High-pressure evolution of a tholeiitic dike system from Egersund, SW Norway and implications for mantle source composition and continental basalt genesis.

摘要

Models for continental basaltic (CB) magmatism must account for the range in compositions observed within and between provinces. Compositional variation can reflect the processes involved in the generation of magmas and in modification of the magma after segregation. Models to account for the compositional variations of the primary magma include: (A) melting of mantle plumes formed by ascent of deep mantle material; (B) melting of the subcontinental lithosphere induced by lithospheric thinning, or mantle plume activity; and (C) modification through interaction with subcontinental lithosphere.;The focus of this dissertation is a petrologic and geochemical study of a suite of basaltic dikes from SW Norway to test and constrain aspects to models for the petrogenesis and evolution of CBs. The Egersund Dolerites, located in SW Norway, intrude Precambrian gneisses and anorthosites. The Dolerites vary in thickness from 0.3 to 30m and individual dikes can be traced over distances of 60km. In relation to regional tectonic events, the Dolerites were likely emplaced in association with magmatism related to rifting of a Precambrian supercontinent at about 650 Ma, followed by formation of the Iapetus Ocean. The Egersund Dolerites are divided into Ol-Tholeiite, Tholeiite, Transitional, and Trachybasalt groups. The tholeiitic basalts can be related through removal of the assemblage Ol-Plag-Cpx. The tholeiitic samples can not be related to the Transitional and Trachybasalt compositions by fractional crystallization. Based on trace-element and radiogenic isotope geochemistry, the tholeiitic magmas represent derivation from a plume source, whereas the transitional compositions represent smaller degrees of partial melting; trachybasaltic compositions represent a small-degree partial melt of the plume-source mixed with an enriched subcontinental lithospheric component. Mineral-compositional and whole-rock geochemical data suggest that the Dolerites evolved at pressures between 8-10 kb before emplacement. High-pressure fractional crystallization accompanied by contamination of the more evolved magmas by an enriched source during evolution explains the compositions of these magmas. Comparison with other rift-related Dolerites in Scandinavia reveals a change in magma source, from enriched plume-derived toward MORB-like depleted magmas, through time. Emplacement of many of the Caledonian Dolerites into arkose-filled rift basins indicates uplift and extension prior to magmatism, consistent with active plume-initiated rifting.