Petrology and geochemistry of pyroxenites in the Lanzo ultramafic massif, Northwestern Italy

Abstract

Combined field, microtextural, and geochemical observations are presented for pyroxenites in the Lanzo ultramafic massif in order to place chemical and petrological constraints both locally on the geologic history of the massif and more generally on the role of magmatic rifting and the rift-to-drift transition in ultraslow-spreading ridge environments. Two separate generations of pyroxenites have been distinguished. A young set of primitive Cr-diopside websterites of MORB affinity are related to melt infiltrations and melt-rock reactions during rifting and Jurassic exhumation, while a much older generation of websterites likely of subcontinental origin were inherited and pre-date continental rifting and exhumation of the massif. The regional compositional and textural variation observed in the latter group of pyroxenites is not due to primary heterogeneities but rather to differences in subsequent metamorphic histories. Pyroxenites from the northern domain experienced a colder exhumation history and physical isolation from the rest of the massif by means of a high temperature shear zone related to the rifting. These websterites preserve garnet pseudomorphs and elevated REE which are textural and geochemical evidence for the prior coexistence of clinopyroxene and garnet. The implications of this study are that the Lanzo massif was indeed subcontinental lithosphere prior its exhumation and existence as an Ocean Continent Transition Zone (OCTZ) and that high temperature shear zones do seem to serve as effective permeability barriers and melt-focusing zones in rifting systems.