.19
3.88 2.78 3.18 0.33 136
Pb 94 15 47 <5 <5
ZrlHt 86 72 54 54 61
cm and lengthsr anging from 7.01 to 9.75 cm (4-DJ7, Nb/Ta zo 15 18 12 59
Th/U 0.89 0.26 077 1 3 0.18
3-DJ7, and 6-DJ7). Samplel ocations are shown in Figure
Rb/Ba 0.22 0.27 024 0.075 0.11
2. Samples8 4-l-RSS-A and -B areg rabs amplesc ollected
Note: Ba, Rb, Sr, Nb, Y, and Zt delerminedb y energy-dispersiveX -ray
from the talus slope.
fluorescences pectroscopy by J. Evans. V, Cu, and Pb determined by
According to the modal classificationo f alkaline rocks atomic absorptionb y M Doughton All other elementsd eterminedb y
presentedb y Sorensen( 1974), 4-DJ7, l-1668, l-143A, instrumentanl eutron-activatioann alysisb y J. Mee.
and 3-DJ7 are melteigites,a nd the rest of the samplesa re
ijolites (Table 3). By definition, ijolites and melteigites
contain between1 00/aon d 700/on epheline( Sorensen1, 974). lized primary igneous minerals and volatile-rich fluids.
Two of the ijolite xenoliths do not contain any nepheline Thesef luids may be residual fluids from the ijolites them-
(Table 3) owing to replacementb y cancrinite and natro- selves or be derived from external sources.)D iopsidic
lite. For the sake of simplicity and to be consistentw ith clinopyroxene, nepheline, and garnets of Groups I and
the terminology used by Erickson and Blade (1963), we IIA (definedi n the section entitled Garnets)a re identified
will collectively refer to the suite of samples as ijolites. as the primary rock-forming minerals. Primary accessory
Photomicrographs (Figs. 4a4d) illustrate the varied minerals, which occur as inclusions within diopside,
modes and textures found in the ijolite xenoliths and in- nepheline, and/or garnets of Groups I and IIA, are pe-
ner-ring ijolites. rovskite, equant apatite, and magnetite. Progressiver e-
Despite the variable modes found in the ijolite xeno- placemento f nephelineb y minerals formed during meta-
liths, mineral compositions indicate that theser ocks form somatism is observed. Total replacement of nepheline
a coherent group. The xenoliths experiencedv arying de- occursi n some of the ijolite xenoliths (Table 3). Replace-
greeso f metasomatism resulting in the textural and, to ment of nepheline commencesb y the formation of can-
some extent, the modal variations seena mong samples. crinite along fractures within the nepheline. In more ad-
Metasomatism is also responsiblef or some of the com- vanced stageso f replacement, intergrown grains of
positional differencesf ound in the minerals, both within cancrinite and, usually, subordinate amounts of natrolite
individual samplesa nd among different samples. and calcite are found, and only small patcheso f nepheline
Primary igneousm inerals are distinguishedf rom later- remain. Magmatic textures are preservedi n ijolite xeno-
formed metasomatic minerals by using textural criteria. liths 6-DJ7 and 5-DJ7 that contain only partially replaced
(Metasomaticm inerals are here defined as those minerals nepheline.I n xenolith 6-DJT,laths of diopside are ophit-
formed as the result of reaction between earlier-crvstal- ically to subophitically enclosedb y nepheline,a nd 5-DJ7
FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS 1t'7
hasa n intergranulart o subophitic texture.A s replacement
of nepheline by hydrous silicates and calcite progresses,
diopside is replacedb y biotite. Biotite is never observed
o
replacing diopside where the latter is in contact with un- =
altered nepheline. Similarly, low-Ti garnets (Groups III al o0
and IV) never occur as overgrowths or rims on the high- so
Ti schorlomite or melanite garnets( Groups I or IIA) where o
these primary garnets are in contact with diopside or o
g
nepheline;r ather garnetso f Groups III and IV formed by
o.
reaction between primary garnet and metasomatic fluid. E
The melanite garnetso f Group II are somewhatp roblem- @6ro
atic in their petrogenesis,a s they rim primary Group I
schorlomite garnets and also form subhedral grains in-
5
tergrown with metasomatically formed phases.G arnets
of Group II may representa transitional stageo f garnet Fig.3 . Chondrite-normalizeradr,e -earth-elemeprat tternsfo r
crystallizationA. secondg enerationo f apatitef orms acic- ijolite xenoliths( I - I 43, I - I 66,a nd5 -DJ7)a ndi nner-ringij olites
ular to tabular grains within masseso f cancrinite, natro- (garneijto lite8 5-l3B-RSSan db iotite-garniejot lite8 5-16-RSS).
