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On the southern side of the Mont Pilat (eastern French Massif Central), an orthoclase-rich granite, which intrudes orthogneisses overlying a granito-migmatitic complex, gives a Dinantian age (RbSr age: 353±21 Ma; UPb age: 341+8/−5 Ma). Such a dating confirms the reality of a ‘northwestern Vivarais’ geological unit, whose metamorphic and granitic evolution cannot be assigned to a Late-Carboniferous event. These results impose a re-examination of both the ‘granite du Velay’ problematic and the geodynamic evolution of the eastern French Massif Central. To cite this article: B. Briand et al., C. R. Geoscience 334 (2002) 741–747.

The De Pas Batholith, in New Quebec, is a mass of granitoids elongated in a N-S direction, covering 7000 km2 in the southeastern part of the Archean Rae province. It is located between the Paleoproterozoic New Quebec Orogen in the West and the Torngat Orogen in the East. The batholith is composed, in its south half, of two parallel plutonic units: a charnockitic intrusion in the West, enclosed in granulitic gneisses, and a granitic unit in the East intruding an Archean orthogneiss complex migmatized at amphibolite facies grade. Field evidence indicates that the charnockitic unit is younger than the granitic unit. Both plutonic units are differentiated. The granitic unit ranges in composition from pyroxenite and diorite to the dominant porphyritic granodiorite. The charnockitic unit varies from norite to the dominant porphyritic opdalite. Late intrusive granites are present. The granitic unit and charnockitic unit show calc-alkalic evolutionary trends, granodioritic for the the granitic unit and monzonitic for the charnockitic unit. Similar major elements patterns suggest a common source area. Compatible trace element and heavy rare earth abundances are similar in both units. However, incompatible elements (Rb, Th, and U) and light rare earth are less abundant in the charnockitic unit. This poverty in certain large-ion lithophile elements and high field strength elements is a primary characteristic of the charnockitic magma. The overall geochemical characteristics of the De Pas Batholith are similar to those of a plutonic arc in a subduction setting. Only the late granites show evidence of a collisional setting. The differences in trace elements of the granitic unit and the charnockitic unit might be explained by contrasting styles of contamination of the magmas by continental materials in different intrusive conditions and geological settings. The granitic magma was intruded into Archean (?) crust of amphibolite facies, with which the granitic unit was in physical equilibrium. The charnockitic magma crystallised in the deeper granulitic crust, poor in highly lithophile elements. The De Pas Batholith occurs in the hinterland of the New Quebec Orogen, and represents an early subductional magmatic arc formed in two stages separated by a period of crustal thickening in the arc zone.

Metamorphic rocks of the Pilat mountain area of the Central Massif near St. Etienne exhibit three periods of metamorphic effects involving anomalies. Period I, the oldest, is represented by mesozonal gneiss and schist (amphibolite-almandine facies) cropping out along the NW side of the area. They are part of the pre-Hercynian metamorphics of the south flank of the Lyonnais mountains. Period II involved epizonal retromorphism and recrystallization. These rocks crop out in a belt along the SE margin of the Period II series. Period III involved thermal, and probably contact metamorphism, resulting in zonation of a belt with chlorite, biotite, cordierite, andalusite, and sillimanite zones in succession SE to the Pilat mountain granite. Period III probably is younger than II. The chemical composition of the granite is anomalous. This may be the metamorphism under deficient shearing stress described by Harker, or Buchan type metamorphism, as described by Read and used by Forestier to explain the schists of the upper Allier river area of the massif.

Muscovite granites of the Pilat massif (French Central Massif) can be divided into two large groups. Those belonging to the first group appear in the so-called metamorphic series of the Pilat massif and are associated with granulites of rhyolitic origin from which they were directly derived by anatexis. Those of the second group are doubtless of similar origin, although this is more difficult to establish. All these granites have a constant chemical composition. Their magmatic paragenesis includes, in addition to quartz, potassic feldspar and albite, some biotite, a little muscovite, and occasionally andalusite. The data seem to indicate crystallization under conditions of low pressure and temperatures less than 700 degrees C. Subsequently, secondary muscovitization developed at the expense of andalusite. The muscovite granites of the Pilat massif do not present a mineralization sequence. Their mode of formation was quite different from that of the late hololeucocratic granites resulting from a differentiation of granitoid assemblages.

Three newly-discovered occurrences of upper Dogger (?) (middle Jurassic) dolerite in upper Lias to Dogger (Jurassic) formations of the Priego de Cordoba area of southern Spain resemble other dolerites of the region. The Las Navas dike and the Buitron sills are in Dogger marl, and the Las Lomas sill is in upper Lias (lower Jurassic) limestone. The contact metamorphic zones are proportional to the thickness of the tabular bodies, and exhibit only thermal characteristics. The thick Las Lomas sill transformed the initial six meters of overlying limestone to garnetiferous marble. Leucocratic enclaves of pegmatite are characterized by about 58 percent SiO 2 as contrasted with about 50 percent in the dolerite.

Amphibolites among the crystalline schists of northern Limousin, France, represent an original spilite-keratophyre association. A major volcanic episode contemporaneous with geosynclinal sedimentation can now be considered well established. Calcareous facies are essentially absent in the schist complex; the only marble known in the region may be the metamorphic product of sediments deposited in an environment locally enriched in lime as a result of extrusion of particularly calcic lavas and tuffs.

The crystalline schists of northern Limousin, France, are polymetamorphic in origin. Two phases of migmatization have been distinguished, of different chemical nature and separated by a long time interval. This double metamorphism is not limited to northern Limousin, but is a general characteristic of other parts of the Central Massif and of the metamorphic terrain of Brittany. The classic scheme of metamorphic zoning of migmatites does not hold; rather than an advancing front of migmatization two distinct processes took place at different times--granitization, consisting essentially of partial or total remobilization of preexisting materials, and regional feldspathization, involving K-metasomatism, both represented by independent facies.