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American Mineralogist, Volume 69, pages 4l-56, 1984
Ti-phlogopites of the Shaw's Cove minette: a comparison with micas of otherlamprophyres, potassic rocks, kimberlites, and mantle xenoliths
SnenoN W. BecHINsKI AND EveNNe L. SltvtpsoNl
Department of Geology, (Jniversity of New BrunswickFredericton. New Brunswick. E3B 5A3 Canada
Abstract
The two generations of micas in the Shaw's Cove minette (mica-potash feldsparlamprophyre) of northern New Brunswick, Canada, are TiO2-rich phlogopites with variablebut low silica and potash contents, low soda, and efectively all their aluminum intetrahedral sites. A majority of the micas have an excess of Y cations and a deficiency of Xcations; there are positive correlations between excess Y ions and number of Al ions in Zsites, between K2O and SiO2 contents, and between K2O and volatile-free totals; yet themicas optically appear fresh, do not respond to hydration or glycolation, and X-raydiffraction patterns show none of the peaks of talc, montmorillonites, vermiculite, or 7- orl4-A chlorites. This suggests that some normally octahedrally coordinated cations andwater or hydronium ions are located in the interlayer X sites. Phenocryst micas are sharplyzoned. Cores are paler colored and have high mg [atomic ratio Mg/(Mg + Fe{2)] (0.81-0.88), Cr2O3 (i : 0.88 wt.Vo), and NiO (i : 0.08 wt.%), but relatively low TiO2 $ = 2.93wt.%), and negligible BaO. Phenocryst rims and groundmass micas are darker reddishbrown and have lower mg (0.68-0.79), nil CrzOr and NiO, but high TiO2 (X = 5.40 wt.%)and BaO (X = 0.38 wt.%). Micas in minettes from other localities (62 analyses) arefrequently richer in silica and potash than the Shaw's Cove samples but otherwise aregenerally chemically comparable. Chemical zoning or bimodalism is nearly universal inminette micas, though the boundary between high-zg-low-TiOz (phenocryst) and low-mg-high-TiOz (groundmass) groups varies from province to province and even from minette tominette within a swarm.
Comparison of analyses of minette micas with 353 analyses of micas from other mafic(largely potassic) rocks indicates that micas chemically indistinguishable from those ofminettes can be found within some other lamprophyres, (ultra)potassic rocks, kimberlites,and high-pressure xenoliths in alkaline volcanic rocks and kimberlites, though each ofthesegroups also contains micas unlike those in minettes. Those kimberlite phlogopites thatmatch minette micas are predominantly secondary rims of phenocrysts (or megacrysts) orgroundmass crystals (mainly "Type II"), but some are unzoned pre-fluidization pheno-crysts. While primary, primary-metasomatic, and uaruo-suite phlogopites of mantlexenoliths are unlike minette micas (higher mg and SiOz, lower TiO2 and AlzOr), some of thesecondary-metasomatic phlogopites in sheared garnetiferous mantle xenoliths are whollyminettelike.
The chemical identity of minette-mica phenocryst cores and some phenocrysts of diversemantle-derived rocks implies crystallization under similar conditions. By analogy with thechemistry of phlogopites produced experimentally in potassic systems at high pressuresand studies of equilibration conditions of natural phlogopites, it is concluded thatphenocrystic phlogopites in minettes may form at temperatures and pressures up to at least-1250"C and -40 kbar and probably under /o. conditions between the NNO and HMbuffers: higher /o, than for primary, primary-metasomatic, and uanto-suite micas inkimberlites and their included xenoliths.
Introduction
Magnesium-rich trioctahedral micas are critical phasesin most lamprophyres but especially in minettes (biotite-
rPresent address: Department of Geology, University ofRegina, Regina, Saskatchewan, Canada.
0003-004)0E4l0102-0Ml$02.00 4l
K-feldspar lamprophyres) and kersantites (biotite-plagio-clase lamprophyres) in which they are the dominantferromagnesian mineral. Studies of the mineral chemistryof lamprophyres are relatively few in number; this un-doubtedly contributes to the present lack of agreement ontheir petrogenesis. If minettes are in fact mantle-derived
42 BACHINSKI AND SIMPSON: TI.PHL)GqPITES oF THE SHAv,S CovE MINETTE
(Bachinski and Scott, 1979, 1980), then detailed examina-tion of the chemistry of their micas plus a comparisonwith the micas of other potassic mantle-derived rocksmay yield useful information relating to the distributionand movement of water and large lithophile ions in thesubcontinental upper mantle as well as the chemicaland/or genetic relationships, if any, between minettemagma and other potassic magmas.
Micas of minettes and other lamprophyres are nearlyuniversally zoned discontinuously (see, e.g., Velde,1969): phenocrysts have lighter-colored cores surroundedby darker rims generally equivalent in color to thegroundmass micas. M6tais et al. (962) were the first toconfirm that this color zonation is indeed due to differingchemical composition. They semi-quantitatively demon-strated that in the micas of a minette (and a kersantite) thepale.r cores are richer in magnesium and chromium andare poorer in iron, titanium, manganese, barium, andcerium than the darker rims. Similar zoning has sincebeen observed in the micas of ultrapotassic rocks andlamproites (e.g., Carmichael, 1967; Nicholls, 1969:Velde, 1969, 1975; Mitchell, 1981), carbonatites (e.g.,Heinrich, 1966; Rimsaite,1969; Gaspar and Wyllie, l9g2),kimberlites (e.g., Emeleus and Andrews, 1975: Boettcheret al.,1977,1979; Smith et al.,1978; Mitchell and Meyer,1980; Boctor and Boyd, 1982), and ultramafic mantlexenoliths in both basaltic rocks (e.g., Dawson et al.,1970) and kimberlites (e.g., Boettcher et al., 1977, 1979;Rawlinson and Dawson, 1979; Jones et al..l98Z).
Phlogopites of the Shaw's Cove minette
A l5-meter-thick minette sill crops out on the southernshore of the Restigouche River at Shaw's Cove, west ofthe town of Dalhousie, in northernmost New Brunswick,Canada, where it has been emplaced along a conglomer-ate-sandstone bedding plane of the lowermost MiddleDevonian Campbellton Formation. On stratigraphicgrounds the minette is presumed to be no younger thanLate Devonian in age. Within the minette, phenocrysts ofphlogopite, diopsidic clinopyroxene, and yellowish-greenphyllosilicate pseudomorphs after olivine occur in agroundmass composed ofa second generation ofphlogo-pite and clinopyroxene poikilitically enclosed by crystalsof sanidine. Accessory phases include apatite, titanomag-netite, sphene, and analcime.
Phenocrystic phlogopites of the Shaw's Cove minettehave pale reddish brown cores surrounded by darkerrims. The core boundaries are frequently resorbed in amanner similar to that described by Mitchell and Meyer(1980). The rims correspond in color and other opticalproperties to the smaller phlogopites in the groundmass,both of which show normal absorption. Fifty-one micasfrom eight samples from a section through the minettewere analyzed (by E. L. Simpson) using the CambridgeMark V electron microprobe and Ortech energy-disper-sive system of the Department of Geology, DalhousieUniversity. The accelerating voltage was 15 kV, beam
current was 5 nanoamps., spot size was as small aspossible (generally about I or 1.5 microns), resolutionwas 159 eV, and the software package EDATA (Smith andGold, 1976) was employed. Standards were those ofDalhousie University: BIO LP6, KK (a kaersutite), natu-ral sanidine and albite, Will (a willemite), ol 174.1, Crmetal, Ni metal, Ilm 96189, and Opx R62.
