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Two Roberts victor cumulate eclogites and their re-equilibration
27 TWO ROBERTS VICTOR CUMULATE ECLOGITES AND THEIR RE-EQUILIBRATION By
M. A. LAPPIN* and 1. B. DAWSONt
ABSTRACT Microprobe and bulk chemical analyses are presented for two eclogites. Both contam relatively Ca-nch 0 0 garnets (25 45 0 grossular) and Na-rich (approximately 50 0 jadeite) tschermakite-poor clmopyroxenes. One specimen IS foliated and layered. The layers consist of garnet--elinopyroxene, garnet--elinopyroxene-{kyanite), where all garnets from overgrowths on kyanite, and garnet -clinopyroxene-kyamte The garnets of the latter layer are ncher in Ca and poorer m Mg than those of the other layers though within these layers ~ cryptic mineral vanation IS present. Some garnets of the garnet-clinopyroxene layer are zoned. The second specimen IS a graphite eclogite. The garnet overgrowths between kyamte and clmopyroxene are considered as part of a general closed system reactIOn cpx, ± gt, ± ky -> cpX 2 + gt 2 ± kyo The compositIOn of cpx, for the overgrowth layer is calculated from modal and mmeral compositIOns. The garnet-clinopyroxene layer could have been formed from cpx, and smaller amounts of garnet of rather simIlar composition The garnet--clmopyroxenekyamte layer probably contamed different clmopyroxenes and garnets. Textures. mmeral compositIOns and calculated mineral compositions, distribution functions, and appropnate expenmental eqUllibnum curves, allow three stages m the T-P evolution of these rocks to be traced. They were probably formed at 1400-1500' C, 26-28 kbar by cumulate processes from an evolved high P magma. They were subJccted to tcctomc processes then eqUllibratmg at 75Q.-900T and 28- 32 kbar. The third stage of evolutIOn. con'ldered to represent reaction, partial meltmg and chemical exchange With an ascending kimberlite magma or fluid. is prOVisionally assessed at 1000 lIOOC, 8 12 kbar.
INTRODUCTION Two eclogite nodules were collected by one of us (1. B. D.) from the Roberts Victor pipe. One of them (6913, Aberdeen University Collection) figured by DAWSON (1968) is noteworthy for its foliated and layered character. The layers consist of a 3.5-cm thick garnetclinopyroxene unit with a variable garnet/clinopyroxene ratio followed by a l-cm-thick garnet-clinopyroxene umt in which all garnets form overgrowths (Fig. 1) on kyanite in a fashion similar to the diablastic intergrowths of SOBOLEV et al. (1968) and then another 3-cm thick layer consisting of garnet-clinopyroxene-kyanite. In the latter layer only one thin overgrowth was seen about I cm from the overgrowth layer (analysis 9A Table 1). The garnets average about 5 mm in diameter and their forms range from subhedral (Fig. I) within the overgrowth layer and the garnet-clinopyroxene layer, to anhedral crystals forming elongate aggregates within the garnet--clinopyroxene-kyanite layer. The clino-
* Department of Geology and Mmcralogy. Marischal College, Umverslty of Aberdeen, Aberdeen AB9 IAS, Scotland t Department of Geology, UnIversity of St Andrews, SI. Andrews, Fife KY16 9TS, Scotland. 351
Z Y X
SI Al[4] All"] TI Fe Mn Mg Ca Na
Total
Si0 2 A1 2 0 3 Ti0 2 FeO MnO MgO CaO Na 2 0
3.00 2.00 3.02
2.947 0.053 1.997 0.014 0.941 0.023 1.289 0.739 0.017
99.6
-- -
39.5 23.3 0.26 15.1 0.36 11.6 9.26 0.12
I
,
I
1
---
3.00 1.96 3.09
UiL'
39.7 23.4 0.22 15.2 0.35 11.5 106 0.15
3
3.00 1.96 309
1.964 0.012 0.935 0.022 1.262 0.835 0.021
~.~92~ 0.073
101.1
----
I
2.9331 0.067 1.958 0.014 0.934 0.022 1.259 0.846 0.017
102.0
1-------
40.1 23.5 0.25 15.3 0.35 11.6 10.8 0.12
2
6913 gt-cpx
3.00 1.91 3.15
2.980 0.020 1.907 0.013 0.976 0.014 1.242 0.881 0.020
I
101.1
~.12
40.8 22.2 0.04 15.6 0.24 10.9 11.2
7
100.7
40.0 21.8 0.11 14.3 0.22 8.36 15.7 0.14 -
101.
--
2.982 0.018 1.910 0.014 0.986 0.018 1.223 0.879 0.019
t 3.01 1.93 3.08
1.934 0.002 0.963 0.015 1.200 0.885 0.017
3.011
3.00 1.92 3.13
2.996 0.004 1.919 0.006 0.894 0.014 0.933 1.259 0.020
40.4 21.6 0.19 13.8 0.24 7.51 17.4 0.14
9B
I
40.7 21.9 0.15 13.8 0.19 744 17.3 0.13
10
6913 gt--epx-ky
I
39.5 22.5 0.24 142 0.27 7.80 173 -0.15
11
~_!
39.8 22.1 0.19 14.4 0.26 8.34 162
12
42.5 20.5 0.13 9.24 0.19 11.6 17.2 ·0:06
A
I
B
42.5 20.6 0.11 9.18 0.20 11.9 16.2 0.07
6914 gt--epx
1.9 6 0.00 8 0.8 7 0.0 2 0.8 I 1.3 5 0.0 9
3.0 7
-
3.01W' 3.02 1 90 1.9 2 3.12 3.09
1.899 0.011 0.857 0.015 0.834 1.386 0.020
3.014
U 3.01 1.91 3.12
--
1907 0.010 0.9 8 0.016 1.180 0.915 0.019
3.008
--
3.00 1.90 3.18
2.935 0.065 1.903 0.013 0.880 0.017 0.863 1.380 0.022
~-
3.00 1.91 3.16
2.964 0.036 1.906 0.011 0.896 0.016 0.922 1.290 0.023
3.09 1.75 3.19
3.10 1.77 3.14
1.767 0.006 0.560 0.012 1.288 1.265 O.Ole
-
1.752 0.007 0.561 0.012 1.259 1.343 0.008
3.103
3.089
- - -
~~131101.61Jill.0_~ 1~008
40. 9 22. o O. 8 16. o O. 25 10. 8 I 11. 6 O. 3
9A
--
Cations on a baSIS of 12 oxygens
101.1
40.2 22.1 0.25 15.9 0.29 11.1 11.1 0.13
6
8
Garnets
ANALYSED GARNETS AND CLINOPYROXENES
3.00 L[.OO 1.96 1.91 3.06 3.14 _
1.956 0.012 0.898 0.011 1.240 0.876 0.019
3.003
g<)~0
401 22.1 0.22 14.3 0.18 11.1 10.9
5
~£:tO[]
100.4
40.0 21.9 0.23 15.7 0.22 11.2 11.0
4
1.
6913 overgrowth layer
TABLE
w
VI
oz
:E
:>
1:1
t:J:I
:--
:> z 1:1
Z
'tI 'tI
:>
t""'
:>
:::::
tv
X
y
z
TI Fe Mn Mg Ca Na
Sl AI[4j AI( 6 )
Total
-
Si0 2 AI 2 0 3 TiO z FeO MnO MgO CaO NazO
2.00 1.07 0.88
1.945 0.055 0.651 0.006 0.069 0.001 0.341 0.380 0.498
2.00 1.06 0.88
.-
1.939 0.061 0.648 0.006 0.069 0.001 0.341 0.390 0.490
100.5
7.31
735
99.3
56.0 17.4 0.25 2.37 0.03 6.62
IQ5
2
55.6 17.1 0.23 2.36 0.03 6.55
IQI
J
-
2.00 1.06 0.89
1.934 0.066 0.670 0.006 0.063 0.001 0.320 0.366 0.528
00.7
7.90
~9
56.1 18.1 0.24 2.19 0.02 6.23
3 {
2.00 1.03 0.92
1.968 0.032 0.653 0.006 0.059 0.000 0.313 0.381 0.536
I 100.5
8.01
IQ3
569 168 0.25 2.05 0.01 6.08
4
-
-
100.3
99.2
99.9
100.0
2.00 1.00 0.91
~~
_. 1.963 0.037 0.618 0.007 0.063 0.001 0.316 0.380 0.530
57.3 18.0 0.06 1.87 0.00 5 17 9.97 7.92
56.9 164 0.08 2.27 0.01 6.29 10.5 6.75
56.5 15.9 0.30 2.49 0.01 6.42 10.5 7.76
56.5 16.9 0.26 2.17 0.03 6.10 10.2 7.89
2.00 1.04 0.91
1.971 0.029 0.626 0.008 0.073 0.000 0.334 0.391 0.524
~ . _
2.00 1.05 0.85
1985 0.015 0.659 0.002 0.066 0.000 0.327 0.392 0.456
~--
.
2.00 1.05 0.90
1.975 0.025 0.706 0.002 0.054 0.000 0.265 0.368 0.529
Cations on a basIs of 6 oxygens
8
6
5
Clinopyroxenes
-
2.00 1.06 0.85
1.979 0.021 0.683 0.005 0.062 0.001 0.311 0.353 0.495
100. 3
57. 5 17. 3 O. 20 2. 17 O. 03 6. 07 9. 58 7. 42
-~
9 -
2.00 1.04 0.86
----
1.991 0.009 0.700 0.003 0.058 0.000 0.277 0.366 0.493
-_.. 100.3
5.40 9.94 7.40
tr
57.9 17.5 0.12 2.01
10
2.00 1.04 0.89
1.957 0.043 0.701 0.003 0.059 0.001 0.274 0.371 0.515
99.6
56.3 182 0.13 2.02 0.04 5.29 9.97 7.65
II
--
2.00 1.04 0.88
1.968 0.032 0.697 0.004 0.060 0.002 0.274 0.371 0.512
------
100.2
57.0 179 0.17 2.10 0.06 5.33 10.0 7.67
12
2.00 1.05 0.97
1.914 0.086 0.609 0.001 0.052 0.000 0.384 0.408 0.564
100.2
54.8 16.9 0.05 1.77 0.01 7.40 10.9 8.34
A
2.00 1.03 0.99
1.910 0.090 0.607 0.002 0.045 0.001 0.377 0.423 0.569
100.1
54.6 16.9 0.07 1.55 0.02 7.24 11.3 8.39
B
\.;.)
V1
\.;.)
BJ
::J
§
m m n
-l
;..-
ct"'"
~
n
o~
n -l
-<
;;J
~
~ m
~
o<
-l
354
M.
