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Pyroxene (re-)equilibration in the Precambrian terrain of SW Norway between 1030--990 Ma and reinterpretation of events during regional cooling (M3 stage) FRANS J. M. RIElMEIJER
Rietmeijer F. J. M.: Pyroxene (re-)equilibration in the Precambrian terrain of SW Norway between 1030--990 Ma and reinterpretation of events during regional cooling (M3 stagc). Norsk Geologisk Tidsskrift, Vol. 64, pp. 7 - 20. Oslo 1984. JSSN 0029-196 X.
Pyroxenes in the Precambrian terrain of SW Norway include metamorphic ortho- and Ca-rich clinopyroxenes and reequilibrated igenous pyroxenes. The compositions indicate a thermal regime of c. 750" to c. 590 •c. They betong to a period of regional cooling (M3) in the metamorphic history of the area around 1030--990 Ma. Events during the M3 stage are reinterpreted and indicate that Rb-Sr whole rock and zircon U-Pb isotope systems may have become reset. This may be coeval with the formation of halogenbearing amphiboles. Intrusion of the (Quartz) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion took place between c. 1050-1035 Ma and c. 1030--990 Ma.
F. J. M. Rietmeijer, Afdeling Petrologie, lnstituut voor Aardwetenschappen, Budapestlaan 4, 3508 TA Utrecht, the Netherlands. Present address: Mai/ Code SN4, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
The Precambrian migmatite terrain in Rogaland (SW Norway) has been intruded by a large igneous complex consisting of several anorthosite massifs and the Bjerkreim-Sokndal Layered Intrusion (Michot & Michot 1969)_
In gametiferous migmatites (mainly metapelites) textural arrangements and mineral compositions indicate four successive metamorphic stages (Kars et al. 1980).
The earliest stage (Ml) probably affected the entire Precambrian basement of Rogaland. Mineral assemblages are indicative for upper-amphibolite facies conditions_ Isotopic ages of about 1200 Ma are assigned to this stage (Wielens et al. 1981) .
The so-called granulite-facies stage (M2 ) occurs in a c. 15-45 km wide zone surrounding the igneous complex. From the contact with the igneous complex, the metamorphic grade in the migmatite complex gradually decreases towards the east from granulite-facies to amphibolite-facies conditions. This change is documented by the pigeonite-in isograde in (Jeuco-) charnockite (Rietmeijer 1980, in Jansen & Maijer 1980), the osumilite-in isograde in metapelites (Meijer et al. 1981) and the hypersthene-in isograde in (leuco) granite (Hermans et al. 1975) (Fig. 1). Also the gradual change of optical and chemical properties of amphiboles (Dekker 1978) and phase relations and compositions in siliceous dolomites
(Sauter 1981) are consistent with this regional pattern.
Metamorphic temperatures at the pigeonite-in and osumilite-in isogrades are 1000 ± 50 °C and >800 °C , respectively. The isogrades closely follow the boundary of the igneous complex (Fig. 1). Kars et al. (1980) and Maijer et al. (1981) suggested that the M2 stage may essentially be a thermo-metamorphic event induced by intrusion of the Leuconoritic Phase of the BjerkreimSokndal Layered lntrusion. This phase crystallised at c. 1200 oc - c. 920 oc (cf. Table l) at about 9 kbar (Rietmeijer & Champness 1982). The intrusion reset U-Pb isotopic ages of zircons from the migmatite complex to 1045-1030 Ma, which may be the age of intrusion (Wielens et al. 1981).
The M3 stage of medium-grade metamorphic conditions represents a period of regional cooling (Kars et al. 1980) between c. 990-850 Ma, indicated by isotopic ages for osumilite (Maijer et al. 1981), hornblendes (Dekker 1978) and brown biotite (Verschure et al. 1980). Isotopic closure temperatures of these minerals indicate regional temperatures between c. 550 oc and 400 oc_
Mosaic microstructures in igneous Ca-rich clinopyroxenes from the Bjerkreim-Sokndal Layered Intrusion (Rietmeijer 1979) and types and compositions of fluid inclusions in quartz from the migmatite complex (Swanenberg 1980)
8 F. J. M. Rietmeijer
59°
58°45'
6500
6490 58°30'
6460
LEGE ND
L2] Ouaternary
INTRUSIVE COMPLEX
� Anorthosite
� Leuconoritic Phase
� (Ouartz-) Monzonitic Phase
MET AMORPHIC COMPLEX
D Caledonides
58°15' D Precambrian
5°30'
NORSK GEOLOGISK TIDSSKRIFr l (1984)
Tonstad
10 km
so
Fig. l. Simplified geological map of southwestern Norway (after Hermans et al. 1975, Rietmeijer 1979). South of dented line the igneous nature of the (Quartz-) Monzonitic Phase obliterates. Charnockitic migmatites occur west of the hypersthene-in isograde.
NORSK GEOLOGISK TIDSSKRIFT l {1984)
indicate a virtually isobaric cooling path at a total pressure slightly in excess of 5 kbar between c. 900-500 oc.
Maijer et al. (1981) argued that only locally in the migmatite terrain adjacent to the igneous complex, formation of M3 mineral assemblages and resetting of some Rb-Sr whole rock and UPb zircon systems may have taken place.
Wielens et al. (1981) postulated a metamorphic event of low-granulite to upper-amphibolite facies conditions at about 950 Ma, during which the (Quartz-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion was emplaced. Crystallisation conditions for this phase are c. 950 oc - c. 850 oc at 5-7 kbar (Rietmeijer 1979).
Maijer et al. (1981) pointed out that the model of Wielens et al. (1981) results in an enigmatic relationship between the intrusion age and the regionally prevailing K-Ar isotopic ages of hornblendes. The latter age of 953 ± 10 Ma is present in the igneous and metamorphic complexes (Dekker 1978). The recalculated Rb-Sr whole-rock isochron age of the (Quartz-) Monzonitic Phase, 928 ±50 Ma, has been interpreted as its intrusion age (Wielens et al. 1981). The new value is higher than the whole-rock isochron age of 857 ± 21 Ma obtained by Pasteels et aL (1979). U-Pb zircon ages for two rocks of the (Quartz-) Monzonitic Phase, viz. 946 ± 14 Ma and 932 ± 5 Ma (Pasteels et al. 1979), are similar to the recalculated whole-rock isochron value.
Finally, a stage of retrograde prehnite-pumpellyite facies metamorphism (M4) is locally observed throughout the area. Circa 400 Ma old green biotite belonging to this stage is linked to the Caledonian orogenesis (Verschure et al. 1980). A similar age is indicated by the lower intercepts of zircon discordia (Wielens et al. 1981).
I will try to evaluate the temperature regime of the M3-stage, its bearing on the intrusion age of the (Quartz-) Monzonitic Phase and to reinterpret isotopic age data.
