Download THE PETROLOGY OF THE PYROXENE

THE PETROLOGY
OF THE PYROXENE-GRANULITE FACIES
ROCKSOFJOTUNHEIMEN,NORWAY
M. HUGH BATIEY & W. DAVID McRITCHIE
Battey, M. H. & McRitchie, W. D.: The petrology of the pyroxene-granulite
facies rocks of Jotunheimen, Norway. Norsk Geologisk Tidsskrift, Vol. 55,
pp. 1-49. Oslo 1975.
The petrography of the peridotites, plagioclase-pyroxene-gneisses, jotunites
and mangerites and chemical analyses of the rocks and their constituent
minerals are presented. An original differentiation by fractional crystallisation
of a melt of generally basaltic composition, in the olivine + plagioclase
stability field, can be inferred from bulk chemistry considerations and ele­
ment partitioning between present assemblages. Later thorough recrystallisa­
tion under stress produced strong foliation and lineation with progressive
elimination by reaction of coexisting olivine and plagioclase in favour of
pyroxene. Initial pressures of recrystallisation exceeded 8 kb (above olivine
+ plagioclase stability limit) and the main metamorphism was concluded
4 kb to preserve granulite facies mineralogy. Partial
under pressures >
melting of the feldspathic fraction during a pressure drop in the later stages
resulted in the formation of transgressive mesoperthosites and assisted meta­
morphic segregation. Finally the massif was more or less passively uplifted.
M. H. Battey, Geology Department, University of Newcastle upon Tyne,
Newcastle upon Tyne, NEJ 7RU, England.
W. D. McRitchie, Geological Survey of Manitoba, Box 20, 139 Tuxedo
Boulevard, Winnipeg 29, Manitoba, Canada.
The igneous rocks of Jotunheimen were grouped by Goldschmidt
(1916)
into the Jotun kindred, ranging in silica percentage from ultrabasic to acid,
and characterised by a persistence of pyroxene in the more acid members
and by a notable richness in potassium feldspar associated with relatively
calcic plagioclase in basic and intermediate members. Goldschmidt's studies
have formed the foundation of all later work.
Subsequent petrological studies by C. W. Carstens
tites and by Hødal
(1945)
(1920)
on the perido­
on the anorthosites of Vossestrand have con­
siderably expanded our knowledge of particular aspects. Gjelsvik
(1946)
has
given a careful account of the gamet-bearing anorthosite-magnerite-pyrox­
enite group of the neighbouring Heidal area in terms of Eskola's facies clas­
sification. Dietrichson
(1953, 1955, 1958, 1960)
studied pseudotachylytes,
lamprophyres and recrystallised cataclasites, and concluded from chemical
analyses and variation diagrams that the Jotunheim massif was originally a
stratiform igneous body.
Observations on the ultramafic bodies and on the layered structure of the
massif were reported by Battey
ping are summarised by Battey
(1960, 1965), and results of geological map­
& McRitchie (1973). In this paper it is pro-
2
M. H. BATfEY & W. D. McRITCHIE
posed to separate the rocks of Jotunheimen into a marginal group of gabbros
with relict igneous textures, associated with thrust sheets of a wide range
of composition metamorphosed in the amphibolite facies, from a central
tract of layered and unlayered pyroxene-granulite facies rocks.
The present paper gives an account of the petrology of the pyroxene­
granulite facies rocks of the central high mountain tract of Jotunheimen
between the Tyin-Gjende Fault on the south and the N.W. Boundary Fault
on the north (Battey
&
McRitchie
1973:
figs.
2 & 3).
Throughout this area the prevailing rock is a gneiss with dark folia, or
spindle-shaped lines, of granular orthopyroxene, clinopyroxene, magnetite
and more or less biotite, in a grey mosaic of plagioclase or plagioclase and
potash feldspar. Porphyroblasts of a mesoperthitic intergrowth of inter­
mediate plagioclase and potash feldspar are often present. Apatite is a con­
stant accessory and zircon appears in the more feldspathic members. The
colour index of these gneisses extends continuously from almost wholly
feldspathic members to melanocratic types in which the layers or streaks of
dark minerals attain centimetre thickness and tend to coalesce. Over wide
areas, especially around Mjolkedalsvatn, Saga, Uranostind and Falketind,
layering of rocks of different colour index, parallel to the foliation, pro­
duces conspicuous topographic featuring (Figs.
1-2).
At the melanocratic limit of the feldspar-bearing gneisses, a mineralogical
discontinuity enters. Heralded by an increasing thickness of mafic mineral
Fig. 1. Uranostind: view southeastward from Uranosi ridge, showing mineralogical
layering. Photo: Ulrik Lunn.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
3
Fig. 2. Saga: view eastwards from Uranosi across Skogadalsbreen, showing folded
mineralogical layering. Photo: Ulrik Lunn.
layers, olivine enters in the cores of the mafic streaks. With its entry the
rock becomes ultramafic, because feldspar is not found in contact with oli­
vine. These two minerals are always separated by a reaction rim resulting
from metamorphic conversion of olivine + plagioclase to pyroxene + spinel
(Battey 1960: 204, Griffin 1971b). The existence of the rim relationship
implies an earlier conjunction of olivine and plagioclase established at lower
pressure followed by re-equilibration above 8 to 11 kb (Emslie & Lindsley
1969,
Green & Hibberson
1970).
The ultramafic bodies are sheet-like, commonly stratified with olivine- and
pyroxene-rich layers, and reach thicknesses of tens of metres. The sheets lie
parallel with the foliation of the pyroxene-feldspar gneisses and, as they are
particularly resistant to erosion and weather to a rusty brown colour, they
contribute notably to the display of the layered structure in the gneisses as
a whole. Fig. 3 shows features of the lensoid body on Mjølkedalstind for
which modal mineral compositions are given in Table l.
The rock types mentioned in the last two paragraphs are the principal con­
stituents of the complex. The petrography and mineralogy of these will
now be described, beginning with the ultrabasic members. Homblende-rich
amphibolites, which are widespread but occur in zones of shearing, are re­
garded as retrogressively metamorphosed derivatives and will not be dealt
with here.
A table setting out the rock classification used in the descriptions appears
as an Appendix to this paper.
M. H. BATIEY & W. D. McRITCHIE
4
Table 1. Modal analyses (vol.
%)
of ultramafic body on MjØlkedalstind.
Height
Spee. No.
above base
(m)
Olivine
Ortho-
Clino-
pyroxene pyroxene
Hornblende
Spinel
Ore
1.5
4.3
(Top 21)
Ml
19.5
M2
M3
61.9
12.9
18.4
1.0
16.5
62.3
12.6
10.1
5.4
13.5
58.0
12.0
20.6
3.9
M4
10.7
65.6
11.8
9.9
5.5
MS
7.5
15.8
15.7
66.6
0.3
0.9
M6
4.5
38.9
19.0
32.4
4.8
5.0
9.6
5.5
7.2
Ultramafic rocks
MINERALOGICAL COMPOSITION
The ultramafic rocks include mineral assemblages definitive of dunite, harz­
burgite, wehrlite, lherzolite and websterite, interstratified in more or less
regular layers. Modal analyses of rocks collected on traverses across two
ultramafic bodies (Tables l, 2 & 3) show the range in proportions of the
minerals. The principal constituents are olivine and clinopyroxene in vary­
ing proportions. Orthopyro:xene is present in most rocks, but is subordinate
to clinopyroxene: it does not exceed 25 vol. per cent except where plagio­
clase is present. The ore content is lower in pyroxene-rich rocks. The Langs­
kavlen body (Table 3) is exceptional in having olivine + plagioclase.
Table 2. Modal analyses (vol. %) of ultramafic body S.E. of Uranosvatn.
Spee. No.
Height
above base
(m)
Olivine
OrthoClinopyroxene pyroxene
Hornblende
Spinel
Ore
64/79
69
14.8
14.2
58.7
11.3
64/78
60
81.3
6.8
2.0
4.6
5.3
64/77
53
82.8
9.0
1.7
5.0
1.5
64176
45
81.0
7.0
3.1
2.4
6.5
64/75*
39
85.9
4.2
1.3
4.3
4.3
1.1
64/74
30
86.5
2.9
2.0
5.9
2.7
64/73
27
86.5
2.4
2.3
4.7
4.1
64/72
22.5
85.4
2.0
2.6
3.3
64/71
18
69.4
5.4
24.0
O.l
64170
15
7.5
85.6
6.9
64/69
13.5
64/68
9
9.3
82.3
8.2
64/67
5.7
5.6
89.4
5.0
64/66**
o
14.2
67.8
8.0
* trace of biotite
**
O.l
of biotite
92.0
8.1
5.3
6.7
1.2
0.4
2.3
0.2
2.0
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
5
Table 3. Modal analyses (vol. %) across a belt including an ultramafic layer, S. end Langskavlen.
Distance
Spee. No.
Spinel
E. from
Plagio-
datum
elase
Olivine
Ortho-
Clinopyroxene pyroxene
Hornblende
Biotite
(m)
Ore
Apatite
1.3
0.3
plectite
63/128
o
49.5
19.8
7.2
22.0
63/129
2.7
53.1
26.6
0.5
19.1
63/130
6.0
12.0
13.6
31.3
2.3
24.6
63/131
9.1
7.0
34.6
24.6
1.6
27.1
34.0
9.2
4.0
25.3
13.1
6.6
63/132*
10.6
28.7
24.2
5.1
31.5
63/133
16.5
52.0
20.5
0.8
24.3
63/136
+
Sym-
0.2
2.4
O.l
O.l
O.l
0.6
15.8
l.O
2.4
0.4
13.8
3.4
l.O
O.l
2.4
0.4
tr
O.l
O.l
O.l
* also 6.2 sphene
The only systematic change that is observed across the ultramafic bodies is
the tendency for the marginal zones to be pyroxene-rich and, in some cases,
for these zones to contain plagioclase in addition to ferromagnesian minerals.
Conspicuous marginal zones of clinopyroxenite up to a metre or more wide
border some ultramafic bodies and have sharp contacts with olivine-rich rock
on one side and feldspathic gneiss on the other. In other cases centimetre­
scale reaction rims with the sequence olivine-pyroxene rockforthopyroxene­
spinelf (amphibole-) clinopyroxene-spinel symplectitefplagioclase-pyroxene
rock separate peridotite from feldspathic gneiss.
Ultramafic bodies may show regular dyke-like form (Fig. 4), but in other
cases interfingering relations are seen and pyroxenite layers only a few centi-
Fig. 3. Sharp contact between ultramafic rock and pyroxene-feldspar gneiss inter­
fingering with it, in the lens SE of MjØlkedalstind.
6
M. H. BATIEY & W. D. McRITCHIE
Fig. 4. Sharply-bounded sheet of ultramafic rock in pyroxene-feldspar gneiss cut
by a typical flat-lying lag fault with a pegmatite vein along it.
metres thick occur (Fig. 5). In some examples strong deformation has pro­
duced a complex relationship between ultramafic and feldspathic gneiss, with
layers and pods of the one lying in a matrix of the other (Fig. 6: see also
Fig. 5. Pyroxenite layers a few cm thick in pyroxene-feldspar gneiss. Ridge 1.5 km N
of Skauthøe.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
7
Battey 1960). In such examples, pyroxenic reaction zones always separate
olivine from plagioclase-bearing rock. An example of one of these reaction
zones has been studied in detail by Griffin (1971b), who attributes the ab­
sence of garnet to an episode of rapid uplift and pressure-drop during
cooling.
Garnet is, in fact, present in small amounts in pyroxenite border zones of
ultramafic bodies (Fig. 9B) and in dark feldspathic gneisses near them. It
forms around ore grains, and biotite is very often associated with it. Its status
is uncertain, in as much as garnet also forms in retrograde assemblages in­
cluding amphibole, biotite, epidote and scapolite, produced by late shearing
movements. Along these movement planes pegmatites often form and
pegmatitic feldspar rocks with garnet porphyroblasts are occasionally seen.
Garnet also replaces plagioclase in the igneoustextured marginal gabbros
and rare dykes within the massif.