A. LAPPIN AND J.
B.
DAWSON
FIG. 1. Textures in the gt--epx and overgrowth layers of 6913. Dotted ornament altered clinopyroxene with clear areas of rehct chnopyroxene. The garnets of the gt -epx layer have alteration cracks and fractures. Those of the overgrowth layer have been left clear to emphasize the pattern of kyanite mcluslOns. The ruled hnes m kyamte are the cleavage traces whIlst the dashed lines represent the stram bands and the black areas zones of alteratIOn. IndIVldual garnets from the overgrowth layer are also shown. OccasIOnal grams of rutile in chnopyroxene are black.
pyroxenes throughout the sample are extremely altered but tend to form interstitiallensoid aggregates elongated parallel to layer interfaces and to the foliation. The kyanite laths are up to 4 mm long and are also elongated within the foliation. The second specimen, 6914, consists of subhedral. 5-mm diameter garnets, interstitial clinopyroxene and occasional hexagonal plates of apparently primary graphite. Rutile and phlogopite occur in both samples. Both nodules show extensive alteration, particularly of clinopyroxenes, the detailed mineralogy and textures of which match almost completely the descriptions by SWITZER and MELSON (1969). The textures of the overgrowth layer provide some evidence as to the relationships of stress to stages of mineral growth. Most of the kyanite crystals have kink or strain bands. The overgrowth garnets form deep embayments in kyanite in the vicinity of these strain bands (Fig. 1). Strain bands have a limited range of crystallographic orientations and it seems an unlikely coincidence that favourable crystallographic orientations always occur in the vicinity of overgrowth embayments, i.e. that the strain bands postdate the development of overgrowth textures. A more likely explanation is that the strain bands act as sites of preferential garnet nucleation and growth and that the stress predates overgrowth development.
355
TWO ROBERTS VICTOR CUMULATE ECLOGITES
MINERAL CHEMISTRY Garnets and clinopyroxenes (Table 1) from specimens 6913 and 6914 were analyzed at the University of Aberdeen with a Geoscan microprobe using the technique of SWEATMAN and LONG (1969). In 6913 twelve garnet/clinopyroxene pairs were analysed across the layering. The garnet analyses were all made within 50 J1 of the crystal edge because of the possibility of zoning.
GARNETS The compositions of the garnets in 6913 range from grossular 25 pyrope 4 2 almandine33 to grossular 44 .3 pyrope 27 . 7 almandine 28 . o ' The average composition of garnet from 6914 is grossular 4 1.6' pyrope 40 . 6 , almandine 1 7.6' Other end-member molecules are unlikely to be present in large amount, apart perhaps from andradite, since Ti0 2 and MnO occur in limited amounts and Cr 2 0 3, measured in some garnets, was uniformly low (0.01-0.06 ~~) as was NiO (0.004-0.02 ~~) and K 2 0 (0.007-0.02 ~~). The garnets in 6913 were analysed sequentially (l ~ 12) together with clinopyroxenes. Almost all garnets show high oxide per cent totals whilst the clinopyroxenes have acceptable totals. Three garnets (9B, 10, 11) were thus re-analysed and oxygen was also determined (quartz used as a standard). Oxygen was also determined in garnet 12. In each case the variation for individual elements was small (Table 2) and the element per cent totals were improved though remaining high. The average element per cent for these four garnets is thus quoted in Table 2 together with structural formulae calculated upon a basis of both available and also stoichiometric oxygens. High oxide per cent totals may be the result of oxygen deficiency (about 1.7 ~~ in Table 2). TABLE
2.
AVERAGE ATOM
0 0
VALUE FOR FOUR GARNETS TOGETHER WITH STRUCTURAL FORMULAE
MaXImum range of OXIde ~.~ also shown
---------
--------
--------
----
Cations on a basis of 12 oxygens ------
Weight °0 atoms -------
°
------
42.1 18.8 11.6 0.12 10.9 0.19 4.85 11.9 0.11
SI Al TI Fe Mn Mg Ca Na
----
Average 91J,10,11,12
---
Total
-----
100.6
Measured 0
y
X
- ~ - -
3.050 1.964 0.011 0.891 0.016 0.908 1362 0.022
----
Z
Maximum range ofvalues in reanalysed garnets 98,10,11
StOichIOmetrIC 0
------
SI AI[4) AII6] TI Fe Mn Mg Ca Na
------ -
---------
SI0 2 AI 2 O, Ti0 2 FeO MnO MgO CaO Na 2 0 -
-
3.05 1.96 3.21
--------- -
----
2.997 0.003 1.927 0.011 0.875 0.016 0.892 1.338 0.021
---
0.28 (9B) 0.44 (11) 0.01 (10,11) 0.17 (II) 0.01 (10) 0.53 (10) 0.44 (11) 0.01 (9B,II)
---- ------
- - ~
3.00 1.93 3.15 ----
--- ------
The analysed garnets lie within the general field of compositions of kimberlite eclogite garnets (RICKWOOD et al., 1968) though 6914 is more extreme than garnets previously recorded from the Roberts Victor pipe (MACGREGOR and CARTER, 1970) having high MgO and CaO.
356
M.
A. LAPPIN AND
J. B.
DAWSON
ZONING IN GARNETS Tests were made for zonation only by traverses across individual grains. Zoning appears to be confined to the garnet-clinopyroxene layer (in about IO/~ of the traverses) and was not found in the overgrowth or garnet-clinopyroxene-kyanite layer. The zoned garnets have cores between 100-400 J1 wide depleted in Ca and enriched in Mg in a sometimes asymmetric, flat-bottomed, saucer-shaped pattern (Fig. 5). The pattern for Fe varies for in some cases there is a slight enrichment or depletion in the core whilst in others no variation could be detected. There were no significant variations in the Mn pattern. A partial analysis of one zoned garnet gave a core composition of grossular 19.2 pyrope 44 . 9 almandine 35 . 9 and a rim of grossular 25 pyrope 42 almandine 33 (Fig. 3A). One garnet (Fig. 5) showed a larger scale asymmetric depletion with respect to Ca though without apparent variation in Mg, Fe and Mn. The anomalous composition of this garnet is mentioned in the following section. The zoning differs from that previously recorded in other eclogites (DUDLEY, 1969; BRYHNI and GRIFFIN, 1971).
CRYPTIC VARIATION OF GARNETS An initial survey of garnets in 6913 suggested considerable variations in garnet composition across the layering. These analyses, however, were of rather poor quality and a second set of analyses was completed in 1973 from a second slice of 6913 (Table 1). For comparative purposes, however, both sets of analyses are plotted in Fig. 2. gt-cpx-ky
gt-cpx
18
9810
9A
,P------ 12
16
,/
I
CaO 14 &
,/
FeO I
12
MgO
11
I
I
I
I
D
I
,~ --~{}---- ----iJ
I
4
10
8
o
4 2 Distance perpendicular to layenng
6cm
FIG. 2. Garnet composItIOns (OXIde ~/~) across the layenng in 6913. The first traverse IS shown WIth dashed lines. The second traverse by solId lInes (some partIal analyses are included within the latter). The analyses from Table I are numbered In the CaO traverse. The pOSition. but not the analySIS. of garnet 9A IS shown
TWO ROBERTS VICTOR CUMULATE ECLOGITES
357
At the edge of the garnet-clinopyroxene layer both traverses begin with a relatively Ca-poor, Mg-rich garnet (I) and in both there is a fairly regular increase ofCa (2-5) towards the overgrowth layer. The most notable exception to this trend is analysis 2 which has an unusually high CaO content. This garnet has a Ca-depleted core, a feature which could account for its anomalous composition. MgO and FeO show less regular patterns of variation, most probably showing a slight increase of FeO and a slight decrease of MgO towards the overgrowth layer interface. The garnets of the overgrowth layer (6,7) and the overgrowth rim within the garnet-clinopyroxene-kyanite layer (9A) are richer in CaO and FeO and poorer in MgO than garnets 1-5 of the garnet-elinopyroxene layer. The garnets of the garnet-elinopyroxene-kyanite layer are much richer in CaO and poorer in MgO than those of the other layers. Within this layer also there is evidence for cryptic variation since the garnet close to the overgrowth layer interface (8) is poorer in CaO and slightly richer in MgO than the garnets from the centre of this layer (9B, 10, 11). Garnet 12 at the edge of the specimen is similar to garnet (8) as was the equivalent garnet from the first traverse (Fig. 2). The distinctive character of garnets at either end of the traverse (I and 12) suggests that the fragment may have in fact broken close to boundaries of chemicalj mineralogical inhomogeneity.
~
~
MgO
grosspydite
A.
-If>
FeO.~,<>~
•
8.
XRV3
/
~,o
6:~:'\\
'98-11
~
E~
/i~2
~~9A /
,
;' I
f
0
~,~rim I: I 1
d
.core
I
°XRV4
FIG 3. Parts of the CaO, MgO, FeO diagram (mol'/~) as shown in the mset. A. Tie Ime relationships for the analysed garnets (filled circles) and clinopyroxenes (unfilled squares). IndiVidual or average garnets are identified by numbers. Bulk compositions (averages of A-E, Table 3) are unfilled diamonds. The compositional trend in 6913 is shown as a thick line. The Roberts Victor grosspydite garnet (MACGREGOR and CARTER, 1970) and the garnet compositional trends (dashed lines) of other layered eclogites (XRV 3 and 4) (RICKWOOD et al., 1968) are also shown. B. PossIble high T-P tie lines (heavy lines) drawn on the assumptIOn that cpx, of the overgrowth layer (C In Fig 3A) is stable in both the gt--{;px and gt-cpx-ky layers. Bulk analyses have been modified to allow for alteration and some of the tie lines of Fig. 3A are shown as dashed lines.
358
M.
A. LAPPIN AND
J.
B. DAWSON
The garnets, or where appropriate, average values, are plotted in Figs. 3A and 4A. They have a linear trend on Fig. 3A similar to XRV 3, another Roberts Victor layered eclogite (RICKWOOD et ai., 1968). Another specimen described by these authors (XRV 4) shows a similar trend in garnets of lower FejMg ratio though of a less extensive range. The rare grosspydites from Roberts Victor (Fig. 3, MACGREGOR and CARTER, 1970) are the culmination of this trend of garnet compositions. I
I
A
B.
