Abstract
In the Precambrian granulite facies terrain of Rogaland, SW Norway,
some small occurrences of marbles are present. They are mainly exposed
at three locations A, Band C, at increasing distance from the anorthositic
and monzonitic intrusions. The Precambrian basement in Rogaland
has undergone several high-grade metamorphic events: MI around
1200 Ma, the granulite facies M2
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around 1050 Ma and M3 around 950 Ma.
Late retrogressive events M4a and M4b have a Caledonian age.
The marbles belong to the Faurefjell formation, which furthermore
consists mainly of quartzites, quartz-diopside gneisses, diopside
rocks, alkalifeldspar-rich rocks and norites (Chapter I). The marbles
are mainly forsterite marbles with the mineral association Fo-Cc-Phl±
Do±Di±Sp. The mineral associations in the forsterite marbles indicate
metamorphic temperatures of about 7000 C for location C to higher temperatures
for location A. Some diopside marbles occur, showin~ the
mineral association Di-Phl-Cc. These rocks and the diopside rocks
(Di, Di-Phl) typically show high-variance assemblages, indicative of
a metasomatic origin. The minerals in the marbles show a high-Mg
chemistry. ~1g is higher than 0.85 for all minerals. With increasing
XMo the order of minerals is: Sp>Cc>Fo>Phl~Di~Do.
~Diopside (Chapter 2) can be strongly zoned, showing an Al-, Ti- and
Fe3+-rich core and Mg- and Si-rich rims. The maximum range observed
in one crystal (containing the highest Al content) is 2.5 to 8.5 wt%
A1 203' The highest Al contents in diopside are found in spinel-bearing
marbles. A late, low-AI stage of diopside formation has taken place
as a result of metasomatic reactions, presumably during M3.
The calcites in the marbles show a considerable amount of dolomite
exsolution (Chapter 3). Dolomite exsolution in the form of tablets,
rhombohedra and symplectites is the most common. The tablet exsolution
type formed during the post-metamorphic cooling stage, the rhombohedral
and symplectitic types presumably during Caledonian M4a and M4b
retrogressive events. The exsolution of dolomite has resulted in a
decrease of the Mg content of calcite. Recalculated Mg contents reveal
temperatures of about 7000 C for locations Band C. Because of
the absence of dolomite, the higher temperature marbles of location A
show lower Mg contents in calcite.
The carbon and oxygen isotope ratios of calcite show a trend from
d I3C=-4 (PDB) and d I80=+21 (SMOW) for location A, values close to
those of unmetamorphosed Precambrian carbonate rocks, to d I3C=-7.5
and d I80=+14 for location C. Isotopic compositions of veins and Caledonian
marbles close to or just above the Caledonian boundary, indicate
that the isotopic depletion in the Precambrian marbles mainly is
the result of interaction with Caledonian retrogressive fluids.
The chemistry of forsterite, phlogopite and spinel is fairly constant
within a sample (Chapter 4). They show the Mg-rich chemistry common in siliceous dolomites. Forsterite shows comparatively high Mn
contents (up to 0.8 wt% MuO). Phlogopite shows high Ba contents, up
to 4.8 ,vt% BaO in a normal phloBOpi te, and about 20% BaO and 12 wt%
Ti02 in a single occurrence of Ba-phlogopite. Spinel contains Zn, up
to 2.4 wt% ZnO.
Mg-Fe distribution between forsterite and diopside is mainly dependent
on the Al content of diopside. A single occurrence of diopsideenstatite
in location A suggests high (9000 C) equilibration temperatures.
Forsterite and spinel show extensive reequilibration chemistry.
Geothermometry for these Cr-free spinel-olivine pairs is not reliable.
Textural and chemical data for the minerals indicate a polymetamorphic,
partly metasomatic history of the Rogaland marbles. Several main
minerals were formed pre-MI, and during MI, M2 and M3. The formation
of retrograde minerals, like clinohumite, tremolite and chlorite, took
place from late-M3 to M4b stages.
The whole rock chemistry of the Faurefjell rocks (Chapter 5) shows
a trend from Ca-Mg rich, Si-poor compositions in the forsterite
marbles, towards compositions higher in Si, Al and alkalies in diopside(-
phlogopite) rocks, diopside-alkalifeldspar rocks, quartz-diopside
gneisses and quartz-alkalifeldspar rocks. Chemical characteristics
of the marbles, especially the Ca/Mg and the Sr/Ca ratios, indicate
sedimentary affinities for these rocks, while anomalous high contents
of, in particular, Ba and Si indicate possibly early hydrothermal
activity as well as higher-grade metasomatism
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