Tentatively we may separate the retrograde paragenesis from the garnet­
iferous rocks near ultramafic contacts. In the latter, the reaction by which
garnet forms between ore and plagioclase in the feldspathic gneisses may be
analogous to that in garnetiferous basic gneisses with less than 5 per cent of
garnet described from Scourie by O'Hara (1961). It does not appear to be
the reaction of aluminous pyroxene + spinel + plagioclase to give low-A1
pyroxene + garnet. Within the ultramafic border zones themselves it is not
certain whether pyroxene enters into the reaction or not.
Fig. 6. Detached slivers of pyroxene-feldspar gneiss in ultramafic rock. W. flank of
ridge N. of Skauthøe.
8
M. H. BATIEY & W. D. McRITCHIE
TEXTURES
Uncrushed ultramafics consist of a mosaic of anhedral, equigranular olivine
and pyroxene grains ranging up to 4 mm in diameter, united by simple
curving junctions. A typical rock has pyroxene plates about 2 mm across.
Magnetite and hercynite granules are disposed along the margins of the sili­
cate grains.
Cataclastic effects are widespread, indeed almost universal, and have
produced a range of grainsize down to 20 microns. The , crushing begins
around grain margins and extends as lanes between relict crystal cores.
Olivine is more readily crushed than pyroxene and, beginning with the
development of translation lamellae of slightly differing extinction position, it
is ruptured and reduced to a paste of fine fragments. Fig. 7 shows a boulder
of peridotite breccia with lumps of unreduced pyroxenite in a matrix of
crushed olivine. The same preferential crushing of olivine is seen on the
microscopic scale.
The pervasive cataclasis has not been accompanied by retrograde meta­
morphism and the high temperature minerals, olivine and pyroxene, persist
with hardly any serpentinisation in most places. On the other hand, there is
also localised shearing along discrete planes in many places, where the
minerals dose to the plane of movement have been converted to an epidote­
amphibolite facies assemblage (Battey & McRitchie 1973: fig. 6). Whereas
the first type of mechanical break-down must have taken place above 400°C
Fig. 7. Tectonic conglomerate of pyroxenite lumps in a matrix of dunite. SW side of
Store MiØlkedalsvatn.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
9
or in the virtual absence of water, the second represents a late-stage high­
leve! shearing episode in the history of the rocks.
DETAILED MINERALOGY OF ULTRAMAFICS
Olivine
Olivine is in rounded equant grains, clear except for specks of magnetite
along cracks, and is rarely serpentinised except in one or two localities near
the Tyin-Gjende Fault. Olivine is usually separated from associated clino­
pyroxene by a thin rim of orthopyroxene. Replacement of olivine by ortho­
pyroxene has been observed, with trails of relict olivine passing into the
orthopyroxene.
The olivines have a rather narrow range of composition, mostly between
Fa1s and F�5· Table 4 presents data on olivines from rocks of one ultramafic
body some 2.5 km north-east of Spiterstulen Hotel. Compositions have been
determined from the refractive index � using the curves of Deer et al. ( 1966:
fig. 3). Averages of 2V measurements generally support the values given, but
indicate a wider spread of composition: they are regarded as subject to much
greater experimental errors than refractive index measurements. The ranges
of both, however, suggest that there is some local variation in composition.
Compositions determined from the d174 X-ray spacing (Jambor & Smith
1964) are in reasonable agreement with those from optical data, though
generally a little more iron-rich (McRitchie 1965). Of 20 olivines from widely
separated ultramafic bodies determined by X-ray, 16 lie in the range Fa1s
to Fa25, the extremes being Fa14 and Fa29. Checks of measurements on film
by diffractometer measurements and by using other powder lines gave satis­
factory agreement suggesting that the range of compositions is not due to
experimental error.
A chemical analysis of one olivine, Fa21, is given in Table 5, anal. 1. Grif­
fin {1971b) reports that numerous electron-probe analyses of olivines in
58{63 (2.5 km on bearing 32° from Spiterstulen) varied little from Fa23•
(Numbers refer to specimens in the collection of the University of Newcastle
upon Tyne Department of Geology.)
No strong evidence has been found of systematic differences between the
olivines of different ultramafic bodies. It may be noted that the three most
magnesian examples (Fa14, Fa15, Fa16) all come from specimens taken across
a body at the south-west end of Uranosvatn, approximately 7.5 km on a
bearing of 285° from Eidsbugarden Hotel. This is the most westerly mass
sampled. However, the Koldedalsnosi body, along strike, has more ferri­
ferous olivine.
Nor is there any systematic change within a single body. Indeed, the re­
fractive index range in one rock (J32, Table 4) is as great as that of the
whole body of which it is a part.
The olivine compositions fall within the range of those forming cumulates
from gabbroic magmas (Fig. 8). But olivine in Jotunheimen has, in general,
l
l
l
l
l
l
l
l
l
J32
J33
J34
J35B
141
142
146
147
l
J30
Rock No.
(6)
19
18
-
19
24
24
23
25
d174
Fa
%
Mol.
l
1.692
(7)
1.690
(11)
1.693
1.689-
1.690
(12)
1.690
(6)
1.692
(7)
1.692
(6)
1.689
(7)
1.689
(5)
1.691
av.
1.688-
1.691
1.689-
1.693
1.690-
1.694
1.690-
1.690
1.688-
1.690
1.687-
1.692
1.690-
range
y
Orthopyroxene
Number of determinations in brackets
Fa16 (little present)
(5)
1.692
=
1.691
1.689-
2V90°
21
20
22
22
21
21
Fa
%
Mol.
(traccs only)
1.694
1.694
( 7)
1.693-
1.692
1.693
(7)
1.692-
1.695
1.694-
(6)
1.696
1.697
1.695
(6)
1.694-
1.694
1.690
(5)
1.691
1.697-
1.694
av.
1.695-
range
�
Olivine
20
20
20
21
21
19
19
21
Fs
%
Mol.
l
l
(2)
1.687
av.
1.683-
1.687
(7)
1.6861.688
(6)
1.686
56
1.684
1.682-
55
(4)
(5)
(6)
1.688
(lO)
1.686
1.686
1.689
(l)
1.684-
(5)
59
(3)
1.685
(4)
55
1.687
1.6841.689
55
(l)
1.687
(little present)
1.687
1.686-
range
( 4)
(1)
55
(4)
5672
av.
�
39-41
40--44
40-46
39-43
36-43
Z:c
range
Mg41
Fe12
Ca47
Mg/Fe = 77123
General composition of clinopyroxene
54-58
54-55
56-64
55-56
54-55
56-57
range
2Vy
Clinopyroxene
Table 4. Optical properties of minerals from the ultramafic body 2.1 km on 50° from Spiterstulen Hotel.
(5)
40
(4)
42
(5)
43
(5)
41
(4)
40
av.
l
l
1.649
�
1.656 y 1.670
1.649
�
1.660 y 1.668
beige Y, Z pinkish brown
2Vy 85° Z: c 20° X pale
2Vy 87° � 1.658 X pale
beige, Y, Z greenish
yellow-brown
Z:c 19°
X pale yellow, Y pale
beige, Z pinkish beige
a
brown Z:c 1772 °
X pale yellow, Y pinkish
yellow-brown, Z green-
a
Amphibole
.....
�
......
...,
()
::r:
tTl
�
�
(")
�
Ro
�><:
>
t:l:l
t:r.:
�
o
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
11
Jotun heim
nf---Scourie
32
(HT-
•HA)
-------+ to 96
30
-toiOO
28
26
nf-
---•to96
nf --­
nf--f
nf ---
20
18
------ - - - -- -
16
6
4
2
---- ---- ----to 83
nf
12
8
-
nf--
14
ro
?
nfnf
nfnf
nf -nfnf
nf-if
nf
10
20
40
30
50
60
Mol. per cent fayalite
Fig. 8. Olivine compositions from some igneous intrusions compared with those from
Jotunheim ultramafics. nf
=
non-feldspathic, if
=
interstitial feldspar, f
=
feldspathic.
1. SE Alaska ultramafics: Taylor &
20. Cuillins, Skye, Hebrides: ibid., p. 415
Noble 1960
2. Stillwater, Montana: Jackson 1961
21. Belhelvie, Aberdeenshire: Wadsworth,
3. Dun Mt. etc. New Zealand: Challis
22. Kapalagulu, Tanzania: Wager &
1965
4. Gt. Dyke, S. Rhodesia: Worst 1958
5. Webster-Addie, N. Carolina: Miller
23. Duke I. Alaska: op. cit., p. 512
24. Muskox, Canada: Smith, C. H. 1962
1953
6. Red Hills, New Zealand: Walcott 1968
7. Hebridean picrites: Drever &
Johnston 1958
8. Union Bay, Alaska: Ruckmick &
Noble 1959
9. Troodos, Cyprus: Wager & Brown
1968, p. 508
10. Bay of Islands, Newfoundland: op.
c:t., p. 453
Stewart & Rothstein 1966
Brown 1968, p. 469
25. lnsch, Aberdeenshire: Clarke &
Wadsworth 1970
26. Kaerven, E. Greenland: Wager &
Brown 1968, p. 435
27. Carlingford, Eire: Le Bas 1960
28. Kap Edvard Holm, E. Greenland:
Wager & Brown 1968, p. 427
29. Skaergaard, E. Greenland: op. cit., pp.
27, 35
30. Phenai Mata (lowermost 250 ft),
11. Lizard, Cornwall: ibid., p. 505
Gujarat, India: Sukheswala & Sethna
12. Rhum, Hebrides: ibid., p. 255
1969
13. Stjernøy, Norway: ibid., p. 506
14. Baltimore, Maryland: ibid., p. 509
31. Ashland, Wisconsin: Wager & Brown
1968, p. 510
15. Huntly, Aberdeenshire: Weedon 1970
32. Kiglapait, Labrador: op. cit., p. 457
16. Moxie, Maine: Wager & Brown 1968,
33. Duluth, Minnesota: ibid., p. 447
Scourie (metamorphic), Sutherland:
p. 509
17. Ard Mheall, Rhum: Wadsworth 1961
18. Insizwa, S. Africa: Wager & Brown
1968, p. 525
19. Bushveld (Basal Zone), S. Africa: op.
cit., p. 358
O'Hara 1961, p. 269
HT Hawaiian tholeiite,
HA Hawaiian alkali basalt
12
M. H. BATfEY & W. D. McRITCHIE
Table 5. Chemical analyses of olivine and orthopyroxenes.
Si02
Ti02
Al20a
F�Oa
FeO
Mn O
MgO
CaO
Na20
Sum
l
2
3
4
5
6
7
8
9
39.10
0.01
n.d.
3.92
15.48
0.60
40.30
0.80
0.20
100.41
53.72
0.06
2.78
2.74
9.72
0.20
29.40
1.40
0.30
100.32
53.01
0.16
4.49
0.03
13.62
0.45
28.20
0.20
0.10
100.26
55.60
0.03
2.00
4.54
10.10
0.75
26.05
1.00
0.60
100.67
52.89
0.14
3.56
0.02
13.70
0.28
28.07
0.61
0.16
99.77
51.69
0.10
4.57
0.32
15.88
0.44
25.80
1.40
0.20
100.40
51.60
0.13
3.72
1.30
16.51
0.65
25.23
1.44
0.16
100.74
52.30
0.12
5.37
0.20
16.29
0.68
23.24
1.86
0.35
100.41
49.90
0.21
6.30
0.25
17.68
0.73
23.90
1.28
0.15
100.40
Cation proportions
to 4 oxygens
Si
Al
lO
50.87
0.05
6.05
3.10
15.46
0.65
22.02
1.15
0.004
99.35
Cation proportions to
6 oxygens
0.992
1.900
0.100
1.806
0.114
1.972
0.028
1.903
0.097
1.862
0.138
1.878
0.122
1.900
0.100
1.821
0.179
1.863
0.137
0.992
2.000
2.000
2.000
2.000
2.000
2.000
2.000
2.000
2.000
0.015
0.074
0.006
0.057
0.054
0.002
0.039
0.004
0.035
0.501
0.020
1.373
0.057
0.013
0.131
0.004
0.004
0.493
0.022
1.269
0.072
0.026
0.093
0.006
0.009
0.539
0.022
1.308
0.050
0.009
0.127
2.021
2.036
0.076
0.328
0.012
1.532
0.021
0.009
0.072
0.286
0.006
1.560
0.053
0.021
0.403
O.D15
1.504
0.009
0.009
0.119
0.298
0.023
1.385
0.038
0.043
0.410
0.009
1.514
0.024
0.013
0.056
0.002
0.009
0.475
0.013
1.392
0.054
0.013
1.978
2.015
2.020
1.963
2.026
2.014
2.042
Atomic ratios
78.7 Ca 2.7
Mg
21.3 Mg 79.1
Fe
Fe 18.2
0.4
78.4
21.2
2.0
1.2
3.9
2.6
2.5
77.7
69.9
69.0
68.6
66.0
21.7
21.0
2.8
72.1
25.1
2.8
76.3
27.3
27.0
28.8
31.5
Cell dimensions (A)
a
b
c
18.279
8.830
5.195
18.259
8.828
5.198
18.290
8.878
5.203
18.277
8.844
5.197
18.292
8.879
5.201
Al
Ti
Fe3+
Fe2+
Mn
Mg
Ca
Na
18.262
8.828
5.194
l. Olivine from hornblende-bearing harzburgite 621112, E. side of Langeskavlen (Includes P205 0.01).
