The petrogenesis of the St. Malo migmatite belt, North-Eastern Brittany, France

Abstract

The St. Malo migmatite belt is located along the north Brittany coast between the towns of N. D. du Guildo (Cotes-du-Nord) and Cancale (Ille-et-Vilaine). The belt, which has a north-east trend and structural grain, has been assigned to the Hercynian (Abrard, 1923), Caledonian (Graindor, 1962) and Cadomian (Leutwein, 1968 and Jeannette, 1971) earth movements. Detailed structural and metamorphic studies have shown, however, that rocks within the St. Malo migmatite belt have a longer structural history and a higher metamorphic grade than adjacent Brioverian supracrustal rocks (Brown, Barber & Roach, 1971). The St. Malo migmatite belt represents part of the Pentevrian basement within the Armorican Massif.
Included within the St. Malo migmatite belt are metasediments, metatexites, diatexites and, locally, sheets of granite and trondhjemite. The terms metatexite and diatexite are used according to the definitions proposed by Brown (1973). Metatexis is defined as "... the process of segregation (usually of quartz and feldspar) by metamorphic differentiation and partial fusion" and a metatexite is therefore a
rock "... produced by metatexis and in which the migmatitic banding is evident". Diatexis is defined as "... high-grade anatexis in which fusion may be complete" and a diatexite is therefore a rock "... produced by diatexis and in which there is no continuous migmatitic banding".
The western half of the St. Malo migmatite belt comprises approximately equal proportions of metatexite and diatexite at the present level of erosion. Mesoscopic patches of diatexite are common within the metatexite and often fragment the metatexite to give schollen
structure. Granite sheets are common within the metatexites. The eastern half of the St. Malo migmatite belt is dominated by diatexite at the level now exposed. The diatexite forms a core to the migmatite belt and on its south-eastern side has punched through the metatexite envelope to intrude Pentevrian metasediments. The River Rance provides a transverse section through the St. Malo migmatite belt and allows the transition from Pentevrian metasediment to metatexite to diatexite to be studied in detail. Similar rocks are exposed around St. Cast and upstream along the Rance near Oinan.
Brioverian metasediments are exposed on the north-western side of the St. Malo migmatite belt. Th~comprise a sequence of turbidites. Shear belts of Cadomian age cut the Pentevrian basement.
The major element chemistry of the diatexites shows a closer affinity with sediments of greywacke type rather than with sediments of arkose type and one sample of Pentevrian metasediment is similar to calcium-poor greywacke. The St. Malo rocks have K/Rb ratios near the average for upper crustal rocks and Th/U ratios similar to amphibolite facies granite goeisses. Twenty four separated biotites have been analysed for a selection of major and trace elements and have allowed an estimate to be made of the prevailing temperature and fugacity of water during diatexis.
Four episodes of deformation are recognized within the Pentevrian metasediments prior to the Cadomian orogeny. Initial metatexis was broadly concomitant with D1 whilst later diatexis was broadly concomitant with D3. Three episodes of deformation are recognized within the Brioverian supracrustal rocks and within shear belts developed in the Pentevrian basement in response to the Cadomian orogenic episode.
Mesonormative Q-Ab-Or ratios of the diatexites show a close relationship with points of minimum melt composition determined experimentally within the NaAlSi3O8-KA1Si3O8-CaAl2Si2O8-SiO2-H2O system and projected from An onto the Q-Ab-Or face of the Q-Ab-Or-An tetrahedron. Mesonormative Ab-Or-An ratios of the diatexites lie within the plagioclase feldspar field on the ternary feldspar face of the Q-Ab-Or-An tetrahedron. A variety of evidence suggests that the diatexites crystallised under variable water vapour pressures in the range 2-5 kb. Consideration of the water saturated phase relations in the Q-Ab-Or-An tetrahedron shows that the mesonormative compositions of the diatexites generally lie within the primary phase volume of quartz close to the quartz-feldspar surface. Crystallization of the diatexites would have proceeded with the early appearance of quartz and plagioclase feldspar which would have moved the liquid composition down the quartz-feldspar surface to the intersection with the two feldspar surface when K feldspar would have appeared. This is consistent with the observed textural relationships between the felsic phases. P-T conditions during diatexis were within the stability field of sillimanite and probably above the stability of muscovite (breakdown possibly by the vapour absent reaction plagioclase + muscovite + quartz = sillimanite + K-feldspar + liquid) at the level now exposed and probably above the stability of biotite (breakdown possibly by the vapour absent reaction biotite + sillimanite + quartz = cordierite + K- feldspar + liquid) at deeper levels within the belt.