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

The article describes the petrochemical evolution of oceanicrocks in terms of plate tectonics with special reference toIceland. The compositional variation along the rift zone isrelated to different production rates of mantle-derived olivinetholeiite of invariant composition which is added to the crustfrom below and modified by mixing with anatectic melts in thecrust and concomitant crystal fractionation. The kinematic processes of crustal accretion cause rocks depositedin the rift zone to subside towards higher temperatures wherethey suffer hydration and progressive metamorphism before becominga part of the stable crustal plate. Rocks deposited near therift-centre assume the highest metamorphic grade (greatest depth)while rocks deposited at the rift-margins follow a shallow pathbefore being carried towards lower temperatures in the stableplate. The material transport through stationary metamorphiczones produces the layering of the oceanic crust. As the hydrated rocks cross their solidus isotherm, silicicmagma is formed by incongruent partial melting. The meltingcontinues until rocks crossing the boundary between the amphiboliteand granulite fades are finally dehydrated by the break-downof amphibole. This reaction boundary defines the surface ofthe upper mantle. The segregation and retention of crust-derived magmas withinthe rift zone results in chemical fractionation in the oceaniccrust, for its lower sections are depleted in elements enteringthe early melt fractions, which are silicic and enriched inthe dispersed elements. The last melt-increments from the samesubsiding pile are ne-normative basalts. The rift-zone rocks are shown to be mantle-derived olivine tholeiitemodified by minor amounts of crustal rhyolite and nephelinebasalt, while volcanism outside the rift zone is dominated bythe crust-derived magmas themselves. All mixtures undergo furthermineralogical evolution towards invariant compositions in thebasalt system, resembling the olivine tholeiite, quartz tholeiite,and nepheline basalt of synthetic systems. The dispersed-elementgeochemistry of the oceanic rocks is but slightly modified bycrystal fractionation, and reflects the mixing ratios of theolivine tholeiite and the different crustal magmas. The geochemistry of radiogenic isotopes is controlled by continuousprocesses of crustal fractionation separating mother and daughterelements. The oxygen-isotope geochemistry can be referred tomagma mixing, for rhyolites formed by anatexis in the hydratedcrust are enriched in light oxygen relative to the mantle-derivedmelts.