The role of crustal fertility in the generation of large silicic magmatic systems triggered by intrusion of mantle magma in the deep crust

The Sesia magmatic system of northwest Italy allows direct study of the links between silicic plutonism and volcanism in the upper crust and the coeval interaction of mafic intrusions with the deep crust. In this paper, we focus on the chemical stratigraphy of the pre-intrusion crust, which can be inferred from the compositions of crustal-contaminated mafic plutonic rocks, restitic crustal material incorporated by the complex, and granitic rocks crystallized from anatectic melts. These data sources independently indicate that the crust was compositionally stratified prior to the intrusion of an 8-km-thick gabbroic to dioritic body known as the Mafic Complex, with mica and K-feldspar abundance decreasing with depth and increasing metamorphic grade. Reconsideration of published zircon age data suggest that the igneous evolution initiated with sporadic pulses at around 295 Ma, when mafic sills intruded deep granulites which provided a minor amount of depleted crustal contaminant, very poor in LIL elements. With accelerated rates of the intrusion, between 292 and 286 m.y, mafic magmas invaded significantly more fertile, amphibolite-facies paragneisses, resulting in increased contamination and generating hybrid rocks with distinct chemistry. At this point, increased anatexis produced a large amount of silicic hybrid melts that fed the incremental growth of upper-crustal plutons and volcanic activity, while the disaggregated restite was largely assimilated once ingested by the growing Mafic Complex. This “igneous climax” was coincident with an increasing rate of intrusion, when the upper Mafic Complex began growing according to the “gabbro glacier” model and, at about the same time, volcanic activity initiated. Cooling lasted millions of years. In the coupled magmatic evolution of the deep and upper crust, the Mafic Complex should be considered more as a large reservoir of heat rather than a source of upper-crustal magma, while the fertility of “under/intra-plated” crust plays a crucial role in governing the generation of large volumes of continental silicic magmas.