2.-10. Orthopyroxenes from the following: 2. Lherzolite 63/M6b, S.E. MjØlkedalstind;
3. Hornblende-pyroxenite 62143, near summit of HøgbrothØgdi; 4. Hornblende-harzburgite 621112, E. side of Langeskavlen; 5. Hornblende-harzburgite 133, 2.1 km on 50°
from Spiterstulen Hotel; 6. Pyroxene-gneiss 62140, HØgbrothØgdi; 7. Pyroxene-rich
jotunite 62147, HøgbrothØgdi; 8. Pyroxene-gneiss 62/53, HØgbrothØgdi; 9. Jotunite
62/36, Høgbrothøgdi; 10. Jotunite 62/48, HøgbrothØgdi summit.
Analyst: W. D. McRitchie, except no. 5, M. H. Battey.
only survived the metamorphism where it is in feldspar-free rocks. On the as­
sumption that the ultramafics are cognate with the surrounding gneisses, we
are denied knowledge of the full composition range that may once have
existed, because the olivine possibly once present in gabbroic rocks would
0.084
0.473
0.021
1.211
0.046
1.962
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
13
have disappeared in the metamorphism. Comparisons should therefore be
made with non-feldspathic cumulates of gabbroic magma, or non-feldspathic
peridotites of other origins. When this is done, the Jotunheimen olivines are
seen to be unusually ferriferous (cf. Goldschmidt 1916: 24-25). They are
of a composition that would be expected to occur in feldspar-bearing rather
than non-feldspathic cumulates. McRitchie (1965), however, finds a similar
range of olivine compositions in ultramafics south of the Tyin-Gjende Fault,
which have not been subjected to granulite facies metamorphism.
The olivine-bearing rocks of Langskavlen (Table 3) appear to represent
rocks from which plagioclase is being eliminated by reaction, as witnessed
by the high proportion of symplectite, though it has not yet been entirely
destroyed. Such rocks may represent the original mineralogical composition
of other ultramafics now free of feldspar. Fig. 9 shows interstitial patches in
a pyroxenite of the border zone of an ultramafic body illustrating the elimi­
nation of basic plagioclase feldspar and the development of garnet b)l meta­
morphic reaction. Garnet is never abundant but is often found in small
amounts in dark rocks transitional to feldspathic gneisses near the margins
of ultramafic bodies.
Fig. 9. Stages in border reactions of ultramafics.
A. Interstitial plagioclase (striped), hypersthene (stippled), hornblende (cross-hatched)
and ore (black), in plates of clinopyroxene enclosing a few apatites. Garnet only as
a thin pellicle around the plagioclase tongue and ore granule in the extreme south.
B. Garnet (stippled) surrounding ore + green spinel (black) amidst plates of clino­
pyroxene (cpx). h
=
hypersthene, pl
=
residual plagioclase, hb
=
hornblende.
Section JJ37 (72133 Harker Coll. Camb.) marginal pyroxenite, ultramafic body 1100 m
on 320° from Juvasshøe.
M. H. BATTEY & W. D. McRITCHIE
14
The proportion of spinel in the main masses of ultramafic rock is not great
enough, however, to suggest the elimination of any large amount of original
plagioclase. The importance of the reaction phenomena is their indication
that olivine + plagioclase was originally the stable assemblage, presumably
under magmatic conditions (cf. Rothstein 1964).
Orthopyroxene
This mineral occurs in widely varying amounts in the ultramafic bodies.
There seems to be no regularity in its proportions relative to the other major
minerals (Tables 1-3). Orthopyroxene in olivine-rich rocks is interstitial to
olivine and somewhat less pleochroic than in pyroxenites and feldspathic
gneisses. Occasionally it poikilitically encloses olivine. Perfectly spherical crys­
tals (Fs24) with smooth, finely-pitted surfaces, but no sign of marginal altera­
tion, have been separated from one specimen (62/112). Their shape is perhaps
30
20
Q)
c
>
,.
l
c
/
/
�
/
o
u..
10
/
«""
··
'<o
,.c
l
l
v
""'
/
/
/.
/
"
/
l
l
l
l
l
""'
10
20
Fs
%
30
in orthopyroxene
Fig. 10. Content of iron silicate in coexisting olivine and orthopyroxene in ultramafic
rocks. Broken line shows main field of olivine-orthopyroxene pairs from layered gab­
broic intrusions, after O'Hara (1963).
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
15
due to same kind of resorption, but no records of such spherical forms have
been found in the literature.
In the pyroxenites, orthopyroxene is rather more pleochroic, and carries
regularly arranged, rectangular, red-brown inclusions supposed to be rutile.
Clinopyroxene is aften exsolved in crude lamellae parallel to (100) and
sometimes in more irregular fashion. Small clear crystals of secondary ortho­
pyroxene develop in streams running between the larger crystals and passing
through cracks in the larger grains.
Table 4 gives values of the refractive index for orthopyroxenes of rocks
collected across the body 2.5 km north-east of Spiterstulen, with com­
positions, Fs19-21, read from the curves of Deer et al. (1966: fig. 41). The
composition range is very small, with no systematic change across the body.
Chemical analyses of four orthopyroxenes from other ultramafic bodies
are presented in Table 5, anals. 2 to 5. They yield ratios of Fs1s to Fs-æ.
Using the microprobe, Griffin (1971b) finds orthopyroxene from a reaction
zone between peridotite and gneiss to be Fs21• Optical determinations on
orthopyroxenes from other ultramafics give values in the range Fs18 to Fs26•
Table 5 gives the cell dimensions of the analysed orthopyroxenes.
The composition range of the orthopyroxenes, like that of the olivines, is
rather restricted. Comparison of the molecular ratio FeOfFeO + MgO of
coexisting olivines and orthopyroxenes (Table 6) reveals only small differ­
ences between the members of each pair. Fig. 10 (cf. O'Hara 1963) shows
the relationship of this 1:1 ratio of Fa per cent to Fs per cent to the equi­
librium ratio determined by Bowen & Schairer (1935).
OlivinejCa-poor pyroxene pairs of igneous rocks show either the 1:1 ratio,
or a greater concentration of FeO in pyroxene than in olivine, a feature at-
Table 6. Compositions of coexisting olivine and orthopyroxene.
130
132
133
134
135B
J41
J46
58/63
62/194
63/M6b
62/ 181
63/137
62/1 12
Olivine
Fa%
Orthopyroxene
Fs%
Difference
21
21
22
22
20
21
19
19
20
19
21
29
22
21
19
19
21
21
20
20
19
18
19
18
26
22
o
-2
-3
-l
+l
-l
+l
o
-2
o
-3
-3
o
The first 12 determined by optics or d174 of olivine. Orthopyroxene checked in some
cases by electron microprobe. 621112 by wet chemical analysis.
16
M. H. BATfEY & W. D. McRITCHIE
tributed to lack of equilibrium in cooling melts. In metamorphic rocks, like
those of Jotunheimen, a doser approach to the equilibrium ratio would be
expe�ted. Since this is not observed, it may be concluded either that the as­
semblage is a disequilibrium igneous one and that the metamorphism has not
been high enough to allow the attainment of equilibrium: or that the equilib­
rium ratio of Bowen and Schairer is inappropriate to natural conditions.
In any case it may be said that the partition of Fe and Mg between olivine
and orthopyroxene in the Jotunheim ultramafics is no different from that
found in slowly cooled mafic igneous rocks.
Clinopyroxene
Widely varying amounts of clinopyroxene are present in different layers of
the ultramafic bodies. Little regularity in its change of proportion relative to
other phases is shown by Tables 1-3. Nevertheless, it has been observed that
clinopyroxene-rich rocks tend to occur at the margins of the bodies (cf.
Carstens 1920) though the thickness of these marginal layers is extremely
variable. There is, however, always a pyroxenic rim of some kind between
olivine-bearing and feldspathic rocks.
The clinopyroxene of the ultramafics is in equant grains which are very
commonly full of opaque and brown inclusions, often zonally arranged and
concentrated centrally. Orthopyroxene is exsolved in pools and occasionally
along the 100 planes (including the twin plane in the limited number of cases
where twinning is shown).
0·1 mm
Fig. 11. Plagioclase and ore granules enclosed along the 100 partings of clinopyroxene.
Section J29B/1, hornblende-websterite, E side of Visdalen, NNE of Spiterstulen.
•
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
17
Table 7. Chemical analyses of clinopyroxenes.
l
2
3
4
5
6
Si0 2
49.02
44.00
47.00
47.60
46.29
46.00
Ti02
0.37
0.52
0.15
0.50
0.51
0.36
A120a
6.88
9.46
6.77
6.34
4.92
6.57
2.51
3.60
3.59
3.85
3.87
2.80
4.61
5.31
6.16
6.46
0.16
0.19
0 30
Fe<&Oa
FeO
3.14
4.63
Mn O
0.30
0.27
MgO
16.14
16.20
15.70
15.00
15.67
15.50
Ca O
20.56
21.10
21.30
20.60
21.30
21.70
0.74
0.60
1.30
0.50
0.50
0.05
0.10
99.91
99.51
Na20
K20
0.10
99.67*
•
100.48
nil
nil
100.42
.
1.00
nil
100.69
includes P205 0.01
Cation proportions to 6 oxygens
Si
1.808
1.638
1.747
1.770
1.750
1.721
Al
0.192
0.362
0.253
0.230
0.218
0.279
2.000
2.000
2.000
2.000
1.968
2.000
Al
0.104
0.044
0.037
0.046
0.000
0.004
Ti
0.011
0.016
0.004
0.027
0.014
0.011
Fe3+
0.071
0.103
0.098
0.107
0.109
0.081
Fe2+
0.097
0.143
0.143
0.165
0.195
0.202
Mn
0.009
0.009
0.004
0.007
0.009
Mg
0.891
0.904
0.876
0.836
0.890
0.871
Ca
0.812
0.841
0.848
0.821
0.864
0.871
Na
0.004
0.045
0.094
0.036
0.036
0.072
K
0.004
0.005
1.999
2.109
Ca
43.4
42.0
43.1
42.5
41.7
42.9
Mg
47.5
45.2
44.5
43.3
43.0
42.9
9.1
12.8
12.4
14.2
15.3
14.2
Fe
2.100
2.042
2.120
2.121
Cell constants
a sin �
9.334
9.349
9.351
9.353
9.354
9.363
b
8.874
8.888
8.890
3.900
3.899
8.915
c sin�
5.0098
5.0166
5.0219
5.0256
5.0202
5.0280
Source rocks
1. Peridotite 63/M6b, S.E. slope MjØlkedalstind.
2. Pyroxenite 62143, near summit of HØgbrothØgdi.
3. Pyroxene-rich pyroxene-feldspar gneiss 62140, HøgbrothØgdi.
4. Ditto, 62153, HøgbrothØgdi.
5. Mangerite-jotunite 62136, HøgbrothØgdi.
6. Jotunite, 62151, W. foot of HØgbrothØgdi.
Analyst: W. D. McRitchie.
18
M. H. BAITEY & W. D. McRITCHIE
In rare cases plagioclase may occur in the heart of a pyroxene grain, as a
nest of rounded blebs enclosing dactylitic granules of pyroxene in optical
continuity with the surrounding crystal. Plagioclase may occur along the 100
parting of clinopyroxene, as thin plates and beaded streaks associated with
magnetite and granules of pleochroic hypersthene (Fig. 11).
