The petrology of a metamorphosed netveined acid-basic intrusion from the Østfold area, Southeast Norway

Abstract

A 1325-1015 Ma old minor netveined acid-basic intrusion within the Sveconorwegian basement terrain of SE Norway is described. The intrusion is well preserved in spite of a superimposed Sveconorwegian amphibolite facies metamorphism (around 1015 Ma). In the netveined intrusion the acid and the basic material represent a potassic quartz monzonite of rapakivi type and a highly fractionated tholeiite enriched in FeOx, TiO2, K2O, P2O5 and incompatible trace elements. These two materials have existed side by side as magmas of essentially the same temperature. Interaction between the materials is demonstrated, as there is evidence of: (1) selective element diffusion comprising Sr (and probably K and Rb), (2) phenocryst capturing, where the basic magma well above its solidus temperature at least captured alkali feldspar and quartz phenocrysts from the potassic quartz monzonitic magma, (3) formation of basic hybrids with an emulsion-like structure, and (4) formation of mechanical mixed hybrids. Based on Sr isotope mixing calculations it appears that the two melts existed side by side 1300 Ma ago. It is argued that the potassic quartz-monzonite magma was hot (> 1100°C) and dry (< 0.6% H2O). It was possibly formed by total melting of a material of similar composition which could have been the basic member of the Moss-Filtvet augen gneiss (granite) which outcrops in the neighbourhood, or alternatively by partial melting of older intermediate or basic rocks at a deeper level in the crust. The heat necessary for the melting was presumably introduced by the injection of the tholeiitic magma that also was the source of the basic component of the netveined intrusion. The formation of the netveined structure of the intrusion has presumably been guided by development of curviplanar joints in the basic material, when the potassic quartz monzonite was still very mobile.