3. RESULTS
3.3 W HOLE - ROCK GEOCHEMISTRY
3.3.1 Major element geochemistry
The samples that were chosen for geochemical analyses have SiO2-content of 37,42-49,35 wt.
% and a MgO-content of 33,82 to 46,35 wt. %. The two major elements are clearly anti-correlated, when the SiO2-content is high the MgO-content is lower and vice versa. This relationship fits fairly well with the observed serpentinization degree in thin sections and observations made in the field. In samples showing high degree of serpentinization the SiO2 -content is higher than the -content, in samples showing little serpentinization the MgO-content is higher than the SiO2-content. The MgO-content is leaching form the meta-peridotite during hydration, in a similar way to that described from ultramafic clasts in the
conglomerates of the Solund basin (Beinlich et al., 2010). Another characteristic feature is the low content of Al2O3 (0,08-1,40 wt. %) and low content of CaO (0-4,45 wt.% the most
common is a content below 0,015wt.%). All the data from the analyses can be seen in Table 6.
When Al2O3, Fe2O3, Mn3O4 and Cr2O3 are plotted against SiO2 in Harker diagrams (Fig. 45) it can be seen that with increasing SiO2 levels the Mn3O4-content decreases, it can also be seen a weak correlation between Al2O3 and SiO2, when the Si levels increase the Al-content also increase. The relationship between Ni and Si are also seen to have a positive correlation, as can the Mg and Si-content.
The samples that are analysed from Vetle Raudberget (a-sel-34-14 and a-sel-40-14) show wide range of SiO2-content (49,35 and 38,68wt.%) and MgO-content (33,82 and 46,43wt.%).
The Fe2O3-content show very little alteration (7,23-7,27wt.%). The content of Al2O3 (0,41 and 0,08wt.%), Mn3O4 (0,07 and 0,15wt.%), CaO (0,00 and 0,05wt.%), Cr2O3 (0,54 and
0,42wt.%) and NiO (0,25 and 0,37wt.%) are very low.
75 The samples analysed from Raudberget also show small variation in SiO2content (36,38 -41,51wt.%) and a bit more variation in the MgO-content (34,26-46,35wt.% with average 41,38wt.%). The Fe2O3-content is fairly low (6,61-12,42wt.% with average 8,17wt.%). The content of Mn3O4 (0,15-0,07wt.%), Al2O3 (0,13-1,40wt.% with average 0,52wt.%), Cr2O3
(0,41-2,36wt.% with average 0,72wt.%), CaO (0,02-4,45wt.% with average 0,60wt.%) and NiO (0,29-0,51wt.% with average 0,35wt.%) are very low.
Figure 45. Selected oxides plotted against SiO2. In A, D and F a positive correlation can be observed. In C a negative correlation can be observed.
76 Table 6 Showing the major and minor elements from the XRF analyses carried out on some samples from Raudberget and Vetle Raudberget. The
sample number 40 and 34 are taken at the Vetle Raudberget peridotite, while the rest of the samples are from Raudberget
a-sel-56-14 a-sel-40-14 a-sel-06-14 a-sel-52-14 a-sel-49-14 a-sel-34-14 a-sel-20-14 a-sel-09-14 a-sel-02-14 a-sel-11-14
SiO2 37,42 49,35 39,83 40,13 40,82 38,68 36,39 38,40 36,38 41,51
TiO2 0,00 0,00 0,00 0,00 0,00 0,00 0,01 0,00 0,00 0,00
Al2O3 0,29 0,41 0,34 0,60 0,64 0,08 0,47 0,13 0,32 1,40
Fe2O3 8,75 7,23 7,23 7,88 6,61 7,27 6,86 7,27 12,42 8,32
Mn3O4 0,15 0,07 0,10 0,13 0,10 0,15 0,12 0,11 0,17 0,07
MgO 43,38 33,82 42,83 42,50 42,24 46,43 39,60 46,35 39,91 34,26
CaO 0,02 0,00 0,08 0,15 0,09 0,05 0,06 0,01 0,00 4,45
Na2O 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 -0,01
Cr2O3 0,41 0,54 0,41 0,44 0,38 0,42 0,39 0,42 2,36 0,95
NiO 0,33 0,25 0,32 0,31 0,31 0,37 0,29 0,36 0,34 0,51
LOI % 9,43 8,60 9,22 8,30 9,21 6,97 16,18 7,32 8,19 8,62
Total 100,17 100,27 100,36 100,46 100,40 100,42 100,37 100,38 100,08 100,10
77 3.3.2 Trace element geochemistry
Five of the samples chosen for major and minor analyses were also chosen for trace element analyses at the ICP-MS. The samples showing most differences in the major and minor elements were chosen for the trace element analyses.
