In the southwestern part of the Aiguilles Rouges massif (pre-Alpine
basement of the Helvetic realm, western Alps), a metavolcanic sequence,
newly defined as the ``Greenstone Unit'',is exposed in two NS trending
belts of several 100 metres in thickness. It consists of epidote
amphibolites, partly epidote and/or calcic amphibole-bearing
greenschists, and small amounts of alkali feldspar-bearing greenschists,
which underwent low- to medium-grade metamorphism during Visean oblique
collision. Metamorphic calcic amphiboles and epidotes show strong
chemical zoning, whereas metamorphic plagioclase is exclusively albitic
in composition (An 1-3). The SiO2 content of the subalkaline tholeiitic
to calc-alkaline suite ranges continuously from 44 wt% to 73 wt%,but
andesitic rocks predominate. The majority of samples have chemical
compositions close to recent subduction-related lavas; some are even
restricted to recent oceanic arcs (extremely low Ta and Nb contents,
high La/Nb and Th/Ta ratios). But several basaltic to basalto-andesitic
samples resemble continental tholeiites (low Th/Ta, La/Nb ratio). As it
is very probable that both lava types are to some extent
contemporaneous, it is proposed that the Greenstone Unit represents a
former oceanic volcanic are which temporarily underwent extension during
which emplacement of continental tholeiite-like rocks occurred. The
cause of the extension remains ambiguous. Considering palaeotectonic
significance and age of other metavolcanic units in the Aiguilles Rouges
massif, the Greenstone Unit most likely formed in the Early Palaeozoic.