Kaersutite-bearing mylonitic gabbro from the Lanzo-peridotite (Western Italian Alps)

Abstract

A mylonitic gabbro, intruding and partly re-equilibrating the Lanzo peridotite in Val di Viù, is described. It consists of Ol + Opx + Cpx + Ti-rich Ho porphyroclasts included in a fine-grained matrix, where primary plagioclase is replaced by the Jd + Zo + Qz assemblage. Geothermobarometric calculations have given T: 1000 'C and P : 0.5 GPa (5 kbar) for the gabbro crystallization. Both gabbro and host spinel/plagioclase lherzolite are crosscut by mm-sized mylonitic veins of brown Ho + Ilm + Ap + Plag. Geologic and petrologic considerations suggest that these veins crystallized from a highly differentiated tholeiitic magma, introduced - most likely immediately after gabbro crystallization - into both gabbro and peridotite. Mineralogic and petrologic evidences indicate that both gabbro and host lherzolite experienced a subsolidus polyphase HT deformation and recrystallization from hornblende granulite- to amphibolite-facies conditions. The metamorphic re-equilibrations, characterized by initial Hp mineral assemblages and late greenschist-facies parageneses, indicate an Alpine orogenic history consistent with that inferred elsewhere for the internal Western Alps.