Representative analyses of these titaniferous phlogo-pites ("titanphlogopites" in the terminology recommend-ed by Rock, 1982) are presented in Table la. All the micasare trioctahedral and have mg (= atomic ratio Mg/(Mg +Fe'f2)) values equal to or greater than 0.68, high titania,low silica, and moderate alumina. Structural formulaecalculated on the basis of 22 oxygens (Simpson, 1980)show that the number of Si ions ranges from 4.824 to5.624 and Al from 2.999 to 2.987 per formula unit.Aluminum is essentially confined to tetrahedrally coordi-nated, or Z, sites: only three of the 5l micas contain anyoctahedrally coordinated Al. As Si + Al is less than 8.000for nearly all the micas, the Z sites must be completed byeither titanium or ferric iron. Since the Shaw's Coveminette contains important amounts of Fe2O3 (Simpson,1980), it is possible that its micas have a "tetraferriphlo-gopite" component. However, ferric iron intheZ sites ofphlogopites is believed to produce reverse pleochroism(Hogarth et al.,1970; Farmer and Boettcher, l98l) whichwe did not observe; also, tetraferriphlogopites have onlybeen reported from rocks with very low alumina contents(e.9., Wendlandt, 1977), much lower than those of min-ettes. Farmer and Boettcher (1981) have suggested thatthe sequence oftetrahedral site preference in phlogopitesis Si > Al > Ti > Fe+3, since all normal phlogopites haveSi + Al + Ti greater than 8 while all known reversephlogopites have this sum less than 8. The Shaw's Covemicas all have Si + Al + Ti greater than 8. We thereforeconclude that Ti rather than Fe*3 occupies the tetrahedralsites unfilled by Si and Al.
The range of values for any given oxide in the micaanalyses is relatively broad but similar to literature valuesfor micas in other minettes (see below). Several pointsconcerning the data, however, warrant special comment.
TiO2, mg, and trace elements
The analytical results for TiO2, MgO, FeO, and thetrace elements in the micas are markedly bimodal (Tablelb and Figure la,b,c,). A plot of wt.Vo TiO2 against mgdefines two chemically distinct groups of micas: Group l,with high mg ,low TiOz, and significant concentrations ofCr and Ni but negligible Ba, and Group 2, with low mg,high TiO2, no Cr or Ni but important amounts of Ba. Themicas of Group 2, in fact, are the major carrier of Ba inthe minette as the maximum amount of BaO in thefeldspar is 0.20 wt.% (Simpson, 1980).
With few exceptions, light-colored phenocryst corescomprise Group l, darker phenocryst rims and ground-mass crystals Group 2. (We did not detect any reverselyzoned micas as are occasionally seen in minettes else-
BACHINSKI AND SIMPSON: T|-PHLOGOPITES OF THE SHAW'S COVE MINETTE
Table la. Representative analyses of phlogopites from the Shaw's Cove minette (oxides in wt.%)
SC25r9 6c25rIO sC26-1 sc26-5 sC25-13 sC27b.-L sc27b-1'2a SC3o-II SC30-I7
P R I C G G G G G P R G
43
',t}pe*
totaLi* 92.7L
sr%
Tio2
Nzo3
lho
FeOT
r'@
co
N%o
K20
NiO
*2-3
Bd
? < l a
4-82
14.68
0 .00
a o ?
20.49
0 .36
0 . 2 3
7 .02
0 .00
0 .00
0 .04
38.32
? ] a
14.40
c .00
) . / 6
22.77
0 , 0 0
0 .24
8 .33
0 ,07
0 .60
0. 16
93.69
0 .88
se36-4
PR
? q t q
5 .32
15 .63
0 .06
13.08
0 .48
E q l
0 .00
n R t
94. 33
32. 13
6 ? O
16.27
0 .50
11.90
19 .41
0 . 4 3
0- 30
3 .4 r
0 .00
0 .00
o . 4 2
90.74
0 .74
33.88
4.90
14. 85
0 .05
13 .39
15 .63
0 .00
0 .20
0 .00
0 .00
0 . 5 1
89. 73
35.1r
s .00
15,27
n t q
I0.78
L9.74
n ? n
1 n ?
o . r o
0 .00
0 .00
u . o f ,
9 3 .48
0 . 7 7
sc38-7
PC
39.30
J. ZU
14. 18
0 .00
5 .60
2L.89
0 .00
0 .29
9 .48
0 .00
0 .99
0 .00
94.95
0 .87
34. 30
5.01
Is.83
0 .00
10 .94
l a o t
0 .07
0 . 4 7
5 .74
0 .00
0 .00
0 .32
q l I e
u. / )
sc3&r0
36.69
5 . 5 5
14.68
0 . 0 0
10.41
1 a Q 1
0.00
0 .5 r
8 .70
0 .00
0 .00
0 .80
94.42
0 .75
34.89 35. 19
6 .43 5 .76
14.38 14.61
0 .00 0 .00
11.75 r t .95
16.58 17.59
0 .00 0 .08
n ? t d 2 0
7.56 7.65
0.00 0.00
0 .00 0 .00
o .25 0 .57
91.95 94.24
mf
lype
sio2
Tio2
dzo3
!,rc
FeOT
!4p
(5
N"f
K2o
Nio
*2-3
BS
total
!9
n ? o
sc36-2
35.56
6 . 2 5
L4.7I
0 .00
L L . Z '
18. 12
0. r0
u . o J
6 .20
0 .00
0 .00
0 .39
93.67
0 .74
36.82
2 .95
15.09
0 .00
7 .87
z z . a L
0. t0
0 .41
4 4 1
0 .07
0 .54
0.09
92.32
0 .84
35. LZ
s .82
14.59
0.00
10.65
18. r0
0 .00
0 .40
0 .00
0 .00
0 .49
92 .31
0 .7s
M
36.86
o - z L
L4.57
0 .00
LL. ZZ
I7.55
0 .00
6 . O Z
0 .00
0 .00
U . ) U
95 .55
0 .74
36. 14
4 .75
15.14
0 .00
L2.70
15.44
0.00
0 .78
o . z 6
0 .00
0 .00
0 .34
94.23
0.70
36. 14
1 . A )
15 .08
0 .13
7 .45
22 .83
0 .25
0 .14
) . L l
0.00
1.86
0 .00
91.89
0 .8s
n 7 q
sc46-10
PC
36.02
? n ?
L4.77
0. 14
7.11
22 .93
0.39
0 .29
6 . 2 8
0 .00
0 .43
0. t3
9L.44
0 . 85
sc37-2 SC37-4
r c ! E
sc38-12 SC46-8
G l t
* P = $lmcq.st, M = nicrcphencryst, c = grcllrdtEs, C = core, R = rim
** P2O5 and Fozo,e:ch,are betn the limits of detection
BACHINSKI AND SIMPSON: Ti-PHLOGOPITES OF THE SHAW'S COVE MINETTE
Table lb. Chemical characteristics of the two groups of micasin the Shaw's Cove minette
rege x
eop I: hit'r-gg-1*Tio2 Srctp (!tsU)
mass micas; such a relationship is displayed by, e.9.,Velde's (1969) analyses of phenocrysts and groundmassmicas in a French minette (Table II, anaI. 12 & l3), butthe reverse can also apply in other minettes (c/. anal. l8 &19 of her Table II).
Overall mean values for K2O (5.59 wt.Vo) and SiO2(34.18 wt.Vo) are not balanced by correspondingly higherNazO (0.36 wt.Vo) and Al2O3 (15.23 wt.Vo) and are signifi-cantly lower than previously published values for micasin lamprophyres. For analyses of minette micas in theliterature of the last two decades for which both potash
5 6
f l A T i O a +
oi. l& + Fct
Fig. l. Chemistry of phlogopites in the Shaw's Cove minette.(a) Covariation of mg and wt.Vo TiO2. @) Covariation of mg and,wt.Vo Al2O3. (c) Atomic proportions of Ti, Al, and (Mg+Fef 2).
st.t rio2 2.56 - 3.20
!C 0 .8 I - 0 .88
wt.t BaO 0.00 - 0.28
wt.l &2o3 0.28 - I.86
wt.g Nio 0.00 - 0.r9
, o t
0 . 8 5
0.09
0.88
0.08
croup IIr fclg-irigiFrio2 grcup (1ts40)
wt.t Tio2 4.03 - 7.96 5.40
rrg 0.68 - 0.79 0.73
wt.t BaO 0.00 - 0.80 0.38
ft.8 e2o3 0.00 - 0.05 0.00
wt.C NiO 0.00 - 0.05 0.00
where; e.9., Velde, 1969, p.207; Roden and Smith, 1979,p. 372). There are, however, anomalies that seem to berelated to the position of the sample within the sill. Threeanalyses of micas identified as groundmass crystals fromsamples near the upper margin of the sill fall within Groupl, although other groundmass micas from the same sam-ples are not anomalous, while three analyses of pheno-cryst cores from the central coarser-grained portion ofthesill have the low mg and high TiOz content of Group 2micas. The chemical group into which microphenocrystsfall varies with their location: those from near the marginsare in Group l, those from well within the sill are inGroup 2. Whole-rock analyses of samples from a sectionthrough the Shaw's Cove sill (Simpson, 1980) do notshow any significant differences in bulk TiO2 content ormg value between the minette's central and marginalportions that could account for these anomalies. Thislack, however, does not preclude the possibility of small-scale gradients in Mg, Fe, and Ti existing within the liquidat the time of mica crystallization. The anomalies mayalso be attributable to simple mislabelling. Slower coolingin the center of the sill has allowed groundmass micas togrow nearly to phenocryst size. Phenocryst phlogopites,especially those with poorly developed or unusuallynarrow rims, from the sill's margins may have beenmisidentified as groundmass crystals because thin sec-tioning made them appear smaller than they are and thepresence of the rim was not noticed using the micro-probe's optical system.