Bjerkreim-Sokndal Layered Intrusion: geologicaVpetrological data
The intrusion is divided in a Leuconoritic Phase and a (Quartz-) Monzonitic Phase (Fig. 1). The synclinally folded Leuconoritic Phase is discordantly overlain by the sub-horizontal (Quartz-)
Pyroxenes in Precambrian terrain 9
Monzonitic Phase. The former, mainly leuconorite and anorthosite, is the most voluminous phase. Its estimated thickness in the axial zone is 5000 metres (Michot & Michot 1969). The latter forms a 350 metre thick sill-like body of twoclinopyroxene monzonite to -syenite (both ± iron-rich olivine) and clinopyroxene-fayalitequartz syenite to -granite (Rietmeijer 1979).
The contact of the intrusion with the anorthosite massifs and charnockite migmatite complex is tectonic (Rietmeijer 1973, 1979, Michot & Michot 1969). Foliation in the (Quartz-) Monzonitic Phase gradually increases towards the contact. Simultaneously the seriate/interlobate inequigranular medium-grained textures become completely granulated (Dekker 1978, Rietmeijer 1973). The medium-grained texture occurs in macroscopically undeformed rocks and is believed to represent the original igneous texture.
Recrystallisation is accompanied by formation of extreme poikiloblasts of iron-rich orthopyroxene (inverted pigeonites) and iron-rich clinoamphibole. Their c-axes are parallel to the foliation (Rietmeijer & Dekker 1978, Dekker 1978). The Ti-content of iron-rich clino-amphiboles indicates that they formed 'at high temperatures of granulite facies conditions' (Dekker 1978).
Pyroxene data
In the present study, published and unpublished electron microprobe data for coexisting orthoand Ca-rich clinopyroxenes have been used. The latter include two unpublished M. Se. theses, viz. Perlin (1980) and van Gaans (1982), and data from the DATA BASE of the Department of Petrology, RU-Utrecht (Tables l and 2 ). Ferric iron is calculated after the method of Hamm & Vieten (1971).
The data set covers every major petrological unit in the area. A detailed petrological study of the area is given by Hermans et al. (1975).
Igneous rocks
Bjerkreim-Sokndal Layered lntrusion
In the Leuconoritic Phase primary orthopyroxene crystallises until its Fe-ratio [ = Fe2+ l (Fe2+ +Mg)] reaches c. 0.4, when it is replaced by pigeonite (Duchesne 1973). Ca-rich clinopy-
Tab
le I
.
SAMP
LE NO
.
S1
02
Al
2o
3
T1
02
Fe
O
Fe
2o
3
MnO
M\1()
Ca
O
Na
2o
64
44
Oil X
cpx
53
.0
7
52
.4
2
1.
43
2
.3
9
0.
18
0
.4
3
20
.6
6
8.
71
0.
37
0
.1
7
23
.4
4
13
.7
3
1.
07
2
1.
57
n1
n
a
64
60
opx
CPX
51
.2
3
51
.7
7
1.
24
2
.1
4
0.
14
0
.4
2
26
.6
0
11
.7
7
0.
55
0
.3
0
18
.9
3
12
.5
7
0.
75
2
0.
56
0.
13
0
.6
6
E2
63
opx
cpx
53
.8
8
52
.3
3
1.
52
2
.4
8
0.
21
0
.5
0
15
.8
4
6.
40
0.
27
0
.1
4
26
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8
14
.9
1
o.
70
2
2.
00
0.
00
0.
44
R2
5
oøx
cox
49
.4
7
50
.8
7
0.
62
1
.3
7
0.
11
0
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0
37
.1
9
16
.3
5
l. 79
o.
75
0
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7
11
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8
8.
93
0.
96
20
.4
0
0.
00
0.
48
R5
3
DPX
CPX
50
.3
6
51
.2
4
0.
51
1
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5
0.
02
0
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0
32
.8
3
14
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4
0.
65
0
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1
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0
10
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0
0.
74
2
0.
26
0.
00
0.
40
R1
86
OPx
CPx
49
.8
3
50
.6
7
0.
87
1
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7
0.
12
0
.1
0
33
.0
4
15
.9
0
o. 70
0
.5
7
13
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3
10
.4
0
1.
05
20
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3
na
n
a
R4
86
OPx
CPx
50
.1
4
51
.4
5
0.
40
1
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0
0.
00
n
o
36.
33
1
6.
78
0.
58
0
.3
7
12
.2
9
9.
34
0.
83
2
1.
06
na
n
a
GA24
8
OPX
CPX
52
.8
9
51
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3
2.
54
3
.8
0
0.
18
0
.6
9
16
.1
5
4.
26
1.
09
3
.1
7
0.
29
0
.1
1
25
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1
14
.3
0
0.
72
2
2.
55
0.
00
0
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8
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0
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CPX
47
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1
49
.0
0
0.
68
1
.0
4
0.
06
0
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0
43
.5
3
23
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5
1.
06
0.
54
5.
88
5
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1
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72
20
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2
0.
00
0
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8
R2
47
R
34
0
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cpx
opx
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47
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7
50
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7
47
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0
49
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0
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91
1
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6
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55
1
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0
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08
0
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1
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20
0
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0
42
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7
22
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8
42
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0
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5
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0
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8
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88
0
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5
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5
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6
7.
10
5
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0
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35
1
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26
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22
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95
1
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65
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00
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. 7
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ta
l 1
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2
99
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2
99
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0
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6
99
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1
99
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0
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4
10
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74
1
00
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7
10
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9
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77
1
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9
99.
74
1
00
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6
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2
99
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8
10
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85
Str
uct
ura
l fo
rmu
lae
calc
ula
ted
to
6 o
xyge
ns
Sl
Al
T1
Fe
2+
Fe
3•
Mn
Mg
Ca
No
to
ta
l
EN
FS
\10
1.
95
9
1.
95
7
1.
96
9
1.
94
7
1.
96
4
1.
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4
1.
98
2
1.
95
5
1.
98
5
1.
98
0
0.
06
2
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10
5
0.
05
5
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09
5
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06
5
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10
9
0.
02
9
0.
062
0
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24
0
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62
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005
0
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12
0
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4
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01
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0
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14
0
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03
0
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06
0
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01
0
.0
06
0.
63
8
0.
27
2
0.
83
6
0.
37
0
0.
48
3
0.
19
9
1.
24
6
0.
52
6
1.
08
2
0.
47
6
0.
05
2
0.
01
2
0.
005
0
.0
18
0
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09
0
.0
08
0.
00
4
0.
02
5
0.
01
2
0.
02
2
0.
01
0
1.
28
9
0.
76
4
1.
08
5
0.
70
5
0.
04
2
0.
86
3
0.
03
1
0.
82
9
0.
01
0
0.
04
7
4.
007
3
.9
78
4
.00
8
4.
01
4
65
.5
4
0.
4
55
.6
3
7 .
o
32
.4
1
4.
4
42
.8
1
9.
4
2.
1
45
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1
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4
3.
6
1.
44
4
0.
82
6
0.
02
7
0.
87
6
0.