The trace element geochemistry show little difference with respect to the lithologies the sample was sampled. There are also little difference with respect to Raudberget and Vetle Raudberget. Most of the trace elements are below or at the detection limit of the ICP-MS.
This is confirmed by Table 7, Figure and Figure The whole rock normalized to chondrite (Figure ) show a negative trend for the LREE while the MREE and HREE are missing since they are below the detection limit of the ICP-MS. Many of the same observations are made for the whole rock normalized to primitive mantle, also in this plot many of the elements are below the detection limit. One trend is however seen in each sample; there is a negative trend between La and Ce.
The low trace element concentrations will be further discussed in the next chapter
78 Table 7 Showing the distribution of trace elements analysed at the ICP-MS. All values are given in ppm. b.d represent the elements that were below the detection limit of the ICP-MS
Analyte symbol Detection limit
79
Figure 46. Primitive mantle plot normalized after values from Sun and McDonough (1989). There are no clear trends, except for the decreasing trend from La to Ce to, see from this plot, since most of the values are at or below the detection limit.
0,0001 0,001 0,01 0,1 1 10 100 1000 10000
Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Y Ho Er Yb Lu
whole rock normalized to primitive mantle
A-SEL-02-14 A-SEL-11-14 A-SEL-20-14 A-SEL-40-14 A-SEL-56-14
80
Figure 47. Whole rock normalized to chondrite after values from Sun and McDonough (1989). The plot shows a negative trend for the LREE, while the MREE and the HREE are missing since they are below the detection limit for the ICP-MS.
0,001 0,01 0,1 1 10 100
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Whole rock normailized chondrite
A-SEL-02-14 A-SEL-11-14 A-SEL-20-14 A-SEL-40-14 A-SEL-56-14
81
3.4 Mineralogy and mineral chemistry
The minerals have been identified in thin sections (25) under the microscope. The sections were then taken to the microprobe to get mineral chemistry from the minerals and also to confirm the identification from the microscopy. The reason for the mineral chemistry was to see if there were any primary minerals preserved in the peridotite body; if present they will be used for further classification. Each mineral group show little variation. All of the analyses of the minerals are shown in appendix B-J. Some of the analyses of each mineral have been removed from the presentation of the mineral chemistry due to bad microscopy analyses.
For the analytical procedure done at the EMP see the methods chapter
3.4.1 Olivine
The olivine (Fig. 28) found in the different lithologies all show a characteristic high MgO-content (49-55Wt. % with average 52,84Wt. %) and the fo-number (90-98) is thus very high.
The SiO2 (37,46-42,10Wt.%) content is also high, the FeO-content (2,65-9,63Wt.% with average 5,69Wt.%) The Al2O3 (0-0,0742Wt. %), Na2O (0-0,06Wt. %), K2O (0-0,03Wt.%), CaO (0-0,08Wt.%), TiO2 (0-0,03Wt%) and Cr2O3 (0-0,19Wt.%) contents are low. The content of MnO (0,12-0,66Wt.% with average 0,24Wt.%) and NiO (0,27-0,57Wt.% with average 0,42) are also fairly low.
As seen in Fig. 48 the olivine grains do not differ in composition depending on, which lithology they were sampled from. The variance in the other elements except MgO and SiO2
are also very low.
82 Figure 48. Plot of Fa vs Fo, the plot is not showing any significant difference with respect
to the position of the grains or if they had a phorphyroclastic or a foam structure.
3.4.2 Diopside
The diopside (Fig. 49) show high content of SiO2 (52,91-56,20Wt.% with average
54,65Wt.%) and high levels of CaO (24,41-26,74Wr.% with average 25,61Wt.%) (Fig. 49) and MgO (17,49-18,77Wt.% with average 18,00Wt.%). The content of Na2O
(0,00-0,36Wt.%), Al2O3 (0,00-1,63Wt.%), K2O (0,00-0,02Wt.%), TiO2 (0,00-0,03Wt.%), FeO (0,59-1,82Wt.%), MnO(0,00-0,19Wt.%), Cr2O3 (0,03-0,68Wt.%) and NiO (0,00-0,10Wt.%) show very low levels.