I
"tie
I
\
\
,
\
\
,
, \
\ \
\
\ \ \
..,...c-
,
-c"'-------"---->L....L----"-------"'-------'''-----'---------'F--''--------''--------''----
cpx
opx
FIG. 4. PortIOns of the A.C.F. diagram (mol 0 0 ) for Roberts Victor eclogites (A = AI 20 3 + Fe203 - Na 20, C = CaO, F = FeO + MgO + MnO). A. Synoptic relationships, mineral composition fields and IImltmg tie lines (dashed lines) after MACGREGOR and CARTER (1970). Tie Ime relationships for the analysed minerals, symbols for minerals and bulk compositions as m Fig. 3A, and the compositional trend m 6913 are also shown B. Possible high T ~P tie line onentations (heavy lines) drawn with the same assumption as Fig. 3B. Cpx l (Table 4) lies on the tie Ime (dashed line) defined by kyanite and the overgrowth layer (C). Both the compositIOn of the overgrowth layer and cpx, have been modified (filled symbols) to allow for their likely Fe 2 0 3 contents. Gt , (open circle) shows the limltmg garnet If composition D IS to remain within the gt ky-epx , compositional field. Some of the tie-line orientatIOns of Fig. 4A are shown by dashed lInes.
CLINOPYROXENES
The clinopyroxenes are jadeite-rich omphacites in which Na occupies around 50~~ of available X sites. They contain limited tschermakite, only about 1 ():, of Al[4l. They closely resemble the clinopyroxenes from Siberian kyanite eclogites and grosspydites (SOBOLEV et ai., 1968) in that they have up to 30~~ Al[6) in excess of that related to Al[4) and Na. Again characteristically they have low cation sums (X + V). The clinopyroxenes have limited amount of other elements, Cr 2 0 3 6913, 0.O1--O.04~.~, 6914, 0.06~~; NiO 6913, 0.02--O.03~;" 6914 0.08 ~;); K 2 0 6913, 0.08--0.09 ~~. They show some traces of cryptic variation especially with respect to FeO and MgO. The amount of these elements decreases through the garnet-clinopyroxene layer, increases slightly in the overgrowth layer and then decreases markedly in the garnet-clinopyroxene-kyanite layer. Within the latter layer clinopyroxene 9, adjacent to the overgrowth garnet 9A, has higher values for these elements resembling values found within the overgrowth layer (6,7). Figure 3A suggests a more or less regular increase in CaOjCaO + MgO + FeO in clinopyroxene with increasing Ca in garnet whilst Fig. 4A shows that excess Al[6l increases with this parameter.
359
TWO ROBERTS VICTOR CUMULATE ECLOGITES
10%FeO 0·5%MnO 0 10'YoMgO 10%CaO 0 10'YoMgO
10%CaO 0
C MgO CaO
10%MgO 10%CaO
0
ky
10%MgQ 1O%CaO
,
200 microns , I
OL...J~------------------'--'
FIG. 5. Zonation m garnets. A. ZonatIOn patterns for FeO. MnO. MgO and CaO along a smgle line of traverse Narrow regions depleted in CaO and enriched m MgO are alteration filled fractures. B. ZonatIOn of MgO and CaO. FeO and MnO shown no sIgnIficant varIation. C. Assymetric zonation with respect CaO. no apparent VarIatIOn m MgO. FeO or MnO (Garnet 2 of Table I and Fig. 2). D. UnIform garnets of the overgrowth layer A traverse across two garnets contammg incluSIOns of kyanite and separated by an area of altered clinopyroxene.
BULK COMPOSITIONS Four analyses are available for 6913 and one for 6914 (Table 3). The garnet-clinopyroxene layer of 6913 was analysed as two portions, A and B, the latter probably including part of the overgrowth layer. The composition of the overgrowth layer (C) was determined from a modal analysis and the average compositions of appropriate garnets and clinopyroxenes (6, 7). A single analysis (0) was made of the garnet-clinopyroxene-kyanite layer. The specimens from 6913 were analysed both by wet chemical methods (University of St. Andrews) and by X-ray fluorescence (University of Aberdeen). Specimen 6914 (E) was analysed only by the latter techniques. The two methods show reasonable correlation except perhaps for a positive bias in X-ray fluorescence SiOz results. The analyses range from hypersthene to nepheline normative, the kyanite eclogites containing normative corundum. All have relatively large amounts of normative anorthite. In Figs. 3A and 4A these analyses are displaced from appropriate tie lines though, for example, in Fig. 4A all lie within appropriate two- or three-phase regions. This displacement is not likely to be the result of analytical error for both X.R.F. and wet chemical results generally plot within the area of the appropriate symbol. It may in part be due to Fe z0 3 not determined in microprobe analyses. The most plausible reason, however, is the extensive alteration within these rocks. Figures 3A and 4A suggest that the rocks have become depleted in Fe and Ca and enriched in Mg as a result of reactions with kimberlite magma or
360
M. A. LAPPIN AND J. B. DAWSON
fluids. Inspection of bulk analyses suggests that the rocks have become enriched in KzO, HzO and COz during this process. TABLE 3. BULK ANALYSES AND CALCULATED BULK ANALYSES OF ECLOGITES TOGETHER WITH THEIR c.I.P.W. NORMS A and B are two parts of the gt-epx layer of 6913. C is the calculated composItion of the overgrowth layer and D an analysis of the gt-cpx-ky layer. E is the analysis of 6914. St. A. and Ab. indicate analyses done by wet chemistry and X.R.F. techniques at the UnIversities of St. Andrews and Aberdeen respectIvely. ----
~------
A -~----
B
C
E
D ---_.
-
-----
St.A.
Ab.
St.A.
Ab.
calculated
St.A.
Ab.
Ab.
Si0 2 Ti0 2 Al 20 3 Fe 20 3 Cr 20 3 FeO MnO MgO CaO Na20 K 20 P 2O, H 2O+ H 2OCO 2
46.9 0.53 17.7 1.65 0.Q3 8.09 0.20 9.20 8.89 3.56 0.86 0.14 1.46 0.55 n.d.
47.7 0.45 18.9 1.95 n.d. 7.61 0.25 9.42 8.83 3.20 0.94 0.09 1.41 n.d 0.20
44.4 0.50 19.4 1.36 tr 10.3 0.30 9.84 9.03 2.36 1.01 0.10 1.22 0.36 n.d.
45.4 0.47 20.0 2.29 n.d 9.21 0.31 10.0 9.06 2.10 1.02 0.06 1.25 n.d. 0.35
45.5 0.14 23.1
41.7 0.30 29.7 1.74 0.03 5.82 0.18 5.68 9.14 219 0.83 0.18 1.83 0.49 0.52
42.3 0.45 299 1.54 nd. 582 0.18 5.67 8.94 2.25 0.91 0.10 1.65 n.d. 0.52
45.6 0.11 21.1 1.88 n.d. 4.61 0.11 9.81 12.9 3.15 0.21 0.09 1.62 n.d. 0.51
Totals
99.8
10.3 0.14 870 10.1 2.43
-~~~~----
100.9
100.2
100.5
100.4
100.2
100.3
101.7
c.I.P.W. norms based upon average analyses recalculated H 20. CO 2 , K 20 and P 20, free. ab
ne an di hy 01 mt il co Totals
28.82 0.28 35.58 7.66
19.39 44.48 0.89 5.64 25.79 2.78 0.91
23.77 2.78 0.91
99.88
99.80 -
- - - - - - - -
18.08 1.30 50.32
19.39 46.63
0.30 0.71
12.92 8.75 2.55 0.76 10.00
100.41
100.80
29.70
11.53 8.24 43.92 16.40 17.00 278 0.15 100.08
--------~--
CONDITIONS OF EQUILIBRATION Tie lines for coexisting mineral pairs plotted in Figs. 3A and 4A suggest a reasonably dose approach to equilibrium despite the crossing ties of 9B-ll and 6914. 6914 is distinguished from 6913 largely in terms of the lower FejMg ratio in its minerals and the high Ca content of garnet, unusual in a kyanite-free paragenesis. The distribution function K' = (FejMg) gtj(FejMg) cpx, modified so that all iron is given as Fe2+, ranges from average values of 3.8 within the garnet-clinopyroxene and overgrowth layers to an average of 4.7 within the garnet-clinopyroxene-kyanite layer. 6914 gives a value of 3.5. The K' values suggest equilibration temperatures lower than those implied by the group of Roberts
TWO ROBERTS VICTOR CUMULATE ECLOGITES
361
Victor eclogites studied by KUSHIRO and AOKI (1968) which have an average K' value, modified in the manner outlined above, of 2.6. Equilibration temperatures derived from these K' values using curves of BANNO (1970) and MYSEN and HEIER (1972) give temperatures in the range 75G--900°C with a mean value of about 850°C. Equilibration pressures are more difficult to assess. Minimum pressures are obviously defined by eclogite equilibrium curves and here the compositions studied by GREEN (1967) provide reasonable models though his results are essentially synthesis curves. More precise estimation depends on a consideration of the roles of diamond and graphite both of which occur in alumina-rich eclogites. Diamond occurs in association with kyanite in South African eclogites (DAWSON, 1968; SWITZER and MELSON, 1969); the material described by the latter authors is from the Roberts Victor pipe. SOBOLEV (1971) describes the association of primary graphite and corundum in material from Yakutia. The implied equilibration temperatures for kyanite and corundum eclogites from Yakutia are not too different from those described here. For example the average K' value for four kyanite eclogites associated with grosspydites is 4.9 whilst a corundum eclogite gives K' = 3.9 (SOBOLEV et al., 1968). Whilst acknowledging that either graphite or diamond may be metastable phases, the associated silicates give an impression of equilibrium assemblages and it is thus suggested that both of these minerals formed within their stability fields and that in general these alumina-rich assemblages equilibrated in the vicinity of the graphite/diamond equilibrium curve (BUNDY et al., 1961). At the temperatures given above the range of pressures would be 22-32 kbar (Fig. 6) though reasons are given later for suggesting that a more limited range, 28-32 kbar, might be appropriate. 40r---------.---~-------____,
solidus
-l»:-:-:::j ... . ~
:;z
kyanlte eclogite
(O'Hara& Yoder,1967)
(fJ
a:
o...J
l"UldUS
20
W
a:
=> ~ w
a:
0..
10
OL-_.L-_.L----:::'::_::_-...I..-~:'::_::_-...I..----:--!:::::-...L-___:~
800
1000 1200 TEMPERATURE 'C
FIG, 6, T -P paths for eclogite specImens. EqUllibnum or synthesIs curves are Identified in the text except for the "garnet in" and "plagioclase out" curves for the kyanite eclogite composition of GREEN (1967) and the watersaturated lherzohte solidus of KUSHIRO et al. (1968). The stippled areas are the three regions of equihbratlOn dIscussed in the text.