K2O and SiO2
K2O and SiO2 contents of most of the micas, bothphenocrysts and groundmass, are lower than normal forphlogopites and are positively correlated. The mean K2Ocontent of groundmass micas is lower than the mean forphenocrysts, although individual groundmass crystalscover virtually the entire range of K2O values, from thelowest to the second highest. For a given silica contenr,the phenocrysts tend to have less K2O than do ground-
II o.rcI
nll
I
@ lo
ot Ti
o€
aqP
:p'^s*v".-;o t F o o
P 6
BACHINSKI AND SIMPSON: T|-PHLOGOPITES OF THE SHAW,S COVE MINETTE 45
and silica values are given, means are 9.2 wt.Vo and 38.6wt.Vo respectively (N = 58); for micas in lamprophyresother than minettes (50 analyses, a little less than half ofwhich are of micas from kersantites) KzO -- 8.4 wt.Vo,SiO2 = 36.8 wt.%. Of the Shaw's Cove micas, TlVo haveKzO < 7.0 wL% and 73Vo have SiO2 < 36.0 wt.%o, whileof the ll2 literature values for micas in other lampro-phyres including minettes, only 8 have less than 7.0 wt.Vopotash and 2l have less than 36.0 wt.Vo silica. Thirty ofour 5l analyses are of groundmass micas. In distinction,only llVo of the published mica analyses for lampro-phyres are microprobe analyses of groundmass crystals.Because of the differing proportions of groundmass micaanalyses in our data set and in the lamprophyre literature,it may not be appropriate to compare mean values. Itshould be noted, though, that 7 ofthe 8 literature analysesof lamprophyric micas that show KzO < 7 .0 wt.To do notrepresent microprobe analyses of groundmass micas butare wet analyses of bulk mica samples; low potash valuesin these samples certainly cannot be attributed to loss ofpotassium from the sample under the electron beam ofthemicroprobe. Volatilization of potassium during analysisof our micas is unlikely since only some micas have lowK2O and identical microprobe conditions prevailed dur-ing all mica analyses. Also, under similar conditions therewas no loss of sodium, an even more volatile element,from other minerals.
Micas with KzO < 7.0 wL% and/or SiO2 < 36.0 wt.Vohave been observed in igneous rocks other than lampro-phyres, including granitic rocks (e.g., Barridre and Cot-ten, 1979), leucite-bearing lavas (e.9., Cundari, 1973;Birch, 1978; Thompson, 1978), high-pressure cognatemegacrysts (Ellis, 1976), monticellite peridotite and tur-jaite (Wendlandt, 1977), carbonatites (Gaspar and Wyllie,1982), ultramafic complexes (Skinner, 1969; Grapes,1975; Donaldson, 1975), and as phenocrysts and ground-mass phases in kimberlites (e.9., Dawson et al., 1970;Giardini et al., 1974: Mitchell, 1978; Smith et al.,1978:Clement et al., 1979; Boctor and Boyd, 1982), as well asin xenoliths in alkaline basalts (Ryabchikov et al., 1982).Some of these micas are especially rich in TiO2 or BaO.But the existence in minettes of equally titanium- orbarium-rich phlogopites with normal amounts of KzO andSiOz (e.9., Nicholls, 1969) indicates that the enrichment intitanium and barium is not necessarily complemented byan impoverishment in potassium and silicon.
It could be hypothesized that there is either a physicalmixture with, interlayering, or an alteration of our mica toan essentially KzO-free, more hydrous phyllosilicate. Therequirement that the second phase, relative to phlogopite,be poorer in SiOz, rich in Al2O3, and not drasticallydifferent in MgO, FeOa, CaO, and Na2O contents and mgratio eliminates serpentine, talc, and smectites (includingsaponitic ones) from consideration. Minerals of appropri-ate composition include chlorites (-clinochlore), sep-techlorites (-amesite), and vermiculite; all have beenreported as weathering or alteration products of phlogopi-tic micas elsewhere (Deer et al., 1962, p.48 & 156; Velde,
1969; McCallum et al., 1975; Mitchell, 1978): for exam-ple, Skinner (1969) records that K2O-poor micas in anultramafic rock contain up to 15% vermiculite.
Several arguments, however, nullify the propositionthat a mixture of phlogopite and some other phyllosilicateexists on anything but a negligible level in the Shaw'sCove micas. First, the micas of this minette microscopi-cally appear fresh and unaltered. Indeed, this is a generalcharacteristic of most micas in lamprophyres (e.9.,Velde, 1969). Second, if one uses the analysis of one ofour phlogopites whose potash and silica contents corre-spond to the mean values for these oxides in minettes ingeneral as one end-member and the analysis of a Ti-bearing clinochlore (Deer er al.,1962, Table 25, anal. 28)as the other, approximately 40 wt.Vo chlorite would haveto be mixed with the phlogopite in order to produce themean, not the lowest, potash and silica values of theShaw's Cove phlogopites! It is inconceivable that such aquantity of chlorite or any similar phase could not bedetected optically. Third, X-ray difraction patterns ofShaw's Cove micas show none of the peaks of either 7-Aor l4-A chlorites, montmorillonites, talc, or vermiculite;the micas do not respond to either hydration or glycola-tion. We conclude, therefore, that the low potash andsilica contents of many of our micas are real and are notan artifact of analytical technique, alteration, or admix-ture.
Structural formulae of our micas indicate that nearlytwo-thirds of the analyses-including phenocrysts andgroundmass crystals-have Y parameters in excess of6.000, the maximum number of octahedral sites (ourhighest Y value is 6.949, the mean is 6.220), and there is astrong negative correlation between Y and X site occu-pancies, the latter of which is always less than 2'000(0.473 to 1.827; i; : 1.261). This suggests that somenormally octahedrally-coordinated cations are in the in-terlayer position. The positive correlation between ex-cess Y ions and the number of tetrahedrally-coordinatedaluminum ions provides further support for this and anexplanation for the positive correlation between wt.VoK2O and SiO2: low amounts of silicon require highamounts of tetrahedrally coordinated aluminum to com-plete the sheet structure, but this leads to a chargeimbalance that may be offset by substitution of divalentcations (".g.,Mg*z, Fe*2) for K*lin the X site. Interest-ingly, Group I micas (high mg, MgO, and FeOl) show agreater number of tetrahedrally-coordinated aluminumions than do Group 2 micas. Of the potash- and silica-poor mica analyses in the literature, only one a "platyphlogopite replacing hornblende" (Deer er al.,1962, anal.6). combines the excess of Y cations with the X-sitedeficiency that characterizes the majority of the Shaw'sCove micas.
However, as even those micas that do not have anexcess of Y cations still have less than 2.000 X cations,many of the mica analyses in Table la have low totals,and there is a rough positive correlation between wt.VoKzO and volatile-free totals, it is possible that there is
46 BACITINSKI AND SIMPSON: Ti-PHLOGOPITES OF THE SHAW'S COVE MINETTE
Table 2. Mean composition of micas in minettes, excludingShaw's Cove micas (N=62)
III, anal. 6-A,-5, bulk sample; Nicholls, 1969, Table 6,anal. 666-P, microprobe analysis of a phenocryst). Inter-estingly, the analyses of the German and Scottish speci-mens, like many of those from Shaw's Cove, have lowK2O and SiO2 values.
Although color zoning and the associated chemicalbimodalism is observed in virtually all those micas ana-lyzed by microprobe, the boundaries of the two chemicalgroups vary from geographical area to area and even fromminette to minette within a swarm. Considering the
r&@
si02
TlO2
Nzo:
-"7
T@
K20
Na20
ry.
