Density-controlled assimilation of underplated crust, Ivrea-Verbano zone, Italy

Abstract

Structural, chemical and density data from the southern Ivrea-Verbano Zone indicate that the evolution of the lower crust may be strongly influenced by evolving density contrasts between mafic magmas and melting country rocks. The Ivrea-Verbano Zone contains a 10 km thick igneous mafic complex that intruded high-grade metamorphic rocks while they were resident in the lower crust. Heat released from the mafic intrusion induced partial melting in the country rocks on a regional scale. Slivers of crustal rocks (septa) are interlayered with igneous mafic/ultramafic rocks deep in the complex and show evidence of an advanced degree of partial melting. The chemical and isotopic composition of the Mafic Complex indicates significant contamination of mantle magma by a component delivered from a crustal source depleted in granophile elements, similar to the septa. In contrast, the present roof rocks cannot represent the source of the main contaminant because they are too rich in incompatible elements. The computed density of the mafic melt at the pressure conditions of the intrusion is intermediate between measured densities of the (lighter) roof rocks and the (heavier) septa. It appears that removal of buoyant anatectic melts increased the density of a restite layer on the roof. After the density exceeded that of the mafic melt, the restite layer was incorporated into the growing igneous body, creating a septum and providing the appropriate source for the contaminant of the Mafic Complex. In the Ivrea-Verbano Zone this process was episodically repeated during the evolution of the Mafic Complex. Worldwide, this process may profoundly influence the chemistry of continental basalts and may create a dense and layered lower crust.