Le Protérozoïque terminal et le Paléozoïque de l'archipel de Saint-Pierre-et-Miquelon

Abstract

Regional setting The main terranes. The Late Proterozoic Cadomian-Avalonian belt (Fig. 1) had a marginal position relative to the West-African shield. In the various reconstructions of the pre-Mesozoic North-Atlantic border (Wilson, 1966; Bond et al., 1988; Murphy and Nance, 1991), the elements of this belt are identified from Florida up to the Bohemian Massif. On the east coast of Canada, they have been included in the Appalachian belt. There, in Newfoundland, Nova Scotia and New-Brunswick (Fig. 2), the belt is composed of four main tectono-stratigraphic units that witness of the creation of this belt between end Proterozoic and Devonian times (Keppie, 1989), i.e. during closure of the Iapetus ocean. From north to south (Fig. 3a, b), they are the Humber Terrane, the Dunnage Terrane, the Gander Terrane and the Avalon Terrane. The Humber Terrane represents the old western continental margin of the ocean. The evolution of this zone began during the latest Proterozoic by rifting of the crystalline Grenville basement, followed during the Early Cambrian by the development of a passive margin and the opening of an oceanic basin that existed at least until the Early Ordovician. Oceanic closure during the Ordovician led to thrusting of the Taconian allochthons and the development of the Taconian foreland basin that, during the Middle Ordovician, was fed by erosion detritus from the nappes The Dunnage Terrane represents the remains of the Iapetus ocean that was sutured during the Taconian orogeny. Cambrian to Middle Ordovician submarine volcanic rocks and early ophiolite suites formed in a subduction environment are found in this zone. However, the evolution of the terrane was much more complex, with several successive volcanic arcs as proved by sub-zones, such as the Notre Dame and Exploits ones, that are described from the Dunnage Terrane. The Gander Terrane represents the Gondwanian part of the Iapetus continental margin. It is characterized by siliciclastic sedimentary rock in which volcanic terms are scarce. The age of the rocks is Cambrian to Ordovician. This terrane is explained as an accretion prism over continental crust (Colman-Sadd, 1992). The Avalon Terrane lies in a very external position relative to the Appalachian orogen. It is bordered to the south by the Minas Fault that forms the contact with the Meguma Terrane. The Avalon Terrane is widely exposed in Nova Scotia and off-shore south of St. Pierre and Miquelon (Fig. 6). During the Early Paleozoic, this domain was independent of the other three terranes, to which it was accreted during the Ordovician to Early Devonian (Keppie, 1989) by oblique sinistral shearing along major crustal discontinuities such as the Dover Fault (Fig.3b). The shearing ended with the Acadian movements and the faults were then intruded by Carboniferous granite (Dallmeyer et al., 1981a). The limits of the Avalon Terrane are different depending on the authors (Raeside and Barr, 1990; Keppie et al., 1991; Figs 3a and b). However, whatever limits are adopted, St-Pierre-and-Miquelon lie within the Avalon Terrane. Placed in the external domain of the Appalachian belt, the Avalon Terrane allows identification of some elements of an older, end Proterozoic (680-550 Ma), Cadomian-Avalonian belt. Although deformation and metamorphism associated with this orogeny were weak in intensity, elements of the belt are recognized in southern Newfoundland (Avallon and Burin peninsulas), in northern Nova Scotia (Cape Breton Island), and in New-Brunswick, Maine and New-England. As a part of the Avalon Terrane, St-Pierre-and-Miquelon show good exposure of Cadomiun rocks. The Meguma Terrane, which is only exposed in the Nova Scotian part of the Appalachian belt, is the outer zone of the belt. It is characterized by a thick turbiditic sequence, Cambrian to Ordovician