38.6
1 3 . 4
0 . 5
0 .82*
34,4 - 4I.7
1 . 4 - 1 1 . 3
9 . 7 - 1 8 . 0
4 . 8 - 1 3 , 1 *
L4.7 - 25.4r
4 . 4 - 1 0 . 6
0 . 0 - r . t
0 .67 - 0 .90*
*wludiBl 6 ilmloE micas fm
the Pqdemis mjrette (@ t*t)
some water or hydronium ion substitution in the interlay-er position.
We believe that these variations in the potash and silicacontents of the Shaw's Cove micas are predominantlyprimary and reflect the relative activities of these andother components that may enter the phlogopite at thetime of its crystallization. That the activities can varywidely in minette liquids is illustrated especially well bythe groundmass phlogopites for which the availability ofcomponents is complicated by the simultaneous crystalli-zation of other groundmass phases.
Comparison with micas from other minettes
Ranges and mean values of the important oxides inanalyses of 62 micas of minettes in the literature arepresented in Table 2 (see Appendix for sources of data)and the covariation of the major atoms that may occur inoctahedrally coordinated sites-Mg, Fe, Ti, and Al-isshown in Figure 2a,b,c. Like the Shaw's Cove micas,micas in other minettes are phlogopites of moderatealuminum and high titanium contents but are richer insilica and potash; the lack of a correlation between rngand Al2O3 @ig. 2b) presumably reflects an absence of Alions in octahedrally-coordinated sites. One of these micas(phenocryst core, mafic minette ring dike, Buell Park,Navajo Indian Reservation, Aizona; Roden and Smith,1979, Table 3, rock BPR-5) is among the most Ti-richmicas (TiO2 = ll.3 wt.To) known to us regardless of rocktype! This specimen also has the lowest SiO2 concentra-tion (33.9 wt.%) of any minette mica but has a normalalumina content.
Analyses of five minette micas are chemically identicalin all respects to those of Shaw's Cove micas; these micasoccur in minettes from southernmost East Germany(Kramer, 1976,TabIe 8, anal. 152, bulk sample), easternFrance (Velde, 1969, Table II, anal. 5, bulk analysis ofphenocrysts), southwestern Scotland (Gallagher, 1963,Table II, anal. 2, bulk sample), and the Navajo IndianReservation of southwestern U.S.A. (Velde. 1969. Table
- o o . * $ 3 @ ^ oo Y - : -
o o o o
o o o o
s
t| 070
mg
ot irg + For
Fig. 2. Chemistry of phlogopites in minettes other than theShaw's Cove minette; see Appendix for sources of data. (a)Covariation of mg with wt.Vo TiO2. (b) Covariation of lzg withwt.Vo Al2O3. (c) Atomic proportions of Ti, Al, and (Mg+Fe.$2).Values for nrg calculated using all iron as FeO.
qf. Ti
BACHINSKI AND SIMPSON: T|-PHLOGOPITES OF THE SHAW'S COVE MINETTE 47
entirety of the minette mica analyses, it is not possible topredict whether a given mica sample will have a relativelyhigh or low TiO2 content merely by knowing whether it isa phenocryst core or a groundmass crystal (or phenocrystrim); the same applies to prediction of the mg value. Forexample, in one minette from the Navajo area a ground-mass mica contains 4.84 wt.%o TiO2 while a mica pheno-cryst from another minette in the same volcanic field has7.69% (Nicholls, 1969, Table 6, anal. 672-GM and 649F-P). Nonetheless, for microprobe analyses of single crys-tals or portions thereof, the following generalizations maybe stated: from core to rim to groundmass, mg decreaseswhile TiO2 increases (except in rare cases of reversezoning of core and rim) and groundmass micas nearlyalways have mg < 0.80 and TiO2 > 4.00 wt.Vo.
Sodium, a minor but ever-present constituent of min-ette micas, is slightly less abundant in the Shaw's Covemicas than in those of other minettes (X : 0.36 and 0.5wt.Vo respectively) though the range of values is nearlyidentical. The overall mean Na2O content in all minettemicas is 0.43 wt.Vo.
Only rarely have minette micas been analyzed for traceelements. NiO and, in particular, Cr2O3 may reach size-able concentrations in phenocryst cores while in pheno-cryst rims and groundmass crystals BaO commonly ex-ceeds 0. I wt.Vo and may be more than an order ofmagnitude greater. Overall means, including Shaw'sCove analyses, are NiO : 0.02 wt.% (N = 76), CrzOz :0.30wt.% (N : 82), BaO : 0.48wr.% (N :86). We havenot analyzed our phlogopites for fluorine, though minettemicas are known to contain significant amounts; litera-ture values (Ndmec, l96E; Nicholls, 1969; Kramer, 1976;Luhr and Carmichael, 1981) have a mean of 1.11 wt.Vo F(N : 24) and fluorine contents may increase or decreasefrom phenocryst core to rim to groundmass (see alsoJones and Smith, 1982). Luhr and Carmichael (1981)report fluorine contents of =1.52 wt.Vo in phlogopitepherlocryst rims, F/(F + OH) : 0. l-0.2 for phenocrysticphlogopites in general, and 4.85 wt.Vo F in a groundmassphlogopite in the minettes of Colima, Mexico. Because ofthe marked variations of all these trace elements (oroxides) between phenocryst cores and rims and ground-mass crystals, mean values are of negligible use.
The unusual peralkaline but silica-oversaturated Pen-dennis minette at Falmouth, Cornwall, England (Hall,1982) contains titaniferous phlogopites whose iron-richrims (Table III, anal. 7-12\ are unlike micas in all otherminettes and all other mafic rocks considered here (seebelow). We have designated them as "anomalous" be-cause they have lower ng values than "normal" minettemicas and Al2O3 contents matching those of the most Al-poor minette micas, but have TiO2 contents that are highbut in the minette-mica range (see Fig. 2a,b). For five ofthese six analyses, mg = 0.48-0.57, AlzOt: 10.20-11.97wt.Vo, and TiO2 : 6.36-7.23 wt.Vq the sixth (anal. #7;mg = 0.63, Al2O3 : 11.39, TiO2 : 4.57) is transitional tonormal minette micas.
Although Hall (1982), Barton (1979), and Arima andEdgar (1981) suggest that the composition, especially thealumina content, of micas in mafic potassic rocks is areflection of the composition of the magma from whichthey crystatlized, the nature of this relationship is notclear for minettes. For the Pendennis minette, molecularproportions KK + NayAll : 1.11, mg = 0.64, and Al2O3= 10.4 wL% (after correcting for CO); yet a metalumin-ous and silica-undersaturated minette in the Navajo vol-canic field with mg value similar to that of the Pendennisminette (mol. prop. (K + NayAU : 0.78, mg : 0.67,Al2O3 = 10.7 wt.%; Nicholls, 1969, Table 10, rock 649F)carries groundmass phlogopite with mg : 0.76 and Al2O3= 10.5 wt.Vo (op. cit., Table 6)-much higher mg but thesame Al2O3 as in the anomalous rims of the Pendennismicas-while the peralkaline but more magnesian Siscominette of Corsica (mol. prop. KK + NayAll: 1.04, mS: 0.73, AlzOr = l2.l wt.Vo; Velde, 1965) is host torelatively aluminum-rich phlogopites with high zg values(AlzO: : 13.8 and 15.0wt.Vo, mC :0.90 and 0.88; Velde,1969, Table II, anal. 2 & 3). The Holsteinsborg minettesare silica-saturated to -oversaturated but more magnesianand even more Al-poor than the Pendennis minette andare remarkably peralkaline (average mol. prop. [(K +Na)/All = 1.39, mg = 0.74, AlzOr : 7.8 wt'Vo aftercorrecting for COz; Scott, 1979, Table 4); their phlogo-pites have high mg values (0.87 and 0.83) and one of them(op. cit., Table 3, anal. 137) has the lowest Al2O3 (9.70wt.Vo) of any minette mica. On the other hand, one of theColima minettes, which have lng values similar to thoseof the Holsteinsborg dikes but more Al2O3 and aremetaluminous rather than peralkaline (mol. prop. [(K +Na)/All = 0.69-0.80, mc : 0.70 - 0.75, Al2O3 = 11.1-12.5 wt.%; Luhr and Carmichael, 1981, Table 3), containsgroundmass phlogopite (AlzOr = 10,8 wt.%, mg : 0.82;op. cit., Table 9, anal. l00B) that has a much higher mgvalue but A12O3 as low as that of the anomalous micas inthe peralkaline Pendennis minette. Furthermore, as theminette magma evolves and the mica's mg vahte de-creases from phenocryst core to rim and/or groundmass,the Al2O3 content of the mica may decrease (e.g.,HaIl,1982; Roden and Smith, 1979, Table 3, micas in rocksBPR-5 and BP-51; Luhr and Carmichael, 1981, Table 9,micas in rock 104; Nicholls, 1969, Table 6, micas in rocks672 and 681), stay the same (Nicholls, 1969, Table 6,micas in rock 649F), increase (this work, Table l, anal'30-ll,l7 and 36-2,4; Roden and Smith, 1979, TabLe 3,micas in rock BP-35), or even first decrease then increase(Nicholls, 1969, Table 6, micas in rock 649C).