Iite, and calcite. Aegirine-augitea nd a secondg eneration
of magnetitea rea ssociatewd ith metasomatics ilicatesa nd
calcitea nd appeart o be contemporaneousw ith thesem in-
erals. + Fe2*+ Fe3*):0 .447 to 0.619,A lrO3: 7.15t o l2.l
wt0/0T, iO, : 1.45t o 3.92 wto/oN, arO : 0.45 to 0.70 wt0/0,
Clinopyroxenes and MnO : 0.17 to 0.45 wt0/0C. aO is uniformly high,
Primary clinopyroxenes occur in all the ijolite xeno- ranging from22.7 to 24.4 wt0/0.P leochroic light- to me-
liths. In all the samples except 6-DJ7, these pyroxenes diumlilac or pink sectorsa re consistently higher in Al,
are subhedral to euhedral, sector zoned, pleochroic, and Ti, and total Fe and lower in Mg than pleochroic light-
partially replaced by biotite. Fine, oscillatory zoning is to medium-green sectors.N o differencesi n the concen-
superimposedo n the sector zoning in some grains. Most trations of Ca, Mn, and Na were detected between sec-
ofthese clinopyroxenesa re 2-5 mm long, although grains tors. A traverseo f one sector-zonedd iopside, 2.6 mm
up to about 2 cm long are found in the coarsests ample, across,f rom sample 84-l-RSS-Al (subhedralg rain shown
4-DJ7 (Fig. 4b). (The nomenclature used in describing in the lower left quadrant of Fig. 4a) was made with the
pyroxenesf ollows that recommendedb y the Internation- microprobe. Count data for Al, Ti, and Si were collected
al Mineralogic Association Subcommittee on Pyroxenes, at l5-pm intervals. The data indicate that the composi-
Morimoto, 1988.)T he sector-zonedc linopyroxenesa re tional boundary between sectors is abrupt, with Ti and
subsilicic aluminian ferrian diopsides( Table 4, nos. l4). Al significantly decreasingin concentrationf rom the out-
The overall compositional rangesa re as follows: Mg/(Mg er to the inner sectorsa nd Si slightly increasing.W ithin
Tlere 3. Modes( in vol%)o f iiolitesf rom the MagnetC ove igneousc omplex,A rkansas
ljolitex enolithsf rom DiamondJ o quarry Inner-riniigo lites
Sample: 4-DJ7- 1-1668- 5-DJ7- 6-DJ7- 84-1- 84-1- 1-1434- 1-1438-3 3-DJ7- 99423171 99423161
1,2 4 1,5 1,3 RSS-AI RSS-B2 1.2 1,2
Clinopyroxene 551 37.8 33.3 287 19.5 19.4 16.6 16.5 10.7 25.8 0
Garnet 2.8 17.4 104 112 26.9 19.7 39 5 34.4 43.9 9.0 16.2
Biotite 164 157 82 8.8 8.4 11.6 12.6 21.5 2.7 11.8
Sphene 28 06 0.8 5.0 1.8 1.5 1.6 0.8 2.8 00
Apatite 39 34 13 2.9 3.4 2.1 1.2 1.1 2.3 no n
Magnetite 02 0 0.2 0.4 trace trace trace 0 0.1 2.3 7.1
Perovskite 06 0 0 08 0 0 03 0 0 5.4 1.9