in age (Schenk, 1970) and possibly overlying a high-grade metamorphic basement (Clarke et al., in Murphy et al., 1992a). The Avalon Terrane. The Avalon type-zone forms a 200 km wide strip in southern Newfoundland (Avalon and Burin peninsulas). Bordered to the west by the Dover and Hermitage Bay Faults, the Avalon Terrane (O'Brien et al., 1990) is characterized by: - homogeneous Precambrian series at the regional scale, - abundant Precambrian, ,felsic rocks, - a regional infra-Cambrian unconformity and a continuous passage between the Vendian and the earliest Cambrian (new international stratotype of the Precambrian-Cambrian limit at the southeast tip of the Burin Peninsula, near Fortune), - Cambrian to Early Ordovician platform deposits characterized by a trilobite fauna of Acadian-Baltic affinity. The same characteristics are recognized from Newfoundland to Maine and New-England and correlation charts have been proposed (Fig. 4,5). The evolution of the Avalon Terrane during the Cadomian cycle (Nance, 1990; Nance et al., 1991; Murphy et al., I992b) was similar to that of an arc/back-arc system (Fig. 4), developed in a oblique subduction framework resorbed along sinistral transform faults and auving not led to a continental collision. In addition to those criteria, which define the Avalon Terrane along the east edge of the North American continent, Devonian to Carboniferous deposits and magmatic rocks that seal the Acadian deformation are described from the Avalon and Burin peninsulas in Newfoundland. The main steps of the geological evolution of this area are shown in Table 1. In Newfoundland, the Avalon Terrane is divided in four units (Keppie et al., 199I), which are from north to south (Figs. 3b and 4b): - the Burgeo Unit, in contact with the Gander Terrane along the Bay d'Est and Dragon Bay Faults. According to Raeside and Barr (1990) and Barr et al., (1990), this unit, an equivalent of the Bras d'Or Terrane in Cape Breton Island, was separated form the Avalon Terrane at least until the Early Ordovician; - the Fortune Unit, bordered to the north by the Dover and Hermitage Bay Faults; - the Burin Unit, which is limited by the Paradise Sound Fault; - the Conception Unit that forms the most part of the Avalon Terrane at the East of Placentia and Trinity Bays. The Burgeo Unit is made up of gneiss and migmatite (Grey River gneiss and Cinq Cerf gneiss), the protolith of which is dated at 686 + 33 -15 Ma and the metamorphismat 579 ± 10 Ma (Dunning and O'Brien, 1989). The suite is intruded by tonalite and granodiorite dated at 563 ± 3 and 499 + 3- 2 Ma (Dunninp and O'Brien, 1989). The lithology of the unit is very similar to that of the metamorphic Miquelon Group, but the recently ages of 615 Ma obtained on the Cap Blanc trondhjemite (Rabu et al., 1993c; this paper) clearly distinguish two groups. The Fortune Unit outcrops on the Burin Peninsula and on St. Pierre and Miquelon (Fig. 4a and 5). It consists of volcanic rocks (O'Brien et al., 1990), mainly felsic with minor mafic terms in the lower parts of the Marystown, Love Cove, Connaigre Bay and Long Harbour groups. The base of these groups is unknown. Ages obtained on the different groups are 608 ± 2.5 Ma for the Marystown Group und 590 ± 30 Ma for the Love Cove Group, which is probably cogenetic with the Swift Current granite dated at 580 ± 20 Ma (Dallmeyer et al., 1981b). They are either conformably overlain by Latest Precambrian epiclastic turbidites (Connecting Point Croup over Love Cove Group), or unconforma- bly overlain (Cadomian orogeny effect) by Latest Precambrian to Earliest Cambrian sedimentary rock (Rencontre, Chapel Islands and Random Fms over the Marystown Group). The Fortune Unit also contains formations attributed to a Devonian-Carboniferous age, which are composed of sub-aerial volcanic rocks and clastic-continental or lacustrine deposits (Terrenceville and Spanish Room Formations). All formations of this