Table 4 gives the optical properties of clinopyroxenes from a series of
rocks collected across one ultramafic body. Analyses 1 and 2 of Table 7 re­
present the compositions of clinopyroxenes from ultramafic rocks deter­
mined by conventional chemical analysis, while analyses 7, 8 and 10 of
Table 8 are partial analyses by electron microprobe. The microprobe results
give slightly higher CaO values which agree hetter with the 2V measure­
ments. As the microprobe analyses were made on selected homogeneous
grains they should be free from interfering effects of included exsolution
lamellae of a lime-poor phase which may affect the bulk analyses. Griffin
(1971b) analysed clinopyroxenes from a peridotite, 58/61, and its reaction
zone with the surrounding gneiss. The two are virtually identical, their
composition being C145Mg44.sFe10.5 in reasonable agreement with the other
results.
The relationship between coexisting clinopyroxenes and orthopyroxenes
on these rocks is considered below in conjunction with analyses of pyroxene
pairs from feldspathic pyroxene-gneisses.
Amphibole
Many of the ultramafic rocks carry a pinkish-brown hornblende forming
anhedral to subhedral crystals with sharp boundaries towards the other
minerals and of comparable grain-size with them. They show every appear­
ance of primary grains. The optical properties of examples of these amphi­
boles are given in Table 4 and an analysis of one of them in Table 9.
Marginal zones of ultramafic bodies and the whole of some small ultra­
mafic lenses may sometimes be recrystallised to greenish-black amphibolite.
The amphibole replacing the clinopyroxene and orthopyroxene is normally
pleochroic in greens. This change is restricted to zones of shearing traversing
the granulite facies rocks and is regarded as representing a late retrograde
metamorphic event.
Spinel
Green spinel is a common accessory mineral of the ultramafic rocks. It
becomes particularly conspicuous in the reaction zones between ultramafic
and feldspar-bearing rocks. Its refractive index is between 1.78 and 1.79 and
the cell-edge
a
FeOfMgO
1.8, so that on the diagram of Deer et al. (1966: 431) it falls
=
=
8.132, while Griffin's (1971b) microprobe analysis gives
between hercynite and the field of pleonaste.
The spinel usually forms composite grains with magnetite. The two phases
occur as separate portions of the composite grain, united along a smoothly­
curving boundary. No regular crystallographically controlled intergrowths
45!
38!
16
21.6
13.1
9.9
Cpx
56.35
0.61
2!
56!
41
1.2
18.1
23.9
1
2
58
40
2
45
39
16
21.4
13.2
9.8
Cpx
56.79
0.63
1.0
19.0
23.3
Opx
3
47
38
15
21.3
12.6
8.9
Cpx
52.68
0.59
2
64
34
1.0
20.2
19.2
Opx
4
Cpx
2
65
33
-
-
42
41
17
0.75 18.9
21.5 13.3
9.6
19.4
Opx
5.
4.
3.
2.
tind.
58/21 Mangerite Eidsbugarden-Olavsbu track at beginning of descent
to Rauddalen.
58123 Jotunite Rauddalen valley floor S.E. of Olavsbu on Eidsbugarden track.
58124 Jotunite Semmeldalsmunnen, Olavsbu-Leirvassbu track, 0.5
km S.S.W. of Langvatn.
58/26 Pyroxene-gneiss Presten Ridge, N. of Leirvassbu.
1. 58/20 Mangerite 350 m E. of ultramafic body E. face MjØlkedals-
Whole Rock
Si02
FeO/
(MgO+FeO)
Atomic ratios
Ca
Mg
Fe
Weight per cent
CaO
MgO
Total Fe
as FeO
Opx
2
60
38
5
6
46
39
15
21.3
12.9
8.6
Cpx
49.81
0.56
2
64
34
1.1
21.1
20.0
Opx
1
73
27
0.5
24.8
16.7
Opx
-
7
47
43
10
22.5
14.6
5.7
Cpx
1
78
21
8
44
44
12
20.4
14.6
6.7
Cpx
38.73
0.33
0.5
27.0
13.0
Opx
1
68
31
-
0.6
22.8
18.7
Opx
9
48
39
13
22.3
12.9
7.5
Cpx
-
1
77!
21!
0.7
27.3
13.3
Opx
Cpx
46
45
9
2 1.8
15.1
5.5
10
Analyst: M. H. Battey.
6. 58/38 Mafic jotunite N.E. ridge of GaldhØpiggen.
7. 120 Biotite-pyroxenite N. ridge of Skauthøe, E. of Visdalen.
8. 133 Homblende-harzburgite E. side Visdalen, N.N.E. of Spiterstulen.
9. 62/53 Mafic pyroxene-gneiss W. face of HØgbrothøgdi N. of Eidsbugarden.
10. 134 Wehrlite E. side Visdalen, N.N.E. of Spiterstulen.
45
40
15
21.5
13.3
9.0
Cpx
48.00
0.53
0.9
19.4
22.0
Opx
Table 8. Partial analyses of related pyroxenes (analysed by electron microprobe).
\0
......
Cll
:;.;;
o
(")
�
Cll
"'1
>
(")
......
1:!1
>
z
c:::
1:""
......
...,
1:!1
6:;.;;
-<
:;.;;
o
:><
1:!1
z
"Ø
o
"'1
1:!1
...,
:;.;;
o
1:""
o
o
-<
"Ø
20
M. H. BATIEY & W. D. McRITCHIE
Table 9. Analysis of amphibole.
Si02
Ti02
Al20:J
F�O:J
FeO
MgO
CaO
Na20
K20
H20+
H2o­
P205
41.20
2.04
14.83
2.41
7.21
15.08
11.85
2.70
1.24
1.07
n.d.
0.18
99.81
From hornblende-harzburgite, 133, E. wall of Vis­
dalen, N.N.E. of Spiterstulen (cf. Table 12).
Analyst: M. H. Battey.
are observed, and the relationship suggests metasomatic change rather than
exsolution at the solvus known from experiments in the system hercynite­
magnetite (Turnock 1959).
Biotite
Usually sparsely distributed in shreds between the other minerals, biotite
may sometimes form more prominent flakes 1 to 2 mm in length. It is a pale­
brown phlogopitic variety, in contrast to the darker biotite of the feldspathic
gneisses discussed below.
Carbonate
Traces of carbonate in shapeless interstitial crystals occur in some of the
ultramafic rocks. The presence of this mineral is presumably due to a slight
degree of alteration by weathering, though on the whole the primary minerals
are remarkable for their freshness, and it is not clear which of them has pro­
vided the materials of the calcite.
Pyroxen�-feldspar gneisses (pyroxene-granulites)
Rocks of this group are overwhelmingly dominant in the area under con­
sideration. The ultramafic layers and lenses are conspicuous but quantita­
tiv�ly quite subordinate.
A recent discussion (Behr et al. 1971) has shown considerable divergence
of opinion on the meaning to be attached to the term granulite. In particular,
a number take the view that granulites, strictly speaking, should contain
platy aggregates of quartz. The question of definition is not yet resolved, but
as the Jotunheim rocks do not usually contain quartz they are referred to
here as pyroxene-feldspar gneisses belonging to the granulite facies.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
21
MINERALOGICAL COMPOSITION
They are typically composed of feldspar, orthopyroxene, clinopyroxene, bia­
tite and opaque ore, with accessory apatite in most specimens and accessory
zircon in the more feldspathic examples. Hornblende is often present, more
notably in the less feldspathic rocks, but it is replacing pyroxene and is
regarded as a product of retrograde metamorphism. Traces of garnet may
sometimes be seen.
Four varieties of feldspar may be distinguished, namely, plagioclase:
plagioclase with sparse antiperthitic lamellae: mesoperthite in which plagio­
clase (andesine) is intergrown with a roughly equal amount of potash feid­
spar: and homogeneous, or weakly perthitic, orthoclase. Micrometric analysis
of 130 feldspar-bearing rocks gave the following result:
plagioclase only
orthoclase + perthite < plagioclase
orthoclase + perthite > plagioclase
orthoclase + perthite only
59
37
29
5
In a further 64 examples examined qualitatively 23 have plagioclase only, 34
have plagioclase + potash feldspar and none have potash feldspar only. Two­
feldspar gneisses thus predominate over gneisses with plagioclase only in the
ratio of about 5:4.
As the colour index of the rock falls, the proportion of potash feldspar
usually rises; but this is not always so, and there are some quite leucocratic
rocks (colour indices 0.3, 6, 11, 13, 16) in which the feldspar is all plagio­
clase. Large bodies of anorthosite have not been found, however, and in this
respect the area contrasts with that farther west around the head of Sogne­
fjorden (Hødal 1945, Griffin 1971a).
In the direction of increasing colour index, neither orthoclase nor con­
spicuous perthite have been seen in rocks with a colour index greater than
44. McRitchie (1965) finds, moreover, that potash feldspar is incompatible
with hornblende.
Most of the rocks in this group contain both clinopyroxene and ortho­
pyroxene, though the fine-granular texture of the spindle-shaped pyroxene
clusters makes distinction between them difficult at times. Micrometric mea­
surement gave the following distribution in 130 rocks:
clinopyroxene > orthopyroxene
orthopyroxene > clinopyroxene
clinopyroxene only
orthopyroxene only
58
39
33
4
In a further qualitative sample 45 rocks have both pyroxenes, 9 have clino­
pyroxene only and l has orthopyroxene only.
Biotite is usually present and may occur in amounts as high as 35 vol. per
cent. Though variable, it is present on average to about half the amount of
22
M. H. BA TIEY & W. D. McRITCHIE
the total pyroxene and makes up less than a third of the dark minerals when
ore is counted.
Ore averages 3.4 vol. per cent, ranging from a few tenths of a per cent to
7 per cent, with exceptional values of 9, 11 and 22 per cent.
Apatite is often relatively coarse and occurs in bluntly oval, anhedral
grains.
Garnet is a mineral occurring in accessory or trace amounts, in a small
proportion of the rocks. It is recorded in 32 out of 130 micrometrically
analysed rocks and in 16 out of 56 others. It is always associated with ore
and nearly always with biotite.
Within the range of these generalisations, the gneisses show a continuous
variation in mineralogy. Formal subdivision can be made by micrometric
analysis, but it is difficult to recognise macroscopically the varieties so estab­
lished. Colour index is the chief parameter that can be used to separate the
rocks in the field.
Fig. 12 shows the proportions of dark minerals plotted against feldspar
content for a sample of olivine-free rocks. This presentation shows: the break
in continuity between feldspar-free rocks and those with more than 15 vol.
per cent feldspar; the relative scarcity of feldspar-bearing rocks with less
than 50 vol. per cent feldspar; the fairly steady diminution, on average, of
both types of pyroxene and biotite with increase in feldspar beyond 50 vol.
per cent; the uneven distribution of hornblende; and that, in any set of
classes by feldspar content, the variation of the biotite content of a class is
likely to be greater than that of the clinopyroxene or orthopyroxene content.
The fairly well-marked break in the trend of mineralogical variation at 50
vol. per cent feldspar seems to be significant. A rock containing 50 per cent
feldspar, 15 per cent clinopyroxene, 12 per cent orthopyroxene and 20 per
cent biotite (all of compositions corresponding to those determined from
these rocks) would contain about 50 wt. per cent Si0.2• There is thus a
smooth mineralogical variation, matched by the chemical variation described
below, that extends from an acid composition to that of basalt and may re­
present a liquid line of descent. The scatter of more basic mineralogical
compositions in Fig. 12 would represent accumulative rocks.