Comparison with micas in other igneous rocks
Minette micas are unlike those of common granitoids,their extrusive equivalents, pegmatites, or metamorphicrocks (e.g., Velde, 1969; Kramer,1976) and none of theserocks is likely to be directly connected to minettesgenetically. Although the petrogenesis of lamprophyres
BACHINSKI AND SIMPSON: Ti-PHLOGOPITES OF THE SHAW,S COVE MINETTE
sensu lato is still controversial, recent studies increasing-ly indicate that the liquids from which minettes and atleast some other lamprophyres have crystallized are fromthe mantle and may be generated by processes, underconditions, and/or from source rocks related (to varyingdegrees) to those responsible for (ultra)potassic rocks andkimberlites (e.g., Bachinski and Scott, 1979, 1980; Ehren-berg, 1979, 1982; Roden and Smith, 1979; Luhr andCarmichael, 1981; Mitchell, 1981; Paul and potts, l98l;Roden, 1981; Hall, 1982;Platt and Mitchell, 1982; Rogerset al., 1982). It is therefore of interest to compare thephlogopites of minettes with the micas of these rocks andxenoliths from the mantle and lower crust.
For this comparison 353 mica analyses from rocksother than minettes were collected from the literature of1966-1982 and a few older papers. Subdivided by rocktype into five groups, these analyses include the follow-ing: 50 from lamprophyres other than minettes; 68 from avariety of ultramafic to intermediate, intrusive and extru-sive, potassic and ultrapotassic rocks (madupites, oren-dites, wyomingites, fitzroyites, cedricites, woldingites,jumillites, verites, fortunites, leucitites, shoshonites,shonkinites, biotitites, etc.);2 ll3 from kimberlites; and122 from high-pressure xenoliths (51 of these are inminettes, nephelinites, basanites, and other alkaline vol-canic rocks, while 7l are in kimberlites and diamonds).Combined with the 113 minette phlogopites, includingthose of Shaw's Cove, 466 mica analyses are consideredhere.
The analyses are presented in a series of mg us. wt.VoTiO2 diagrams (Figs. 3 to 7) like those used above for theminette micas. On each of the diagrams the superimposedIong curved lines indicate the field in which plot micasfrom all minettes (N = 107) including the Shaw's Covebut excluding the anomalous Pendennis micas of Figuresla and 2a: the broken line encloses the field containing93Vo,the solid line encloses 100% of these: the field of thefive anomalous and one transitional pendennis micas isindicated separately. To be conservative, the followingcomparisons are presented with reference to the brokenlines (main minette mica field) unless explicitly statedotherwise. Diagrams of mg us. wt.Vo Al2O3 and atomicproportions of (Mg + Fe+2)-Ti-AI were also constructedfor all the sets of analyses. Although these latter two setsoffigures are not presented here for the sake ofeconomy,when we say below that certain micas fall ..within theminette field" or make equivalent statements we arereferring to the minette field on all three diagrams. Interms of the four components encompassed by these setsof diagrams as well as wt.Vo SiO2, K2O, Na2O, and traceelement concentrations, it is important to note that noneof the five groups of micas is wholly distinct from minettemicas. (Considering the errors associated with analyzingfor Na2O by electron microprobe, especially when it is
2 For convenience this group will be referred to hereinafter as"potassic rocks".
present in such minimal amounts as in phlogopites,differences in Na2O contents may not be significant). Itmust be stressed, though, that each of the five groups ofrocks also contains micas that difer from those of min-ettes and/or have differing trends of compositional evolu-tion (see also Mitchell, 1981, and Arima and Edgar, 1982).
Lamprophyres other than minettes (Fig. 3)
The micas of other lamprophyres tend to have lowermg values, the same or slightly lower TiO2, the sameNa2O (although micas of kersantites as a distinct sub-group have slightly higher), and somewhat higher Al2O3.Of those analyses (26) that plot within the minette field onall three diagrams, about half are of micas from kersan-tites (Metais, et al.,1962,Table2, anal. l; Velde, 1969,Table 4, anal .4,7,8,9,12, 15; Strong and Harr is , 1974,Table 2, anal .3; Kramer, 1976,Table 8, anal .2A5,2 l7,271). The others are from monchiquites and camptonites(Nicholls, 1969, Table 6, anal. 618-P: McHone. 1978.Table III-4, anal. BU-1, BU-2; Rock, 1978, Table 9, anal.2; Cooper, 1979, Table l, anal. LG-core and O-core),damkjernite (monchiquite?); (Griffin and Taylor, 1975,Table 3, anal. 7), three un-named lamprophyres (Rim-saite, 1971, Table l, anal. 8-PH,9-PH; Janse, 1977,Tablel, anal. 4), and Precambrian alnciitic ultrabasic lampro-phyres (Platt and Mitchell, 1982, Table 3, anal. 1,2,3,6).Approximately half of these minette-like micas have thelow SiO2 and K2O contents commonly seen in minettemicas. Three minette-like micas, however, do not trulymatch minette micas if Na2O and trace element contentsare taken into consideration: the two camptonite speci-mens from New Zealand (Cooper, 1979) contain signifi-cantly more Na2O than any minette mica while the micaof the biotite monchiquite from Arizona (Nicholls, 1969)has a BaO content (6.20 wt.%) over twice as great as the
Fig. 3. Covariation of mg and wt.Vo TiO2 in micas oflamprophyres other than minettes (total iron calculated as FeO);see Appendix for sources of data. Excluding the 6 anomalousPendennis micas, the larger broken line indicates the fielddefined by 93Vo of the minette micas of Fig. la and 2a, the largersolid line encloses l00Vo of these; the anomalous Pendennismicas' field is the small area defined by the solid line below themain minette-mica field; see text.
I6 - a a ^ 6 - - - o
. - A A
sl TiO?+
BACHINSKI AND SIMPSON: Ti-PHLOGOPITES OF THE SHAW'S COVE MINETTE 49
maximum reported for a mica from a minette. (Thehighest value known to us is 10.3 wt.% BaO in the core ofa phlogopite from the Jacupiranga carbonatite's scivitefacies; Gaspar and Wyllie, 1982).
Potassic rocks (Fig.4)
Because ofthe diversity ofthis group ofrocks, generalcomparisons between their micas and those of minettesare not possible; instead reference must be made tosubgroups. Two of these, biotit i tes (2 analyses;Boettcher, 1967, Table 3) and shonkinites (5 analyses;Nash and Wilkinson, 1970, Table 5; Langworthy andBlack, 1978, Table l) carry micas that are not at all likethose of minettes. The micas of the other subgroups areminette-like to varying degrees. Of the 13 mica analysesthat fall within the minette field, one shows higher BaOthan the phlogopites of minettes and another higherNa2O.
Ultrapotassic rocks (lamproites) (40 analyses). Themicas of ultrapotassic volcanic rocks or lamproites are ofparticular interest because these rocks are geochemicallymuch like minettes (e.B'., Carmichael et al.,1974; Mitch-ell, 1981). The micas of two of the ultrapotassic lavas ofsoutheastern Spain (verite and fortunite; Velde, 1969,Table 3, anal. 12-4-5 and l4-A-7) are identical to minettemicas, but the jumillite micas (Borley, 1967, Table II;Carmichael, 1967, Table 6), although as rich in TiO2 asthe most titaniferous minette micas. are Al-deficient. Incontrast, the micas from the famous Leucite Hills, Wyo-ming (U.S.A). locality (Cross, 1897; Carmichael, 1967,Table 6; see also Barton, 1979) are wholly distinct fromminette micas though similar in Na2O and trace elementcontents: orendite and wyomingite micas have higher mgvalues and lower TiO2 while the madupite mica has lowerA12O3. In the equally famous West Kimberley province ofAustralia (Carmichael, 1967, Table 6; Mitchell, 1981) themicas are very TiO2-rich, have higher mg values thanmost minette micas of equivalent TiO2 content, and plot
within or above the borders of the minette field of Figure4; they all, however, have Al2O3 contents lower than orequal to the most Al-poor minette micas, and traceelements in the same concentrations as in minette micas.(Nearly three-quarters of the analyses of micas fromultrapotassic rocks plotted on Fig. 4 are from WestKimberley).
