Metamorphic heterogeneities within a single HP unit: Overprint effect or metamorphic mix?

Eclogites and eclogites-facies rocks in mountain belts provide evidence that material in subduction zones material can return from depths of more than 100 km to the surface in the early stages of orogenic processes. Their relationship with lower metamorphic grade rock of i.e. blueschist and greenschist facies can provide information on the late stages of orogenic processes, but are often matter of debate. Petrography of metamorphic rocks in mountain belts has mainly focused on mafic rocks systems. Nevertheless, metamorphic domains of recent mountain belts like the Alps are not only constituted by mafic rocks but also by metasediments that continuously outcrop over very large areas. Such mountain belts are made of a large part of low-temperature metasediments devoid of index minerals classically observed in mafic and quartzo-feldspathic rocks systems, allowing a direct comparison to be made. These metasediments also have various chemical and mineralogical compositions that represent an important geothermobarometric potential. Thus we propose to study relationships between eclogites and less metamorphosed rocks for a significant case that has been long debate (Entrelor area, Western Alps). Despite metasediments continuously outcropping over the whole area, all previous metamorphic studies were carried out on dismembered sequences of mafic rocks. They have evidenced two kind of metamorphic evolution. Three explanations are generally proposed to interpret this feature: incertainties of the thermodynamic data, the loss of HP mineralogy during the exhumation and a late-stage tectonic juxtaposition. By studying simultaneously both eclogites and metasedimentary contry-rocks, we show that rocks association in the Entrelor area can be interpreted as a metamorphic mix. This area consists of eclogites rocks embedded in a blueschist facies matrix mainly made of metapelites. Exhumation of HP metamorphic rocks reveals that different pressure peaks (1.2 GPa at 450 degrees C vs. 2.3 GPa at 550 degrees C) were contemporaneous. The different types of rocks have been juxtaposed at a shallow crustal level within a subduction channel. Due to the fact the Western Alps do not reach the mature stage of a colliding belt, as the Central or as the Eastern Alps, the rocks of the Entrelor area can be viewed as an exhumed part of a frozen subduction channel attributed to a metamorphic mixing of rocks having different metamorphic evolution and accreted at great depths. (c) 2007 Elsevier B.V. All rights reserved.