The lowest values of the Al2O3-content are seen in samples with severe serpentinization, while the highest values are observed in samples with less serpentinization and in contact with the most FeO rich olivines.
83 The content of Na2O and Cr2O3 from the diopside has been used in a plot to classify if the dunite has a continental lithosphere origin or oceanic lithosphere origin (Kornprobst et al.,, 1981). The plot shows an oceanic origin as seen in Fig. 50 This will be further discussed in the discussion.
Figure 49. Classification scheme of Clinopyroxene. As can be seen from the plot the Pyroxene plots on top or above the diopside field due to the high content of CaO. See chapter 4 for further discussion of their formation and metamorphism.
Figure 50. Plot of the Cr*1000 vs Na*1000 to classify the peridotite as oceanic lithosphere or continental lithosphere. After (Kornprobst et al., 1981).
84 3.4.3 Magnetite/Ferrian chromite
The magnetite rims (Fig. 31B) are characterized by a very high content of FeO (68, 66-87,84Wt.%) and varies a bit. The content of SiO2 (0,00-0,33Wt.%), Na2O (0,00-0,06Wt.%), Al2O3 (0,00-0,03Wt.%), K2O (0,00-0,03Wt.%), CaO (0,00-0,04Wt.%), TiO2
(0,00-0,09Wt.%), MnO (0,05-0,51Wt.%), MgO (0,51-3,63Wt.%), Cr2O3 (1,02-12,16Wt.%) and NiO (0,61-1,72Wt.%) are quite low.
The FeO-content and Cr2O3 content varies a bit in the rims, this imply that the magnetite is not a pure magnetite, but do have a little Cr2O3 content in the rims as well (Fig. 51).
The ferrian-chromite (Fig 31B) has high content of Cr2O3 (34,84-51,07Wt%) and high levels of FeO (21,11-55,49Wt.%). The Al2O3-content (0,19-21,61Wt.%) and MgO
(1,63-14,32Wt.%) varies a bit. The SiO2 (0,03-0,40Wt.%), Na2O, K2O , CaO and TiO2-content are low, below 0,05Wt.%. The MnO (0,42-1,24Wt.%) and NiO (0,02-0,35Wt.%) are quite low.
Figure 51. Classification scheme of the different spinel minerals. It can be shown that the most abundant is magnetite, which in most cases represents the outer rims of zoned minerals with ferrian-chromite in the cores. The Al2O3, Cr2O3 and FeO content is calculated by dividing the respective element on the sum of Al2O3+FeO+Cr2O3 . Notice also that one point has been classified as Cr-magnetite, this imply that the rims of magnetite is not always pure magnetite.
85 3.4.4 Carbonates
In this work there has been observed two kinds of carbonates in the rocks from Raudberget and Vetle Raudberget; calcite and dolomite. In NGU reports there have been seen in addition magnesite and breunnerite (Bakke, 1985).
The calcite is characterized by a CaO (52,83-55,56Wt.%) content with very little variation.
The SiO2 (0,0038-0,034Wt.%), FeO (0,00-0,11Wt.%), MnO (0,04-0,11Wt.%) and MgO (0,00-1,57Wt.%) are low in the calcite minerals
The dolomite is characterized by a lower content of CaO (30,50-31,22Wt.%) than the calcite minerals and moderate level of MgO (20,02-23,74Wt.%). The content of SiO2
(0,008-0,025Wt.%), FeO (0,22-0.52Wt.%) and MnO (0.14-0.19Wt.%) are very low.
The magnesite is characterized by a very low content of CaO (0-0.41Wt.%), MnO (0.08-0.64Wt.%). The FeO-content is fairly low (1.40-8.28Wt.%) and the MgO
(40.40-48.33Wt).% is high (Bakke, 1985).
The breunnerite is characterized by a low CaO (0.12-0.41 Wt.%) and MnO (0.32-0.83 Wt.%). The FeO-content is fairly low (10.83-13.32 Wt.%) and the MgO (38.58-42.72 Wt.%) (Bakke, 1985).