The chemistry of the lamproites at Smoky Butte,Montana (U.S.A.) is similar to that of the Australian WestKimberley rocks (Velde, 1975). A bulk analysis of SmokyButte phlogopites from a "Type III" lamproite (op. cit.,Table 3) is identical to analyses of high-TiOz minettemicas. The other two analyses are of a core and rim ofextremely Ti-rich (TiO2 = 11.3 and ll.l wt.%) phlogo-pite; the rim is poorer in Al2O3 (8.9 wt.Vo) and the corehas a higher mg value (0.86) than equally titaniferousminette micas. None of these micas was analyzed fortrace elements. A lamproite (called a "potash trachyte"by Nicholls, 1969, rock 643) in the Navajo area of Arizonais probably an extrusive felsic minette, so it was predict-able that its phlogopite phenocryst would be identical tominette micas in all respects.
Shoshonites (2 analyses). Micas from two shoshoniticvolcanic rocks (latites) are minetteJike though one ofthem has higher NazO than minette micas; trace elementdata are not given (Joplin et al.,1972, Table V).
Leucite-bearing lavas (7 analyses). All but one ofthesemicas are rich in TiO2. Five of the 7 difer considerablyfrom the micas of minettes (Cundari , 1973 , Table V, anal.BQ-PEGM and BEH-B; Birch, 1978, Table 5; Thompson,1978, Table 2, anal.5), but the other two (Cundari, 1973,anal. BEH-A and CND-6) are identical to high-TiO2minette micas and both have Na2O and trace elementconcentrations within the minette-mica range.
Other potassic rocks (12 analyses). Three of the sixanalyzed phlogopites from Precambrian hypabyssal po-tassic ultramafic rocks (phlogopite pyroxenites andphlogopite peridotites) in the Gardar province of southGreenland (Upton and Thomas, 1973, Table 3, anal. 1,2,6) match high-TiOz minette micas in their major andminor oxides and Cr2O3 and BaO contents (there are noanalyses for NazO). The remaining three phlogopites areminette-like in all respects save that they are impover-ished in FeOl and consequently have marginally higherlzg values (0.90 and 0.91) than minette micas of equiva-lent TiOz content. Upton and Thomas believe that theserocks-which in their trace element chemistry are remi-niscent of ultrapotassic rocks, minettes, and kimber-Iites-crystallized from magmas formed by the fraction-ation of olivine from kimberlitic liquids.
One of the three analyses of barian phlogopites inmonticellite peridotites and turjaites at Haystack Butte,in the potassic Highwood Mountains province of Mon-tana (Wendlandt, 1977, Table 25) plots within the minettefield but contains far too much BaO (3.91 wt.%) to beminette-like; the other two micas are unlike minettemicas.
too
oao
II o.rc
09@
q0
GOo | 2 3 { 5 6 7 6 9 p i l t 2
r l ! 6 T i O z +
Fig.4. Covariationof mg andwt.VoTiO2in micas of ultramaficto intermediate potassic and ultrapotassic igneous rocks otherthan kimberlites and lamprophyres and xenoliths therein; seeAppendix for sources of data. Curved lines indicate extent ofminette-mica field as explained in caption to Fig. 3 and text.
50 BACHINSKI AND SIMPSON: T|-PHLOGOPITES OF THE SHAW'S COVE MINETTE
Lastly, three representative analyses of phlogopitemegacrysts (phenocrysts) from an alnditic breccia in theSolomon Islands (Dawson e t al., 197 8, T able 2, anal. l, 2,3) are chemically indistinguishable from phlogopites inminettes on the basis of all oxides for which data arepresented, though their analysis #l plots right on theborder of the minette field of Figure 4.
Kimberlites (Fig. 5)
Generally, kimberlite phlogopites have higher mg val-ues and SiO2, lower TiO2, and similar or lower Al2O3contents than minette micas and many plot outside theminette field, some even of the range of the diagrams.Strictly minette-like micas, however, occur in kimberlitesfrom Somerset Island, Northwest Territories, Canada(Clarke and Mitchell,1975, Table l. anal.5; Mitchell,1978, Table 3, anal. 2; Mitchell, 1979,Table 3, anal 2,3,4, 5); Ontario, Canada (Rimsaite, 1971, Table l, anal. ll-PH, and Table 2, anal. II, III; Smith, et aI.,1978, Table 3,anal. 5, and Table 4, anal. 8a to 80: Arkansas. U.S.A.(Giardini et al., 1974, Table l, brown mica from lightgreen kimberlite); western Greenland (Emeleus and An-drews, 1975, Table 2, anal.3965llMl2; Scott, 1979, Table3, anal. 129); and various localities in South Africa(Smith, et al., 1978, Table 3, anal. 6b, 7c, and Table 4,anal. 5b, 6b,7c, l0f, and Table 5, anal. lb, 4a; Boettcheret al., 1977 , and 1979, Table l, anal. KB-5-l-A-rim, Kb-5-l-B-rim, Kb-s-l-C, Kb-5-13-rim; Clement et al., 1979,Table 3, anal.7; Boctorand Boyd, 1982, Table 2, anal. 4).Nearly all the kimberlites containing minetteJike micaare micaceous kimberlites or, in the terminology recom-mended by Skinner and Clement (1979), phlogopite kim-berlites.
Groundmass crystals (including both "Type I" and"Type II" of Smith et al., 1978) and secondary rims ofIarge phlogopites account for approximately three-quar-ters of the minette-like micas in kimberlites, but unzonedpre-fluidization phenocrysts (or megacrysts) of the Cana-dian Arctic's kimberlites (Clarke and Mitchell. 1975. loc.
' f f o ,
$
o I 2 3 u * , 0 n 0 , , o " 1 * t "
s P n e
Fig. 5. Covariation of mg and wt.Vo TiO2 in micas ofkimberlites (excluding xenoliths therein); see Appendix forsources of data. Curved lines indicate extent of minette-micafield as defined in caption to Fig. 3 and text.
rie. o. co'u.i*,-, t;J# ir* rrt,," ,.,*. ; r,;pressure xenoliths in mafic alkaline volcanic rocks other thankimberlites; see Appendix for sources of data. Curved linesindicate extent of minette-mica field as defined in caption to Fig.3 and text.
cir; Mitchell, 1978, loc. cir; Mitchell, 1979, loc. cit.) andone from the South African DeBeers pipe (Boettcher eral., 1979, Table l. anal. Kb-S-l-C) also plot within theminette field. It would appear that the micas being soughtby Smith et al.,(1978,p.970) and Mitchell (1979,p.165)that are chemically comparable to their kimberlite micasare to be found in minettes.
Xenoliths in alkaline rocks (Fig.6)
Most of the 5l micas in this category have zg valueslower than or in the same range as minette micas (fivehave higher); all the other oxides of importance are withinthe minette range with the exception of TiO2 in sixmegacrysts from Mongolian alkaline basalts (Ryabchikovet al., 1982): these micas have 9.0-12.2 wt.Vo TiO2 andare the most Ti-rich micas known to us from any rocktype anywhere. Micas matching those in minettes arefound in peridotite and pyroxenite xenoliths in alkalinelavas or pyroclastic rocks from southwest Uganda (Lloydand Bailey, 1975, Table 2, anal. 523-214, 523-209), Vic-toria, Australia (Ellis, 1976, ̂ table 3, anal. 7), the WestEifel area of West Germany (Aoki and Kushiro, 1968,Table 4, anal. 2; Becker, 1977, Table 3, anal. GM29,GM17, MEll), Paramore Crater, Arizona, U.S.A. (Ev-ans and Nash, 1979, Table 4, anal. PC76-2), and theLashaine Volcano, Tanzania (Dawson et al.,1970,Table7, anal. 2\.
