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Time scales on melt extraction derived from garnet xenocrysts in felsic plutonic rocks

Othmar Müntener , Benjamin Z. Klein, Arnaud Devoir

Abstract

Volcanism in Earth's continental crust is dominantly fed by transcrustal magmatic plumbing systems but one of the fundamental issues of such systems is the timescales by which magmas ascend through the crust. Here we present major and trace element data and maps to document timescales of a multiple growth history for garnet xenocrysts entrained in an upper crustal magma body from the Sesia magmatic system (Ivrea zone, northern Italy). Based on phase diagram calculations and garnet compositional variations, metamorphic cores of garnet in the plutonic rocks either originate from granulite facies lower crustal rocks (ca. 8-9 kbar), or metasedimentary enclaves at mid crustal levels (ca. 6.8 +/- 1.2 kbar). The contrasting metamorphic conditions indicate that the Borgosesia monzogranite has accumulated xenocrysts from different depth along its ascent path from the lower crust to the emplacement level at ~3 kbar. Rhyolite-MELTS modeling was used to explore decompression and cooling paths of hydrous rhyodacitic magmas and its effects on the density and viscosity evolution of the system. Simple calculations using hindered settling indicate garnet grain settling velocities of ca. 0.5 -- 0.8 m.yr-1 which correspond to maximum time scales of magma ascent of ~20-30 kyr. This is the same order of magnitude as derived from multicomponent diffusion calculations in garnet (~5 - 12 kyr). Garnet xenocrysts provide strong constraints on ascent rates of moderately hydrous siliceous magmas derived from the lower crust.