Nepheline 0 0.6 28.2 21I 0.5 11.4 0.3 0 0.1 52.4- 54.4+
Cancrinite 13.4 5.4 137 19.0 to./ 26.3 11.4 20.8 0 0.4
Natrolite 0.2 155 29 0 16.0 9.5 15.0 11. 7 11.7 00
Carbonate 42 2.6 U.C U5 5.0 la 18 1.9 o.2 4.2
Sulfide 0.2 1.0 03 0.4 0.8 o.2 0.4 trace 0.1 u,o J.c
Other 0.1 0 0.1 04 0 o.2 02 0.2 0.4 o.4 0.4
99.9 100 99 I 100 99.9 99.9 99.9 100 100 oaa ooo
No points 985 907 957 979 968 r0 07 975 931 908 1030 944
t Thin sectionsw ere obtainedf rom the F. C. Calkinsc ollectiono f 1918,c uratedb y the NationalM useum,W ashington,D .C. Sample9 942317is a
garneti jolitea nd sample9 9423/6i s a biotite-garneitjo lite
. Sum of altered( 3 2%) and unaltered( 49.2'/"]'nepheline
i Sum of altered( 50 1%) and unaltered( 4 3%) nepheline
I l8 FLOHR AND ROSS:M ETASOMATIZED IJOLITE XENOLITHS
a
'i ii a9 - !:
{
Fig 4. Photomicrographso f thin sectionso f ijolites taken in calcite, and euhedrala patite (especiallya bundant in lower right,
plane-polarizedli ght to illustratev aried modesa nd textures.F ield near edgeo f section).I nner-ring ijolites are shown in (c) and (d).
ofview is 2. I cm acrossi n all photomicrographs.X enoliths from (c) Garnet ijolite 99423/7: laths ofclinop).roxene are interstitial
Diamond Jo quarry are shown in (a) and (b). (a) Sample 84-l- to and included in equant (white) nepheline. Black areas are
RSS-AI: euhedral to subhedral "atoll"-structured schorlomite magnetite replacing perovskite. Clinopyroxene, together with
garnets( black) with euhedral, tabular, sector-zonedc linopyrox- minor garnet and biotite, is also intergrown with the oxides
enes marginally replaced by biotite in a light-colored ground- forming clotso r aggregate(sd. ) Garnet ijolite 85-l3B-RSS:l aths
mass of cancrinite, natrolite, and calcite, with minor melanite of diopside (gray) are interstitial to nepheline (white) in lower
garnet (dark) and acicular apatite (lower-left quadrant). (b) Sam- left and upper right and form at aggregatein central part of the
ple 4-DJ7- 2: clinopyroxene (abundant tabular, subhedralt o eu- photograph. Large black grains are garnet; smaller black grains
hedral gray crystals)i n a light-colored groundmasso f cancrinite, are gamet and sulfide.
both sectors,T i and Al show minor fluctuations in con- They are continuously zoned. Some laths are rimmed by
centration. aegirine-augitea nd are not partially replaced by biotite
In 6-DJ7, the finest-grained ijolite xenolith, primary as are the diopsides from the other xenoliths.
