STABLE-ISOTOPIC CONSTRAINTS ON THE ORIGIN OF AN S-TYPE GRANITE IN THE SESIA VALLEY OF THE NORTHWESTERN ITALIAN ALPS

An opportunity to study the origin of S-type granite exists in the Sesia Valley where a bimodal magmatic system transects a remarkably coherent, >25-km-thick crustal section. Formed during widespread Permian silicic volcanism associated with transtensional tectonics, the “Sesia magmatic system” was tilted by the Alpine Orogeny, exposing a nearly complete crustal section. Capping the section is a silicic volcanic complex containing the >14-km-diameter Sesia caldera. Intruding these volcanic rocks is the >6-km-thick, peraluminous, Valle Mosso granitic pluton, which is rooted in paragneiss of the Kinzigite Formation at mid-crustal levels. An 8-km-thick section of coeval gabbro, referred to as the Mafic Complex, crystallized from mantle-derived melts that, intruded the Kinzigite Formation at depths >15 km. Generation of anatectic melts in the Kinzigite Formation is vividly displayed in migmatites above the Mafic Complex, and mafic enclaves and andesitic volcanism demonstrate that magma from the underplated gabbro reached the upper crust. Sesia volcanic rocks were strongly affected by low temperature hydrothermal alteration (Δ_{quartz-whole rock} <0, minimum -5.4), while underlying metamorphic and plutonic rocks were less affected with Δ_{quartz-whole rock}»0 at the contact and slightly increasing with depth to a maximum Δ_{quartz-whole rock}=+1.8 so that their oxygen isotopic compositions provide robust constraints on the evolution of the magmatic system. Paragneiss is dominated by amphibolite- to granulite-facies metawacke and metapelite with δ¹⁸O = 6.5 to 18; the distribution of δ¹⁸O values is crudely bimodal with peaks at +11.5 and 14.0-14.5. Whole-rock δ¹⁸O in the Mafic Complex ranges from 7 to 10, but is remarkably uniform, ranging from 7.6 to 9.2, mean δ¹⁸O=8.4 through most of its volume, consistent with near steady-state assimilation of the paragneiss. The elevated δ¹⁸O of these rocks require ~30% contamination by crustal material. Quartz δ¹⁸O of the granitic rocks ranges from 10 to 11.5, intermediate between the Kinzigite Formation and the Mafic Complex, suggesting that they formed by the hybridization of peraluminous, anatectic granitic melts from the Kinzigite Formation and late-stage differentiates from the Mafic Complex, which masks clear evidence of a primitive mantle-derived component.