362
M.
A. LAPPIN AND
J. B.
DAWSON
PROCESSES PRIOR TO SUBSOLIDUS EQUILIBRAnON The garnet overgrowths on kyanite and the zonation in garnet may provide information about processes prior to this subsolidus equilibration. The overgrowth garnets (6,7, 9A) cannot form by reaction between kyanite and the analysed clinopyroxenes (Fig. 4B). This reaction texture can be interpreted as an open system reaction with say a fluid which in this process has become enriched in CaO relative to MgO + FeO (Fig. 4B) and also, for example, NazO. It may also be interpreted as a closed-system reaction. In the latter case the most likely reaction would be cpx l
+ ky -+ cpX 2 + gt + ky
(1)
It seems unlikely, however, that the whole specimen has equilibrated with respect to such a fluid for the overgrowth texture is virtually confined to the overgrowth layer (the area in which garnet 9A occurs is thought to be a local system with a composition akin to the overgrowth layer) even though the clinopyroxenes of rather similar composition occur in the garnet-clinopyroxene-kyanite layer. On this basis the closed-system reaction is to be preferred. Some supporting evidence for this hypothesis is provided by the cryptic mineral variations and the overgrowth textures themselves, all of which suggest limited diffusional volumes more in keeping with closed system reactions than equilibration with respect to a fluid of particular chemical potential. TABLE
4.
CALCULATED COMPOSITION OF
cpx,
Cations on a basis of 6 oxygens Si0 2
Ti0 2 FeO MnO MgO CaO Na 2 0
46.3 18.0 0.16 11.5 0.16 9.7 11.3 2.72
Tolal
99.9
Al 2 0 3
------
Sl
1.698
Al(4) 0.302 Al1 6 1 0.477
Ti Fe Mn Mg Ca Na
0.004 0.352 0.004 0.530 0.444 0.189
Z = 2.000 Y = 1.367 X = 0.633
- - - - - - - -- - - - - - ' - - - - - - - - - -
The composition of CPX 1 (Table 4) in the above calculation was determined from the calculated bulk analysis of the overgrowth layer assuming that this clinopyroxene is approximately stoichiometric and that there is no solid solution with kyanite. Both assumptions can be questioned. However, the high P-T synthetic clinopyroxenes fully analysed by BULTITUDE and GREEN (1971) are reasonably stoichiometric; in particular they do not contain the large amounts of excess AI!61 (cf. Table 1). O'HARA and YODER (1967) note that kyanite disappears by beginning of melting at 30 kbar and also that orientated intergrowths of kyanite and clinopyroxene occur in kimberlites. It may thus be possible that the two phases of equation (1) may be represented by a single composite phase. Other authors, however, suggest that kyanite is a primary phase (RICKWOOD et al., 1968; MACGREGOR and CARTER, 1970). Such a conclusion is certainly in keeping with the textural relations in 6913. It seems reasonable to suggest that CPX 1 or a similar early clinopyroxene may have a more general significance in terms of the early history of specimen 6913. Tie lines from
TWO ROBERTS VICTOR CUMULATE ECLOGITES
363
cpx 1 through bulk compositions should give the composition a possible associated phase. The bulk compositions however have been affected by alteration processes. In Figs. 3B and 4B an attempt has been made to allow for this alteration and for the presence of Fe z0 3 in cpx l . In the garnet-clinopyroxene layer cpX 1 could occur in association with garnets of compositions ranging from those found in Ca-depleted cores to those recorded in Table 1 (Fig. 3B). Here the modal amounts of cpX 1 would be much larger than the amount of clinopyroxene in the present assemblage. The situation is more complex within the garnet-clinopyroxene-kyanite layer. The tie line (Fig. 3B) between cpX 1 and the modified bulk composition suggests that any possible associated garnet is Ca-rich (as does Fig. 4B) with a somewhat lower Fe/Mg ratio than the analysed garnets. However, mass balance calculations suggest that any early pyroxene must have been somewhat different to cpx I' One possible solution would be a clinopyroxene somewhat richer in jadeite than cpx l , a grossular-rich garnet and kyanite. Precise calculation of compositions is impossible. The general reaction for this layered eclogite is thus (2)
The reaction involves a marked decrease in the amount of clinopyroxene and a marked increase in the amount of garnet (except perhaps for the garnet-clinopyroxene-kyanite layer) together with a smaller decrease in the amount of kyanite, if present. Cpx 1 is tschermakite- and orthopyroxene-rich and jadeite-poor relative to the analysed clinopyroxenes and thus has high T-P characteristics. The distribution coefficient K' for the possible early assemblage cpxdcore garnet is small (1.2) and compares with the values (1.1-1.8) recorded from quenched high P experimental systems (BULTITUDE and GREEN, 1971) and supports the high T origins of this assemblage. The conditions under which this early assemblage might form can be assessed relative to the subsolidus equilibration. Increasing proportions of garnet relative to clinopyroxene would be favoured by decreasing temperature and increasing pressure (GREEN, 1967). The increasing amount of jadeite in the pyroxenes could result from isobaric cooling (KUSHIRO, 1969) but would be favoured by an increment of pressure. T-P conditions of 140(}-1500'C at 26-28 kbar (Fig. 6) seem reasonably appropriate since eclogites of broadly similar bulk composition are stable at the solidus (GREEN, 1967) and such conditions provide for an increment ofpressure providing the higher-pressure estimates for subsolidus equilibration are accepted. The trend towards enrichment in Ca seen in the garnets of the layered eclogites and culminating in the very rare grosspydites of Roberts Victor is considered to be an essentially igneous trend with some minor subsolidus adjustment of Ca/Fe/Mg ratios. The commoner occurrence of grosspydites in Yakutia (SOBOLEV et al., 1968) probably indicates rather higher pressures when compared with Roberts Victor materials or the commoner development of the magma from which such rocks crystallize.
THE T-P HISTORY OF THE ECLOGITES Three T P regions are shown in Fig. 6 where it is suggested that these eclogites were stable. The line joining these regions is the T-P path which reflects the geological history of the specimens.
364
M.
A. LAPPIN AND
J. B.
DAWSON
T-P conditions of 1400-1500°C, 26-28 kbar, are such that these specimens formed from high P liquids. MACGREGOR and CARTER (1970) have suggested that the layering and fabric of their Type 1 eclogites indicate a cumulate origin. We reject the second criterion on the grounds that in 6913 the best-formed garnets (overgrowth layer) are of subsolidus origin. In more general terms, however, it seems likely that considerable changes in the proportions of garnet/clinopyroxene will occur during subsolidus equilibration and thus that the mutual fabric of these minerals cannot reflect igneous processes. The gross layering of 6913, together with the cryptic mineral variations which are considered to reflect gradual compositional variations within the layering, provides stronger evidence for a cumulate model. The sequence of primary cumulate minerals would be clinopyroxene and minor garnet, clinopyroxene and minor kyanite and then clinopyroxene, kyanite and garnet. There seem to be significant changes of clinopyroxene/garnet associated with the extensive precipitation of kyanite. Little can be said about the magma from which these phases crystallized except that the relatively high Fe/Mg ratio of the early pyroxene and the variable but high Ca contents of possible early garnets suggest an evolved magma relative to possible highpressure primitive partial melts. The second region is one of subsolidus equilibration at 75G-900°C, 28-32 kbar (Fig. 6). The suggested T- P path involves increasing pressure and is considered to be tectonically controlled, for during this stage strain banding in kyanite develops and probably also the foliated fabric of 6913. The cryptic mineralogical and smaller scale compositional variations might develop also at this stage through solid-state differentiation or diffusional processes. In this case, however, an explanation must still be offered for the gross layering. The third stage of evolution (Fig. 6) represents alteration of primary eclogite minerals which SWITZER and MELSON (1969) ascribe to partial melting and quenching during rapid emplacement of the kimberlite magma. The typical quench assemblage AI-rich augite-plagioclase-hornblende suggests relatively high pressures provided the first phase is not metastable. On the other hand, mullite reported by SWITZER and MELSON (1969) as an alteration product of kyanite along with corundum is probably a low P phase though again a metastable origin cannot be ruled out. A provisional estimate 100G-l100°C and 8-12 kbar is suggested because such conditions lie close to the water-saturated lherzolite solidus (Fig. 6) which may be a simple analogue to a natural kimberlite magma. SWITZER and MELSON (1969) note that water may have an important role in this partial melting and in such a case the provisional temperatures would be maximum ones. The analytical data presented earlier suggest considerable chemical exchange between eclogite and kimberlite through either magma or residual fluids. This exchange may be a reflection of still later processes of alteration at lower temperatures and pressures.
ACKNOWLEDGEMENTS We should like to thank Mr. G. Taylor and Mr. A. Hawkesworth for their care and skill in obtaining microprobe analyses and Miss 1. R. Baldwin and Mr. R. Maitland for chemical and X.R.F. analyses.