clinopyroxenesfo rm laths up to 2 mm long, someo f which Pyroxenes,w hich are the products of metasomatism,
are twinned. Clinopyroxenesf rom 6-DJ7 arc also subsil- are found in several of the ijolite xenoliths. In 5-DJ7
icic aluminian ferrian diopsides (Table 4, no. 5) but are, (Table 4, no. 6) and 4-DJ7, pale- to medium-greenp leo-
on the average,m ore magnesiant han the sector-zoned chroic aegirine-augitei s intergrown with and is appar-
diopsides from the other ijolite xenoliths, with Mg/(Mg ently replacing bioite, which itself has replaced primary
* Fe2* + Fe3*): 0.555 to 0.729. Thesed iopsidesa re diopside. Aegirine-augite having a similar composition
also slightly less aluminous than the sector-zonedd iop- occursa s singleg rains or aggregateisn the groundmasso f
sides and have AlrO, ranging from 5.61 to 9.07 wtVo. 3-DJ7. Ti-rich aegirine( Table 4, no. 7) in 4-DJ7 occurs
FLOHR AND ROSS:M ETASOMATIZED IJOLITE XENOLITHS l 19
TaaLe4 . Clinopyroxenefsr om the ijolitex enoliths,D iamondJ o quarry
Qamnro. 84-1-RSS-82 5-DJ7-24 6-DJ7-1 5-DJ7-2A 4-DJ7-2
Weight percent
sio, 39.9 41.8 41.5 43 9 45.5 50.9 51I
Tio, 3.42 251 2.30 1.85 2.38 0.18 3.22
Alros 11.2 9.78 8.80 777 6.21 0.89 1.52
V.O. nd no 0.19 0.20 0.12 0.09 u.5b
Fer03. 8.37 845 10.5 823 I -31 9.32 26.6
FeO- 4.39 4.20 4.06 527 5.11 8.35 1.20
MnO o22 0.23 0.30 038 0.40 0.46 0.17
Mgo 811 8.81 8.25 9.11 9.99 7.32 0.60
CaO 229 232 23.2 23.1 22.7 1Q R 0.70
Naro 0.52 0.51 0.59 0.58 0.87 3.38 13.2
Sum 99 03 994 9 99.69 100.4 100.8 99.39 99.57
Cations
Si 1.543 1.604 1 603 1.673 1. 719 1.961 1.977
AI 0.457 0.396 0 397 0.327 0.279 0.039 0.023
2.000 2.000 2.000 2.000 1.995 2.000 2.000
AI 0.053 0.046 0.003 0.022 0 0.001 0.045
Ti 0.099 o.072 0.067 0.053 0 068 0.005 0.092
0 0 0 006 0.006 0.004 0 003 0.017
Fe3* 0.244 o.244 0 305 0.236 0.214 o 270 o 764
Fe,* 0.142 0.135 0.131 0.168 0.161 0.269 0.038
Mn 0 0 0.010 0 0 0.015 0.005
Mg 0.462 0 503 0.475 0.515 0.553 o 421 0.034
1.000 1 000 0.997 1 000 1.000 0 984 0.995
Mn 0.007 0 007 0 0 012 0.013 0 0
M9 0.005 0 001 0 0 002 0.010 0 0
0 949 0.954 n oEo 0.943 0.918 0.763 0.029
Na 0.039 0 038 0.044 0 043 0.064 0.253 0.976
1.000 1 000 1.003 1.000 1.005 1. 016 1.005
No pts- averaged I o 7 4 5
Nofe. nd : not detected. Column headingsa s follows: 1, 2-Sector-zoned grain with no. 1 from sector richer in Al and Ti than sector characterized
in no 2; 3, 4-Sector-zoned grainw ith no 3 from sectorr icheri n Al and Ti than sectorc haracterizedin no. 4; S-Lath enclosedw ithinn epheline6; -
Secondarya egirine-augitien tergrownw ith and rimmingb iotite;7 -Fibrous aegirinein tergrownw ith magnetiteg, arnet,a nd calcitei n groundmass.
* Fe3*a nd Fe,* calculatedb y assuming ideal stoichiometryo f 4 cations per 6 oxygens.
as fibrous massesi ntergrown with pale-pink garnet, cal- (Tables 5, 6) is given in Appendix l. The data plotted in
cite, cancrinite, apatite, and titanomagnetite and also Figure 6 representt he complete data set (Groups I-IV),
forms clusters of fibers in the cores of apatite. It is the whereast he data plotted in Figure 7 are only those of the
only pyroxenea nalyzedc ontaining significanta mounts of F-bearing garnets of Group III. Andradite garnets con-
Y,0.47 to 1.04w to/oV rO.. taining > l5 wt0/0T iO, are classifieda s schorlomites,a nd
In the primary clinopyroxenes,A l primarily substitutes those containing < 15 wt0/0,a s melanites, following the
for Si in the tetrahedral site, while Ti enters the octahe- criterion of Deer et al. (1982).
