REVISITING THE LANZO PERIDOTITE (NW-ITALY): 'ASTHENOSPHERIZATION' OF ANCIENT MANTLE LITHOSPHERE

Abstract

In this paper, we compiled previous investigations and summarize recent results to show that the Lanzo peridotite records a sequence of deep lithospheric igneous events that largely obliterated the previous subcontinental history. Field, microtextural and trace element data indicate the progressive evolution of an igneous system that forms above an upwelling asthenosphere, most probably during the transition from a rifted system to the establishment of an (ultra-) slow spreading ridge of the Piedmont Ligurian ocean. Pervasive porous flow of melt forming ‘reactive’ spinel peridotites followed by plagioclase peridotite is later replaced by focused porous flow, reflected by a system of replacive harzburgite and dunites. Cooling of the Lanzo peridotite leads to the interstitial crystallization of clinopyroxene and plagioclase, and finally to the formation of gabbro dikes. Thus, the Lanzo peridotites may indicate that extension-driven ‘asthenospherization’ of lithospheric mantle is an important process in the development of embryonic ocean basins.