Twelve of 19 analyses of phlogopites from garnetlherzolite and megacrystalline nodules in minette plugs ordiatremes in the Navajo field of northwestern Arizonaand contiguous parts of New Mexico, U.S.A. (Ehren-berg, 1979, Table 3, anal. l44TH, l75TH; Ehrenberg,19E2, Table 8, anal. YA78,710,l7l,174, X078-1, X078-2,8712,F077,249, R078) are wholly minettelike. The otherseven phlogopites narrowly miss being minetteJike byhaving zg values marginally higher than those of minettemicas of equivalent TiO2 and Al2O3 contents (two haveless titania than any minette mica).
tl o -
m9
o00
tI o ^
n0
o@
qo
m
BACHINSKI AND SIMPSON: T|-PHLOGOPITES OF THE SHAW'S COVE MINETTE 5 l
Of all the micas in xenoliths in alkaline rocks, thephlogopites in the xenoliths in minettes, on average, havesimultaneously the lowest TiO2 and A12O3 contents andhighest ng values (in this latter respect they are virtuallythe "least evolved" micas in this group). When comparedto the micas in kimberlites (previous section) the micasin xenoliths in minettes are seen to be identical to manyType Il-groundmass micas (Smith et al.,1978, Table 4)and secondary rims on phenocrysts or megacrysts (e.g.,Boettcher et al.,1979, Table l, anal. Kb-5-l-rim and Kb-5-13-rim). They also are comparable to many of thephlogopites occurring in peridotite xenoliths in kimber-lites (next section).
Xenoliths in kimberlites (Fie.7)
Three of these analyses are of micas included indiamonds (Giardini et al., 1974;Prinz et al.,1975; Gurneyet al., 1979) and are dissimilar to analyses of minettemlcas.
Most of the micas in high-pressure xenoliths in kimber-lites are wholly distinct from minette micas: they havelower TiO2 and Al2O3, higher SiO2, and higher lng values.These chemical characteristics are typical of "primary"phlogopites (e.9., Carswell, 1975, Table l) and phlogo-pites in the uenrn suite (Aoki, l974,Table l; Dawson andSmith, 1977, Table 2).3 Some micas in xenoliths inkimberlites, however. are identical to minette micas in allrespects. These minette-like phlogopites are predomi-nantly described as being of "secondary" mantle oigin("secondary-textured") and commonly occur as rims onearlier phlogopites or other minerals in eclogites or de-formed (sheared) garnet peridotites. One of these micasoccurs in a xenolith included in the Peuyuk C kimberliteof Somerset Island, Canada (Clarke and Mitchell, 1975,Table l, anal.6), the others are in xenoliths in SouthAfrican kimberlites (Boettcher et al., 1979, Table l, anal.Kb-8-8-rim, Kb-9-9-A-rim, Kb-9-9-B-rim, 8-16-7; Dan-chin, 1979, Table 2, anal. RVD-152, RVD-501; Rawlinsonand Dawson, 1979, Table 6, anal. 8D2027-core; Jones etal.,1982, Table 5, anal. l0c, lla, l2a, l2b).
An additional two micas, one in an eclogite nodule(Aoki and Kushiro, 1968, Table 4, anal. 4) and oneforming a secondary rim on an earlier phlogopite in aperidotite xenolith (Boettcher et al.,1979, Table 1, anal.Kb-5-10-rim), qualify as minettelike on Figure 7 andthe mg vs. wt.Vo Al2O3 diagram but fall just outside thelowerJeft boundary of the minette field of the atomic(Mg+ Fe{2lTi-Al triangular diagram.
Three minette-like micas are found not in clearly man-tle-derived xenoliths but in two lower-crustal garnetgranulites and a garnet websterite in kimberlites in north-eastern Lesotho where they appear to be in equilibrium
'It should be noted, however, that MARrD micas cannot beseparated from minette micas on the NiO vs. mg or Cr2O3 vs.FeOl diagrams used by Dawson and Smith (1977, Fie. 6).
p I t 2
Fig. 7. Covariation of mg and wt.Vo TiO2 in micas of high-pressure xenoliths in kimberlites; see Appendix for sources ofdata. Curved lines indicate extent of minette-mica field asdefined in caption to Fig. 3 and text.
with the anhydrous phases in the xenoliths (Grifrn et a/.,197 9, T able 4, anal. K3, LQ I, LT2). A mica from a garnetwebsterite xenolith from the Monastery Mine, SouthAftica (loc . cit. , anal PHN2630/1) plots in the field of theanomalous micas of the Pendennis minette and matchesthese micas in all respects save that it has an elevatedAl2O3 content.
Discussion
As micas whose compositions are essentially identicalmay have formed under similar physical and/or chemicalconditions, it is of petrog€netic significance that minettemicas match many micas in the diverse groups of mantle-derived rocks discussed above; the chemical identity isyet another piece of evidence that the liquids from whichminettes crystallize are themselves mantle-derived.Those minette mica analyses that are clearly identified asrepresenting the cores ofphenocrysts have TiO2 concen-trations of 2.0-11.3 wt.Vo andrng values of 0.74 to 0.90.We assume that these cores crystallized at elevatedsubcrustal pressures.
High-pressure experiments of, e.8., Edgar et al., (1976,1980), Ruddock and Hamilton (1978, and pers. comm.,1978) Ryabchikov and Green (1978), and Barton andHamilton (1982) have demonstrated that phlogopites areIiquidus or early-crystallizing phases in a variety ofpotassic hydrous + carbonate-bearing systems (includinga model minette) at pressures of 10-35 kbar, temperaturesof 1150-13fi)"C, /6, between the NNO and HM buffers,and molar COzl(CO2+HzO) ratios of 0.00 to 0.50. Phlogo-pites produced in these studies have mg values of 0.82-0.90 and TiOz contents of I .8 to 8.1 wt.Vo (the phlogopitewith the highest titania content was produced underconditions of T: 13fi)'C, P = 30 kbar,2.4wt.VoH2O and5.9 wt.% CO2, CO2|(CO2+H2O) : 0.50). Some of theprimary (as opposed to quench) phlogopites crystallizedby Edgar et al.,(1976, Tables 3 & 5; 1980, Table 8) andRyabchikov and Green (1978, Table 3) are chemically
1
4 5 6 7
w i T o T i O z +
52 BACHINSKI AND SIMPSON: Ti-PHLOGOPITES OF THE SHAW'S COVE MINETTE
identical to cores of minette-mica phenocrysts (as well asother minette micas). In attempting to ascertain theconditions under which minette micas may have crystal-lized in the light of these experimental studies, the maindifficulty lies in separating the effects of temperature,pressure, for, and fluid and liquid compositions. Thesestudies demonstrate that, under mantle conditions, phlog-opite crystallizes early only from H2O-rich, COz-poorsystems and the TiO2 content of the phlogopite rncreaseswith increasing /e, (Fe"3 content of meltl, decreasingpressure,4 decreasing water content of the system, anddecreasing mg valte of the crystals and liquid; increasingCO2 content of the system lowers the temperature atwhich phlogopites crystallize. The consensus is thattemperature and, perhaps more importantly, /o, are themajor controls on TiO2 content (see also Arima andEdgar, 1981). It therefore would appear that minettemicas, which are notably rich in TiO2-as well as some ofthe micas in certain other lamprophyres, potassic rocks,kimberlites (phenocryst rims and groundmass, largely),and high-pressure xenoliths in alkaline volcanic rocks,and "secondary metasomatic" micas-crystallized underhigher /e, conditions and/or higher temperatures thanmost "primary," "primary metasomatic," and ueruo-suite micas in kimberlites and xenoliths in kimberlites.
Luhr and Carmichael (1981) report equilibration tem-peratures (based on the partitioning of oxygen isotopesbetween olivine, pyroxene, and magma) of 1070' to1200"C andfo, values (calculated) between the NNO andHM oxygen buffers for petrographically pristine late-Quaternary minette lavas intimately associated spatially(and probably genetically) with leucite basanites in west-ern Mexico. These values of temperature and /6. are inagreement with those under which phlogopite was crys-tallized in the high-pressure experiments.
Garnet peridotite xenoliths in the Navajo minertescalled The Thumb and Ship Rock equilibrated at depthsof -ll0 km and at temperatures ranging from 950' to1230'C (Ehrenberg, 1979, 1982), nearly the same condi-tions (T = 1150'C, P = 42 kbar) obtained by Mercier(1976) for a diopside xenocryst in the Ship Rock minette.It is improbable that the Navajo minette magmas formedat shallower depths than the xenoliths they bear. As thexenoliths contain phlogopites that are chemically identi-cal or very similar to minette micas, it is clear that,irrespective of whether there has been interaction be-tween the xenoliths and minette magma, such micas canform in the upper mantle at temperatures as great as1250'C and pressures of 40 kbar or more, conditionsessentially equivalent to those under which ultrapotassicrocks and kimberlites originate.