unit have an equivalent on the St. Pierre and Miquelon islands (Rabu and Rabottin, 1992; Rabu et al., 1992; Rabu et al., 1993a, b, c). The Burin Unit forms a narrow strip on the south coast of Burin Peninsula. It is characterized by mafic rock (Burin Group; Strong et al., 1978), showing a MORB affinity (Strong and Dostal, 1980) and dated at 763 ± 2 Ma (Krogh et al., 1987). These rocks were the basis of circum-Atlantic correlations with the Pan-African Bou Azzer ophiolites (Leblanc, 1981). No equivalent rocks are known in St. Pierre and Miquelon that would correlate with the poor offshore extension of the very well-defined geophysical anomaly of the Burin Unit. The Conception Unit shows a similar succession to that of the Fortune Unit, but is more complete and diversified. The felsic and mafic suites, equivalent to the Marystown and Love Cove groups, are present in the Harbour Main Croup, but clearly older (631 ± 2 to 606 ± 3 Ma; Krogh et al., 1987). They are generally conformably overlain by marine sedimentary rock that filled a basin with turbiditic, deltaic, alluvial and then continental deposits of the superposed Conception, St. John's and Signal Hill groups. This sedimentation ended around 570 Ma (O'Brien et al., 1990) and included Ediacara faunas (Anderson und Conway-Morris, 1982). Between 565 und 550 Ma, a brief rifting phase produced, in the Avalon Peninsula, the bimodal volcanic suite of the Musgravetown Group, unconformably overlain by the latest Precambrian to Early Cambrian Rencontre, Chapel Island and Random Fms, and then by the Middle Cambriun Adeyton and Harcourt groups. Off shore the Avalon Peninsula, no discontinuity is known during the Silurian.. Inland, the Acadian orogeny in the Avalon Peninsula was mainly identified by plutonism, tilted-blocks and open folds, in places associated with very weak cleavage and low-grade metamorphism (prhenite-pumpelleyite to chlorite). In the western part of the Avalon Terrane, the Acadian tectonics produced thrust and strike-slip faults with penetrative fabrics (Strong et al., 1978). 40Ar/39Ar ages obtained on minerals from the plutonic rocks are between 356 and 352 Ma (Dallmeyer et al., 1983), but the Taconian movements, which are well-developed in the inner part of the Appalachian belt (Humber and Dunnage Terranes), are unknown here. The most important effect of Acadian deformation is the unconformity between Devonian-Carboniferous rocks and the Avalonian basement. Sedimentation and volcamism of that period were controlled by major dislocation zones. The deposits reworked detritus from Gander Terrane forma- tions (O'Brien et al., 1990) that gives a first evaluation on the suturing of the two zones. Similar rocks could be exposed on the north and west coasts of Langlade (see day 4). The opening of the North-Atlantic Ocean at the beginning of the Mesozoic was associated with the intrusion of a doleritic dyke swarm dated at 201 ± 3 Ma (Ilodych and Ilayatsu, I980), the traces of which are exposed on the archipelago. Geophysical lines across the continental margin south of the archipelago (Fig. 6), show that the opening of the Atlantic Ocean in places reused previous weakness zones in the basement, such as the limit between the Avalon and the Meguma Terranes. It was along this limit that Jurassic to Miocene deposits were accumulated on the continental shelf (McLean and Wade, 1992). The main units on St. Pierre and Miquelon. On the east coast of Canada, 22 km south of Newfoundland, the St. Pierre and Miquelon archipelago is composed of three main islands (Fig. 24): St. Pierre in the southeast, Miquelon in the north and Langlade at the south, plus several islets, all covering 242 sq. km. The coasts are generally steep and poorly accessible. The highest point of the archipelago, located on the Morne de la Grande Montagne on Miquelon island, dominates a peneplain strongly marked by the passage of Quaternary inlandsis. Draining