TWO ROBERTS VICTOR CUMULATE ECLOGITES
365
REFERENCES BANNO, S (1970) Eclogltic types and partitIOn of Mg, Fe, and Mn between chnopyroxene and garnet. Proc Japan Acad. 41, 716-21. BRYHNI, I. and GRIFFIN. W L. (1971) Zonmg m eclogite garnets from Nordfjord, West Norway Contr. Minerai. Petrol. 32, 112-25 BULTITUDE, R. J and GREEN, D. H. (1971) Expenmental study ofcrystal-hquid relationships at high pressures m olivine nephehlllte and basalllte compositIOns. J. Petrol. 12, 121-48. BUNDY, F. P., BOVENKERK, H. M. and WENTORF, R. M. (1961) Diamond-graphite eqUlhbrium Ime from growth and graphitisatlOn of diamond. J. Chern. Phys. 35. 383 -91. DAWSON, J. B. (1968) Recent researches on kimberhte and diamond geology. Econ. Geol. 63, 504-11. DUDLEY. P. P (1969) Electron microprobe analyses of garnet III glaucophane schists and associated eclogites Amer. Mineral. 54, 1139-50 GREEN, T. H. (1967) An expenmentalmvestlgatlOn of sub-solidus assemblages formed at high pressure in highalumma basalt, kyanite eclogite and grosspydlte compositIOns Contr. Minerai. Petrol. 16,84-114. KUSHIRO, I. and AOKI, K. (1968) Origin of some eclogite inclusions in kimberlite. Amer. Mineral. 53,134767 KUSHIRO, I., SYONO, Y. and AKIMOTO, S. (1968) Melting of peridotIte at high pressures and high water pressures. J. Geophys. Res. 73, 6023-9. KUSHIRO, I. (1969) Chnopyroxene sohd solutIOns formed by reactions between dlOpside and plagioclase at high pressures. Miner. Soc. Amer. Special Paper 2, 179- 191 MACGREGOR, I. D. and CARTER. J. L. (1970) The chemistry of chnopyroxenes and garnets of eclogite and pendotlte xenoliths from the Robert Victor Mme, South Africa. Phys. Earth Planet. InterIOrs 3,391 -7 MYSEN, B. 0 and HEIER, K S (1972) PetrogenesIs of eclogites m high grade metamorphic terrains as exemphfied by the Hareldland eclogite. Western Norway. Contr. Mineral. Petrol. 36. 73-94 O'HARA, M J. and YODER, H. S., Jr. (1967) Formation and fractIOnation of basic magmas at high pressures. Scot. J Geol. 3, 67 117. RICKWOOD, P. C , MATHIAS, M and SIEBERT, J. C (1968) A study of garnets from eclogite and pendotlte xenohth, found in kimberlite Contr Mineral. Petrol. 19.271-301. SOBOLEV, N. V, Jr, KUZNETSOVA, I. K and ZYUZIN, N. I. (1968) The petrology of grosspydlte xenohths from Zagadochanya kimberlite pipe m YakutJa. 1. Petrol. 9. 253-80 SOBOLEV, N. V., Jr. (1971) Some speCific features of dlstnbution and transportatIOn of xenoliths in the klmberhte pipes ofYakutta. J Geophys Res. 76, 1309-14. SWEATMAN, T. R. and LONG. 1. V. P. ( 1969) Quantitative electron probe micro-analysIs of rock-formmg mmerals J. Petrol. 10, 332-79. SWITZER, G. and MELSON. W. G. (1969) Partially melted kyalllte eclogite from the Roberts Victor mme. South Afnca. Smlthsoman Contr. Earth SCI. 1. 1-9.
M. A. LAPPIN* and 1. B. DAWSONt
ABSTRACT Microprobe and bulk chemical analyses are presented for two eclogites. Both contam relatively Ca-nch 0 0 garnets (25 45 0 grossular) and Na-rich (approximately 50 0 jadeite) tschermakite-poor clmopyroxenes. One specimen IS foliated and layered. The layers consist of garnet--elinopyroxene, garnet--elinopyroxene-{kyanite), where all garnets from overgrowths on kyanite, and garnet -clinopyroxene-kyamte The garnets of the latter layer are ncher in Ca and poorer m Mg than those of the other layers though within these layers ~ cryptic mineral vanation IS present. Some garnets of the garnet-clinopyroxene layer are zoned. The second specimen IS a graphite eclogite. The garnet overgrowths between kyamte and clmopyroxene are considered as part of a general closed system reactIOn cpx, ± gt, ± ky -> cpX 2 + gt 2 ± kyo The compositIOn of cpx, for the overgrowth layer is calculated from modal and mmeral compositIOns. The garnet-clinopyroxene layer could have been formed from cpx, and smaller amounts of garnet of rather simIlar composition The garnet--clmopyroxenekyamte layer probably contamed different clmopyroxenes and garnets. Textures. mmeral compositIOns and calculated mineral compositions, distribution functions, and appropnate expenmental eqUllibnum curves, allow three stages m the T-P evolution of these rocks to be traced. They were probably formed at 1400-1500' C, 26-28 kbar by cumulate processes from an evolved high P magma. They were subJccted to tcctomc processes then eqUllibratmg at 75Q.-900T and 28- 32 kbar. The third stage of evolutIOn. con'ldered to represent reaction, partial meltmg and chemical exchange With an ascending kimberlite magma or fluid. is prOVisionally assessed at 1000 lIOOC, 8 12 kbar.
INTRODUCTION Two eclogite nodules were collected by one of us (1. B. D.) from the Roberts Victor pipe. One of them (6913, Aberdeen University Collection) figured by DAWSON (1968) is noteworthy for its foliated and layered character. The layers consist of a 3.5-cm thick garnetclinopyroxene unit with a variable garnet/clinopyroxene ratio followed by a l-cm-thick garnet-clinopyroxene umt in which all garnets form overgrowths (Fig. 1) on kyanite in a fashion similar to the diablastic intergrowths of SOBOLEV et al. (1968) and then another 3-cm thick layer consisting of garnet-clinopyroxene-kyanite. In the latter layer only one thin overgrowth was seen about I cm from the overgrowth layer (analysis 9A Table 1). The garnets average about 5 mm in diameter and their forms range from subhedral (Fig. I) within the overgrowth layer and the garnet-clinopyroxene layer, to anhedral crystals forming elongate aggregates within the garnet--clinopyroxene-kyanite layer. The clino-
* Department of Geology and Mmcralogy. Marischal College, Umverslty of Aberdeen, Aberdeen AB9 IAS, Scotland t Department of Geology, UnIversity of St Andrews, SI. Andrews, Fife KY16 9TS, Scotland. 351
Z Y X
SI Al[4] All"] TI Fe Mn Mg Ca Na
Total
Si0 2 A1 2 0 3 Ti0 2 FeO MnO MgO CaO Na 2 0
3.00 2.00 3.02
2.947 0.053 1.997 0.014 0.941 0.023 1.289 0.739 0.017
99.6
-- -
39.5 23.3 0.26 15.1 0.36 11.6 9.26 0.12
I
,
I
1
---
3.00 1.96 3.09
UiL'
39.7 23.4 0.22 15.2 0.35 11.5 106 0.15
3
3.00 1.96 309
1.964 0.012 0.935 0.022 1.262 0.835 0.021
~.~92~ 0.073
101.1
----
I
2.9331 0.067 1.958 0.014 0.934 0.022 1.259 0.846 0.017
102.0
1-------
40.1 23.5 0.25 15.3 0.35 11.6 10.8 0.12
2
6913 gt-cpx
3.00 1.91 3.15
2.980 0.020 1.907 0.013 0.976 0.014 1.242 0.881 0.020
I
101.1
~.12
40.8 22.2 0.04 15.6 0.24 10.9 11.2
7
100.7
40.0 21.8 0.11 14.3 0.22 8.36 15.7 0.14 -
101.
--
2.982 0.018 1.910 0.014 0.986 0.018 1.223 0.879 0.019
t 3.01 1.93 3.08
1.934 0.002 0.963 0.015 1.200 0.885 0.017
3.011
3.00 1.92 3.13
2.996 0.004 1.919 0.006 0.894 0.014 0.933 1.259 0.020
40.4 21.6 0.19 13.8 0.24 7.51 17.4 0.14
9B
I
40.7 21.9 0.15 13.8 0.19 744 17.3 0.13
10
6913 gt--epx-ky
I
39.5 22.5 0.24 142 0.27 7.80 173 -0.15
11
~_!
39.8 22.1 0.19 14.4 0.26 8.34 162
12
42.5 20.5 0.13 9.24 0.19 11.6 17.2 ·0:06
A
I
B
42.5 20.6 0.11 9.18 0.20 11.9 16.2 0.07
6914 gt--epx
1.9 6 0.00 8 0.8 7 0.0 2 0.8 I 1.3 5 0.0 9
3.0 7
-
3.01W' 3.02 1 90 1.9 2 3.12 3.09
1.899 0.011 0.857 0.015 0.834 1.386 0.020
3.014
U 3.01 1.91 3.12
--
1907 0.010 0.9 8 0.016 1.180 0.915 0.019
3.008
--
3.00 1.90 3.18
2.935 0.065 1.903 0.013 0.880 0.017 0.863 1.380 0.022
~-
3.00 1.91 3.16
2.964 0.036 1.906 0.011 0.896 0.016 0.922 1.290 0.023
3.09 1.75 3.19
3.10 1.77 3.14
1.767 0.006 0.560 0.012 1.288 1.265 O.Ole
-
1.752 0.007 0.561 0.012 1.259 1.343 0.008
3.103
3.089
- - -
~~131101.61Jill.0_~ 1~008
40. 9 22. o O. 8 16. o O. 25 10. 8 I 11. 6 O. 3
9A
--
Cations on a baSIS of 12 oxygens
101.1
40.2 22.1 0.25 15.9 0.29 11.1 11.1 0.13
6
8
Garnets
ANALYSED GARNETS AND CLINOPYROXENES
3.00 L[.OO 1.96 1.91 3.06 3.14 _
1.956 0.012 0.898 0.011 1.240 0.876 0.019
3.003
g<)~0
401 22.1 0.22 14.3 0.18 11.1 10.9
5
~£:tO[]
100.4
40.0 21.9 0.23 15.7 0.22 11.2 11.0
4
1.
6913 overgrowth layer
TABLE
w
VI
oz
:E
:>
1:1
t:J:I
:--
:> z 1:1
Z
'tI 'tI
:>
t""'
:>
:::::
tv
X
y
z
TI Fe Mn Mg Ca Na
Sl AI[4j AI( 6 )
Total
-
Si0 2 AI 2 0 3 TiO z FeO MnO MgO CaO NazO
2.00 1.07 0.88
1.945 0.055 0.651 0.006 0.069 0.001 0.341 0.380 0.498
2.00 1.06 0.88
.-
1.939 0.061 0.648 0.006 0.069 0.001 0.341 0.390 0.490
100.5
7.31
735
99.3
56.0 17.4 0.25 2.37 0.03 6.62
IQ5
2
55.6 17.1 0.23 2.36 0.03 6.55
IQI
J
-
2.00 1.06 0.89
1.934 0.066 0.670 0.006 0.063 0.001 0.320 0.366 0.528
00.7
7.90
~9
56.1 18.1 0.24 2.19 0.02 6.23
3 {
2.00 1.03 0.92
1.968 0.032 0.653 0.006 0.059 0.000 0.313 0.381 0.536
I 100.5
8.01
IQ3
569 168 0.25 2.05 0.01 6.08
4
-
-
100.3
99.2
99.9
100.0
2.00 1.00 0.91
~~
_. 1.963 0.037 0.618 0.007 0.063 0.001 0.316 0.380 0.530
57.3 18.0 0.06 1.87 0.00 5 17 9.97 7.92
56.9 164 0.08 2.27 0.01 6.29 10.5 6.75
56.5 15.9 0.30 2.49 0.01 6.42 10.5 7.76
56.5 16.9 0.26 2.17 0.03 6.10 10.2 7.89
2.00 1.04 0.91
1.971 0.029 0.626 0.008 0.073 0.000 0.334 0.391 0.524
~ . _
2.00 1.05 0.85
1985 0.015 0.659 0.002 0.066 0.000 0.327 0.392 0.456
~--
.
