Metamorphism of siliceous dolomites in the high-grade Precambrian of Rogaland, SW Norway

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 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.