000
0
.0
32
3.
99
7
4.
004
73
.9
4
3.
5
24
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1
0.
5
1.
4
46
.0
2
0.
67
3
0.
51
2
0.
85
8
0.
59
6
0.
04
1
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84
0
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39
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0
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36
0
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99
9
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34
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2
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3
43
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3
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2
63
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2
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0
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2
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9
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1
44
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1
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9
8
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4
1.
95
2
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99
1
1.
97
8
1.
92
4
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87
9
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6
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96
0
1.
97
5
1.
98
8
1.
97
7
1.
96
5
0.
04
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08
5
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01
9
0.
05
9
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10
9
0.
16
4
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03
3
0.
05
0
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09
5
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08
3
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02
7
0.
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0
0.
00
4
0.
00
3
0.
000
0
.00
5
0.
01
9
0.
002
0
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5
0.
00
3
0.
00
8
0.
006
0
.0
06
1.
09
4
0.
51
2
1.
20
7
0.
54
0
0.
49
1
0.
13
1
1.
52
0
0.
77
8
1.
47
5
0.
74
3
1.
48
0
0.
74
9
0.
03
0
0.
08
7
0.
02
3
0.
01
9
0.
02
0
0.
01
2
0.
00
9
0.
00
3
0.
03
7
0.
01
8
0.
04
2
0.
01
5
0.
03
1
0.
01
5
0.
82
2
0.
59
7
0.
04
5
0.
83
5
4.
01
3
4.
00
3
41
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3
0.
7
55
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2
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3
2.
3
43
.0
o.
72
7
0.
53
5
0.
03
5
0.
86
8
3.
99
9
3.
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2
36
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2
7.
5
61
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2
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8
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44
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40
5
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78
2
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02
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0.
88
6
0.
000
0
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4.
00
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9
73
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4
3.
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25
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5
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36
6
0.
31
0
0.
03
2
0.
87
7
0.
000
0
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22
3.
98
6
4.
02
0
19
.1
1
5.
8
79
.2
3
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6
1.
7
44
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0.
31
8
0.
29
8
0.
44
0
0.
34
6
0.
06
0
0.
81
5
0.
04
2
0.
82
9
0.
01
8
0.
03
8
0.
000
0
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53
3.
98
6
3.
98
8
4.
003
4
.0
23
17
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1
6.
1
79
.6
4
0.
0
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2
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22
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0
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3
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9
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o
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;.:l � j
Tab
le l
con
td.
SAIIP
LE
IK!.
R394
oø
x
cp
x
SID z
47.00
51
.50
A1 2o 3
1. 75
1.
00
Ho 2
o.1o
o.
1o
Fell
44.6
0 22
.70
F�2o 3
MnO
1.
00
0.40
14g0
5.10
4.
30
CaD
1.40
20
.00
Na2o
0.40
o.
oo
073
OØ
lll.
CP
lll.
46.7
4 47
.16
0.34
1.
35
0.08
0.
14
44.84
21
.14
4.16
0.76
0.
72
5.43
5.
23
0.82
19
.52
na
0.
00
F107
0
oo
x
co
x
50.8
1 52
.21
0.65
1.
20
0.28
0.
28
30.7
0 12
.92
1.09
0.
41
0546
OD
X C
PX
50.5
5 52
.42
o. 74
1.
53
0.14
0.
21
37.3
0 18
.25
0.45
0.
26
15.4
5 ll
.39
11.5
4 9.
60
0.76
21
.74
0.95
20
.32
o.oo
0.
60
na
n
a
0521
oo
x
co
x
52.3
5 52
.41
0.67
1.
27
0.13
0.
20
32.4
9 14
.17
0.61
0.
25
15.7
4 ll
.25
0.69
21
.19
na
n
a
To
tal
101.
35 1
00.00
99
.01
99.4
2 99
.74
100.
47
101.
67 1
02.5
9 10
2.68
10
0.74
Stru
ctur
al f
orm
ula
e ca
lcu
late
d t
o 6
oxyg
ens
Sl
Al
Tl Fe
2•
F�3+
Mn
l4g
Ca
No to
ta
l
EN
FS
MO
1.94
6 2.
032
0.08
5 0.
047
0.00
3 0.
003
1.54
5 o.
749
1.98
5 1.
904
0.01
7 0.
064
0.00
3 0.
004
1.59
2 0.
714
0.12
8
1.98
4 1.
971
0.03
0 0.
053
0.00
8 0.
008
1.00
3 0.
408
0.03
5 0.
013
0.02
7 0.
025
0.03
6 0.
013
0.31
5 0.
253
0.34
4 0.
315
0.89
9 0.
641
0.06
2 0.
845
0.03
7 0.
844
0.03
2 0.
879
0.03
2 0.
000
0.00
0 0.
000
0.02
2
4.02
3 3.
942
4.00
3 3.
998
3.99
2 3.
995
16.4
13
.7
80.4
40
.6
3.2
45.7
17.4
16
.8
80.7
38
.1
1.9
45.1
46.5
33
.2
51.9
21
.2
1.6
45.6
1.990
1.
973
0.03
4 0.
068
0.00
4 0.
006
1.22
8 0.
574
0.01
5 0.
008
0.67
7 0.
538
0.04
0 0.
819
3.98
8 3.
986
34.9
28
.1
63.1
29
.7
2.0
42.2
1.98
8 1.
979
0.03
0 0.
057
0.004
0.
006
1.03
2 0.
447
0.02
0 0.
008
0.89
1 0.
633
0.02
8 0.
857
3.99
3 3.
987
45.6
32
.5
52.8
23
.2
1.6
44.3
0555
OP
X
CO
X
46.04
49
.31
0.65
1.
37
0.13
0.
19
39.0
2 20
.53
2.41
0.47
0.
22
8.03
6.
67
1.00
20
.13
na
n
a
97.7
5 98
.42
l. 95
2 1.
970
0.03
2 0.
065
0.00
4 0.
006
1.35
4 0.
686
0.07
7
0.01
7 0.
007
0.50
7 0.
397
0.04
5 0.
862
3.98
8 3.
993
26.6
20
.5
71.0
35
.4
2.4
44.1
F475
oø
x
co
x
50.2
9 50
.83
0.99
1.
76
0.11
0.
21
30.9
8 13
.43
0.45
0.
22
15.8
5 ll
.52
0.63
21
.34
na
n
a
99.3
0 99
.31
1.97
1 1.
948
0.04
5 0.
080
0.00
3 0.
006
1.01
5 0.
431
0.01
5 0.
007
0.92
6 0.
658
0.02
6 0.
876
4.00
1 4.
006
47 .
o 33
.5
51.5
21
.8
1.5
44.7
F106
F1
66
oo
x
co
x
47.7
0 48
.37
1.06
1.
77
O.ll
0.
24
29.6
5 14
.02
2.51
oo
x
co
x
49.7
6 49
.27
1.20
2.
29
0.16
0.
38
35.2
3 16
.07
0.22
0.