is mediocre and the many peat bogs reduce the number of rocky outcrops. The geology of St. Pierre and Miquelon, is very similar to that of the Burin Peninsula (Newfoundland). The first geological data on SPM date from 1667, when de Denouville described porphyry occurrences on St. Pierre. Other information was given by Bachelot de la Pylaie (I825), who reported coal formations on Langlade, and Gautier (1866) who was the first to recognize a glacial origin for the surficial deposits of the archipelago; later, de Tromelin (1877) mentioned the metamorphic rocks of the Cap de Miquelon. In 1894, de Launay assigned a Cambrian age to a red detrital formation unconformably overlying the Precambrian rocks. In 1928, Howley showed the archipelago on his geological map of Newfoundland. The geology of St. Pierre and Miquelon as such was first described in Aubert de la Rüe's publications, who drew the first geological map of the archipelago (Aubert de la Rüe, 1951) after having given a mining overview (Aubert de la Rüe, 1932, 1933). This author recognized the main lithological units (Aubert de la Rüe, 1933, 1950) and pointed out the presence of a Cambrian trilobite fauna (Aubert de la Rüe, 1935). Glacial and peri-glacial deposits have been dated Middle to late Wisconsinian by Tucker (1979) and Tucker and McCann (1980). In the regional framework of the Cadomian-Avalonian belt (see above), St. Pierre and Miquelon belong either to the Avalon Terrane (Williams, 1979; O'Brien et al., 1990; Raeside and Barr, 1990; Fig. 3a), or to the Fortune subzone within the Avalon Composite Terrane (Keppie, 1985; Keppie et al., 1991; Fig. 3b). South of St. Pierre, the continental margin has recently been described by McLean and Wade (1992), who specifically studied the post-Jurassic formations and the limit between the Meguma and Avalon terranes. Synthesis maps of the Nova Scotia and Newfoundland margins (Ross and coll., 1991) show the archipelago in the geological setting of eastern Canada, but unfortunately the geological outlines on the archipelago (Sanford et al., 1991; Jansa, 1991) are a clear regression from Aubert de la Rüe's work (1951). Until recently, the stratigraphic constraints were poor and only based on the presence of Paradoxides davidis on Langlade (Aubert de la Rüe, 1935). According to this author (Aubert de la Rüe, 1933, 1951), St. Pierre and Miquelon was composed of three main geological units: - a volcanic unit attributed to the Precambrian and made up of rhyolite, breccia and rhyolitic tuffs, associated with subordinated andesite and andesite breccia; - a metamorphic series attributed to the Precambrian and including paragneiss, metaquartzite, amphibolite and migmatite, intruded by granite and diorite plutons (Aubert de la Rüe, 1950, 1951); more recently, these intrusions have been attributed to the Devonian on the 1 :1,000,000-scale geological map of Newfoundland (Colman-Sadd et al., 1990); - a composite sedimentary unit, including a Cambrian series, the stratigraphic succession of which has recently been modified (Rabu et al., 1993b), overlain by a post-Cambrian Paleozoic series. As a result of geologial mapping by BRGM (French Geological Survey), which started in 1992 (Rabu and Rabottin, 1991; Rabu et al., 1992; Rabu et al., 1993a, Rabu et al., 1993b, Rabu et al., 1993c), five main units (Fig. 7), not counting surficial deposits, have been described. - The Cap de Miquelon Group consists of metasedimentary and mafic metavolcanic rock, intruded and metamorphosed by dioritic to trondhjemitic plutons dated at 614 Ma. No equivalent is known in Newfoundland, but this group does not represent the crystalline basement of the Avalon Terrane. - The St. Pierre Group is mainly composed of felsic rock dated at 581 Ma; basaltic and andesitic terms are subordinate, but characteristic, as is the felsic suite, of an arc/back-arc environment. This group could be an equivalent of the Marystown-Love