Extract

Although volumetrically minor, granulites and related rocks are commonly exposed throughout Variscan Europe. Pin & Vielzeuf (1983) recognized two main types among these granulites.

1 ‘Type I’ granulites are high-pressure (HP) rocks, associated with eclogites, which result from the earliest stage of evolution of the Hercynian orogeny (c. 430–400 Ma).

2 ‘Type II’ granulites display medium-P to low-P parageneses and are characteristic of the late Hercynian (320–280 Ma) lower crust.

New data obtained on these rocks are discussed in the light of this partitioning. The geodynamic implications are also explored.

Early high-P granulites

Geology and petrology

High-P granulites occur in the internal parts of the orogen (Fig. 1). They outcrop as lenses of variable size (from dm³ to km³) within medium- to high-grade series of paragneisses, ortho-amphibolites and felsic gneisses of supracrustal origin. There is growing evidence that they are allochthonous units belonging to thrust nappes emplaced at various stages during the tectonic evolution of the belt. These granulitic lenses are rimmed by retrograde, often migmatitic envelopes with amphibolite-facies assemblages. Although these rocks come from widely separated areas, their parageneses are strikingly similar.

High-P equilibration in these rocks is shown by the systematic occurrence of kyanite in aluminous rocks, and the Cpx-Grt-Qtz association in mafic lithologies. A review of P-T estimates available up to 1982 is given in Pin & Vielzeuf (1983). In order to give a coherent picture of the pressure of crystallization of the aluminous, supracrustal granulites, all along the Hercynian belt, a comparative study has been done