19
0.64
0.
23
14.6
7 10
.28
13.8
0 9.
83
0.68
20
.23
0.84
20
.08
na
n
a
na
n
a
96.6
0 95
.10
101.
63
98.1
5
1.94
6 1.
946
0.05
1 0.
084
0.00
3 0.
007
0.98
0 0.
472
0.07
7
0.00
8 0.
006
0.89
2 0.
616
0.03
0 0.
872
3.98
7 4.
003
46.9
31
.6
51.5
24
.0
1.6
44.4
1.94
7 1.
933
0.05
5 0.
106
0.00
5 O.
Oll
1.15
3 0.
527
0.02
1 0.
008
0.80
5 0.
575
0.03
5 0.
844
4.02
1 4.
004
41.7
29
.6
56.2
27
.2
2.1
43.2
L40
0
oo
x
co
x
48.3
3 48
.58
0.96
1.
66
0.11
0.
21
33.8
1 14
.65
0.48
0.
21
Lll9
oox
c
øx
49.2
5 49
.13
0.85
1.
41
0.10
0.
19
36.0
5 15
.88
0.62
0.
16
5031
oox
c
px
52.1
8 50
.48
1.16
1.
92
O.ll
0.
25
22.6
4 8.
40
13.8
3 10
.20
12.1
3 9.
25
o. 9
0 20
.34
1.00
20
.09
0.46
0.
17
21.6
3 13
.74
0.51
21
.69
••
n
a
••
•
•
na
n
a
98.4
2 95
.85
100.
00
96.1
1 98
.69
96.6
5
1.94
9 1.
945
0.04
6 0.
078
0.00
3 0.
006
1.14
0 0.
491
0.01
6 0.
007
0.83
1 0.
609
0.03
9 0.
873
4.02
4 4.
009
41.3
31
.0
56.7
24
.9
2.0
44.1
1.97
0
0.04
0
0.00
3
1.206
0.02
1
0.72
3
0.04
3
4.00
6
36.5
61.4
2.1
1.96
8 1.
973
1.94
6
0.06
7 0.
052
0.08
7
0.006
0.
003
0.00
7
0.53
2 o
. 71
6 0.
271
0.00
5 0.
015
0.006
o. 55
2 1.
219
o. 79
0
0.86
2 0.
021
0.89
6
3.99
2 3.
999
4.00
3
28.4
62
.2
40.4
27.3
36
.7
13.8
44.3
1.
1 45
.8
z
o
"'
"'
;o::
Cl
rn
o
5
Cl
v;
;o::
::l
o
"'
Vl
;o::
"' �
� J � � ;:
:
�
s·
� � <::)- �- �
.,
�
s
·
......
......
Tab
le l
con
td.
SAHI'LE
NO
.
s1
o2
A
12
o3
H
o2
F.O
Fe
2o
3
HnO
HgO
CaO
No 20
F3
/7
0
OPX
CPX
49
.60
4
9.
76
0.
70
1
.7
1
0.
18
0
.3
7
32
.5
7
16
.3
4
0.
63
0
.1
1
14
.7
6
10
.4
9
0.
87
1
8.
79
na
n
a
ll
6A
OPX
c
pic
48
.4
7
49
.6
2
0.
63
1
.2
7
O.
ll
0
.2
0
33
.2
5
14
.2
8
0.
80
0
.2
4
13.
71
1
0.
29
0.
81
2
0.
88
na
n
a
05
13
op
x
co
x
49
.3
2
48
.8
7
1.
07
1
.9
9
0.
10
0
.2
7
34
.6
1
15
.4
8
0.
56
O
.l
l
13
.0
7
9.
65
0,
78
1
9.
62
na
n
a
V1
87
OPX
COX
49
.8
0
50
.9
2
o.
71
1
.5
7
0.
15
0
.2
0
35
.7
2
15
.0
7
o. 7
2
0.
29
13
.0
7
10
.�
0.
93
2
1.
96
0.
07
0
.5
1
R4
0
OPX
COX
48
.8
0
49.
49
1.
80
1
.1
0
0.
31
0
.2
2
43
.6
0
22.
73
no
1}.
58
5.
36
4
.8
9
1.
05
2
0.
67
no
0.
00
R9
5
OOX
co
x
o.�
4
9.
�
1.
90
1
.2
2
0.
10
0
.1
9
«.
64
�
.�
1.
23
0
.5
3
2.
65
3
.0
3
0.
69
1
9.
46
1.
04
0
.6
1
Til
OOX
co
x
47
.5
8
48
.8
6
0.
47
1
.0
9
0.
19
0
.2
2
44
.1
8
24
.2
6
1.
12
0
.5
3
5.
48
5
.0
8
0.
77
1
9.
77
na
n
a
R9
8
OOX
co
x
47
.0
1
49
.2
3
0.
55
0
.7
0
0.
13
0
.2
2
45
.1
6
26
.0
9
1.
02
0
.6
2
2.
85
2
.9
8
2.
64
1
9.
38
no
0
.6
5
R3
40
OP
X
48
.00
0.
90
0.
00
43
.8
0
1.
20
5.
40
0.
80
0.
00
CPX
50
.1
0
1.
10
0.
00
21
.5
0
0.
60
5.
30
21
.5
0
0.
00
R3
94
OOX
co
x
47
.2
0
51
.5
0
0.
50
1
.00
0.
00
0
.1
0
45
.1
0
22
.7
0
1.
10
0
.4
0
5.
20
4
.3
0
0.
85
2
0.
00
0.
00
0.
00
R4
0
OOX
co
x
47
.5
5
49
.9
1
0.
65
0
.9
4
0.
10
0
.1
2
42
.6
0
21
.4
7
1.
05
0
.5
3
5.
64
5
.3
7
0.
97
2
0.
73
0.
00
0.
08
R1
53
oo
x c
px
47
.7
4
49
.6
4
0.
40
0
.9
2
0.
20
0
.1
8
42
.0
1
20
.4
0
0.
98
0
.4
7
7.
49
6
.3
2
0.
67
2
0.
15
0.
02
0
.4
5
Tot
al
99
.3
1
97
.5
7
97
.7
8
96
.7
8
99
.5
1
94
.9
9
10
1.
17
1
00
.5
8
10
0.
92
9
9.
68
9
9.
50
9
9.
97
9
9.
79
9
9.
82
9
9.
36
9
9.
86
1
00
.1
0
10
0.
10
9
9.
95
1
00
.00
9
8.
56
9
9.
15
9
9.
51
9
8.
53
Str
uct
ura
l fo
rmu
lae
calc
ula
ted
to
6 o
xyge
ns
Si Al
Ti
Fo
2+
Fe
3+
""
Hg
Co
No
toto
l
EN
FS
NO
N
1.
96
6
1.
95
6
1.
96
5
1.
96
3
0.
03
2
0.
07
9
0.
03
0
0.
05
9
0.
005
0
.0
11
0
.00
3
0.
008
1.