The chemical identity of minette micas and many of the"secondary metasomatic" micas of kimberlites and man-
a Note, however, that Robert (1976) suggests that high-tem-perature-high-Ti phlogopites may be of either high- or low-pressure ongrn.
tle xenoliths indicates that metasomatism and consequentmantle heterogeneity are important factors in the origin ofboth minettes and kimberlites (but see Wyllie and Sekine,1982, for an alternative explanation). On the basis of alarge number of experimental and theoretical studies inrecent years on peridotite-H2o-Co2 systems, it appearsthat source-rock composition-as influenced by metaso-matism, partial melting, and reaction between liquids andsolids-as well as the absolute and relative abundances ofH2O, CO2, and halogens at the time of magma generationare decisive factors in determining whether the magmaproduced from a K-rich mantle source will be minette-like, kimberlitic, or ultrapotassic (see, e.9., Ryabchikovand Green, 1978; Baldridge et al. , l98l; Luhr and Carmi-chael. 1981).
The inter- and intra-grain chemical variations seen inminette micas are probably due to small-scale magmaticheterogeneities produced by mixing of batches of magmaof similar but differing composition, as suggested bySmith et al., (1978) and Mitchell and Meyer (1980), aswell as high- and low-pressure evolution of minettemagma.
Acknowledgments
We would like to thank D. B. Clarke and R. M. MacKay ofDalhousie University for assistance in the use of the electronmicroprobe facilities. J. W. Nicholls, University of Calgary,kindly sent us corrected versions of the tables from his Ph.D.dissertation for which we are grateful. A. D. Edgar, Universityof Western Ontario, is thanked for supplying us with an Englishtranslation ofRyabchikov and Green (1978). We are grateful forthe helpful comments of R. H. Mitchell in his review of themanuscript. This work was supported in part by grants from theNatural Science and Engineering Research Council of Canada tos . w. B .
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BACHINSKI AND SIMPSON: T|-PHLOGOPITES OF THE SHAW'S COVE MINETTE 53
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Appendix--Source papcrs for mica analyscs
(Full references are given only for those publications not citedin the list of references or earlier in this appendix. Sources arelisted in alphabetical order within rock-type subdivisions).
A. In Minettes.'Ehrenberg, 1979;Callagher, 1963; Hall, 1982;Kramer, 1976; Luhr and Carmichael, l9El; M6tais el al., 1962:N6mec, 1972; Nicholls, 1969; Roden and Smith, 1979; Scott,1979; Velde, 1969; Velde, D., 1971, Les lamprophyres ifeldspath alcalin et biotite: minettes et roches voisines.Contributions to Mineralogy and Petrology, 30, 216-239;Wifliams, H., 1936, Pliocene volcanoes of the Navajo-Hopicountry. Bulletin of the Geological Society of America, 47, lll-172.
B. In Kersantites and Other Lamproplryres (other thanminettes): Cooper, 1979; Grapes, 1975; Griffin and Taylor, 1975;Ianse, 1977; Kramer, 1976; Leavy, B. D., and Hermes, O. D.,1979, Mantle xenoliths from southeastern New England. In F. R.Boyd and H. O. A. Meyer, Eds., The Mantle Sample: Inclusionsin Kimberlites and Other Volcanics, p. 374481. AmericanGeophysical Union, Washington, D.C.; McHone, 1978; Metaiset al., 1962; Ndmec, 1972; Nicholls, 1969; Platt and Mitchell,f9E2; Rimsaite, l97l; Rock, l97E; Rock, l9E2; Strong andHarris, 1974; Velde, 1969;
C. In Potassic Roc&s (other than kimberlites, lamprophyres,and xenoliths): Birch, l97E; Boettcher, 1967; Borley, 1967;Carmichael, 1967; Cross, 1897; Cundari, 1973; Dawson et al.,1978; Joplin et al., 1972; Langworthy and Black, 1978; Mitchell,l9El; Nash and Wilkinson, 1970; Nicholls, 1969; Thompson,1978; Upton and Thomas, 1973; Velde, 1969; Velde, 1975;Wendlandt. 1977:
D. In Kimberlites (excluding xenoliths therein): Akella, J.,McCallister, R. H., and Meyer, H. O. A. (1977) Diamondiferouskimberlite of the Wajrakharur area, southern India. ExtendedAbstracts, Second International Kimberlite Conference [nopaginationl; Boctor and Boyd, 1982; Boettcher et al., 1979;Clarke and Mitchell, 1975; Clement et al., 1979; Dawson et al.,1970; Elthon, D., and Ridley, W. I. (1977) Mineral chemistry ofthe Premier Mine kimberlite: Extended Abstracts, SecondInternational Kimberlite Conference; Elthon, D., and Ridley,W. I. (1979), The oxide and silicate mineral chemistry of a kimber-lite from the Premier Mine: implications for the evolution ofkimberlitic rocks. In F. R. Boyd and H. O. A. Meyer, Eds.,Kimberl i tes, Diatremes and Diamonds: Their Geology,Petrology and Geochemis try, p. 206-216. American GeophysicalUnion, Washington, D.C.; Emeleus and Andrews, 1975; Giardiniet al.,1974; Gogineni, S. V., Melton, C. E., and Giardini, A. A.(197E) Some petrological aspects of the Prairie Creek diamond-bearing kimberlite diatreme, Arkansas. Contributions toMineralogy and Petrology, 66,251-261; Lewis, R. D., andMeyer, H. O. A. (1977) Diamond-bearing kimberlite of PrairieCreek, Murfreesboro, Arkansas. Extended Abstracts. SecondInternational Kimberlite Conference [no pagination]; Merrill,R. B., Bickford, M. E., and Irving, A. L (1977) The Hills Pondperidotite, Woodson County, Kansas: a richterite-bearingCretaceous intrusive with kimberlitic affinities. ExtendedAbstracts, Second International Kimberlite Conference [nopaginationl; Meyer, H. O. A., and Kridelbaugh, S. J., 1977,Green Mountain kimberlite, Colorado: mineralogy andpetrology. Extended Abstracts, Second International KimberliteConference [no pagination]; Mitchell, l97E; Mitchell, 1979;Reid,A. M., Donaldson, C. H., Dawson, J. 8., Brown, R. W., andRidley, W. I. (1975) The Igwisi Hills extrusive "kimberlites".Physics and Chemistry of the Earth, 9,199-218. Rimsaite, l97l;
56 BACHINSKI AND SIMPSON: Ti-PHLOGOPITES OF THE SHAW'S COVE MINETTE
Scott, 1979; Smith et al., 1978;E. In Xenoliths in Alkaline Lavas and Kimberlites: AokL 1974:
Aoki and Kushiro, l96E; Arculus, R. J., and Smith, D. (1979)Eclogite, pyroxenite, and amphibolite inclusions in the SullivanButtes latite, Chino Valley, Yavapi County, Arizona. In F. R.Boyd and H. O. A. Meyer, Eds., The Mantle Sample: Inclusionsin Kimberlites and Other Volcanics; p. 309-317. AmericanGeophysical Union, Washington, D.C.; Becker, 1977; Best,M. C. (1975) Amphibole-bearing cumulate inclusions, GrandCanyon, Arizona and their bearing on silica-undersaturatedhydrous magmas in the upper mantle. Journal of Petrology, 16,212-236; Boettcher et al. , 1977 ; Boettcher et al., 1979', Carswell,1975; Chapman, N. A. (1976) Inclusions and megacrysts fromundersaturated tuffs and basanites, east Fife, Scotland. Journalof Petrology, 17, 472-498;' Clarke and Mitchell, 1975; Danchin,R. V. (1979) Mineral and bulk chemistry of garnet lherzolite andgarnet harzburgite xenoliths from the Premier mine, SouthAfrica. In F. R. Boyd and H. O. A. Meyer, Eds., The MantleSample: Inclusions in Kimberlites and Other Volcanics, p. lM-126. American Geophysical Union, Washington, D.C.; Dawsonand Smith, 1977; Dawson et al., 1970; Ehrenberg, 1979;
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