dral site, resulting in the positive correlation betweenA l Group I garnets are dark-red to black schorlomites
and Ti seen in Figure 5a. Clinopyroxenes from 6-DJ7 containing 15.5-18.2w to/oT iO, and occur in all samples
contain amounts of Ti comparable with the other xeno- except 5-DJ7. Group I garnetsh ave morphologies rang-
lith pyroxenes,b ut generallyh ave slightly lower amounts ing from poikilitic with an "atoll"' structure (Fig. 4a) to
of Al. Both Al (Fig. 5b) and Ti increasew ith decreasing anhedral to subhedral grains up to 7 mm across. Com-
Mg/(Mg + Fe2+* Fe3*)i n the primary clinopyroxenes. positions are uniform among samples,a nd zoning within
Diopsides from 5-DJ7 cluster at the low Al and Ti end single crystals is slight. In contrast, garnetsi n 5-DJ7 are
of the main group of pyroxenes and also are at the low melanites containing 8-10 wto/oT iO,. Compositionally
end of the rangeo f Mg/(Mg * Fe2* + Fe3*)f or the group. these garnets belong to Group II (melanites); however,
because they are morphologically different from other
Garnets
Group II garnets, they are differentiated as Group IIA.
Garnets are classifiedi nto four groups (I to IV) based Garnets ofGroup IIA form anhedral to subhedralg rains
on compositions. These groups indicate the crystalliza- up to 5 mm acrossa nd commonly contain abundant apa-
tion sequencefo r the garnets.R epresentativem icroprobe tite inclusions.
analysesa re presentedi n Tables 5 and 6, and data are Melanite garnets (Group II) have a much wider com-
plotted in Figures 6 and 7. A detailed explanation of the positional ranget han garnetso f Group I. They form light-
methods used to calculatet he formulae of all the garnets red to brown rims, up to 50 pm wide, on some of the
t20 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS
1-5
a.
+ Group I
I Group tl
+lr[ih.;r+ x Group llA
+1r+
1,O O Group lll
A Group lV
F
,, +f*f*++ o.5
rI- oAt44--Dr-JRzs Xar-a-rr4Bo
-rr li:--3j;"'-'
,rr1*- l r"'."",, o
1.5
o.o2 0.o4 0.o6 0.o6 0.to o"t2 b.
Ti
1.o
o.5 +'t4fi+'f
, ,+*-++ +#+
r
o
2.O
o-
o.4 0.5 0.o o.7 0,4 o.9
Mg,z(Mg+ F#++ Fe3+) + 1.5
a)
o I'l
Fig. 5. Variation of total Al with (a) Ti and (b) Mgi(Mg + E Hi+*;"1
Fe'?+* Fe3+)in primary clinopyroxenes( cations per 6 oxygens)
1.O
from ijolite xenoliths and garnet ijolite99423/7. Symbol key for
both plots is given in (a).
o.5
schorlomites and occur as small subhedral grains in the 2.O 2.25 2.5 2.7 5 3.O
groundmass.M elanites contain slightly more Mn and Ca si
and less Al, Mg, and Fe2* on the average than do the Fig. 6. Variationo f (a) Ti, (b) Al, and (c) Fe3*w ith Si in
schorlomites. garnets( cationsp er 12 oxygensf)r om ijolite xenoliths.I n (a),
Group III garnets (Table 6) are the most unusual be- GroupI II garnetsd o not lie alongt he trendd efinedb y Groups
causet hey contain as much as 3.56 wto/oF . They form I, il, IIA, andI V andh encea ren ot plotteds oa sn ot to obscure
colorlesst o pale-yellow, slightly birefringent over- this trend;o nly the field whereG roupI II garnetsw ould plot is
growths, which range in thickness from a few microme- indicated.G roupsa re definedi n the text. Symbolk ey for all
ters to about 40 um. on melanitesa nd schorlomites.T hese threep lotsi s giveni n (a).