07
9
0.
53
7
1.
12
7
0.
47
3
0.
02
1
0.
004
0.
87
2
0.
61
5
0.
03
7
o. 7
92
4.
01
2
3.
99
4
43
.7
3
1.
6
54
.3
2
7.
6
2.
0
40
.8
0.
02
7
0.
008
0.
82
8
0.
60
7
0.
03
5
0.
88
5
4.
01
5
4.
003
43
.0
3
1.
0
54
.9
2
3.
9
2.
1
45
.1
1.
96
7
1.
95
3
1.
96
4
1.
94
9
0.
05
0
0.
09
4
0.
03
3
0.
07
1
o.
oo3
0
.00
8
o.00
4
o.�
1.
15
5
0.
51
7
1.
17
8
0.
48
2
1.
99
5
1.
97
8
0.
08
8
0.
05
2
0.
01
0
0.
008
1.
490
o.
76
0
0.
01
9
0.
00
4
0.
77
7
0.
57
5
0.
03
3
0.
84
0
4.
004
3.
99
1
39
.5
2
9.
8
58
.8
2
6.
8
1.
7
43
.4
0.
02
4
0.
009
0
.0
20
o. 7
68
0
.5
74
0
.3
28
0
.29
0
0.
03
9
0.
90
1
0.
04
5
0.
88
5
0.
00
5
0.
03
8
0.
000
4.
01
5
4.
03
0
3.
95
6
3.
99
3
38
.7
2
9.
3
17
.6
1
5.
0
59
.3
2
4.
6
80
.0
3
9.3
2,
0
46
.1
2
.4
4
5.
7
1.
99
5
2.
000
0.
09
5
0.
05
8
0.
00
2
0.
00
5
1.
57
8
0.
84
0
0.
04
5
0.
01
8
0.
16
8
0.
18
3
0.
03
0
0.
83
8
0.
08
5
0.
04
8
3.
99
8
3.
99
0
9.
5
9.
8
88
.9
4
5.
1
1.
6
45
.1
1.
99
5
1.
96
1
0.
02
3
0.
05
2
o.�
o
.oo
1
1.
55
0
0.
81
4
0.
04
0
0.
01
8
0.
34
3
0.
30
4
0.
03
5
0.
85
0
3.
99
2
4.�
17
.8
1
5.
5
80
.4
4
1.
3
1.
8
43
.2
2.
00
2
1.
99
3
0.
02
7
0.
03
3
0.
005
0
.00
8
1.
60
7
0.
88
3
0.
03
8
0.
02
0
0.
18
0
0.
18
0
0.
12
0
0.
84
0
0.
05
3
3.
97
9
4.
01
0
9.
4
9.
5
84
.3
4
6.
4
6.
3
44
.1
1.
99
8
0.
04
4
0.
000
1.
52
5
0.
04
2
0.
33
5
0.
03
6
0.
000
3.
98
0
17
.7
80
.4
1.
9
1.
98
3
0.
05
1
0.
000
o. 7
12
0.
02
0
0.
31
3
0.
91
2
0.
00
0
3.
99
1
16
.1
36
.8
47
.1
1.
98
6
2.
03
0
0.
02
5
0.
047
0.
000
0.
003
1.
58
7
0.
74
9
0.
03
9
0.
01
3
0.
32
6
0.
25
3
0.
03
8
0.
84
5
0.
000
0
.00
0
4.
001
3
.9
40
16
.7
1
3.
7
81
.3
4
0.
6
2.
0
45
.7
2.
005
1
.9
93
0.
03
3
0.
04
5
0.
003
0
.0
02
1.
50
2
o. 7
18
0.
03
8
0.
01
7
0.
35
5
0.
32
0
0.
04
5
0.
88
8
0.
000
0
.0
05
3.
98
1
3,
98
8
18
.7
1
6.
6
79
.0
3
7.
3
2.
3
46
.1
1.
98
4
1.
98
5
0.
02
0
0.
04
3
o.�
o
.o
o5
1.
46
0
0.
68
2
0.
03
5
0.
01
6
0.
46
4
0.
37
7
0.
03
0
0.
86
3
0.
00
1
0.
03
5
4.
000
4.
�
23
.7
1
9.
6
74
.7
3
5.
5
1.
6
44
.9
.....
N
�
�
�
:::tl �· "' ..::: ·
<�>"
....
z o
::0
"'
::-: Cl
m
o
r
o
Cl
c;;
::-:
::l
o
"'
"'
::-:
::0 � .....
i
NORSK GEOLOGISK TIDSSKRIFT l (1984)
� <
>< ... u
K ... o
K ... o
g o
�
:ll o
"' (!; ' o
� "'
o
"' "' "' 8 � � o o o
Pyroxenes in Precambrian terrain 13
� "'
,.;
O M \D M .., "' ... M M In ,.... s � � � �
o " e o"": o :;; � 8 �
- o o o O .... CC O
o o o o ..
" .. o
� � � � � � � � �OOsj Occ c:ic:i l.C \() M 0 C7l CP\ N � g å o �
M � ..
c:i o N
� 2 � 2 o W'l o o
o o o o N M Y"' lt'l
... .., O ._ CIO O
o o o o N
.., "' " ... ���a
o Cl) o o !å8� N M M M
8 � (!; 8 o o o o
8 o -o o- ...
"' M Ln o ..n I.I'J ��i å 8 � ... N
" e
o ..
o o N
�. � 8 � o o c ei o o o ...
"' K ... o
"'� � o
o o
; o
....
roxene forms small anhedral crystals or partial rims on Ca-poor pyroxene (Rietmeijer 1973, van Gaans 1982). Subhedral primary orthopyroxene has exsolution lamellae of Ca-rich clino-pyroxene /1100. Elongated crystals (//c-axis) of (optically) homogeneous primary orthopyroxene and Ca-rich clinopyroxene are locally present. The Fe-ratio of elongated pyroxenes is 0.25-0.66 (van Gaans 1982).
In the charnockite migmatite complex of Rogaland elongated to acicular primary orthopyroxene of similar compositions is typical in dehydrated rims of intercalated amphibolite lenses (Rietmeijer 1979).
The grain size of Ca-rich clinopyroxenes in the felsic rocks shows a bimodal distribution (Rietmeijer 1979):
l) large(> l mm) subhedral crystals with up to four generations of exsolution lamellae //'001', '100' and 100 (Rietmeijer & Champness 1980a, 1982) ('hk!' means approximately //hk!). The initial wollastonite con tent [ =Ca/(Fe2+ + Mg+Ca) xlOO] is c. 35-40 mole%.
2) small ( < l mm) anhedral crystals with exsolution lamellae //'001' only. The initial wollastonite content is 40-45 mole%.
��ss o o o o
Primary orthopyroxene also has two distinct ha bits:
l) subhedral crystals (0.5-1.0 mm) with exsolution lamellae of Ca-rich clinopyroxene 11100. In addition Fe-Ti oxide plates and/or needles //(100) may be present.