TEXTURES
Texturally the rocks are made up of either folia or spindle-shaped aggregates
of anhedral pyroxene grains, passing by attenuation into trains of granules,
separated by layers or lanes of anhedral feldspar. The foliation may be well
marked, but often it is rather crude, and a linear structure defined by trains
of dark minerals is dominant. There is rarely any fissility along the folia
(except along late-formed movement planes with retrogressive minerals), but
the outcrops as a whole weather into ledges controlled by mineralogical
layering.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
30
Zircon
present
Biotite
(93/130}
z
z z
z z z zz
o
o
10 �
lO
o
o
å
t
50
4Q
Hornblende
o
o o
o
20 ;
QO
10 �
fo
b
50 �o
�
o
Clinopyroxene
30
�
o
o
Orthopyroxene
o
o
o
20
o
o
oo 8 c:Po
q, o�o'b o
0
Oo � o o
cfl
0 o
o
00�
O
0Cb O �c{?.�
� o Cl;fb
ciJ
o oo o
o
o
O
o
o
o
o
o
o oo
n
o
o
o
o 00
oo
o oo oocfP
o
c€
o
o ooo �oo o .
o
o
0
o
o�Oo8
oo o
0
o
g;,'?l o oao oOo o
�o
o
o
20{)
1
o Do
191/1301
o
o o
o
o
o
o
z
z z zz
zzz z z
zz zzz z z zzz z
z z z
zzz zzz z z
o
o
o
o
oi
10
c:P
(1231130)
�
o
ol
40
o
00
o o
oO
o
o
g
40 {P
o
30
o
20 �
o
n
o
o
o
o
(47/130)
o
o
30 ;
o
10
o
o o
o oo o
o
20
z
z z
zzz
23
oo
�
m-l:
h:Jo
00
o
o o
o
o
40
Vol.
n
percent
o
o
(B O
o O
O
QO
0
o
� cfJ 3-g�n
· �l '-"'
-' 0
fJ
80
ex;> 0 c&:Pcf�.-�a
60
feldspor
Fig. 12. Mineral composition of olivine-free gneisses. Percentages (by vol.) of dark
minerals plotted against vol. per cent feldspar. Fractions (e.g. 91/130) denote number of
rocks, out of 130 measured, that contain the mineral in question.
Within the pyroxenic streaks, clinopyroxene and orthopyroxene may occur
as independent grains. Sometimes, however, the orthopyroxene occurs as
granules margining clusters of clinopyroxene crystals.
A remarkable texture of clinopyroxene aften observed may be called
festoon texture (Fig. 13). This consists of rings of tiny dactylitic clinopyro­
xene granules surrounding areas (like lagoons) of clear plagioclase feldspar,
while outside the rings a mosaic of plagioclase and orthoclase occurs, or
plagioclase and mesoperthite coexist. Often the dactylitic pyroxene forms
what might be called a barrier reef around islands of pyroxene, the lagoon
represented by plagioclase and the ocean by a mosaic of plagioclase and
24
M. H. BATTEY & W. D. McRITCHIE
orthoclase. The granules are so small that optical determination is difficult,
but sometimes they are seen in near-continuity with identifiable clinopyro­
xene, and Griffin (1971b) has analysed some by microprobe and determined
them as clinopyroxene.
Ore forms amoeboid blots always closely associated with pyroxene clusters,
generally around their borders.
Biotite is, in turn, often moulded upon ore, though it also occurs inde­
pendently.
Garnet, when present, forms narrow pellicles around ore grains in dose
association with biotite.
The feldspar grains are invariably anhedral and equant. When plagioclase
and mesoperthite or plagioclase and orthoclase are present together, the
mesoperthite or orthoclase sometimes seems to shun the pyroxene spindles or
folia, being separated from them by zones of plagioclase alone.
l
mm.
Fig. 13. Festoon texture in pyroxene-feldspar gneiss. Clinopyroxene (stippled), ore
(black), plagioclase (blank) and mesoperthite (wavy lines). Section JS 23, 1100 m on
7° from Skauthøe, and 50 cm from a l m thick pyroxenite sheet.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
25
In the plagioclase-orthoclase mosaic, viewed microscopically with re­
stricted illumination, the orthoclase appears as lanes and angular cuspate
embayments amongst plagioclase crystals, like leads of open water in dispers­
ing pack ice. The larger porphyroblasts of mesoperthite are commonly found
in the centres of the feldspathic lanes or folia between the pyroxene trains.
They are anhedral and generally oval in shape.
Apatite and zircon, when present, are found in the feldspathic !anes and
folia.
DETAILED MINERALOGY OF THE PYROXENE-FELDSPAR
GNEISSES
Pyroxenes
Tables 5, 7 and 8 present data on the composition of pyroxenes from ultra­
mafic and feldspathic rocks of the Jotun-kindred. Both ortho- and clino­
pyroxenes are commonly clouded by minute particles of opaque exsolved
minerals. The bulk analyses thus represent an original pyroxene composition
before this exsolution took place.
In Fig. 14 coexisting orthopyroxene-clinopyroxene pairs from the whole
rock series are plotted together. The small difference, shown in this figure,
between wet-chemical and microprobe values of CaO has already been men­
tioned.
The trend of iron-enrichment shown by the pyroxenes correlates with in­
creasing feldspar content, increasing Si� and increasing FeOjFeO + MgO
in the whole rock composition. Though it covers only a short range of iron­
enrichment, this trend is consistent with differentiation by fractional crystal­
lisation.
On the basis of the CaO contents determined by rnicroprobe (on the right­
hand section of Fig. 14) a wider solvus is indicated than that given by bulk
compositions of pyroxene pairs in plutonic igneous rocks (e.g. Wager &
Brown 1968: fig. 214, p. 391). This suggests that the Jotunheim pyroxenes
equilibrated at low temperatures during metamorphism, to compositions
comparable with those resulting from exsolution in plutonic pyroxenes de­
scribed by Boyd & Brown (1969: 86). The implied temperatures are a little
lower than those of Boyd & Brown's examples from the Bushveld Complex.
The AW3 contents of both pyroxenes are high and the high content of
CaAklSi06 and low NaAlS�06 place them clearly in the granulite facies
(White 1964). The Ab03 content of the orthopyroxene appears to increase
with increasing feldspar content of the rock, with the exception of specimen
62/43. This tendency is not shown, however, in the clinopyroxene.
In the special texture, described above, in which festoons of fine-granular
pyroxene encircle folia of coarser pyroxene, Griffin (1971b) has shown that
the festoon pyroxenes have lower AW3 than the islands of pyroxene within.
He attributes this to a partial recrystallisation of the rock after a drop in
pressure from the conditions of deep seated metamorphism.The symplectitic
26
M. H. BAITEY & W. D. McRITCHIE
Co· existing pyroxenes. Jotunheimen
50
/l
40
/lo
cJ
Mg
2 3 45
lO
Atoms%
20
Fe+Mn-
Mg
14
10
Atoms%
'J
{'\'\
40
50
Fe-
Fig. 14. Compositions of co-existing pyroxenes, Jotunheimen. Key to rock specimens:
1, 63/M6b (Table 5 no. 2 & 7 no. 1) - 2, 62/43 (5 no. 3 & 7 no. 2) - 3, 62/40 (5 no.
6 & 7 no. 3) - 4, 62/53 (5 no. 8 & 7 no. 4) - 5, 62/36 (5 no. 9 & 7 no. 5) - 6, J34* (8
no. 10) - 7, J33* (8 no. 8 & 5 no. 5) - 8, J20 (8 no. 7) - 9, 62/53 (8 no. 9) - 10,
58/24 (8 no. 4) - 11, 58/38 (8 no. 6) - 12, 58/23 (8 no. 3) - 13, 58/26 (8 no. 5)
- 14, 58/21 (8 no. 2) - 15, 58/20 (8 no. 1).
* co-existing olivine measured by X-ray.
character of the fine-granular pyroxene suggests to him rapid reaction follow­
ed by chilling in a shallower environment. It may be observed, however, that
the plagioclase within the festoons, also presumably a product of the reac­
tion, is not conspicuously fine-grained. While it seems to the present writers
very doubtful whether the proportions of plagioclase and symplectitic pyro­
xene in the festoons are appropriate to a simple breakdown reaction of the
earlier pyroxene, the lower AbOs of the symplectitic pyroxene must be ac­
cepted as indicating a lower pressure regime during its crystallisation.
The partition coefficient for magnesium and ferrous iron between coexisting
pyroxenes has been used by Kretz (1961, 1963) and Bartholome (1961) as
an index of PT conditions of crystallisation and to discriminate between
igneous and metamorphic pyroxenes. Kretz (1963) finds values of his distri­
bution function Kn of 0.86 to 0.65 for igneous rocks and 0.65 to 0.51 in
metamorphics. Fig. 15 shows the values for coexisting pyroxenes from
Jotunheimen which, on this basis, have equilibrated under metamorphic
conditions. It has been noted above that the pyroxenes show clouding which
is attributed to exsolution of Fe, Ti and Mn. When the function �Kn is cal­
culated, using (Fe2+ + Fe3+ + Mn) in place of Fe2+, its value is greater in
most cases than Kn and lies close to the curve of Kn
0.73 in Fig. 19. It
may be concluded that original crystallisation probably took place under
igneous conditions, with subsequent re-equilibration under a metamorphic
regime.
=
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
27
The evidence of the pyroxenes, therefore, suggests a primary segregation
by fractional crystallisation. If the rocks were differentiated by other means
(e.g. if they were originally sedimentary), it is difficult to see why the ratio
FejFe + Mg should increase with feldspar and Si0-2 content. This differ­
entiation was followed by recrystallisation under stress, to produce the meta­
morphic textures. Equilibration during this process led to a more complete
segregation of Ca into clinopyroxene than has occurred in the bulk clino­
pyroxene fraction of annealed, igneous-textured plutonic bodies. The di­
stribution of magnesium and ferrous iron between coexisting pyroxenes, and
their high AW3 content will have been established (or preserved) in the
phase of high pressure recrystallisation. Symplectitic, low Al20a, festoon
pyroxene is then believed by Griffin to have formed during partial recrystal­
lisation at lower pressure.
1·0
0·9
Q)
c:
Q)
)(
o
...
>..
Q.
o
.J::.
...
o
i""
0·8
Q)
I.L.
+
Cl
�
Cl
�
0·7
0·6
Mg/(Mg + Fe2+) clinopyroxene
Fig. 15. Distribution of Mg and Fe between co-existing pyroxenes (after Kretz 1963).
M. H. BATTEY & W. D. McRITCHIE
28
Feldspars
Optical study of the gneisses shows that, broadly speaking, the anorthite
content of the plagioclase decreases with increase in feldspar content, and
that rocks with potash feldspar in addition to plagioclase have more sodic
plagioclase than those from which potash feldspar is absent.
Detailed optical study of the plagioclase seemed likely to be inconclusive,
however, because shadowy extinction is common, whether due to strain or
compositional change. The former is often to be suspected, but there is also
a suggestion that plagioclase is more calcic near pyroxene clusters than in the
feldspathic lanes and folia. In rocks with corona structures around the pyro­
xene clusters it seems clear that equilibrium has not been attained. The
variation in plagioclase composition would in these circumstances, probably
be progressive and would be best studied by microprobe as part of a study of
the corona reaction.
The frequent occurrence of mesoperthite or antiperthite and the intimate
mosaic of plagioclase and potash feldspar prevents separation of represen­
tative feldspar for bulk chemical analysis.
From the point of view of petrogenesis, the structural state of the average
feldspars of the rocks should indicate temperature limits within which other
reactions have taken place, and information on this was sought be X-ray
powder methods. The feldspars were separated from a sample of
8 gneisses,
and the bulk feldspar separated further into a light and a heavy fraction,
representing potash feldspar and plagioclase respectively. The parameters
d201 and axial obliquity from the 130 and 131 reflexions (MacKenzie
1954) were determined for the potash feldspar and the functions r and B
(Smith & Gay 1958) for the plagioclase, by X-ray diffractometer. The results
are given in Table 10.
Table 10.
K feldspar
Rock
58/20
58/21
58/23
58/24
28 201
Obliquity
Cu Ka. from 130 from 131
20.97
21.02
20.99
20.99
Nil
Nil
Nil
Nil
58/26
None present
58/38
64/2
62/120
None present
20.97
Nil
20.97
?**
*
Nil
Nil
Nil
Nil
Nil
Nil
Plagioclase
2Vx
r
B
45 ( 7)
44!(16)
42 (12)
0.46
0.50
0.65
0.54
0.55
0.54
0.54
0.10
0.36
0.88
0.91
0.89
0.86
0.90
0.86*
0.90
0.96
0.91
{
% An
(ext. angles)
Ana6
Ana5-37
An50-58
An43-45
An4s
An50
Value obtained before biotite and doubtless some calcic feldspar were sunk out of
the sample in heavy liquid.