overgrowths are zoned and compositionally range from
andraditest o grossulars( grandite series);w e refer to them comm., 1986),l esst han that indicatedb y the low sums
as fluoro-hydrograndites. Initial microprobe analyses of the microprobe analyseso f other colorlessg arnets( Ta-
yielded low oxide sums even after analyzing for F, sug- ble 6). Microprobe data show a strong negative correla-
gesting the presenceo f a hydrogarnet component. The tion between oxide sums and F contents of the fluoro-
presenceo f OH was verified by Anne Hofmeister (Geo- hydrograndites,s uggestingth at the OH content increases
physical Laboratory), who obtained a mid-IR spectrum as the F content increases.G iven that the IR spectrum
ofa garnetg rain from l-1668. This grain was approxi- representsa n averageo fthe analyzedg rain and that mi-
mately 30 pm across,w as composedo f nearly all colorless croprobe data indicate that the fluoro-hydrogranditesa re
garnet with two small red areas (probably melanite gar- significantly zoned with respectt o F (Table 6, nos. 4, 5)
net), and appearedo ptically to be very similar to the col- and probably OH, the low amount of HrO indicated by
orlesso vergrowth on a melanite garnet from I - 16 68 (Fig. the IR data is not totally unreasonableo r inconsistent
8). The IR spectrum representsa n averageo f the entire with the microprobe data. The grain from which the IR
crystal, including the small amounts of red melanite gar- spectrum was obtained was unfortunately lost before it
net. The amount of OH (calculateda s water) indicated by could be afialyzedb y microprobe.
the IR spectrum is 0.1-O.5 wtolo( A. Hofmeister, written The garnetso f Group IV are low-Ti melanitesa nd con-
FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS t2l
tain little or no F. Theseg arnetsa re the most Fe-rich and 3.O
Al-poor garnets found in the ijolites and occur both as
rims on fluoro-hydrograndite overgrowths and as pale-
pink skeletal grains in the groundmasso f 4-DJ7 (Table
.
5, no. 7). Enrichment of V occurs in the Group IV gar- u, 2-7 'j.
nets; over I wto/oV 2O3w as found in some overgrowths ''aG3t .
(Table 5, no. 8). "l\..-.
A strong negative correlation exists betweenT i and Si
in the Group II, IIA, and IV melanites as well as in the 2.5 o.25 o.5 o,7 5
F
less Ti-rich schorlomites (Fig. 6a). No correlation be-
tween Al and Si exists in garnetso f Groups I, II, and IV,
though one does exist in Group III (Fig. 6b). However, . "llt.
there is a generalt rend of slightly increasingA l with de- i -t'-.t!.1--'
-
creasingS i from Groups IV to II to I. Group IIA mela- t
nites have Al contents that are similar to those of Group i. r.!'.
sf-A
I and are generally higher than those of the other mela-
t
nites.W ithin the more Ti-rich schorlomites,F er* decreases
with increasing Si, reflecting Fe3* substitution for Si in
o-o.zs o.5 o.75
the tetrahedral site. In the melanites, there is a strong F
positive correlation between Fe3* and Si, indicating in- jq?..- G.
creasingF e3" substitution for Ti in the octahedral site of
the melanites. Zoning, with F both increasing and de-
creasingf rom outer to inner rim, is found in Group III
fluoro-hydrograndites.T he following trends are found in
theseg arnets:S i decreasesa nd Al increasesa s F increases,
and Al is inversely correlated with Fe3* (Fig. 7). These
trends indicate that as the fluoro-hydrogrossular com- ooffi"
ponent increasest,h e andraditec omponentd ecreasesT. he
Group III garnetss how a small variation in Ti content, Fe3*
and no correlation of Ti4t with F was found. Group IV Fig. 7. Variation of (a) Si and (b) Al with F (per formula
garnets lie on the extensions of the trends defined by unit) and (c) variation of A1 with Fe3* for Group III fluoro-
Groups I and II (Fie. 6). hydrograndites( cationsp er 12 anions; seeA pp. I for the method
of cation calculation) from ijolite xenoliths.
