Pyroxenes from iron-rich igneous rocks in Rogaland, SW. Norway

Abstract

An igneous complex, consisting of several anorthosite massifs and the layered intrusion(lopolith) of Bjerkreim-Sokndal, is surrounded by high-grade metamorphic sveconorwegian migmatites. The upper part of the lopolith or (quartz-)monzonitic phase(QfW) consists of iron-rich pyroxene monzonites and -syenites + fayalite + Fe-Ti oxides. It may be subdivided in a lower stage(l) characterised by primary orthopyroxene and an upper stage(2) characterised by (inverted/decomposed) pigeonite. Two massifs mainly consisting of similar rocks are concordantly intercalated in the migmatite terrane: the Botnavatnet and Gloppurdi Igneous Complexes(BIC and GIC). The BIC is mainly comparable with stage 2 of the QMP. Textural evidence and iron-enrichment in pyroxenes and olivines suggest that stage 1 of the QMP must be affixed to the leuconoritic phase of the layered lopolith. Pyroxene crystallization temperatures in the QMP and BIC are comparable: ca 1050- ca 900oC. The total pressure during pyroxene crystallization varied between 5-7 kb in the QMP and from 7-10 kb in the BIC. Isotopic ages are about 950 Ma and> 1060 respectively. During crystallization and subsequent cooling the water pressure was low. A great number of microprobe analyses of pyroxenes and olivines are presented. The low amount of 'non-quadrilateral components' makes these phases suitable for use as geobaro- and geothermometers. Textural data were obtained with a light-optical microscope and an electron microscope. Exsolution of pyroxenes proceeded by heterogeneous nucleation and growth. Texture and scale appear to be largely diffusion controlled. In clinopyroxenes cation diffusion is easier in the c- than in the adirection. Clinopyroxene is capable of stabilising thin pigeonite lamellae //"(001)" to below its transition temperature. Lamellae which exceed the critical lamellar thickness invert to the Rogaland inverted pigeonite lamellae.Anhydrous conditions during crystallization and cooling hampered the nucleation of orthopyroxene, which provided the conditions for the pigeonite-orthopyroxene transition to take place as a massive transformation. The transition did not take place by inversion but rather by decomposition.