** A slight shoulder on the high angle side of the peak.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
29
The potash feldspar proves to be free of any sodium component with
d101
=
0.423
nm (Bowen & Tuttle
with no splitting of
130
or
131
1950)
and is monoclinic in symmetry
reflexions. Measurement of
2V
gives varying
results partly because of the low birefringence and partly because of undu­
lose extinction. The range of values (Table
10)
indicates compositions of
Orso to Or10o with a mean of Or9i�· The values agree with those reported by
Heier
( 196 1: 133
and fig.
3a)
for granulite facies rocks. The absence of any
obliquity in feldspar of this composition indicates crystallisation at a tem­
perature above 500°C (MacKenzie & Smith
1961).
For the plagioclase, an approximate idea of the An content can be ob­
tained from extinction angles (Table
10) .
The measured values of r and B
may then be plotted on the diagrams of Smith & Gay
where they
(1958)
fall in the region occupied by plutonic and metamorphic plagioclases. The
structural state indicated is one of relatively high cation ordering, though
somewhat less than complete order. Perhaps no doser approach to the fully
ordered state can be expected above the orthoclase-microcline inversion.
THE PROBLEM OF MESOPERTHITIC INTERGROWTHS
The distinguishing features of mesoperthite are that the plagioclase com­
ponent is not albitic, as in ordinary perthite, but is an intermediate or acid­
intermediate plagioclase; and that the plagioclase and orthoclase components
of the intergrowth are roughly equal in volume.
The coexistence of plagioclase of intermediate anorthite content with
potassium feldspar places a strict lower limit on the temperature of meta­
morphism, if the two feldspars are in equilibrium (Morse
(1967)
1968).
De Waard
uses the presence of mesoperthite as an indicator of the minimum
temperature of granulite facies metamorphism, on the assumption that the
mesoperthite represents exsolution of originally homogeneous feldspar.
Table 11 gives the modal and normative feldspar compositions of some
gneisses in which modal feldspar is over 80 vol. per cent. Allowance has
been made for the presence of biotite in calculating the normative feldspar.
The results are plotted in Fig.
16
along with the normative feldspar for other
rocks in which the rather small biotite correction has been neglected. From
Fig.
16
it may be seen that the total feldspar is comparable with, though
more calcic than, the Kiglapait mesoperthite studied by Morse
(1968),
and
would yield a mean solvus, projected from anorthite, similar to the Kiglapait
example. This does not, however, constitute evidence that the Jotunheim
feldspar was originally a single homogeneous phase, and we have to turn to
textural evidence (as indeed Morse does in the Kiglapait example) to try to
decide this question.
Though the textural relations are complicated and variable, the following
statements apply to many examples.
In the more mafic rocks, plagioclase is the sole feldspar, or is over­
whelmingly predominant. In some examples the crystals may contain thin
2.77
2.76
2.73
2.72
2.70
2.79
2.72
62/56
62162
62/27
62130
62/35
62/41
62/49
69.9
48.8
56.7
45.2
91.0
50.3
71.8
Or.+
perth.
Analyst: W. D. McRitchie.
S.G.
Rock
17.8
35.7
27.9
45.6
0.5
34.3
18.9
87.7
84.5
84.6
90.8
91.5
84.6
90.7
Modes (vol. %)
Plag.
Tot.fsp.
8.1
3.4
2.2
0.5
2.1
1.5
0.9
Cpx.
-
6.4
11.6
3.4
O.l
4.3
-
Bio.
Table 11. Feldspar compositions from whole rock chemical analyses.
81.4
79.5
80.1
86.8
86.4
71.5
85.7
Mod.fsp.
(wt. %)
24.4
24.5
26.1
19.9
27.2
15.7
21.1
Or
35.6
35.6
38.8
45.6
44.5
39.5
44.0
Ab
17.5
20.6
20.3
17.8
13.3
22.2
20.3
An
77.5
80.7
85.2
83.3
85.0
77.4
85.4
�
Norm. feldspar (wt.
%)
33
37
34
28
23
36
32
An %
Plag
31.5
30.4
30.6
23.9
32.0
20.3
24.7
45.9
44.1
45.5
54.7
52.4
51.0
5 1.5
22.6
25.5
23.8
21.4
15.6
28.7
23.8
Norm. fsp. to 100
An
Ab
Or
æ
g
�
�
�
n
�
....::
fi:o
�
l:l:l
;z:
�
w
o
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
31
An
l
•
l
•
<50 wt.0/0 norm. feidspor
o
50-80 ..
6.
>80
.Å
>80vol (o-75 wt) '}o feldspar
corr'd for biotite
•
l
.o
l
.
o
o o
•
L Op
C?o
r: 0:
C,.OJ
�P o
pi:;)Op
l
0
0
p
oP
0oP
,A; P
p �f:€P
.1Ji. P
.Åp p
OP
..t.P
,AP
P_�
��p
p
'62/35 p
LI
.Å
Fig. 16. Normative feldspar components of Jotun rocks. p indicates conspicuously
perthitic feldspar. The boundary of the equilibrium feldspar field and the line of the
low-melting trough (after Kleeman 1965) are also shown.
parallel antiperthitic lamellae, indicating unusually high solubility of potas­
sium in intermediate plagioclase: A large dass of more feldspathic gneisses
has both orthoclase and plagioclase. In these, the common relationship is an
even-grained mosaic of the two feldspars in independent crystals (Fig.
17).
In
the mosaic some plagioclase crystals may have sparse orthoclase lamellae,
while the orthoclase may sometimes contain small blocks or shreds of plagio­
clase. The plagioclase in orthoclase has the appearance of relics left after a
process of replacement by orthoclase.
Large, shapeless, indefinitely-bounded crystals of mesoperthite Iie in the
centres of the feldspathic lanes or layers of feldspar mosaic (Fig.
18).
The
two components of the mesoperthite may be intergrown in several ways.
Some crystals are divided into wavy, but broadly parallel lamellae of the two
feldspars (Fig.
19);
others have two sets of orthoclase lamellae approximately
at right angles, following
(010)
and
(001)
(Fig.
20);
and others again have a
more irregular intergrowth in which the components form lobate masses,
rounded masses or curving streaks, rather than regular lamellae (Figs.
27).
21, 22,
These different textures may coexist in the same rock. The boundaries
32
Fig.
M. H. BATI'EY & W. D. McRITCHIE
17.
Jotunite,
58/40,
Breagrovein,
Spiterstulen.
Plagioclase-orthoclase
mosaic
between pyroxene grains (dark). Plane polarised light with restricted illumination.
Scale bar
Fig.
=
l mm.
18. Jotunite, 58/40, NE ridge of GaldhØpiggen. Mesoperthite intergrowths in
centres of feldspar areas merge into plagioclase-orthoclase mosaic. Pyroxene is sur­
rounded by plagioclase only. Plane polarised light. Scale bar
=
l mm.
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
33
Fig. 19. Jotunite, 58/30, Breagrovein, Spiterstulen. Mesoperthite crystal with parallel
lamellae of orthoclase in plagioclase. Crossed polars. Scale bar
0.5 mm.
=
Fig. 20. Jotunite, 58/49, ridge crest 3.5 km E of GaldhØpiggen. Mesoperthite crystal
with orthoclase lamellae intersecting at right angles. Section approximately normal
to u of host plagioclase (about An35). Crossed polars. Scale bar
0.5 mm.
=
34
M. H. BATTEY & W. D. McRITCHIE
Fig. 21. Jotunite, 58/71, Skautflye, NE of Leirtjern. Mesoperthite with irregular blebs
of orthoclase, as well as regular lamellae. Section normal to 010 of host. Crossed
polars. Scale bar
=
0. 5 mm.
of the intergrown crystals are vague, the orthoclase component passing out
into the surrounding mosaic (Fig.
23).
Moreover, in the same rocks there may be zones of plagioclase against the
pyroxene clusters that are free of mosaic or intergrown orthoclase (Fig. 18).
Such orthoclase-free plagioclase may also Iie inside the festoons of pyroxene
granules already described: The rocks called mesoperthosites by McRitchie
(1965)
have mesoperthite porphyroblasts from
3
mm to
30
mm in length.
They develop in streams, parallel to the main foliation of the rocks, the units
·
varying from a few centimetres to metres in thickness. The coarser meso­
perthosites are lilac-pink in colour and the fractured crystals show schiller
lustre. Some mesoperthosite layers, that carry quartz as well as mesoperthite,
occur near the contacts of ultramafic layers with feldspathic gneisses. Other
mesoperthosites form discordant lenses and some show intrusive relations,
extending amongst broken-up fragments of pyroxenic gneisses to produce
agmatite. Examples of this are seen on the south end of Langeskavlen. Cata­
clastic margins are characteristic of the large mesoperthite crystals of the
mesoperthosites.
From these features the following deductions may be made:
The general porphyroblastic habit of the mesoperthites seems to imply
the growth of large crystals of a temary feldspar followed by unrnixing; for
it is unlikely that large plagioclases grew in the centres of feldspathic lanes
and were then preferentially replaced by orthoclase in lamellar fashion while
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
35
the finer-grained material formed a mosaic of independent crystals; the
presence of crystals with two sets of lamellae at right angles, and irregular
lobate intergrowths suggests the cooperation of a process of replacement and
migration of potash feldspar in producing the present textures. The same
conclusion is suggested by the plagioclase relict blocks in orthoclase. The
discordant mesoperthosite lenses and veins indicate a degree of partial melt­
ing of the feldspathic component of the gneiss after the main layering had
formed, but under depth and temperature conditions that would yield a
temary feldspar. The plagioclase-orthoclase mosaic presumably represents
the products of complete unmixing and recrystallisation of the original
temary feldspar of which the large mesoperthite crystals are relics.
In any attempt to estimate temperature of crystallisation of these temary
feldspars the anorthite content is critical, because the solvus crest rises rapidly
with increasing anorthite. The use of normative bulk feldspar composition
of rocks will lead to erroneous results because of the presence within the
folia of regions with plagioclase alone, which may have formed independently
of the original temary feldspar. Of the samples plotted in Fig. 16, 62f35, a
mesoperthosite, is likely to be most nearly representative of the temary feid­
spar, and it is one of the least calcic examples. Future work should concen­
trate on analysing regularly-lamellated mesoperthite crystals only, perhaps
with a defocused electron microprobe.
The probability is, however, that the mesoperthite crystallised at a tem­
perature of the order of 900°C and the environment must have been exceed­
ingly deficient in water to permit solidification at this temperature.
Fig. 22. Jotunite, 58/40, NE ridge of GaldhØpiggen. Irregular orthoclase lamellae in
outer zone of a plagioclase crystal. Crossed polars. Scale bar
0.5 mm.
=
36
M. H. BATIEY & W. D. McRITCHlE
Fig. 23. The same crystal as in Fig. 20, showing merging of lamellae with surrounding
plagioclase-orthoclase mosaic (cf. also Fig. 18). Plane polarised light. Scale bar
=
0.5 mm.
BULK COMPOSITIONS OF THE ROCKS
The rocks of central Jotunheim are highly differentiated; yet, as the fore­
going account shows, they possess a close mineralogical and textural affinity.
Available for study we have
60
new analyses of rocks from the relatively
restricted area defined above (A selection of analyses typical of the various
petrographic types is presented in Table 12. The full analytical data is avail­
able on request from M. H. Battey). Dietrichson (1958) assembled 18 anal­
yses from a much wider region of rocks assigned to the Jotun kindred, but
this collection lacked the close spatial, textural and probable genetic homo­
geneity of the set presented here.
MAJOR ELEMENT VARIATION
The variation diagram (Fig.
24),
shows a scattered distribution of the major
eletnent oxides in rocks with less than about
53
wt. per cent Si02, attribut­
able to large concentrations of pyroxene, olivine and sometimes hornblende.
There is a small overlap in Si02 content between this group and the series
beginning at about 50 wt. per cent Si02 and extending to
64
per cent Si02
in which CaO, MgO and total iron (as Fe20a) decline, while Na20 and K20
increase, in the manner typical of igneous rock series. A third group with
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
37
Table 12. Chemical analyses of typical Jotunheim rocks.