2.00 1.05 0.90
1.975 0.025 0.706 0.002 0.054 0.000 0.265 0.368 0.529
Cations on a basIs of 6 oxygens
8
6
5
Clinopyroxenes
-
2.00 1.06 0.85
1.979 0.021 0.683 0.005 0.062 0.001 0.311 0.353 0.495
100. 3
57. 5 17. 3 O. 20 2. 17 O. 03 6. 07 9. 58 7. 42
-~
9 -
2.00 1.04 0.86
----
1.991 0.009 0.700 0.003 0.058 0.000 0.277 0.366 0.493
-_.. 100.3
5.40 9.94 7.40
tr
57.9 17.5 0.12 2.01
10
2.00 1.04 0.89
1.957 0.043 0.701 0.003 0.059 0.001 0.274 0.371 0.515
99.6
56.3 182 0.13 2.02 0.04 5.29 9.97 7.65
II
--
2.00 1.04 0.88
1.968 0.032 0.697 0.004 0.060 0.002 0.274 0.371 0.512
------
100.2
57.0 179 0.17 2.10 0.06 5.33 10.0 7.67
12
2.00 1.05 0.97
1.914 0.086 0.609 0.001 0.052 0.000 0.384 0.408 0.564
100.2
54.8 16.9 0.05 1.77 0.01 7.40 10.9 8.34
A
2.00 1.03 0.99
1.910 0.090 0.607 0.002 0.045 0.001 0.377 0.423 0.569
100.1
54.6 16.9 0.07 1.55 0.02 7.24 11.3 8.39
B
\.;.)
V1
\.;.)
BJ
::J
§
m m n
-l
;..-
ct"'"
~
n
o~
n -l
-<
;;J
~
~ m
~
o<
-l
354
M.
A. LAPPIN AND J.
B.
DAWSON
FIG. 1. Textures in the gt--epx and overgrowth layers of 6913. Dotted ornament altered clinopyroxene with clear areas of rehct chnopyroxene. The garnets of the gt -epx layer have alteration cracks and fractures. Those of the overgrowth layer have been left clear to emphasize the pattern of kyanite mcluslOns. The ruled hnes m kyamte are the cleavage traces whIlst the dashed lines represent the stram bands and the black areas zones of alteratIOn. IndIVldual garnets from the overgrowth layer are also shown. OccasIOnal grams of rutile in chnopyroxene are black.
pyroxenes throughout the sample are extremely altered but tend to form interstitiallensoid aggregates elongated parallel to layer interfaces and to the foliation. The kyanite laths are up to 4 mm long and are also elongated within the foliation. The second specimen, 6914, consists of subhedral. 5-mm diameter garnets, interstitial clinopyroxene and occasional hexagonal plates of apparently primary graphite. Rutile and phlogopite occur in both samples. Both nodules show extensive alteration, particularly of clinopyroxenes, the detailed mineralogy and textures of which match almost completely the descriptions by SWITZER and MELSON (1969). The textures of the overgrowth layer provide some evidence as to the relationships of stress to stages of mineral growth. Most of the kyanite crystals have kink or strain bands. The overgrowth garnets form deep embayments in kyanite in the vicinity of these strain bands (Fig. 1). Strain bands have a limited range of crystallographic orientations and it seems an unlikely coincidence that favourable crystallographic orientations always occur in the vicinity of overgrowth embayments, i.e. that the strain bands postdate the development of overgrowth textures. A more likely explanation is that the strain bands act as sites of preferential garnet nucleation and growth and that the stress predates overgrowth development.
355
TWO ROBERTS VICTOR CUMULATE ECLOGITES
MINERAL CHEMISTRY Garnets and clinopyroxenes (Table 1) from specimens 6913 and 6914 were analyzed at the University of Aberdeen with a Geoscan microprobe using the technique of SWEATMAN and LONG (1969). In 6913 twelve garnet/clinopyroxene pairs were analysed across the layering. The garnet analyses were all made within 50 J1 of the crystal edge because of the possibility of zoning.
GARNETS The compositions of the garnets in 6913 range from grossular 25 pyrope 4 2 almandine33 to grossular 44 .3 pyrope 27 . 7 almandine 28 . o ' The average composition of garnet from 6914 is grossular 4 1.6' pyrope 40 . 6 , almandine 1 7.6' Other end-member molecules are unlikely to be present in large amount, apart perhaps from andradite, since Ti0 2 and MnO occur in limited amounts and Cr 2 0 3, measured in some garnets, was uniformly low (0.01-0.06 ~~) as was NiO (0.004-0.02 ~~) and K 2 0 (0.007-0.02 ~~). The garnets in 6913 were analysed sequentially (l ~ 12) together with clinopyroxenes. Almost all garnets show high oxide per cent totals whilst the clinopyroxenes have acceptable totals. Three garnets (9B, 10, 11) were thus re-analysed and oxygen was also determined (quartz used as a standard). Oxygen was also determined in garnet 12. In each case the variation for individual elements was small (Table 2) and the element per cent totals were improved though remaining high. The average element per cent for these four garnets is thus quoted in Table 2 together with structural formulae calculated upon a basis of both available and also stoichiometric oxygens. High oxide per cent totals may be the result of oxygen deficiency (about 1.7 ~~ in Table 2). TABLE
2.
AVERAGE ATOM
0 0
VALUE FOR FOUR GARNETS TOGETHER WITH STRUCTURAL FORMULAE
MaXImum range of OXIde ~.~ also shown
---------
--------
--------
----
Cations on a basis of 12 oxygens ------
Weight °0 atoms -------
°
------
42.1 18.8 11.6 0.12 10.9 0.19 4.85 11.9 0.11
SI Al TI Fe Mn Mg Ca Na
----
Average 91J,10,11,12
---
Total
-----
100.6
Measured 0
y
X
- ~ - -
3.050 1.964 0.011 0.891 0.016 0.908 1362 0.022
----
Z
Maximum range ofvalues in reanalysed garnets 98,10,11
StOichIOmetrIC 0
------
SI AI[4) AII6] TI Fe Mn Mg Ca Na
------ -
---------
SI0 2 AI 2 O, Ti0 2 FeO MnO MgO CaO Na 2 0 -
-
3.05 1.96 3.21
--------- -
----
2.997 0.003 1.927 0.011 0.875 0.016 0.892 1.338 0.021
---
0.28 (9B) 0.44 (11) 0.01 (10,11) 0.17 (II) 0.01 (10) 0.53 (10) 0.44 (11) 0.01 (9B,II)
---- ------
- - ~
3.00 1.93 3.15 ----
--- ------
The analysed garnets lie within the general field of compositions of kimberlite eclogite garnets (RICKWOOD et al., 1968) though 6914 is more extreme than garnets previously recorded from the Roberts Victor pipe (MACGREGOR and CARTER, 1970) having high MgO and CaO.
356
M.
A. LAPPIN AND
J. B.
DAWSON
ZONING IN GARNETS Tests were made for zonation only by traverses across individual grains. Zoning appears to be confined to the garnet-clinopyroxene layer (in about IO/~ of the traverses) and was not found in the overgrowth or garnet-clinopyroxene-kyanite layer. The zoned garnets have cores between 100-400 J1 wide depleted in Ca and enriched in Mg in a sometimes asymmetric, flat-bottomed, saucer-shaped pattern (Fig. 5). The pattern for Fe varies for in some cases there is a slight enrichment or depletion in the core whilst in others no variation could be detected. There were no significant variations in the Mn pattern. A partial analysis of one zoned garnet gave a core composition of grossular 19.2 pyrope 44 . 9 almandine 35 . 9 and a rim of grossular 25 pyrope 42 almandine 33 (Fig. 3A). One garnet (Fig. 5) showed a larger scale asymmetric depletion with respect to Ca though without apparent variation in Mg, Fe and Mn. The anomalous composition of this garnet is mentioned in the following section. The zoning differs from that previously recorded in other eclogites (DUDLEY, 1969; BRYHNI and GRIFFIN, 1971).
CRYPTIC VARIATION OF GARNETS An initial survey of garnets in 6913 suggested considerable variations in garnet composition across the layering. These analyses, however, were of rather poor quality and a second set of analyses was completed in 1973 from a second slice of 6913 (Table 1). For comparative purposes, however, both sets of analyses are plotted in Fig. 2. gt-cpx-ky
gt-cpx
18
9810
9A
,P------ 12
16
,/
I
CaO 14 &
,/
FeO I
12
MgO
11
I
I
I
I
D
I
,~ --~{}---- ----iJ
I
4
10
8
o
4 2 Distance perpendicular to layenng
6cm
FIG. 2. Garnet composItIOns (OXIde ~/~) across the layenng in 6913. The first traverse IS shown WIth dashed lines. The second traverse by solId lInes (some partIal analyses are included within the latter). The analyses from Table I are numbered In the CaO traverse. The pOSition. but not the analySIS. of garnet 9A IS shown
TWO ROBERTS VICTOR CUMULATE ECLOGITES
357
At the edge of the garnet-clinopyroxene layer both traverses begin with a relatively Ca-poor, Mg-rich garnet (I) and in both there is a fairly regular increase ofCa (2-5) towards the overgrowth layer. The most notable exception to this trend is analysis 2 which has an unusually high CaO content. This garnet has a Ca-depleted core, a feature which could account for its anomalous composition. MgO and FeO show less regular patterns of variation, most probably showing a slight increase of FeO and a slight decrease of MgO towards the overgrowth layer interface. The garnets of the overgrowth layer (6,7) and the overgrowth rim within the garnet-clinopyroxene-kyanite layer (9A) are richer in CaO and FeO and poorer in MgO than garnets 1-5 of the garnet-elinopyroxene layer. The garnets of the garnet-elinopyroxene-kyanite layer are much richer in CaO and poorer in MgO than those of the other layers. Within this layer also there is evidence for cryptic variation since the garnet close to the overgrowth layer interface (8) is poorer in CaO and slightly richer in MgO than the garnets from the centre of this layer (9B, 10, 11). Garnet 12 at the edge of the specimen is similar to garnet (8) as was the equivalent garnet from the first traverse (Fig. 2). The distinctive character of garnets at either end of the traverse (I and 12) suggests that the fragment may have in fact broken close to boundaries of chemicalj mineralogical inhomogeneity.
~
~
MgO
grosspydite
A.
-If>
FeO.~,<>~
•
8.
