A Dynamic Model of Rift Zone Petrogenesis and the Regional Petrology of Iceland

Abstract

The article describes the petrochemical evolution of oceanic rocks in terms of plate tectonics with special reference to Iceland. The compositional variation along the rift zone is related to different production rates of mantle-derived olivine tholeiite of invariant composition which is added to the crust from below and modified by mixing with anatectic melts in the crust and concomitant crystal fractionation. The kinematic processes of crustal accretion cause rocks deposited in the rift zone to subside towards higher temperatures where they suffer hydration and progressive metamorphism before becoming a part of the stable crustal plate. Rocks deposited near the rift-centre assume the highest metamorphic grade (greatest depth) while rocks deposited at the rift-margins follow a shallow path before being carried towards lower temperatures in the stable plate. The material transport through stationary metamorphic zones produces the layering of the oceanic crust. As the hydrated rocks cross their solidus isotherm, silicic magma is formed by incongruent partial melting. The melting continues until rocks crossing the boundary between the amphibolite and granulite fades are finally dehydrated by the break-down of amphibole. This reaction boundary defines the surface of the upper mantle. The segregation and retention of crust-derived magmas within the rift zone results in chemical fractionation in the oceanic crust, for its lower sections are depleted in elements entering the early melt fractions, which are silicic and enriched in the dispersed elements. The last melt-increments from the same subsiding pile are ne-normative basalts. The rift-zone rocks are shown to be mantle-derived olivine tholeiite modified by minor amounts of crustal rhyolite and nepheline basalt, while volcanism outside the rift zone is dominated by the crust-derived magmas themselves. All mixtures undergo further mineralogical evolution towards invariant compositions in the basalt system, resembling the olivine tholeiite, quartz tholeiite, and nepheline basalt of synthetic systems. The dispersed-element geochemistry of the oceanic rocks is but slightly modified by crystal fractionation, and reflects the mixing ratios of the olivine tholeiite and the different crustal magmas. The geochemistry of radiogenic isotopes is controlled by continuous processes of crustal fractionation separating mother and daughter elements. The oxygen-isotope geochemistry can be referred to magma mixing, for rhyolites formed by anatexis in the hydrated crust are enriched in light oxygen relative to the mantle-derived melts.