3.4.5 Chlorite
The chlorite (Fig. 42) is characterized by a SiO2 -content of (28.99-34.67Wt.% with average 31.56Wt.%) and MgO (26.77-34.58Wt.% with average 31.38Wt%). It can also be seen that the levels of MgO varies a bit. The content of Al2O3 (12.11-18.90Wt.% with average 15.13Wt.%) is moderate to low and this value also varies a bit. The content of K2O (0.00-0.013Wt.%), CaO (0.014-0.017Wt.%), TiO2 (0.00-0.08Wt.%), MnO (0.00-0.16Wt.%), Cr2O3 (0.05-2.92Wt.%) and NiO (0.16-0.37Wt.%) have very low concentrations.
3.4.6 Serpentine
The serpentine is characterized by a high content of SiO2 (32.70-44.75Wt.% with average 41.82Wt.%) and MgO (22.73-44.56Wt.% with average 39.08Wt.%). It can also be seen that
86 the content of this elements varies a bit. The content of Na2O (0.00-0.036Wt.%), K2O (0.00-0.03Wt.%), CaO (0.00-0.10Wt.%), TiO2 (0.00-0.04Wt.%), MnO (0.00-0.23Wt.%), Cr2O3 (0.00-1.13Wt.%) and NiO (0,00-1.12Wt.%) content are also very low. The FeO (1.25-8.41Wt. %) and Al2O3 (0.00-10.53Wt. %)
There could not be seen any differences between the different locaTiO2ons of serpenTiO2ne e.g. serpentine in veins and serpentine in matrixes.
Any X-ray diffraction (XRD) analyses on the serpentine minerals have not been run; this is the only way to differ between the different serpentine minerals since it is impossible to distinguish between them on the basis of geochemistry.
3.4.7 Talc
The talc (Fig. 26) is distinguished by a high content of SiO2 (58.55-64.75 Wt. % with average 61.07 Wt. %). The MgO (26.44-29.13 Wt. % with average 27.81) is fairly high the content of FeO (2.31-4.20 Wt. % with average 3.36 Wt. %) is low. The Na2O (0-0.05 Wt.%), K2O (0-0.14 Wt.%), Al2O3 (0.05-1.26 Wt.% with average 0.26 Wt.%), CaO (0-0.11 Wt.%), TiO2 (0-0.03 Wt.%), MnO (0-0.09 Wt.%), Cr2O3 (0-0.04 Wt.%) and NiO (0.04-0.25 Wt.%) contents are very low.
3.4.8 Amphiboles
The tremolite is characterized by a high content of SiO2 (51.67-63.24 Wt.% with average 57.74 Wt.%). The MgO (21.72-31.71Wt. % with average 23.53 Wt.%) is fairly high, while the CaO (0.02-14.09Wt.% with average 13.36Wt.%) varies a bit. The FeO (1.58-3.93Wt.%
with average 2.94Wt.%) is low. The Na2O (0-0.03 Wt.%), Al2O3 (0.01-0.42Wt.%), K2 O (0-0.04Wt.%), TiO2 (0-0.03Wt.%), MnO (0.0-0.21Wt.%), Cr2O3 (0-0.10Wt.%) and NiO (0.06-0.20Wt.%)
The actinolite is characterized by a high content of SiO2 (54.53-58.22 Wt.% with average 56.60Wt.%). The MgO (19.61-22.32Wt. % with average 20.97 Wt.%) is fairly low and the CaO (12.64-13.83Wt.% with average 13.01Wt.%) varies a bit. The FeO (4.42-6.64Wt.%
with average 5.39Wt.%) is fairly low. The Na2O (0.02-0.35Wt. %), Al2O3 (0.02-2.69Wt. %),
87 K2O (0.01-0.07Wt.%), TiO2 (0-0.04Wt.%), MnO (0.08-0.41Wt.%), Cr2O3 (0.-0.67Wt.%) and NiO (0-0.17Wt.%) content are very low.
The magnesio-hornblende is characterized by a high SiO2 (45.14-53.04 Wt. % with average 49.95). The MgO (18.01-23.01Wt. % with average 20.65 Wt.%) is fairly low and the CaO (10.93-12.86 Wt.% with average 12.03 Wt.%) is also low. The Na2O (1.22-2.02 Wt. %) and Al2O3 (4.08-9.91 Wt.% with average 6.63 Wt.%) are quite low. K2O (0.13-0.67 Wt.%), TiO2
(0.11-0.66 Wt.%), MnO (0.10-0.34 Wt.%), Cr2O3 (0-0.56Wt.%) and NiO (0.05-0.17 Wt.%) are very low.
