Contrib. Mineral. Petrol. 72, 311-317 (1980) Contributions to Mineralogy and Petrology 9 by Springer-Verlag 1980 180/160 Ratios of Anorthosites and Related Rocks From the Rogaland Complex (SW Norway) D. Demaiffe 1 and M, Javoy a 1 Laboratoire de Min6ralogie-P~trologie, Universit6 Libre de Bruxelles, Avenue Roosevelt 50, 1050 Bruxelles, Belgium 2 Laboratoire de Q6ochimie des Isotopes Stables, D6partcment des Sciences de la Terrc, Universit6 de Paris VII, Paris, France Abstract. The South Rogaland Complex (South West- ern Norway) consists of several anorthositic intru- sions emplaced in granulite facies metamorphic rocks. The anorthosites and related norites and jotunites have 6180 values of 5.2 to 7%0 suggesting a mantle origin for these rocks, in agreement with the stron- tium isotopic evidence. The acidic rocks, mostly char- nockitic, associated with the anorthosites have similar 6180 values and thus a comagmatic relation between these two rock types is inferred. Small departures from mantle values are explained in terms of crustal contamination by surrounding gneisses that have 5 ~ 80 values between 4.3 and 10% o. Locally, this corre- sponds to important anatexis as has been suggested for the Farsund charnockite on the basis of strontium isotope and REE geochemistry. The isotopic tempera- tures calculated from the isotopic fractionations are in the range 500~ ~ C, lower than the orthomag- matic temperatures and probably due to subsolidus isotopic exchange during the slow cooling of these plutonic rocks, either during a late magmatic deutcric stage or during a stow, postorogenic ascent under wet conditions. I. Introduction The genesis of anorthosites has interested many petro- logists, i.e. Bowen, Barth, Buddington .... but impor- tant questions still remain unanswered: (e.g., the na- ture and the source region of the parental magmas) and the relation between these rocks and the spatially related acidic rocks, mostly of charnockitic type 1 Geochemical and isotopic data on anorthosites are unfortunately rather scarce with the exception of the t Streckeisen's (1974) nomenclature of the charnockitic rocks has been adopted in this paper isotopic work of Taylor (1969). However, since the discovery of anorthosites on the Moon (i.e., Wood et al. 1971 ; Taylor 1975), interest in terrestrial anor- thosites has been renewed. This paper is focused on the oxygen isotope geochemistry of the South Roga- land complex of Norway (Michot and Michot 1969; De Waard et al. 1974), a typical example of massif- type anorthosite (Anderson and Morin 1969). This paper complements the previous ones dealing with the trace element geochemistry, especially the REE~ and the Sr isotopic composition of this complex (Du- chesne et al. 1974; Demaiffe 1977; Duchesne and Demaiffe 1978; Demaiffe et al. 1979). II. Geological Setting The Precambrian of the Southern tip of Norway (Fig. 1) is com- monly subdivided into two major lithologic units (Michot 1960): 1. A supracrustal gneissic series of granulite facies metamor- phism. P- T conditions of these rocks have been calculated from the Fe--Mg distribution between coexisting garnet and cordierite at equilibrium or between ortho- and clinopyroxene. 20 determina- tions lead to a mean of 750 ~ C, 6.3 kbar total pressure (Henry 1974; Demaiffe 1977; Jacques de Dixmude 1978). To the North and the East, a transition towards amphibolite facies is observed. 2. The South Rogatand anorthositic complex (Michot and Mi- chot 1969; de Waard et al. 1974) which consists of a sequence of massif-type andesine anorthosite plutons. The following intru~ sire sequence is deduced mainly from field relationships: the large Egersund-Ogna anorthositic body; the thin Lakssevelefjellet noritic sheet; the layered anorthosito-noritic lopolith of Bjerkrem-Sogndal giving rise to charnockitic differentiates (the so-called mangerites and quartz mangerites) at the end of the differentiation process (Michot 1965; Duchesne 1972, 1979). An anatectic process (Michot 1961)givingrise to the Haaland- Helleren and Aana-Sira massifs takes place before the final stage of tile Bjerkrem-Sogndal differentiation. To the East, the main complex is bordered by Ontliers em- placed within the granulite facies metamorphic rocks: the Hidra and Garsaknatt leuconoritic bodies with their jotunitic border facies and associated charnockitic dykes (Demaiffe 1977); 0010-7999/80/0072/0311/$01.40