133
137
199
58126
62120
58/20
62/18
62135
58/50A
62121
38.70
44.0
51.1
48.00
52.64
56.35
58.80
60.4
63.42
68. 8
Al20a
3.88
2.1
4. 3
14.83
18.73
17.02
19.01
19.0
17.28
15.2
Fe20a
1.88
1.40
2.19
6.49
5.46
3.20
2.85
1.9
2.39
0.57
6.55
6.13
4.17
4.77
2.78
2.3
2.06
0.44
Si02
FeO
19.1
12.60
MgO
31.38
29.0
14.57
6.94
2.77
3.11
1.52
2.2
1.28
0.30
CaO
2.48
8.12
18.34
11.44
9.16
6.54
4.38
3.0
3.73
1.30
Na20
0.44
0.24
1.03
2.90
4.90
4.19
4.51
5.25
4.61
3.95
K20
0.26
0.03
0.10
0.80
0.92
3.28
5.69
4.63
4.32
9.05
H20+
0.58
1.83
0.38
0.25
0.24
0.59
0.15
0.13
0.22
0.11
Ti02
0.47
0.27
0.55
0.94
0.78
0.60
0.75
0.50
0.40
0.10
MnO
0.32
0.27
0.21
0.18
0.18
0.15
0.10
0.12
0.12
0.02
P205
0.10
Sum
99.59
0.30
99.8
99.3
0.6
98.90
99.95
4.7
5.4
100.54
99.7
19.4
33.6
27.4
99.80
99.8
99.83
CIPW Norms
2.3
1.5
Q
1.5
0.2
ab
2.7
2.0
8.7
24.5
41.4
35.4
38.1
44.4
39.0
an
7.8
4.6
6.8
25.1
26.4
18.0
14.8
13.1
13.7
ne
0.5
di
hy
C 0.5
0.9
di
2. 9
3.4
1.9
1.3 wo 1.2
0.8
en
1.1
10.5
25.2
9.5
5.5
fs
0.4
3.2
6.7
2.6
1.8
en
6.3
4.1
2.1
1.4
4.3
1.6
5.5
1.9
fs
1.9
1.1
0.6
0.4
3.0
0.7
2.1
0.8
6.0
0.6
fo
54.0
38.8
4.9
4.0
fa
25.4
13.1
1.4
1.2
mt
2.7
2.0
3. 2
9.4
7.9
4.6
4.1
2.8
3.5
il
0.9
0.5
1.0
1. 8
1.5
1.1
1.4
1.0
0.8
ol
1.6
ns
2.4
35.1
7.9
27.8
ac
2.0
14.9
13.2
54.5
2. 9
1.6
wo
13.1
10.6
25. 5
or
0.2
O.l
0. 2
0.7
ap
133 Hornblende-harzburgite, E. side of Visdalen, N.N.E. of Spiterstulen.
137 Wehrlite, slightly serpentinised, same locality.
199 Pyroxenite (fine-grained), at ultramafic margin, same locality.
58126 Pyroxene-gneiss, Presten ridge, Leirvassbu.
62/20 Jotunite, head of tarn in Øvre MjØlkedalen, S.E. of Storegut.
58/20 Jotunite, E. flank MjØlkedalstind, 400 m E. of ultramafic body.
62/18 Mangerite, head of tarn in Øvre Mjølkedalen, S.E. of Storegut.
62135 Mesoperthosite, W. face of HØgbrothØgdi.
58/50A Pyroxene-granite, E. end of ridge from GaldhØpiggen to Spiterstulen.
62121 Pyroxene-granite, W. side of Øvre MjØlkedalen corrie.
Analysts: M. H. Battey, W. D. McRitchie, P. J. Oakley.
over
67
wt. per cent Si02 shows a scattering of Na20, K20 and Al20a values
due to variation in the content of plagioclase and potash feldspar and the
entry of quartz. This third group represents quartz-feldspar layers of limited
extent. The break between it and the preceding group may be exaggerated by
incomplete sampling, but field observation suggests that quartzose members
of the series are relatively rare.
M. H. BATTEY & W. D. McRITCHIE
38
l)
20
•
•
o
•
o
•o o
o•
•
•
e
o
0
8 o o oO 0
cPO
AI203
•
•
•
• •
•
10
..
V
" V
�"
V
20
V
""
6
,.
"'"
"
..
•
Fe2 03
"
Uotall
..
•
""
10
• "l
•
•
l\)"
o •'O
•
Illl"!�
o
o
o
o
g o oooo 0
o
V
l-"
"
V
JO
"
•
••
t.
V
"
..
•
"
.
o
20
..
• •
"
..
•
Wehrlite
Harzburgite
Pyroxenite
Pyroxene -gneiss
)alun ile
o
Man gerite
•
Pyroxene- granite
0
..
•
KEY
Mesoperthosite
Mg O
f-10
•
•
•
•
e • "� '21
g
20
8
(}rP cf>Ore 0
•
...
Ca O
..
"
..
..
10
.
"
"
..
"
l
•
El
"
t.V
• ••
!fo
• 00 oo
-
�o q,oo
o
00
V
5
•
t.Vv
..
""
!JB \!i
o• o
••
• •
o
•
• •
.. .
..
"
"
o
o
CbD:>
5
0
8
0
o
� o
.
.�
.
.
-t.
.
..J.
..
--- t. Y,L--t..!... ..
·
.
50
'CP
percent
•
•
Na20
o
O
o
0
•
�
•
K20
o
o
-
-
..
Wt
•
•
•
•
o&
•
��"
40
0
ooo0o
•
Si02
60
70
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
39
Table 13.
Si� Al20:!
I
51.5
Il
52.5- 17.553.25 18.25
I
6.12 31.96 28.91
or
18.0
ab
an
Fe<.!Oa FeO
MgO
CaO Na20 K20
3.5
6.0
5.0
10.0
3.{}3.5
6.{}6.6
4.{}5.0
9.{}- 2.4510.0 3.15
hy(en
di(wo
en
fs)
8.70
5.00
3.30
1.0
3.8
0.35
0.75
fs)
ol(fo
fa)
mt
2.51 1.00
3.92
2.65
5.10
FRACTIONAL CRYSTALLISATION HYPOTHESIS
The most reasonable interpretation of this series of compositions is that they
represent a sequence of liquids developed by fractional crystallisation from a
parent liquid with about 51 per cent Si02, associated with a group of ac­
cumulative rocks (pyroxenites and peridotites) produced by segregation of
early-formed crystals. The composition of the parent magma, on this inter­
pretation, would be roughly that given as I in Table 13.
Such a composition would precipitate olivine and could give rise to the
sequence of rocks observed. The great predominance of intermediate rocks
in the observed sequence would be in accord with the operation of such a
process.
The parent material suggested does not correspond closely to any of the
proposed basaltic partial melts of mantle origin. On the other hand, its oxide
percentages conform with the maxima of Chayes's (1964) distribution curves
for Cenozoic 'basalts' (volcanics with Si02 less than 54 wt. per cent) from
the world's circum-oceanic regions, Il in Table 13). The only differences
are the lower Si� and somewhat higher alkalis. The suggested parent com­
position is thus comparable with a widespread eruptible mixture found on
the continental side of the great circum-oceanic trenches of the Cenozoic.
These lines of evidence lead to the view that the layered metamorphic
series of central Jotunheim is of originally igneous origin and the range of
composition within it can be ascribed to differentiation by fractional crystal­
lisation. A special feature of the differentiation is the high soda content of
rocks with more than 50 wt. per cent of Si� and the very steep rise in pot­
ash between 50 and 60 per cent Si02, reflecting the increase in alkali feid­
spar especially orthoclase. The pervasive distribution of the feldspar, and the
fact that it is partly in the form of mesoperthite makes its introduction by a
process of partial melting difficult to accept, though it may have been to
some extent redistributed by this means.
Apparently a high temperature and pressure of original equilibration al­
lows the intermediate rocks to store K� as a temary feldspar. Important
Fig. 24. Major oxide percentages of chemically analysed rocks plotted against silica.
40
M. H. BATTEY & W. D. McRITCHIE
local concentrations of K can thus exist in lower crustal rocks, like those
of Jotunheimen, to be released for upward migration during later retrograde
metamorphism or igneous action. This could represent a staging point in the
progressive upward concentration of K in the continental crust disclosed by
the work of Lambert & Heier (1968) and others. The role of K in orogenic
igneous events is gradually being elucidated (Moore 1959, Jakes & White
1970, Ewart & Bryan 1973) and the existence of K stores emplaced in the
deep structure of ancient fold mountain beits may help to explain some
features of its distribution.
If the hypothesis of crystallisation differentiation is accepted to explain
the main compositional features of the massif, the implication is clear that
it was originally a plutonic body, since the extreme ultrabasic products and
those formed from residual liquids are still associated. Oftedahl (1961),
accepting Hamilton's (1960) revival of Daly's old concept of an areal erup­
tion, has suggested that the massif is the result of an areal eruption taking
place on the sea floor. Criteria are lacking to identify such a body with cer­
tainty, though a distant connexion might be traced between this hypothesis
and the imagined events at constructive plate boundaries. Such a model does
not seem to be applicable to the Jotunheim, however, and the idea of plutonic
emplacement is preferred.
The layered arrangement of the rock types, and the persistent zone of
ultrabasic layers from Rusteggi to Gravdalen (Battey & McRitchie 1973)
suggests an original stratiform plutonic complex. However, the smaller bodies
of peridotite and pyroxenite scattered elsewhere throughout the area testify
to drastic disruption of any system of gravitational layering that may once
have existed. Intense small-scale folding of pyroxene-rich laminae also in­
dicates that any original layering has been much modified.
The distribution of N a� and �20 at the acid end of the series is irregular
and indicates segregation of the two alkalis, due probably to the meta­
morphism. Such segregation does not seriously conflict with the hypothesis
of crystallisation differentiation for the series as a whole.
DISCUSSION
It may be argued that any series of analyses of rocks varying in the propor­
tion of light to dark minerals will have the basic oxides decreasing as in
Fig. 24, and hence an original sedimentary differentiation is not excluded.
However, the increase in alkalis, without any depletion in Na20, and the
rarity of quartzose representatives contradict the sedimentary hypothesis.
Moreover, as shown in discussion of the pyroxenes, the increase of Fe{Fe +
Mg with decreasing colour index is a typical igneous feature.
One peculiar rock (62{29, Table 12) must, however, be mentioned here,
that might be suspected of being sedimentary. This forms a pale-coloured
rusty-weathering layer on the south-western slope of Høgbrothøgdi and,
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
41
because of its distinctive colour it is easily visible from the Eidsbugarden­
Olavsbu track. It consists of a mosaic of millimetre-sized perthite grains with
marginal cataclasis, some plagioclase (An10), sutured quartz, pale biotite in
centimetre-long parallel streaks and granular sillimanite. Rounded zircons
are common, arranged in lines reminiscent of heavy mineral layers in a
sediment and associated with leucoxene grains, pyrite is concentrated along
the biotite-sillimanite streaks and tiny needles of probable rutile are present.
Small amounts of iron sulphide are associated with this layer, which is
slightly transgressive to the layering in the rocks above and below. The exact
relations of this limited occurrence to the other layers are not clear and
would repay further study. It is near the Tyin-Gjende fault-zone and may be
some kind of tectonic inclusion or raft. Its occurrence does not seem to war­
rant wholesale reinterpretation of the other members of the Jotunheim series.
The conclusion from the study of the bulk chemical compositions is that
the series was originally differentiated by processes in a melt and has, since
this prim·ary differentiation, undergone extensive disruption coupled with
some remobilisation and segregation of the feldspathic portion.
This subsequent metamorphic history has been complex. There has un­
doubtedly been thorough-going recrystallisation under shearing stress pro­
ducing strong foliation or lineation. We have clear evidence of the following
reactions taking t=Cace during metamorphism:
Reaction between olivine and plagioclase to yield orthopyroxene, clinopyro­
xene and spinel; exsolution of iron from the pyroxenes, especially clino­
pyroxene; reaction producing hercynite from magnetite; crystallisation of
biotite; exsolution of ternary feldspar to produce mesoperthite and plagio­
clase-orthoclase mosaic; reaction between ore and plagioclase to produce
garnet; late amphibolisation of pyroxene, crystallisation of garnet and zoisite,
and the segregation of pegmatite veins.