Figure 52. Classification of amphiboles as can be seen the two lithologies with amphiboles consist of tremolite, actinolite and magnesio hornblende. For a detailed description of their chemistry see section above.
3.4.9 Brucite
The brucite has a high MgO (46.46-47-42 Wt.%) and low levels of FeO (1.85-2.58 Wt.%).
The content of SiO2 (0-0.06 Wt.%), Al2O3 (0-0.03 Wt.%), K2O (0-0.01 Wt.%), CaO (0-0.04 Wt.%), TiO2 (0.007-0.03 Wt.%), MnO (0.38-0.47 Wt.%) and NiO (0.02-0.05 Wt.%) are very low.
88
4. Discussion
The first part of the discussion will present on how to use mineral chemistry, major, minor and trace element geochemistry in classifying peridotites and the problems with the classification due to hydration reactions. Based on this knowledge it will be given an interpretation of the data collected from the studied peridotite. Lastly the origin of the peridotite, as remnants of hyperextended exhumed mantle versus ophiolite will be discussed with respect to the peridotite and the country rock association.
4.1 Mineral geochemistry
4.1.1 Clinopyroxene
The Al, Ti and Na content of CPX can be used when looking at differences between a suboceanic or a subcontinental origin of peridotites (Seyler and Bonatti, 1994). The Al content in CPX varies between 2-8Wt.%, where the average subcontinental CPX has 6-7 Wt.% while the subocceanic has 5 Wt.% (Seyler and Bonatti, 1994). It has also been observed that CPX from suboceanic
mantle has a higher ratio of Al in octahedral position than tetrahedral position while the opposite is shown for the subcontinental lithosphere (Fig. 53).
The Ti-content can also be used in discriminating between the two types of origin (Seyler and Bonatti, 1994).
Kornprobst et al. (1981) showed that the Na-content in subcontinental lithosphere was higher than in the subocceanic lithosphere (Fig. 50). He further suggested that the ratio between Cr and Na in CPX could be used as a
89 and suboceanic mantle. This observation was criticised by Sen (1982) who argued that data from some oceanic and continental xenolithes overlap and that Kornprobst et al. (1981) used both spinel and plagioclase peridotite oceanic samples. The observations concerning Na was confirmed by Seyler and Bonatti (1994).
4.1.2 Spinels
The concentration of Al, Cr, Mg and Fe in spinels can be used when differentiating between abyssal versus continental peridotites and, which peridotite field present (Dick and Bullen, 1984). The Cr# (Cr/ (Cr+Al)) and Mg# (Mg/ (Mg+Fe)) in spinels can also be used when dividing the peridotite into different environments (Fig. 54). It is seen that peridotites from fore-arc environments have higher Cr# and low Mg# while the abyssal peridotite has lower Cr# and higher Mg# (e.g. Okamura et al., 2006).
Figure 54. Showing the distribution of Cr# from mid-ocean ridges to Alpine peridotites. The green area shows the Cr# from the ferrian-chromites from Stølsheimen. Modified from Lee (1999)
4.1.3 Olivine
The Fo-content in olivine can also be used as an indicator for, which environment the peridotite body did form. Bonatti and Michael (1989) looked at the whole rock chemistry
90 and mineral chemistry from peridotites from subduction zones to undepleted continental lithosphere. From their interpretation it can be seen a linear trend from 88 % in the
undepleted continental lithosphere increasing towards 92% Fo in oceanic trenches (Fig 55).
Figure 55. Distribution of fo % from continental undepleted lithosphere to oceanic trench.
An increasing trend from continental to oceanic trench can be observed. The green area mark the most abundant fo-content from Stølsheimen plot, as can be observed those result
91 plot outside the area defined for the different tectonic environments. Modified from Bonatti
and Michael (1989).
4.1.4 Alteration of mineral chemistry due to serpentinisation and hydration
The most basic serpentinization reactions were presented in the introduction page 23. Below follows a more detailed description of the serpentinization process with respect to the
minerals observed in the studied peridotite.
Most of the samples analysed from the two peridotite bodies in Stølsheimen show a high degree of serpentinization, some magnetite, diopside and at one locality some brucite. This fit really well with peridotites metamorphosed in the greenschist facies (Winter, 2010).
