The Geology and Petrology of the Trois Seigneurs Massif Pyrenees, France

In the introduction the criteria are mentioned which enable us to distinguish the products of the synkinematic and post-kinematic phases of the Hercynian metamorphism. A short characteristic has been given of the Silurian and Cambro-Ordovician rocks. The mica-schists, discussed in Chapter I, are exposed in the southeastern, central and northern parts of the mapped area. The grade of metamorphism increases from north to south in this rock-series. A biotite-, an andalusiteand a sillimanite zone can be distinguished. The biotite- and andalusite zones originated during the synkinematic phase. However, the sillimanite of the mica-schists is thought to be post-kinematic. There are indications that the rocks of the sillimanite zone have undergone high-grade metamorphism. In the low-grade biotite-muscovite-schists a marble layer occurs with biotite-gneiss bands, containing fairly basic plagioclase. Three varieties of synkinematic gneiss are distinguished: A) Sillimanitegneisses (migmatites), B) quartz-dioritic gneisses and C) homogeneous biotite-gneisses. The migmatites predominate in a 200 m thick zone which is located structurally below the mica-schists. The migmatization (both lit-par-lit replacement and metamorphic differentiation probably were involved) appears to be late- to post-kinematic. Nearly all the sillimanite, however, is postkinematic. The quartz-dioritic gneisses are supposed to have been formed by a selective sodium metasomatism of lime-bearing pelites. The biotite-gneisses possibly originated from semipelitic or pelitic rocks by sodium metasomatism. The quartz-diorites which occur structurally below the zone of migmatites, mentioned above, occupy most of the area in the north-eastern and southeastern parts of the region investigated. In most cases it was clear that these rocks were formed by rheomorphism at the expense of migmatites. On the other hand, static recrystallization of homogeneous biotite-gneisses may also result in the formation of quartz-diorites. The field observations show that rheomorphism took place on a regional scale in the Trois Seigneurs massif. Nowhere do the quartz-diorites break through the sillimanite-gneiss zone, which forms a continuous envelope around these rocks. Numerous relics of the pre-existing rock series are still recognizable in the autochthonous quartzdiorites. The resisters (Chapter IV) occur as conformable layers or lenses in the synkinematic gneisses. These lenses and layers have been broken into many pieces, due to the flow movements in the rheomorphic quartz-diorites. A series of rock types of sedimentary origin (part I) could be distinguished: magnesian and non-magnesian marbles and lime-silicate rocks, hornblende and biotite-bytownite-gneisses (presumably original marls) and quartzites. They may be intercalated, and usually show a typical sedimentary banding. Transitions between the three different types have been observed. Another group of resisters (part II) show characteristics which indicate that they are of igneous origin. The quartz-gabbros (part III) probably originated at the expense of amphibolites through an extreme static recrystallization. In the registers a synkinematic paragenesis can be distinguished which can be placed in the amphibolite facies. The following minerals: diopside, clinopyroxene, grossularite, wollastonite, forsterite, spinel, calcic plagioclase and green hornblende characterize this paragenesis. Furthermore a high-grade post-kinematic association is present which contains, among others: cummingtonite, light green amphibole, pargasite, cordierite (?), anthophyllite (?), and bytownite. During the post-kinematic phase the green hornblende of the hornblende-bytownite-gneisses apparently remained stable as did the clinopyroxene, grossularite and basic plagioclase of the lime-silicate rocks. In association with late cross-cutting pegmatites a low-grade paragenesis occurs: sericite, pennine, epidote, clinozoisite, prehnite, actinolite and albite. It was not possible to establish, whether the tremolite, epidote and clinozoisite of the lime-silicate rocks originated in association with pegmatites, or as a result of the cooling in the rock series during the latest stages of the metamorphism. In general the resisters probably underwent an isochemical metamorphism. Sills or cross-cutting dykes of pegmatite and leucocratic granitic rocks (Chapter V) are of frequent occurrence in the gneisses of the Trois Seigneurs massif. They originated during many different stages of the Hercynian metamorphism. In the quartz-diorites the early sills reacted as plastic bodies during the stage of rheomorphism and preserved their original parallel position. The muscovite-granite of the Pic d’Estibat is located close above the migmatite boundary. The biotite-muscovite granite of la Ruse (Chapter VI) is possibly intrusive and perhaps originated at the expense of a concentration of pegmatites and leucocratic granites which, as a rule, occur near the migmatite boundary. The biotite-granodiorite of the Pic des Trois Seigneurs is described in Chapter VII. Its intrusive character is clear, mainly from evidence of shouldering aside the enclosing rocks. It is suggested that the original magma of the granodiorite originated at the expense of autochthonous gneisses and quartzdiorites which have become intrusive. There are indications that the granodiorite body narrows downward and that the connection with the autochthonous rocks has more or less been lost. Bodies of chlorite-albite rock (Chapter VIII) occur in many places in the gneisses and quartz-diorites of the mapped area, but also in the biotite-granodiorite of the Pic des Trois Seigneurs, and even in the micaschists. It could be proved that these rocks were formed through a strong sodium metasomatism under hydrothermal conditions. Especially in the field, the original appearance of the pre-existing rocks has been well preserved. Small lenses of chloritite frequently occur in the chlorite-albite rocks. They are original basic resisters. These rocks appear to be later than the late-Hercynian cross-cutting pegmatites. The age of the chlorite-albite rocks could not yet be determined. Possibly they are associated with the late-Hercynian period of faulting or with the Alpine orogeny. The influence of chloritization and albitization is also clear in gneisses and quartz-diorites near important fault zones. The structural relations of the Trois Seigneurs massif with respect to the surrounding mesozoic rocks and the axial zone of the Pyrenees are discussed in the last chapter.