High-Mg Andesite Petrogenesis by Amphibole Crystallization and Ultramafic Crust Assimilation: Evidence from Adamello Hornblendites (Central Alps, Italy)

New insights into the origin of high-Mg andesites are inferred from the mineral chemistry and U-Pb geochronology of Tertiary amphibole-rich ultramafic intrusive rocks (hornblendites) and included clinopyroxene-bearing dunitic clots from the southern Adamello batholith (Central Alps). The hornblendites consist mostly of amphibole grains with brown cores (Ti-pargasite) that grade through brownish-green (Mg-hornblende) to light green (edenite) rims. Brown amphibole contains olivine (Fo=85-87 mol %) and clinopyroxene inclusions with irregular boundaries indicating disequilibrium with the host amphibole. The ultramafic clots are interpreted to represent fragments of older cumulates dismembered by the injection of the amphibole-forming melts, thereby providing evidence for a melt-rock reaction process. Amphibole from the hornblendites shows a marked trace element zoning. From the brown core outward to the brownish-green portion of a single crystal, a significant enrichment is observed in light rare earth elements, Th and U, coupled with a decrease in Ti and heavy rare earth elements. The melt in equilibrium with the brownish-green amphibole has an adakitic trace element signature (e. g. high LaN/YbN and Sr/Y). Based on amphibole/liquid partition coefficients, a fractional crystallization process driven by amphibole could explain most of these chemical variations. However, the outward increase of highly compatible elements in amphibole (e. g. Mg, Ni, Co, and Zn) argues against closed-system fractional crystallization. The assimilation of olivine is considered the most efficient mechanism to supply or buffer the highly compatible elements in the evolving system during amphibole crystallization. In situ U-Pb zircon geochronology of hornblendites and associated amphibole gabbros reveals the occurrence of inherited cores, thereby providing evidence for assimilation of crustal material. We propose that a differentiation process controlled by amphibole crystallization and assimilation of slightly older ultramafic cumulates may produce melts rich in SiO2 and MgO with adakitic trace element signatures.