Cove groups in the Burin Peninsula. - The Fortune Group, in which the Precambrian-Cambrian boundary was recently defined in Newfoundland, includes the Rencontre, Chapel Island and Random formations that unconformably overlain the Marystown Group. In St. Pierre and Miquelon, the lowest part of the group is not exposed, but a Tommotian fauna has been found. - The Langlade Group is an equivalent of the Adeyton and Harcourt groups (Bengston and Fletcher, 1983), and is attributed to the Early to Middle Cambrian. In the archipelago red, grey and black siltstones with calcareous beds of Early Cambrian age are combined into a single group, because, though each is clearly identified in specific points, mainly on the coast, none can be followed inland. -- The Belle-Rivière Group is exposed on the west and northeast coasts of Langlade. It unconformably overlies the Langlade and Fortune groups, and is made up of a bimodal volcanic suite conformably overlain by a red, terrigeneous detrital formation. This group, the deposits of which are fault-controlled, could be compared to the Devonian- Carboniferous Terrenceville Formation of the Burin and Avalon peninsulas. Late dolerite dykes cross intersect all archipelago formations and some produced a contact metamorphism that overprints Acadian cleavage in Cambrian siltstone. A Mesozoic age (Triassic to Early Lias) is probable for these intrusions that could be contemporaneous with the opening of the North Atlantic ocean. Description of the main group. The Cap de Miquelon Group. The Cap de Miquelon Group (Fig. 8) only occurs in the northern part of Miquelon island, in the Cap Peninsula and south of the Grand Etang. In the Cap Peninsula, the best outcrops are along steep cliffs over the shore. Inland, outcrops are poor and covered by wood and thick peat layers. South of the Grand Etang, the only outcrops are found along the shore. The group is mainly composed of non-fossiliferous metagreywacke (stops 3.1 and 3.8) with interbedded mafic volcanic rock and dykes (stop 3.2). Two diorite plutons intruded the series, one along Anse à la Vierge (stops 3.10 and 3.11), the other at Cap Blanc (trondhjemite of Cap Blanc, stop 3.1). Both produced a strong contact metamorphism in the surrounding rocks (biotite-K-feldspar-cordierite micaschist), and garnet was described by Aubert de la Rüe (1951) from south of the Grand Etang. These index-minerals are consistently found in the metamorphic foliation. Muscovite is ubiquitous and associated with sphene andaupatite; it forms large flakes in the metamorphic foliation. The metamorphic foliation is everywhere well-expressed with, locally, a strong stretching lineation (stops 3.5 and 3.6). Metamorphic gradients around the intrusions are controlled by the foliation trends. No previous metamorphism has been observed in these rocks, in which bedding and sedimentary structures are commonly well-preserved (stop 3.3). In Aubert de la Rüe's descriptions of the Cap de Miquelon unit (1950, 1951), migmatite was mentioned between Anse à la Vierge and the tip of Cap de MiqueIon. However, the same lithology has been observed all along the peninsula, with rocks being variably affected by the metamorphism. Near the Anse à la Vierge diorite, indications of partial melting are obvious, but the paragneiss texture with sedimentaly structures is generally preserved. Indications of partial melting are also exposed in other places (stop 3.7). Several types of dykes cross-cut the metamorphic unit: - Pink, aplitic, muscovite-bearing microgranite (stops 3.1 and 3.9) are directly linked to the plutons. They are well-developed in the cliffs above Anse à la Vierge. - Post-metamorphic dolerite with pyroxene and olivine (stops 3.1 and 3.4). - Rhyolite and porphyritic microgranite. South of the Grand Etang, the Cap de Miquelon Group is composed of variably recrystallized amphibolite and quartz-amphibolite, cross-cut by small diorite