Secondary ilvaite in a dolerite dyke from Rogaland, SW Norway

TLDR: Ilvaite is an alteration product of carbonate, which itself replaced clinopyroxene, in a Precambrian tholeiitic dyke from Rogaland, SW Norway.

Abstract: A BST R ACT. llvaite is an alteration product of carbonate, which itself replaced clinopyroxene, in a Precambrian tholeiitic dyke. Additional secondary minerals are ferroactinolite, cummingtonite, chlorite, biotite, prehnite, and epidote. Microprobe analyses demonstrate that the ilvaite is close to the ideal end-member composition [CaFe22" Fe3+Si2Os(OH)] and that only cummingtonites, Mg/(Mg+Fe 2+) 0.77-0.39, and carbonates (solid solutions between calcite and dolomite, dolomite and ankerite, magnesite and siderite) show significant compositional variations. It is estimated that ilvaite formed at temperatures < 470 ~ and at pressures < 2 kbar, probably during a low-grade metamorphic event (or events) associated with the Caledonian orogeny. The fluids involved in the alteration process contained both CO2 and HzO, initial fluids probably being richer in CO2 than later ones. Ilvaite may be more common in hydrothermally altered igneous rocks than previously supposed. ILVAITF, CaFe22+Fea+Si2Os(OH), is a relatively rare mineral which typically occurs in Ca-Fe-Si skarn deposits (Fonteilles, 1962; Bartholom6 and Dimanche, 1967; Burt, 1971a; Verkaeren, 1974; Plimer and Ashley, 1978). It is also found in contact metamorphosed iron formations (Ramdohr, 1927; Dietrich, 1972) and as a late-stage alteration product in some igneous rocks, perhaps the bestknown examples of which are the Upper Border Group of the Skaergaard intrusion (Wager et al., 1957; Naslund et at., 1983), and the Basistoppin sill (Douglas, 1964) which intrudes the upper part of the Skaergaard intrusion. The mineral occurs as an alteration product in a dolerite dyke from Rogaland, SW Norway, and has not been previously reported from Rogaland despite the occurrence in this region of iron-rich metamorphic assemblages containing andradite and hedenbergite (minerals which are frequently associated with ilvaite in skarn deposits). Moreover, its mode of occurrence is quite unlike that in most other igneous rocks in that other extremely Fe-rich silicates are absent. In this paper we discuss the paragenesis and mineral chemistry of the assemblage in the dykes.