2) homogeneous crystals slightly elongated along the c-axis. They are smaller than the former.
Textural evidence indicates that coexisting pyroxene pairs are formed by ortho- and Ca-rich clinopyroxenes from groups l) and 2) (Rietmeijer 1979).
Multiple exsolution patterns are typical for igneous clinopyroxenes, whereas metamorphic clinopyroxenes tend to exhibit simple exsolution patterns (Robinson et al. 1972, Jaffe et al. 1975). Thus group l) pyroxenes are interpreted as the original igneous phases and group 2) as metamorphic phases formed by recrystallisation.
The initial wollastonite contents agree with this interpretation.
Group l) Ca-rich clinopyroxenes may show partial rims of primary orthopyroxene. Rietmeijer (1979) interpreted the texture as an ig-
14 F. J. M. Rietmeijer
20
NORSK GEOLOGISK TIDSSKRIFT l (1984)
10
100
Fet"rosilitr
Fig. 2. Compositions of cocxisting pyroxenes from the Precambrian terrain in Rogaland (SW Norway) plotted on the pyroxene quadrilateral Enstatite-Ferrosilite-Diopside-Hedenbergite (cf. Table 1). The compositions of Ca-rich clinopyroxenes delineate a zone at about 45 mole% CaSi03. For explanation of symbols cf. Table 2.
neous phenomenon. However, Lindsley & Andersen (1983) showed granule exsolution to be important in slowly cooled Ca-rich clinopyroxenes. The resulting microstructures are similar to those observed by Rietmeijer (1979). The orthopyroxene rims are therefore reinterpreted as subsolidus reequilibration of igneous Ca-rich clinopyroxenes.
Botnavatnet and Gloppurdi Igneous Complexes
They are c. 1200 Ma old iron-rich intrusions concordantly intercalated in the migmatites complex. Stratiform mafic layers [ amphibolite and (leuco )norite] form an intrinsic part of the intrusions (Rietmeijer 1979). In some layers iron-rich mafic silicates and Fe-Ti oxides reacted with adjacent plagioclase to garnet (Perlin 1980) (Note: similar garnet is observed in a few samples from the Bjerkreim-Sokndal Layered Intrusion).
In sarnple 073 (garnet-bearing norite) garnet forms rims on primary orthopyroxene, pigeonite (Fe-ratio= 0.81) and Ca-rich clinopyroxene (Feratio= 0.73). The pyroxene assemblage indicates crystallisation conditions at about 935 oc (Table 2).
Pyroxene relations are similar to those observed in the (Quartz-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion (Rietmeijer 1979, Rietmeijer & Champness 1980 b).
The low Al content of orthopyroxene (Al203 = 0.25.-0.50 wt%) and the high iron and calcium content of garnet prohibit their use as geobarometer (cf. Harley & Green 1981). The low Al content is typical for orthopyroxene from the igneous rocks in the area (Rietmeijer 1979).
Folded basic intrusions
Hermans et al. (1975) observed igneous masses and zones of pyroxene dioritic composition concordantly folded with the surrounding migmatites. Modal analysis shows the presence of enderbite, norite and gabbronorite. Textures and colour of constituent amphibole indicate that they reequilibrated under high-grade metamorphic conditions (Dekker 1978). In sample F107-D (apatite-amphibole monzonorite) the coexisting pyroxenes are optically almost homogeneous primary orthopyroxene and small subhedral Carich clinopyroxene with simple exsolution textures. They may be of metamorphic origin (cf. section Bjerkreim-Sokndal Layered Intrusion above).
Metabasites
In the migmatite complex 'mainly banded migmatites' and 'massive parts' can be distinguished
T
(°Cl
SOU
RCE
SA
MP
LE
S RE
MA
RKS
SY
MB
OLS
l.
IG
NE
OUS
R
OC
KS
1.
1
BJE
RKRE
IM
-S
OK
ND
AL
LAY
ERE
D
92
2,
80R
D
ucn
es
ne
1
973
64
44
, 64
60
Thi
rd
r
hyt
hmi
c u
n�t
; P
ig
eo
ni
te
cr
ys
ta
ll
is
at
io
n
6
IN
TR
US
ION
t
em
pe
ra
tu
re
1
13
5
C
(TH
E
LEU
CON
ORI
TIC
P
HA
SE)
92
0 D
ATA
B
ASE
E2
63
Tni
rd
rhy
thm
ic
un
it;
el
on
ga
te
d
pr
im
ar
y
OPX
•
B74
-
795
van
Ga
an
s 1
982
R
25
, R
53
, R
186
, F
ou
rt
n &
Fi
ft
n r
nyt
nm
ic
un
it
s
su
bne
dr
al
OP
X +
•
(av
. 83
2)
R4
B6,
GA24
8 in
te
rg
ro
wn
Fe
-Ti
oxi
des
; a
cicu
la
r0
0P
X;
Pi
ge
on
it
e
cry
st
al
li
sa
ti
on
t
em
per
at
ur
e
11
98
C
874
-
755
Riet
me
ij
er
19
79
; R
40
, R2
47,
R
34
0,
Fi
ft
n r
nyt
nmi
c u
ni
t;
OPX
ri
ms
on
C
a-
CP
X;
Pi
ge
on
it
e
o
(av
. 81
3)
Riet
me
ij
er
a
nd
R3
94
cr
ys
ta
ll
is
at
io
n
tem
per
at
ur
e
920
°c
(cf
.
Ri
et
mei
je
r
Cha
mp
ne
ss
19
82
1
983
)
1.
2
GL
OP
PU
RD
I I
GN
EO
US
C
OMP
LEX
802
P
er1
1n
1
980
07
3 P
i g
eo
n1 t
e cr
.vs
ta
11
1sa
t1
on
t
em
per
at
ur
e 9
35
°c
•
11
.
FOL
DED
B
ASI
C I
NTR
US
ION
S
810
D
ATA
BA
SE
F
l0
7-
D
cf.
D
ekk
er
19
78
•
11
1.
ME
TAB
AS I
TE
S
866
-
806
Ja
cqu
es
de
D
ixm
ud
e
054
6,
052
1,
05
55
, C
or
re
cte
d f
or
fe
rr
ic
1r
on
•
(a
v.
836)
19
78
F4
75
, F
l0
6,
F1
66
, L4
00
, L
11
9,
S0
31
, F
3/
70
, L
16
A,
05
13
740
DAT
A
BA
SE
V1
87
Su
bne
dr
al
Ca
-C
PX
an
d s
li
gnt
ly
e
lo
ng
at
ed
p
ri
ma
ry
OP
X e
(c
f.