More uncertain, but strongly suggested in many thin sections, is a reaction
leading to the corrosion and break-up of larger pyroxene crystals into small
granules in a sea of feldspar, as an extension of the process shown in Fig. 13.
Griffin (1971b) puts this down to the reaction
aluminous clinopyroxene -+ plagioclase + Al-poor clinopyroxene
a reaction which requires the addition of silica. This may be part of the
story. However, examination of a large number of thin sections suggests that
pyroxene is being greatly diminished in some rocks, and that alkalis and
alumina, as well as silica, may well have been added, as part of a process of
metamorphic segregation. The existence of transgressive mesoperthosite
bodies shows that late partial melting of feldspar components has occurred,
at least locally, in the metamorphic history. Migration of feldspathic material
may also have taken place on a more limited scale to help produce the rocks
with festooned pyroxenes. The process envisaged is akin to the limited partial
fusion suggested by Tuttle & Bowen (1958: 122 ff.) in connection with the
advanced metamorphism of sediments. It would undoubtedly assist in the
42
M. H. BATIEY & W. D. McRITCHIE
deformation and extreme attenuation of the pyroxene folia and linear trains
shown in some of these rocks (see Pl. Ill, fig. 2 of Battey 1965). Such a
process would fit reasonably into the cooling scheme discussed below.
COMPARISON WITH SEILAND
The Seiland gabbro province in Finnmark, north Norway, offers many points
of comparison with Jotunheimen. In this region there is an extensive series of
layered gabbros with igneous textures associated with two-pyroxene granu­
lites (gabbro-gneisses) lying to the east in the direction of increasing meta­
morphic grade (Hooper 1971).
On the island of Stjernøy, in the Caledonides of Finnmark, Oosterom
(1963) has described two contiguous rock series. The first includes peridoti­
tes, olivine-melagabbros and layered gabbros which he compares with suc­
cessions from the Stillwater and Bushveld complexes (1963: 188-199, 271277) and regards as of igneous origin. This group differs from the Jotunheim
series in that it has a much more limited range of Si0.2 content (34 to 47 wt.
per cent) and in the coexistence of olivine and plagioclase in some of its
members. This last feature suggests that the rocks have been metamorphosed
under lower pressures than the Jotunheim set (Green & Hibberson 1970).
The second series described by Oosterom (p. 199) comprises dominant
gabbro-gneisses and amphibolites with hypersthene-plagioclase gneiss, sye­
nite-gneiss ( mesoperthosite ), various gamet granulites and some calc­
silicate layers. The rocks of this group have many resemblances to those of
Jotunheimen, but ultramafic members are absent on Stjernøy. The range of
Si02 content is 45 to 71 per cent. Though most of the mafic rocks could
well be of igneous origin, because of the presence of the calc-silicate layers
Oosterom (p. 202) is reluctantly constrained to accept the view of Krauskopf
(1954) that the gabbro-gneiss group represents the metamorphic product of
an immense pile of volcanic ash, with interbedded calcareous sediments.
Oosterom emphasises the long and complex metamorphic history of the
Stjernøy metamorphic complex, including the gabbro-gneiss group, compared
with the group of ultramafics and layered gabbros.
It is interesting to note that, in a similar way, in Jotunheimen, there is a
gabbro group with relict igneous texture which is peripheral to the much
more highly metamorphosed rocks of the central Jotunheim described in this
paper (cf. Battey & McRitchie 1973).
The chemical and mineralogical features of the Stjernøy metamorphic
group and the Jotunheim rocks suggest that they represent tectonically
elevated parts of a deep-seated crustal layer that may extend widely below
the Scandinavian Caledonides. More basic than estimates of the average
continental shield composition (Holland & Lambert 1972), such rocks may
form an important element in the make-up of the deep crust in the fold
mountain belt.
=
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
43
Hooper (1971) dealing briefly with the Seiland two-pyroxene granulites
(gabbro-gneisses) south of Oosterom's area suggests that, in their formation,
meta-sediments and plutonic mafic rocks have been folded together in a
process of intrusion at depth during tectonic movement.
Of particular interest, within the Seiland province, is the concentrically
zoned, but also layered, ultrabasic intrusion of Reinfjord (Bennett 1971,
1974). This body, about 4 km by 6 km across, intrudes earlier layered gab­
bros and bears many resemblances to the zoned ultramafic complexes of
S.E. Alaska and the Ural Mountains (Taylor 1967).
Comparing the relations of the Reinfjord mass and its surrounding gab­
bros with the Jotunheim rocks, it is not difficult to conceive that, if the Rein­
fjord body and its envelope were intensely metamorphosed and folded,
something very like the Jotunheim complex could be produced. It seems
quite likely that these two areas represent the results of similar magmatic
events at different crustal levels and with different subsequent metamorphic
histories.
The hypothesis, presented above, of a simple gravity differentiation as the
early stage in Jotunheimen may well be over-simplified; but the heavy
metamorphic overprint hampers any attempt to greatly refine the original
spatial and temporal relationships.
In the Seiland province, where individual bodies can be separately mapped
and their time relations worked out, there is still an overall unity of mag­
matic process that makes it a distinctive plutonic province. If the suggested
analogies between that area and Jotunheimen are valid, the gabbroic inva­
sion into the roots of the Caledonian fold belt was indeed on an immense
regional scale. Emphasis is aften placed upon granitic invasion of fold­
mountain beits. Here, in the Norwegian Caledonides, we see that, at deeper
levels, there may also be a massive uprise of basic (and same would say
ultrabasic) magma, as well.
Summary and conclusions
The chemistry of the granulite facies rocks of Jotunheimen suggests that they
have formed by fractional crystallisation from a melt. A layered structure is
inferred to have developed during this process. The structural disposition of
the differentiated rocks indicates extensive internal disruption of the rock
units formed by the fractionation, but the fact that the extreme members are
still associated with one another indicates a plutonic site of differentiation
and the tectonic upthrusting of the differentiated body as a large scale unit.
The original fractionation took place under conditions where the plagio­
clase-olivine assemblage was stable, that is at pressures below 10 kb at
temperatures of 1100° to 1200°C. Subsequently, metamorphic reactions at
high pressure led to the elimination of this assemblage and to the widening
of the solvus between orthopyroxene and Ca-rich clinopyroxene. Original
44
M. H. BATTEY & W. D. McRITCHIE
temary feldspar that had crystallised at high pressure exsolved to give either
mesoperthite or, on further segregation, a mosaic of intermediate plagio­
clase of relatively ordered structure with monoclinic potash feldspar.
At a later stage, partial melting has permitted local transgressive move­
ment of feldspathic material and sometimes with agmatitic breakup of the
layered structure, and this partial melt also crystallised to yield a temary
feldspar that gave rise to mesoperthosite.
Fig. 25 illustrates a path through the probable stability fields of critical
mineral assemblages that could lead to the observed products. The scheme
involves high-pressure fractional crystallisation, followed either by isobaric
cooling through the olivine-plagioclase reaction, eliminating this assemblage,
or by a rise in pressure, regardless of temperature, to carry the system across
the boundary from olivine + plagioclase to spinel + clinopyroxene + pla­
gioclase. Herzberg (1972) insists that the boundary is parallel to the temper­
ature axis and that a rise in pressure is essential, but geological evidence (e.g.
Gardner & Robins 1974) does not support this and isobaric cooling is con­
sidered more probable. This is succeeded by a drop in pressure, at more or
less constant temperature, leading to local partial melting of feldspar-rich
fractions, at temperatures depending upon the water content, within the PT
region where mesoperthite would be precipitated on renewed cooling. This
accounts for the transgressive mesoperthites.
The 9 kb pressure of initial crystallisation shown in Fig. 25 is indicated
by the demonstration that the presence of Na and Fe in the system stabilises
olivine + plagioclase to higher pressures than forsterite + anorthite (Emslie
1970).
Apart from questions of the absolute P and T scales, the scheme outlined
is consistent with Griffin's suggestion of a relatively rapid uplift and fall in
pressure during the formation of the gneiss complex.
In the sequence of events suggested for the Jotunheim in our earlier paper
(Battey & McRitchie 1973), the uplift of the granulite facies rocks is regarded
as taking place relatively rapidly, during the Caledonian orogeny. The reason
for this view is that, had the granulite facies rocks been uplifted in the
Precambrian, buried by the Valdres Sparagmite and then participated in the
Caledonian movements, they would in all probability have undergone exten­
sive retrogressive metamorphism. It does not seem possible that granulite
facies rocks can survive such a long history in the near-surface zones of an
orogenic belt. Moreover, rocks of gabbroic composition peripheral to the
granulite facies rocks have been recrystallised in the amphibolite facies.
As the Valdres Sparagmite is now believed to be Eocambrian in age, the
hypothesis of late, rapid, tectonic uplift of the granulites during the Cale­
donian orogeny means that the mesoperthite grains of the Valdres Sparagmite
were not derived from the jotunites of the granulite facies group, or at least
not from the part of them now exposed in the axis of the fold-belt. It seems
preferable to abandon this long-held view of the source of the Valdres meso­
perthite grains rather than to claim a persistence of pyroxene-granulite facies
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
45
40
l
10
---
o\�p
--l
: 14
:1
1
l
�Il
(l
3!� l f
; l
l
l
kb
Gl
6
l
l
2
900
l
ll
l
l
l
E
JO
l
l
Gl
-
- �-�
l
l
:l
8
\
l
km
l
1
1/" l/::
l l+
l l
l l
20
"O
:;)
.!!
V
�
o
Q.
"'
::2
Gl
...
1100
10
1300° c
Fig. 25. Possible cooling path of Jotun gneisses. l. Limit of olivine + plagioclase
stability, after Emslie (1970). 2. Limit of forsterite + anorthite stability, after Herzberg
(1972). 3. 3'. Possible limits of homogeneous temary feldspar, after Morse (1968). K
is the experimental solvus crest for Kiglapait mesoperthite. 4. 4' etc. Schematic curves
of the solidus minimum for alkali feldspar solid solutions, dry and with increasing H20
content. The short-dashed line is a possible cooling path.
rocks at shallow depth throughout the lang run-up to the Caledonian oro­
geny.
Briefly, in order to preserve the central Jotunheimen rocks in their won­
derfully fresh condition, it seems necessary to keep them at depth as lang as
possible and to conceive of most of their extensive deformation as having
taken place at pressures greater than 4 kb - that is, at a depth of 12 km or
more. In an earlier stage, pressures must have exceeded 8 kb.
46
M. H. BATIEY & W. D. McRITCHIE
The work on which this paper is based has been supported by
grants from the Geology Department, University of Newcastle upon Tyne, the Royal
Society of London, Norges Almenvitenskapelige Forskningsråd and Norges Geologisk
Undersøkelse. McRitchie's work was carried out during tenure of a Research Student­
ship of the United Kingdom Department of Scientific & lndustrial Research. To all
these organisations the authors express their grateful thanks. Mr. P. Oakley has carried
out some of the chemical analyses. To Mr. Ulrik Lunn the authors extend their thanks
for his pleasant companionship in the field and for two excellent photographs used as
illustrations.
October 1973
Acknowledgments.
-
APPENDIX
Scheme of rock classification
Main Granulite Facies
Stem
Leucocratic
types
Clinopyroxene
dominant
Orthopyroxene
dominant
l
DUNITE (>90% olivine)
l
Without
feldspar
L HERZOLITE
HARZ­
BURGITE
WE HRLITE
(>5% olivine)
PYROXENITE C< 5% olivine)
Plagioclase 90%
of feldspar
Plagioclase
+ orthoclase
Quartz < 10%
PYROXENE-GNEISS
Plag. >
Orthoclase
(incl.
mesoperthite)
JOTUNITE
Orthoclase >
Plag.
(incl.
mesoperthite)
MANGERITE
MESO­
PERTHO­
SITE
PYROXENE-GRANITE
Plagioclase
+ orthoclase
Quartz > 10%
ANORT HO­
SITE
(>50% An in
plagioclase)
HYPER­
ST HENE­
GRANITE
VALLE­
VARITE
PETROLOGY OF PYROXENE-GRANULITE FACIES ROCKS
47
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