Single-crystalX -ray examination was made of three
garnetc rystalsf rom sample 1-1668. A dark-brown, ho-
mogeneous-appearingc rystal possessesc ubic symmetry within the groundmasso f the ijolite xenoliths. The micas
with a : 12.144 A, a value consistent with those of Ti- arep leochroici n various shadeso fgreen and red or brown.
rich melanites( 12.104 to 12.125A ) and schorlomites Extinction is often patchy. A large range of mica com-
(12.138t o 12.167A , Deer et al, 1982,p . 626-627).A positions is found in the ijolite xenoliths from phlogo-
light-brown crystal gave a -- 12.048 A, suggestiveo f a pites with Mg/(Mg * Fe2*) as high as 0.783 to biotites
Ti-rich andradite (a : 12.029 A, Deer et al., 1982, p. with Me/(Mg * Fe2*) as low as 0.095. Representative
624).The third garnetc rystal examined is nearly colorless analysesa re given in Table 7, and five analysesf rom 84-
and possessessp aceg roup symmetry 1a3d wth a: 12.067 l-RSS-B are presentedt o illustrate the range of compo-
A. ttre very light color and fairty large unit-cell edges ug- sitions in a single sample. Compositions are correlated
gest that the crystal contains a significant amount of F with color: in general,b rown or red-brown grains are more
and OH in the structure. The replacemento f SiO" groups Mg-rich than green grains. However, deep-red biotites
with (OH)4 and Fo groups in the garnet crystal structure (Table 7, no. 6) in 6-DJ7 contain very high Fe and are
is expectedt o expand the lattice; for example, naturally deficient in (Si + Al) in the tetrahedral site, suggesting
occurringg rossularsh ave a dimensionso f I 1.790t o 11.94 that they may contain small amounts of Fe3*i n this site,
A lDeer er al., 1982,p . 60a-607), whereass ynthetic end- as identified by Dyar and Burns (1986) in other biotites.
member hydrogrossular,C a.Alr(OH),r, has an 4 dimen- Ba was detected only in phlogopites from sample 84-l-
sion of 12.5n A(Weiss and Grandjean, 1964).T he X-ray RSS-B.M ost of the micash ave <2.5 wto/oT iOr, but some
data are consistentw ith microprobe analysest hat verified have as much as 4.6 wto/oT iOr. There is no clear corre-
that sample l-1668 containss chorlomite,m elanite,a nd lation of Ti with Me/(Mg * Fe2*),b ut very Fe-rich bio-
fl uoro-hydrograndite garnets. tites generallyc ontain the most Ti. As expected( seeS pear,
1984,a nd Munoz, 1984,f or discussionso f the partition-
Phlogopitesa nd biotites ing of F in micas), F is positively correlatedw ith Me/(Mg
Phlogopite and biotite are most commonly observed * Fe'?*)( Fig. 9) in micas from the ijolite xenoliths, but
replacingc linopyroxene and are rarely seena s aggregates phlogopites from the inner-ring ijolites, with a very nar-
122 FLOHR AND ROSS: METASOMATIZED IJOLITE XENOLITHS
TABLE5 . Garnetsf rom iiolitex enoliths,D iamondJ o quarry
GroupI Groupl lA Group ll Groupl V
Sample: 3-DJ7-2 1-1438-3 5-DJ7-2A 3-DJ7-2 84-1- 1-1438-3 4-DJ7-2 3-DJ7-2
RSS.B1
I 2 4 5 o 7 I
Weight pelcent
sio, 25.4 24.7 302 322 31 0 34.7 34.6 34.8
Tio, 17.8 17.5 10.3 6.51 I 09 2.64 3.17 1.95
ZtO2 na 0.80 0.44 0.26 0 22 0.07 0.18 0.14
Al,03 5.ZO 357 3.89 2.24 2.51 1. 18 0.79 0.85
Vro. 0.59 0.52 0.78 0.54 na 039 0.44 1.18
FerO". 17.3 17.6 17.8 23.8 22j 27.1 27,4 zI -h
FeO. 2.66 1.87 2.58 1.14 0 90 0.08 0.10 0
MnO 0.46 0.33 0.41 0.70 0 59 0.31 0.29 0.16
Mgo 128 142 1.00 0.64 0.89 o.24 0.24 0.24
31.4 31.7 31.7 32.1 32.7 33.2 33.5 33.4