De
kk
er
19
78)
SU
BS
OL
ID
US
PH
AS
ES
(f
or
I
GN
EOU
S
ROC
KS
o
nl
y)
Exs
ol
ve
d
Ca
-CP
X 87
6
-76
9
Du
che
sn
e
19
73
; R4
0,
R9
5,
Til
, o
Ri
etm
ei
je
r 1
97
9;
R9
8,
R34
0,
R39
4
Ri
etm
ei
j e
r
and
C
nam
pn
es
s 1
9R2
Inv
er
te
d P
ig
e�
ni
te
88
7
-7
87
Ri
etm
ei
je
r 1
979
R
40
, R1
53,
R24
7,
o
R31
2,
R788
, A7
, A
l O l
Tabl
e 2. E
quili
brat
ion
tem
per
atur
es a
nd so
urce
s for
met
amor
phi
c an
d re
equi
libra
ted
igne
ous o
rtho
-and
Ca-
rich
clin
opyr
oxen
es u
sed
in F
ig. 2
. Tem
per
atur
es a
re c
alcu
late
d ac
cord
ing
to th
e
pyr
oxen
e ge
othe
rmom
eter
of
Woo
d &
Ban
no (
1973
).
Pige
onit
e cr
ysta
llisa
tion
tem
per
atur
es a
re c
alcu
late
d af
ter
Ishi
i (1
975)
: T
("C
) ==
1270-4
80 X
Fe +
[(10
-5X
Fe).
P);
XFe
= F
e2+/
(Fe2
+ +M
g) p
;g;
P= 9
kba
r (c
f. R
ietm
eije
r an
d C
ham
pne
ss 1
982)
.
Th
e ri
ght-
hand
sid
e co
lum
n p
rovi
des
key
to s
ymbo
ls u
sed
in F
ig.
2.
l
z
o " en
;.:
o
!Tl
o 5 o
c;;
;.:
-1 a en
en
;.: " � j �
.... � � � s·
� 2 3 <::1-
5·
;::: � � s·
....
Vl
16 F. J. M. Rietmeijer
(Hermans et al. 1975). The former is composed of alternating light- and dark coloured layers suggestive of a supracrustal origin. Dark layers in the charnockitic part of the migmatite complex include (leuco)norite, amphibolite, monzonorite and enderbite.
Jacques de Dixmude (1978) suggested that the greater part of the dark layers represent metavolcanics, albeit that they generally show granoblastic textures. Several samples (e.g. V187, amphibole norite) contain large multiple exsolved Carich clinopyroxenes, small anhedral crystals of homogeneous orthopyroxenes and simple exsolved Ca-rich clinopyroxenes. The interpretation of pyroxene phase relations compares with groups l) and 2) in the section Bjerkreim-Sokndal Layered Intrusion above.
To summarise, igneous pyroxenes in the Precambrian basement show evidence of subsolidus reequilibration. Recrystallisation in the migmatite complex (Huijsmans et al. 1981) and in the Bjerkreim-Sokndal Layered Intrusion (cf. section on petrologicallgeological data) took place in the orthopyroxene - Ca-rich clinopyroxene stability field. The wollastonite contents of pyroxenes (Fig. 2) indicate that they (re-) equilibrated on a regional scale (Rietmeijer 1983).
Pyroxene equilibration temperatures
Pyroxene geothermometers by Wood & Banno (1973) or Wells (1977) may be used to calculate equilibration temperatures of coexisting orthoand Ca-rich clinopyroxenes. The accuracy of the geothermometers is claimed to be within ± 70 °C. All pyroxenes used in the present study are low in Ah03, Ti02, MnO, Na20 and K20 ( < ca 3 wt%) and may be treated as 'pure phases'. Thus compositional errors in calculated temperatures will be negligible. Turnock & Lindsley (1981) showed that application of erroneous thermodynamic data necessitate recalibration of geothermometer by Wells (1977). Therefore I will use the Wood and Banno geothermometer.
However, the experimentally studied distribution of Mg, Fe
2+ and Ca between coexisting
ortho- and Ca-rich clinopyroxenes at 750 oc and 800 oc (Fonarev & Graphchikov 1982) indicate that, at these temperatures, results obtained with the Wood and Banno geothermometer will be 115-185 oc too high.
For exsolved Ca-rich clinopyroxenes Rietmeijer & Champness (1980b, 1982) compared
NORSK GEOLOGISK TIDSSKRIFT l (1984)
temperatures calculated using the Wood and Banno geothermometer with temperatures obtained by cell-parameter modelling. They also concluded that the former method produces temperatures which are c. 150 °C too high.
Thus pyroxene equilibration temperatures in Table 2, ranging from 920 oc to 740 oc, must be 'corrected' to c. 770 oc to 590 °C. In a low to intermediate pressure regime, the pressure effect on pyroxene equilibration temperatures will be negligible (Lindsley & Andersen 1983).
Isothermal projections of the orthopyroxenepigeonite and orthopyroxene-Ca-rich clinopyroxene solvi onto the pyroxene quadrilateral plane (Ross & Huebner 1975, Lindsley et al. 1974, Turnock & Lindsley 1981) may also be used to obtain pyroxene equilibration temperatures.
In Rogaland the range of pyroxene equilibration temperatures then becomes c. 700 oc to well below 600 oc (Fig. 3). This is in good agreement with the 'corrected' temperatures.
For comparison, Fig. 3 shows the trends for reequilibrated igneous and metamorphic pyroxenes from the Adirondack Province (U.S.A.) (Boblen & Essene 1978) and the mangerite-charnockite intrusives from the Lofoten-Vesterålen Precambrian terrain (Norway) (Malm & Ormaasen 1978). Metamorphic temperatures in the Adirondack Province are 790 °C - 760 oc using experimentally determined phase relations on the ferrosilite-hedenbergite join (Jaffe et al. 1978). Malm & Ormaasen (1978) calculated pyroxene equilibration temperatures ranging from 900 oc to 650 oc using the geothermometer of Wood & Banno (1973) (Note: these temperatures are to o high).
Rietmeijer (1983) proposed that compositions of orthopyroxenes coexisting with Ca-rich clinopyroxenes may be used to evaluate the extent of regional pyroxene reequilibration, viz. the wollastonite content vs. Fe-ratio for completely reequilibrated orthopyroxenes will define a straight line in a diagram showing these parameters.
The linear correlation (corr. coeff. 0.83) observed for coexisting pyroxenes from Rogaland reflects regional reequilibration at about 600 oc.
The observation that some data points are on the high-temperature side of the reequilibration line was taken as evidence that isolated orthopyroxenes were not able to ad just to changes of physical conditions (Rietmeijer 1983). It is noteworthy that these orthopyroxenes are from an area north of the Bjerkreim-Sokndal Layered lntrusion, where Sauter (1981) reported local occur-
NORSK GEOLOGISK TIDSSKRIFT l (1984)
Diopside
�C!_ _______ _
10 20 50 Enstatite
60
Pyroxenes in Precambrian terrain 17
70
Hedenbefgite 50
BO
10
100 Ferrosilite
Fig. 3. Isothermal projections of the OPX-CaPX solvus at 900 °C (Turnock and Lindsley 1981), at 810 oc (Lindsley et al. 1974) and at 800 °C, 700 oc and 600 oc (Ross & Huebner 1975) plotted on the pyroxene quadrilateral Enstatite-Ferrosilite-DiopsideHedenbergite. Trends of coexisting pyroxenes are given for recrystallised mangerite-charnockite intrusives (Malm and Ormaasen 1978) (1), metamorphic and reequilibrated igneous pyroxenes from the Precambrian terrain in SW Norway (Il) and metamorphic and reequilibrated igneous pyroxenes from the Adirondacks (Boblen & Essene 1978) (Ill).
