Thermal and decompression history of the Lanzo Massif, northern Italy: Implications for the thermal structure near the lithosphere–asthenosphere boundary

Abstract

The lithosphere–asthenosphere boundary (LAB) is a zone where thermal, mechanical, and material interactions take place between the conductive mantle and the underlying convective mantle, and it plays an important role in plate tectonics. In this paper we focus on the thermal aspects of the LAB zone, based on a petrological study of a large peridotite complex that experienced exhumation in the solid state, for the most part. The complex of interest is the Lanzo Massif in the western Alps, northern Italy, where we were able to clarify its thermal and exhumation history and estimate the original thermal structure before exhumation. We examined plagioclase-bearing lherzolites collected from 16 localities, covering the entire massif. All the constituent minerals show compositional heterogeneities on the grain scale. The patterns of Ca, Al, and Cr zoning in pyroxene, the fluorescence-corrected Ca zoning of olivine adjacent to pyroxenes, the Cr and Al zoning of spinel adjacent to plagioclase, and the Ca and Na zoning of plagioclase suggest that an early, nearly isothermal decompression of the Lanzo Massif partly crossing the dry solidus was followed by monotonous cooling through the plagioclase-facies peridotite field. Various deformation microstructures allowed us to specify the timing of the deformation in the framework of the decompression history by carefully observing their relationships with the compositional zoning of the minerals. We show that the deformation took place mainly when effective cooling had started following a period of nearly isothermal exhumation. By applying several geothermometers and evaluating the compositional zoning of the minerals, we were able to quantify the spatial variations in the thermal and decompression history of the Lanzo Massif and constrain the timescales of decompression and cooling. All the estimated temperatures decrease from the southern body towards the northern body. The grain-scale patterns of zoning indicate that the temperatures recorded by the cores of orthopyroxene (1000–1200 °C) indicate a long period of residence in the mantle, whereas those recorded by the rims of pyroxenes and the cores and rims of olivine (600–1100 °C) represent closure temperatures at various times during the decompression. All the closure temperatures decrease from south to north, while the temperature differences between the cores and rims of orthopyroxene increase. This suggests that the cooler and probably shallower northern body cooled at a relatively slow rate than the hotter and probably deeper southern body. The decrease in temperature of ~60 K from south to north, calculated from orthopyroxene cores, may represent the geotherm near the LAB zone. A thermal gradient of ~10 K/km is indicated, which is significantly greater than that estimated for deep subcontinental lithosphere in a steady thermal state. Such a high geotherm might have been caused by thermal perturbation from the underlying hotter asthenospheric mantle.