When brucite is present it has lower concentration than serpentine and will be consumed when reaction with antigorite at approximately 400oC;
20Brc + Atg= 34 Fo + 51H2O
The olivine (forsterite) created at this point is more rich in Mg than the original olivine before the serpentinization process began. The typical peridotite at this point has the
minerals diopside, antigorite and forsterite. It should be mentioned that the diopside minerals should not be taken for primary minerals, this is a metamorphic mineral. If primary CPX is present it is in the form as augite (Bucher and Grapes, 2010). When the temperature reaches approximately 530oC;
8 Di + Atg = 18 Fo + 4 Tr + 7 H2O
This point includes the loss of diopside and the creation of tremolite.
The main role of Al is to stabilize Al rich phase often chlorite, this mineral has been seen in the talcified regions of the peridotite bodies, but is not very abundant.
Some of the metamorphic minerals from the studied bodies differ from the usual geochemistry seen in such minerals. Below follows a discussion of theese minerals.
92 4.1.5 Rodingitization
The diopsides from the Raudberget dunitic body show a high content of CaO (27 Wt. %) and low content of Al2O3 (0-1.63 Wt. %)( see previous chapter). The Al2O3 content in cpx varies from 2-8 Wt.% in peridotite bodies, with average 5 Wt.% in subcontinental lithosphere and 6-7 Wt.% in sub-oceanic lithosphere (Seyler and Bonatti, 1994). The low Al2O3-content implies that the diopsides are not primary minerals, the classification of subcontinental vs sub-oceanic lithosphere made in the previous section may for this reason be debateable since it may not a primary mineral. It can also be seen that the high Ca-content in the diopsides makes the diopside plots at the top or above diopside field in the classification scheme (Fig.
49).
In the Oman ophiolites the diopside occurs as diopisidte dykes in the peridotite section of the ophiolite. The diopside is characterized by a high CaO-content (Python et al.,, 2007) that is very similar to that found in the Raudberget body. The diopside has been suggested to be Ca enriched from seawater that may penetrate into the mantle. The seawater gets enriched in Ca when travelling through the crust (Python et al., 2007). In hyperextended passive margins it are known that seawater travels along detachment faults and serpentinize the mantle (e.g.
Escartin et al., 1997).
Figure 56. Model of how seawater penetrates into the crust and get enriched in Ca, The Ca rich fluid then starts to metasomatize the mantle and recrystallize the diopsides rich in Ca.
93 Diopside is commonly observed in contact with serpentine and secondary olivine ( Fig. 31).
This also points towards serpentinization as a process that can create the diopside. From mafic rocks it is known that rodingites form during serpentinization. The term rodingite was first used by Bell et al., (1911) to described grossular rich veins crosscutting serpentinite.
This has later been defined to be a process that happens when the primary CPX is broken down, when this happens a Ca rich fluid is created (Bilgrami and Howie, 1960, Capedri et al.,, 1978, Dubinska, 1997, Frost et al.,, 2008). This fluid starts to metasomatically alter the mantle. This can explain why the diopsides have such a high Ca-content and also why the diopsides have quite a high content of Si. The high content of Mg and the low content of Al on the other hand cannot be explained by rodingitization. This could be due to this being a ultramafic and not mafic rock (Python et al., 2007). Rodingites normally form by Ca- metasomatism in mafic rocks (e.g. Austrheim and Prestvik, 2008), whereas the diopside recrystallizes in ultramafic rocks. Python et al. (2007) has called the process in ultramafic rocks for “diopsidization”. It is also important to mention that the diopsidization has only been seen to occur in dunitic or harzburgitic rocks. In the Oman ophiolites it can be seen that this take place in the upper dunitic transition zone. It is likely that since the process takes place in an ultramafic rock, that the Al-content is very low and the Mg-content is
correspondingly high.
This process is also known from the Melange unit at Kopparholmen (Fig. 11) structurally below to the Lindås Nappe (Bøe, 1985). Here two amphibolite dykes consist of garnet, diopside, sphene, epidote, calcite and some serpentine occur. The diopside mineral is seen to have a very high content of CaO (47-50 Wt.%). Metasomatic alterations have destroyed every primary minerals and only secondary is left. This represent a complete rodignitization process (Bøe, 1985).
4.1.6 Metamorphism of magnetite and ferian chromite
The content of Al and Cr in primary magnetite is quite distinct from the content in secondary
The content of Al and Cr in primary magnetite is quite distinct from the content in secondary