Ca-rich clinopyroxenes delineate narrow zones at c. 45 mole % CaSi03. Orthopyroxenes plot on lines at the base of the quadrilateral. Tie-lines indicate the range of FeH/(Fe2++Mg) ratios, viz. squares (I), dots (Il) and asterisks (Ill).
rences of sanidine in rocks of the Faurefjell Metasediment Formation.
Discussion In the igneous and migmatite complexes of Rogaland, pyroxene crystallisation took place between c. 750 oc and c. 590 oc (at !east on the scale of electron microprobe analysis). Reequilibration of igneous pyroxenes is of the same order of magnitude. The range of temperatures may be partically explained by incomplete readjustment to·subsequent ambient temperature regimes.
Crystallisation temperatures in igneous rocks are considerably higher than the regional crystallisationlreequilibration temperatures (Table 2). A comparable situation is reported from metamorphosed anorthosites in the Adirondacks (Boblen & Essene 1978).
A Time-Temperature diagram (Fig. 4) summarises data from the introduction. During the thermo-metamorphic stage (M2) at about 1050-1035 Ma, temperatures reached c. 1000-800 oc in migmatites adjacent to the igneous complex. Isotopic ages and closure temperatures for osumilite (>550 °C), hornblendes (ca 550 oq and brown
2 - Geologisk Tidsskr. 1184
biotite ( 450-400 oq indicate that subsequent to the M2 stage the initial cooling rate of c. 6 oc/Ma gradually decreased to c. 1.6 °C/Ma. The thermal regime of regional pyroxene (re-) equilibration existed between c. 1030 - c. 990 Ma (Fig. 4).
Reequilibrated igneous and metamorphic pyroxenes occur in the (Quartz-) Monwnitic Phase, indicating that its solidification was completed before the regional event took place. The structural unconformity between the Leuconoritic and (Quartz-) Monzonitic Phases indicates a discontinuous emplacement history for the Bjerkreim-Sokndal Layered Intrusion. The extent of the hiatus is presently unknown, but it is tentatively bracketed between (1050-1035) Ma and c. (1030-990) Ma. Maijer et al. (1981) argued that the thermal influence caused by intrusion of the (Quartz-) Monzonitic Phase was negligible. The period of regional cooling (M3 stage) is tentatively estimated between c. 1030 to c. 850 Ma.
Recrystallisation (granulation) in the (Quartz) Monzonitic Phase is accompanied by formation of extreme poikiloblasts of iron-rich clino-amphiboles. Amphiboles in the Precambrian basement of Rogaland contain fluorine [F/ (F+OH) = 0.1 -0.19) (Dekker 1978) and chlorine (preliminary
18 F. J. M. Rietmeijer
1200
1100
1000
900
u 800 g_
600
500
NORSK GEOLOGISK TIDSSKRIFT I (1984)
horn blendes
400
1050 1000 950 Time (Ma)
900
Rb-Sr :t!::: 1 brown biotite ..__x-iK-Ar
850
Fig. 4. Time (Ma) vs Temperature ("C) diagram showing the cooling history of the Precambrian charnockitic migmatite terrain in SW Norway between c. 1050 and 850 Ma. The Time- Temperature space for the Leuconoritic Phase of the Bjerkreim-Sokndal Layered Intrusion is indicated in the upper teft-hand corner. Isotopic ages are given for osumilite (Maijer et al. 1981), hornblendes (Dekker 1978) and brown biotite (Verschure et al. 1980). The range of crystallisation temperatures for the (Quart�-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion, c. 950 "C - c. 850 •c, is shaded. Metamorphic pyroxene crystallisation and reequilibration took place in the range bracketed by the dashed horizontal lines.
data by the author). Both elements will stabilise amphiboles to higher temperatures (amongst others, Kearns et al. 1980). The presence of halogens may explain that the amphiboles could have formed in the temperature regime of pyroxene (re-) equilibration.
The sample set used for determination of hornblende K-Ar isotopic ages (Dekker 1978) includes a sample of an extreme poikiloblast. The hornblende isotopic age (953 ± 10 Ma) may thus represent the autometamorphic stage of the Bjerkreim-Sokndal Layered Intrusion suggested by Rietmeijer & Dekker (1978). Alternatively, it may indicate a phase of regional amphibole formation implied by Dekker's (1978) observation
that almost all primary amphiboles in the area seem to be of metamorphic origin.
The recalculated Rb-Sr whole-rock isochron age for the (Quartz-) Monzonitic Phase (928 ±50 Ma) conveniently includes the K-Ar hornblende isotopic age as well as the U-Pb zircon ages (Wielens et al. 1981, Pasteels et al. 1979). Petrological evidence and isotopic ages do not agree with the interpretation of Wielens et al. (1981) that the intrusion age of the (Quartz-) Monzonitic Phase is 928 ± 50 Ma. Instead it seems that Rb-Sr whole-rock and U-Pb zircon isotopic systems have been reset during the M3 stage and may be contemporaneous with the formation of halogen-bearing amphiboles.
NORSK GEOLOGISK TIDSSKRIFT l (1984)
Conclusions l. Metamorphic pyroxene crystallisation and
(re-)equilibration of igneous pyroxenes took place on a regional scale (M3 stage) between c. 750 oc and c. 590 oc around 103�990 Ma.
2. Intrusion of the (Quartz-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion took place between c. 105� 1035 Ma and c. 103�990 Ma.
3. Halogen-bearing amphiboles formed during a period of regional cooling (M3 stage) and/or during a stage of autometamorphism in the Bjerkreim-Sokndal Layered Intrusion.
4. Rb-Sr and K-Ar isotopic systems may have been reset during the M3 stage and be contemporaneous with the formation of halogenbearing amphiboles.
Acknowledgements. - l wish to express my gratitude to the 'Afdeling Petrologie, lnstituut voor Aardwetenschappen' (Utrecht, the Netherlands) for use of its unpublished data. Professor A. C. Tobi, Drs. G. A. E. M. Hermans, P. C. C. Sauter, J. B. W. Wielens and A. G. C. Dekker critically reviewed the manuscript. Special thanks are due to Dr. Cees Maijer, with whom l have spent many hours discussing the possible Time-Temperature schemes for Rogaland.
References
Manuscript received August 1982,
revised April 1983
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Pyroxenes in Precambrian terrain 19
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lshii, T. 1975: The relations between temperature and composition of pigeonite in some lavas and their application to geothermometry. Mineral. J. 8, 48-57.
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