Naro 0.16 0.10 0.06 0.09 0.09 0.12 0.13 0.09
Nb,os na no nd nd na nd nd no
F na no nd nd na no nd 0.18
Sum 100.3 100.1 99.16 100.2 1000 100.0 100.8 100.51
Tetrahedrals ites
Si 2.148 2.096 2.547 2.703 2602 2.912 2.888 2.914
AI 0.325 0.357 0.387 0.222 o.248 0.088 0.078 0.084
Fes* 0.527 0.547 0.066 0.075 0.150 0 0.034 0.002
3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000
Octahedrasli tes
AI 0 0 0 0 0 0.029 0 0
Ti 1 132 1.117 0.653 0.411 0.574 0.16 7 0.199 0.123
Zl 0.038 0.021 0.012 0.010 0.003 0 008 0 007
0.040 0 035 0.0s3 0.036 0.026 0.029 0.079
Fe3* 0.574 0.574 1.062 1.429 1.243 1 713 1.689 1.736
Fe* 0.093 0.056 0.085 0.032 0.062 0.006 0.007 0
Mg 0 161 0.18 0 0.126 0.080 0.111 0.030 0.030 0.030
Mn 0 0 0 0 0 0.022 0.021 0 011
Ca 0 0 0 0 0 0.004 0.017 0.014
2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000
Dodecahedrasli tes
Fe,t 0.095 0.077 0.097 0.048 0 001 0 0 0
Mn 0.033 0 024 0.029 0.050 0.042 0 0 0
Ca 2.845 2.882 2.865 2.887 2.941 2.981 2979 2.986
Na 0.026 0.016 0.010 0.015 0.015 0.020 0.021 0.015
2.999 2.999 3.001 3.000 2.999 3.001 3.000 3.001
No pts averaged 5 4 4 4 4 3 5 4
Notej nd : not detected. na : not analyzed.C olumn headingsa s follows: 1, 2-Euhedral to subhedrals chorlomitesr immed by melanitesa nd fluoro-
hydrograndites3;- Subhedral melanite4; -Light-red-brownz ones urroundings chorlomiten o. 1; 5-Light-red melanitein groundmass6; -Outer zone
of colorfesso vergrowtho n schorlomiten o.2;7-Pale pink skeletalg rain in groundmassi ntergrownw ith magnetitea, edirine,a nd calcite;8 -Outer
zoneo f colorlesso vergrowtho n schorlomiten o. 1.
* Fe3ta nd Fe2*c alculatedb y assuming ideal stoichiometryo f 8 cations per 12 oxygens.
t Oxide sum adjusted for oxygen equivalento f F.
row range of Mg/(Mg + Fe2*),d o not show such a cor- placement rims on perovskite (4-DJ7), and as small an-
relation. Cl was not detectedi n any of the micas analyzed. hedral grains intergrown with biotite and garnet. Sphene
An unusual intergrowth of biotite, pyrrhotite, ilmenite, commonly shows pink to colorlessp leochroism. Sphene
minor sphene,a nd a trace of pyrite occurs in sample 84- compositions are varied among the ijolite xenoliths (Ta-
l-RSS-A2. The biotite, dark olive green to slightly pleo- ble 8). The low oxide sums may be attributed to the pres-
chroic red brown, is Fe-rich and contains 2.48-2.98 wto/o enceo f water and/or unanalyzede lements.F e is reported
V,O, (Table 7, no. 7). Sums for analyseso f this biotite as all Fe3*i n accordancew ith the findings of Higgins and
were consistently high, probably indicating analytical Ribbe (1976). We consistentlyf ound that Fe3*a nd Al are
error, but V-free biotites analyzedd uring the same anal- inversely correlatedw ith Ti, indicating that theset wo ele-
ysis sessiong aveg ood results.B iotites from severalo fthe ments are substituting for Ti in the octahedral site.
other xenoliths were analyzed for V, but it was not de- Oxides. Primary magnetite occurs with diopside and
tected. as inclusionsi n garneti n 6-DJ7 (Table 9, nos. l-3), and
secondary magnetite is intergrown with aegirine in the
Other minerals
groundmasso f 4-DJ7. Rims of primary grains are slightly
Sphene.S pheneo ccurs as subhedral to euhedral crys- enriched in Ti relative to cores. Laths of Ti-free magne-
tals in the groundmasso f several ijolite xenoliths, as re- tite (Table 9, no. 4) within clinopyroxene in 4-DJ7 are