XRV3
/
~,o
6:~:'\\
'98-11
~
E~
/i~2
~~9A /
,
;' I
f
0
~,~rim I: I 1
d
.core
I
°XRV4
FIG 3. Parts of the CaO, MgO, FeO diagram (mol'/~) as shown in the mset. A. Tie Ime relationships for the analysed garnets (filled circles) and clinopyroxenes (unfilled squares). IndiVidual or average garnets are identified by numbers. Bulk compositions (averages of A-E, Table 3) are unfilled diamonds. The compositional trend in 6913 is shown as a thick line. The Roberts Victor grosspydite garnet (MACGREGOR and CARTER, 1970) and the garnet compositional trends (dashed lines) of other layered eclogites (XRV 3 and 4) (RICKWOOD et al., 1968) are also shown. B. PossIble high T-P tie lines (heavy lines) drawn on the assumptIOn that cpx, of the overgrowth layer (C In Fig 3A) is stable in both the gt--{;px and gt-cpx-ky layers. Bulk analyses have been modified to allow for alteration and some of the tie lines of Fig. 3A are shown as dashed lines.
358
M.
A. LAPPIN AND
J.
B. DAWSON
The garnets, or where appropriate, average values, are plotted in Figs. 3A and 4A. They have a linear trend on Fig. 3A similar to XRV 3, another Roberts Victor layered eclogite (RICKWOOD et ai., 1968). Another specimen described by these authors (XRV 4) shows a similar trend in garnets of lower FejMg ratio though of a less extensive range. The rare grosspydites from Roberts Victor (Fig. 3, MACGREGOR and CARTER, 1970) are the culmination of this trend of garnet compositions. I
I
A
B.
I
"tie
I
\
\
,
\
\
,
, \
\ \
\
\ \ \
..,...c-
,
-c"'-------"---->L....L----"-------"'-------'''-----'---------'F--''--------''--------''----
cpx
opx
FIG. 4. PortIOns of the A.C.F. diagram (mol 0 0 ) for Roberts Victor eclogites (A = AI 20 3 + Fe203 - Na 20, C = CaO, F = FeO + MgO + MnO). A. Synoptic relationships, mineral composition fields and IImltmg tie lines (dashed lines) after MACGREGOR and CARTER (1970). Tie Ime relationships for the analysed minerals, symbols for minerals and bulk compositions as m Fig. 3A, and the compositional trend m 6913 are also shown B. Possible high T ~P tie line onentations (heavy lines) drawn with the same assumption as Fig. 3B. Cpx l (Table 4) lies on the tie Ime (dashed line) defined by kyanite and the overgrowth layer (C). Both the compositIOn of the overgrowth layer and cpx, have been modified (filled symbols) to allow for their likely Fe 2 0 3 contents. Gt , (open circle) shows the limltmg garnet If composition D IS to remain within the gt ky-epx , compositional field. Some of the tie-line orientatIOns of Fig. 4A are shown by dashed lInes.
CLINOPYROXENES
The clinopyroxenes are jadeite-rich omphacites in which Na occupies around 50~~ of available X sites. They contain limited tschermakite, only about 1 ():, of Al[4l. They closely resemble the clinopyroxenes from Siberian kyanite eclogites and grosspydites (SOBOLEV et ai., 1968) in that they have up to 30~~ Al[6) in excess of that related to Al[4) and Na. Again characteristically they have low cation sums (X + V). The clinopyroxenes have limited amount of other elements, Cr 2 0 3 6913, 0.O1--O.04~.~, 6914, 0.06~~; NiO 6913, 0.02--O.03~;" 6914 0.08 ~;); K 2 0 6913, 0.08--0.09 ~~. They show some traces of cryptic variation especially with respect to FeO and MgO. The amount of these elements decreases through the garnet-clinopyroxene layer, increases slightly in the overgrowth layer and then decreases markedly in the garnet-clinopyroxene-kyanite layer. Within the latter layer clinopyroxene 9, adjacent to the overgrowth garnet 9A, has higher values for these elements resembling values found within the overgrowth layer (6,7). Figure 3A suggests a more or less regular increase in CaOjCaO + MgO + FeO in clinopyroxene with increasing Ca in garnet whilst Fig. 4A shows that excess Al[6l increases with this parameter.
359
TWO ROBERTS VICTOR CUMULATE ECLOGITES
10%FeO 0·5%MnO 0 10'YoMgO 10%CaO 0 10'YoMgO
10%CaO 0
C MgO CaO
10%MgO 10%CaO
0
ky
10%MgQ 1O%CaO
,
200 microns , I
OL...J~------------------'--'
FIG. 5. Zonation m garnets. A. ZonatIOn patterns for FeO. MnO. MgO and CaO along a smgle line of traverse Narrow regions depleted in CaO and enriched m MgO are alteration filled fractures. B. ZonatIOn of MgO and CaO. FeO and MnO shown no sIgnIficant varIation. C. Assymetric zonation with respect CaO. no apparent VarIatIOn m MgO. FeO or MnO (Garnet 2 of Table I and Fig. 2). D. UnIform garnets of the overgrowth layer A traverse across two garnets contammg incluSIOns of kyanite and separated by an area of altered clinopyroxene.
BULK COMPOSITIONS Four analyses are available for 6913 and one for 6914 (Table 3). The garnet-clinopyroxene layer of 6913 was analysed as two portions, A and B, the latter probably including part of the overgrowth layer. The composition of the overgrowth layer (C) was determined from a modal analysis and the average compositions of appropriate garnets and clinopyroxenes (6, 7). A single analysis (0) was made of the garnet-clinopyroxene-kyanite layer. The specimens from 6913 were analysed both by wet chemical methods (University of St. Andrews) and by X-ray fluorescence (University of Aberdeen). Specimen 6914 (E) was analysed only by the latter techniques. The two methods show reasonable correlation except perhaps for a positive bias in X-ray fluorescence SiOz results. The analyses range from hypersthene to nepheline normative, the kyanite eclogites containing normative corundum. All have relatively large amounts of normative anorthite. In Figs. 3A and 4A these analyses are displaced from appropriate tie lines though, for example, in Fig. 4A all lie within appropriate two- or three-phase regions. This displacement is not likely to be the result of analytical error for both X.R.F. and wet chemical results generally plot within the area of the appropriate symbol. It may in part be due to Fe z0 3 not determined in microprobe analyses. The most plausible reason, however, is the extensive alteration within these rocks. Figures 3A and 4A suggest that the rocks have become depleted in Fe and Ca and enriched in Mg as a result of reactions with kimberlite magma or
360
M. A. LAPPIN AND J. B. DAWSON
fluids. Inspection of bulk analyses suggests that the rocks have become enriched in KzO, HzO and COz during this process. TABLE 3. BULK ANALYSES AND CALCULATED BULK ANALYSES OF ECLOGITES TOGETHER WITH THEIR c.I.P.W. NORMS A and B are two parts of the gt-epx layer of 6913. C is the calculated composItion of the overgrowth layer and D an analysis of the gt-cpx-ky layer. E is the analysis of 6914. St. A. and Ab. indicate analyses done by wet chemistry and X.R.F. techniques at the UnIversities of St. Andrews and Aberdeen respectIvely. ----
~------
A -~----
B
C
E
D ---_.
-
-----
St.A.
Ab.
St.A.
Ab.
calculated
St.A.
Ab.
Ab.
Si0 2 Ti0 2 Al 20 3 Fe 20 3 Cr 20 3 FeO MnO MgO CaO Na20 K 20 P 2O, H 2O+ H 2OCO 2
46.9 0.53 17.7 1.65 0.Q3 8.09 0.20 9.20 8.89 3.56 0.86 0.14 1.46 0.55 n.d.
47.7 0.45 18.9 1.95 n.d. 7.61 0.25 9.42 8.83 3.20 0.94 0.09 1.41 n.d 0.20
44.4 0.50 19.4 1.36 tr 10.3 0.30 9.84 9.03 2.36 1.01 0.10 1.22 0.36 n.d.
45.4 0.47 20.0 2.29 n.d 9.21 0.31 10.0 9.06 2.10 1.02 0.06 1.25 n.d. 0.35
45.5 0.14 23.1
41.7 0.30 29.7 1.74 0.03 5.82 0.18 5.68 9.14 219 0.83 0.18 1.83 0.49 0.52
42.3 0.45 299 1.54 nd. 582 0.18 5.67 8.94 2.25 0.91 0.10 1.65 n.d. 0.52
45.6 0.11 21.1 1.88 n.d. 4.61 0.11 9.81 12.9 3.15 0.21 0.09 1.62 n.d. 0.51
Totals
99.8
10.3 0.14 870 10.1 2.43
-~~~~----
100.9
100.2
100.5
100.4
100.2
100.3
101.7
c.I.P.W. norms based upon average analyses recalculated H 20. CO 2 , K 20 and P 20, free. ab
ne an di hy 01 mt il co Totals
28.82 0.28 35.58 7.66
19.39 44.48 0.89 5.64 25.79 2.78 0.91
23.77 2.78 0.91
99.88
99.80 -
- - - - - - - -
18.08 1.30 50.32
19.39 46.63
0.30 0.71
12.92 8.75 2.55 0.76 10.00
100.41
100.80
29.70
11.53 8.24 43.92 16.40 17.00 278 0.15 100.08
--------~--
CONDITIONS OF EQUILIBRATION Tie lines for coexisting mineral pairs plotted in Figs. 3A and 4A suggest a reasonably dose approach to equilibrium despite the crossing ties of 9B-ll and 6914. 6914 is distinguished from 6913 largely in terms of the lower FejMg ratio in its minerals and the high Ca content of garnet, unusual in a kyanite-free paragenesis. The distribution function K' = (FejMg) gtj(FejMg) cpx, modified so that all iron is given as Fe2+, ranges from average values of 3.8 within the garnet-clinopyroxene and overgrowth layers to an average of 4.7 within the garnet-clinopyroxene-kyanite layer. 6914 gives a value of 3.5. The K' values suggest equilibration temperatures lower than those implied by the group of Roberts
TWO ROBERTS VICTOR CUMULATE ECLOGITES
361
Victor eclogites studied by KUSHIRO and AOKI (1968) which have an average K' value, modified in the manner outlined above, of 2.6. Equilibration temperatures derived from these K' values using curves of BANNO (1970) and MYSEN and HEIER (1972) give temperatures in the range 75G--900°C with a mean value of about 850°C. Equilibration pressures are more difficult to assess. Minimum pressures are obviously defined by eclogite equilibrium curves and here the compositions studied by GREEN (1967) provide reasonable models though his results are essentially synthesis curves. More precise estimation depends on a consideration of the roles of diamond and graphite both of which occur in alumina-rich eclogites. Diamond occurs in association with kyanite in South African eclogites (DAWSON, 1968; SWITZER and MELSON, 1969); the material described by the latter authors is from the Roberts Victor pipe. SOBOLEV (1971) describes the association of primary graphite and corundum in material from Yakutia. The implied equilibration temperatures for kyanite and corundum eclogites from Yakutia are not too different from those described here. For example the average K' value for four kyanite eclogites associated with grosspydites is 4.9 whilst a corundum eclogite gives K' = 3.9 (SOBOLEV et al., 1968). Whilst acknowledging that either graphite or diamond may be metastable phases, the associated silicates give an impression of equilibrium assemblages and it is thus suggested that both of these minerals formed within their stability fields and that in general these alumina-rich assemblages equilibrated in the vicinity of the graphite/diamond equilibrium curve (BUNDY et al., 1961). At the temperatures given above the range of pressures would be 22-32 kbar (Fig. 6) though reasons are given later for suggesting that a more limited range, 28-32 kbar, might be appropriate. 40r---------.---~-------____,
solidus
-l»:-:-:::j ... . ~
:;z
kyanlte eclogite
(O'Hara& Yoder,1967)
(fJ
a:
o...J
l"UldUS
20
W
a:
=> ~ w
a:
0..
