6 - The role of serpentinite melanges in the unroofing of UHPM rocks: An example from the Western Alps of Italy

Summary

Abstract

The ultrahigh pressure metamorphic rocks (UHPM) of the Dora-Maira continental massif are overlain by a stack of oceanic nappes. Metamorphic grade appears to increase downward but with marked discontinuities between each of the nappes, suggesting that section has been removed along the bounding faults. This apparent omission of section is greatest in the lowest oceanic unit where a serpentinite melange containing blocks and slabs of eclogite, metamorphosed at 12–19 kbar, lies on the UHPM rocks. We suggest that this serpentinite melange represents a highly attenuated upper mantle section that structurally overlay the UHP rocks during subduction. Similar serpentinite melanges are known from other high-pressure (HP) and UHPM areas and may have a similar origin.

Introduction

Earlier, we presented evidence that the ultrahigh pressure metamorphic (UHPM) rocks of the continental Dora-Maira massif of the Western Alps, Italy, were structurally overlain by gently dipping oceanic units that were tectonically thinned during or following. Alpine subduction (Blake and Jayko, 1990). Subsequent authors (Philippot, 1990; Avigad, 1992) have reached similar conclusions. However, the timing of metamorphism and uplift remains controversial.

In this chapter, we present new pressure-temperature estimates for eclogitic rocks from several of the oceanic subunits, including blocks from the basal melange. These new data support our earlier suggestion that the serpentinite melange represents a greatly attenuated section of upper mantle that may have once formed the upper plate to the UHPM rocks of the Dora-Maira massif.