10
OL-_.L-_.L----:::'::_::_-...I..-~:'::_::_-...I..----:--!:::::-...L-___:~
800
1000 1200 TEMPERATURE 'C
FIG, 6, T -P paths for eclogite specImens. EqUllibnum or synthesIs curves are Identified in the text except for the "garnet in" and "plagioclase out" curves for the kyanite eclogite composition of GREEN (1967) and the watersaturated lherzohte solidus of KUSHIRO et al. (1968). The stippled areas are the three regions of equihbratlOn dIscussed in the text.
362
M.
A. LAPPIN AND
J. B.
DAWSON
PROCESSES PRIOR TO SUBSOLIDUS EQUILIBRAnON The garnet overgrowths on kyanite and the zonation in garnet may provide information about processes prior to this subsolidus equilibration. The overgrowth garnets (6,7, 9A) cannot form by reaction between kyanite and the analysed clinopyroxenes (Fig. 4B). This reaction texture can be interpreted as an open system reaction with say a fluid which in this process has become enriched in CaO relative to MgO + FeO (Fig. 4B) and also, for example, NazO. It may also be interpreted as a closed-system reaction. In the latter case the most likely reaction would be cpx l
+ ky -+ cpX 2 + gt + ky
(1)
It seems unlikely, however, that the whole specimen has equilibrated with respect to such a fluid for the overgrowth texture is virtually confined to the overgrowth layer (the area in which garnet 9A occurs is thought to be a local system with a composition akin to the overgrowth layer) even though the clinopyroxenes of rather similar composition occur in the garnet-clinopyroxene-kyanite layer. On this basis the closed-system reaction is to be preferred. Some supporting evidence for this hypothesis is provided by the cryptic mineral variations and the overgrowth textures themselves, all of which suggest limited diffusional volumes more in keeping with closed system reactions than equilibration with respect to a fluid of particular chemical potential. TABLE
4.
CALCULATED COMPOSITION OF
cpx,
Cations on a basis of 6 oxygens Si0 2
Ti0 2 FeO MnO MgO CaO Na 2 0
46.3 18.0 0.16 11.5 0.16 9.7 11.3 2.72
Tolal
99.9
Al 2 0 3
------
Sl
1.698
Al(4) 0.302 Al1 6 1 0.477
Ti Fe Mn Mg Ca Na
0.004 0.352 0.004 0.530 0.444 0.189
Z = 2.000 Y = 1.367 X = 0.633
- - - - - - - -- - - - - - ' - - - - - - - - - -
The composition of CPX 1 (Table 4) in the above calculation was determined from the calculated bulk analysis of the overgrowth layer assuming that this clinopyroxene is approximately stoichiometric and that there is no solid solution with kyanite. Both assumptions can be questioned. However, the high P-T synthetic clinopyroxenes fully analysed by BULTITUDE and GREEN (1971) are reasonably stoichiometric; in particular they do not contain the large amounts of excess AI!61 (cf. Table 1). O'HARA and YODER (1967) note that kyanite disappears by beginning of melting at 30 kbar and also that orientated intergrowths of kyanite and clinopyroxene occur in kimberlites. It may thus be possible that the two phases of equation (1) may be represented by a single composite phase. Other authors, however, suggest that kyanite is a primary phase (RICKWOOD et al., 1968; MACGREGOR and CARTER, 1970). Such a conclusion is certainly in keeping with the textural relations in 6913. It seems reasonable to suggest that CPX 1 or a similar early clinopyroxene may have a more general significance in terms of the early history of specimen 6913. Tie lines from
TWO ROBERTS VICTOR CUMULATE ECLOGITES
363
cpx 1 through bulk compositions should give the composition a possible associated phase. The bulk compositions however have been affected by alteration processes. In Figs. 3B and 4B an attempt has been made to allow for this alteration and for the presence of Fe z0 3 in cpx l . In the garnet-clinopyroxene layer cpX 1 could occur in association with garnets of compositions ranging from those found in Ca-depleted cores to those recorded in Table 1 (Fig. 3B). Here the modal amounts of cpX 1 would be much larger than the amount of clinopyroxene in the present assemblage. The situation is more complex within the garnet-clinopyroxene-kyanite layer. The tie line (Fig. 3B) between cpX 1 and the modified bulk composition suggests that any possible associated garnet is Ca-rich (as does Fig. 4B) with a somewhat lower Fe/Mg ratio than the analysed garnets. However, mass balance calculations suggest that any early pyroxene must have been somewhat different to cpx I' One possible solution would be a clinopyroxene somewhat richer in jadeite than cpx l , a grossular-rich garnet and kyanite. Precise calculation of compositions is impossible. The general reaction for this layered eclogite is thus (2)
The reaction involves a marked decrease in the amount of clinopyroxene and a marked increase in the amount of garnet (except perhaps for the garnet-clinopyroxene-kyanite layer) together with a smaller decrease in the amount of kyanite, if present. Cpx 1 is tschermakite- and orthopyroxene-rich and jadeite-poor relative to the analysed clinopyroxenes and thus has high T-P characteristics. The distribution coefficient K' for the possible early assemblage cpxdcore garnet is small (1.2) and compares with the values (1.1-1.8) recorded from quenched high P experimental systems (BULTITUDE and GREEN, 1971) and supports the high T origins of this assemblage. The conditions under which this early assemblage might form can be assessed relative to the subsolidus equilibration. Increasing proportions of garnet relative to clinopyroxene would be favoured by decreasing temperature and increasing pressure (GREEN, 1967). The increasing amount of jadeite in the pyroxenes could result from isobaric cooling (KUSHIRO, 1969) but would be favoured by an increment of pressure. T-P conditions of 140(}-1500'C at 26-28 kbar (Fig. 6) seem reasonably appropriate since eclogites of broadly similar bulk composition are stable at the solidus (GREEN, 1967) and such conditions provide for an increment ofpressure providing the higher-pressure estimates for subsolidus equilibration are accepted. The trend towards enrichment in Ca seen in the garnets of the layered eclogites and culminating in the very rare grosspydites of Roberts Victor is considered to be an essentially igneous trend with some minor subsolidus adjustment of Ca/Fe/Mg ratios. The commoner occurrence of grosspydites in Yakutia (SOBOLEV et al., 1968) probably indicates rather higher pressures when compared with Roberts Victor materials or the commoner development of the magma from which such rocks crystallize.
THE T-P HISTORY OF THE ECLOGITES Three T P regions are shown in Fig. 6 where it is suggested that these eclogites were stable. The line joining these regions is the T-P path which reflects the geological history of the specimens.
364
M.
A. LAPPIN AND
J. B.
DAWSON
T-P conditions of 1400-1500°C, 26-28 kbar, are such that these specimens formed from high P liquids. MACGREGOR and CARTER (1970) have suggested that the layering and fabric of their Type 1 eclogites indicate a cumulate origin. We reject the second criterion on the grounds that in 6913 the best-formed garnets (overgrowth layer) are of subsolidus origin. In more general terms, however, it seems likely that considerable changes in the proportions of garnet/clinopyroxene will occur during subsolidus equilibration and thus that the mutual fabric of these minerals cannot reflect igneous processes. The gross layering of 6913, together with the cryptic mineral variations which are considered to reflect gradual compositional variations within the layering, provides stronger evidence for a cumulate model. The sequence of primary cumulate minerals would be clinopyroxene and minor garnet, clinopyroxene and minor kyanite and then clinopyroxene, kyanite and garnet. There seem to be significant changes of clinopyroxene/garnet associated with the extensive precipitation of kyanite. Little can be said about the magma from which these phases crystallized except that the relatively high Fe/Mg ratio of the early pyroxene and the variable but high Ca contents of possible early garnets suggest an evolved magma relative to possible highpressure primitive partial melts. The second region is one of subsolidus equilibration at 75G-900°C, 28-32 kbar (Fig. 6). The suggested T- P path involves increasing pressure and is considered to be tectonically controlled, for during this stage strain banding in kyanite develops and probably also the foliated fabric of 6913. The cryptic mineralogical and smaller scale compositional variations might develop also at this stage through solid-state differentiation or diffusional processes. In this case, however, an explanation must still be offered for the gross layering. The third stage of evolution (Fig. 6) represents alteration of primary eclogite minerals which SWITZER and MELSON (1969) ascribe to partial melting and quenching during rapid emplacement of the kimberlite magma. The typical quench assemblage AI-rich augite-plagioclase-hornblende suggests relatively high pressures provided the first phase is not metastable. On the other hand, mullite reported by SWITZER and MELSON (1969) as an alteration product of kyanite along with corundum is probably a low P phase though again a metastable origin cannot be ruled out. A provisional estimate 100G-l100°C and 8-12 kbar is suggested because such conditions lie close to the water-saturated lherzolite solidus (Fig. 6) which may be a simple analogue to a natural kimberlite magma. SWITZER and MELSON (1969) note that water may have an important role in this partial melting and in such a case the provisional temperatures would be maximum ones. The analytical data presented earlier suggest considerable chemical exchange between eclogite and kimberlite through either magma or residual fluids. This exchange may be a reflection of still later processes of alteration at lower temperatures and pressures.
ACKNOWLEDGEMENTS We should like to thank Mr. G. Taylor and Mr. A. Hawkesworth for their care and skill in obtaining microprobe analyses and Miss 1. R. Baldwin and Mr. R. Maitland for chemical and X.R.F. analyses.
TWO ROBERTS VICTOR CUMULATE ECLOGITES
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