intrusions. In the less recrystallized rocks, pillow structure is preserved. The rocks are in fault contact with unmetamorphosed felsic pyroclastic rock of the St. Pierre Group, also exposed in the central part of Miquelon. The Cap Blanc trondhjemite occurs in a small massif along the shore of the Cap Blanc and around the Butte du Calvaire. Westward, the same pluton is exposed at the Veaux Marins (Aubert de la Rüe, 1950). Magmatic foliation is present with the medium grain-size, biotite-amphibole facies. Plagioclase and biotite are automorphic, whereas quartz and (scarce) K-feldspar are xenomorphic. The Anse à la Vierge diorite, a melanocratic rock enriched in amphibole and biotite forming two small massifs near the cove (stop 3.10), is locally associated in a magmatic breccia (stop 3.11) with leucocratic rock resembling Cap Blanc trondhjemite. In the heart of the massif, it is an isogranular diorite With amphibole and minor biotite, but towards its borders, brown biotite is more abundant, and amphibole consists of small disorganized paler crystals. Age of the plutonic rocks of the Cap de Miquelon Group. Four small, clear and light-pink zircon grains of the Cap Blanc trondhjemite were dated (Rabu et al., 1993c), first by thermal evaporation of monozircon, then by chemical dissolution. The first gave a 615 + 14 Ma age, calculated with 430 isotopic ratios. Analysis by dissolution gave a discordia with an upper intercept at 614 + 11, -7 Ma. The age of the Cap Blanc trondhjemite is thus well constrained at 615 + 14 Ma. In Newfoundland, a 620 + 2.1, - 1,8 Ma age was proposed by Krogh et al. (1987) for a gabbro-quartzmonzonite and granite complex at Holyrood Beach (Strong Minatidis, 1975). Ages for the Cap de Miquelon Group are in the same range as those of the Marystown and Harbour Main groups, the lavas of which extruded between 630 and 608 Ma on the Burin and Avalon peninsulas. In Nova-Scotia, the Fourchu Group is cross-cut by a pluton dated at 635-600 Ma (Keppie et al., 1990). Geochemistry of the magmatic rocks of the Cap de Miquelon Group. Thieblemont (1993) studied three types of magmatic rock: amphibolite, Anse à la Vierge diorite and Cap Blanc trondhjemite. Dolerite and rhyolite dykes that intrude the unit, show the same characteristics as equivalent rocks from St. Pierre Island. Amphibolite is basaltic (47 % < SiO2 < 52 %) or andesitic (SiO2, # 58 %) in composition. in the Myashiro (1974) diagrams, these rocks either plot in the tholeiitic field or are spread over the tholeiitic and calc-alkaline fields (Fig. 9a, b). In the Th-Hf-Ta and Th-Tb-Ta diagrams (Fig. 10a, b), the two types of amphibolite are clearly separated; quartz-amphibolite, andesitic in composition, plots in the field associated with subduction zones, whereas basaltic amphibolite plots in the back-arc basalt field. The same scheme is proposed for the mafic rocks of the Pointe du Diamant Fm., at the base of the St. Pierre Group (see this paper). The amphibolites were affected by metamorphism developed by a 615 Ma pluton, which means that they represent a volcanic episode older than that of the St. Pierre Group. This argues for the existence, in this part of the Cadomian belt, of an arc/back-arc system for about 40 million years, that is consistent with present-day systems such as the Marianas and Philippines. Diorite and quartz-diorite (Anse à la Vierge type) are mafic to intermediate in composition and are distinguished from thp quartz amphibolite by less Si02,but a higher P205-Al203 content. In the SiO2 vs. FeOtlMgO diagram (Myashiro, 1974), they plot in the tholeiitic ,field (fig. 9a). In the Th-Hf-Ta and Th-Tb-Ta diagram (Fig. 10a, b), the Anse à la Vierge diorite clearly plots in the fïeld of magmatic. rocks associated with subduction zones. The trondhjemite of Cap Blanc shows the chemical characteristics as defined by Barker (1979), i.e. 68 % < Si01 < 75 %, Al203 > 15 % if SiO2#70 %, NiO2 = 4-5.5 %, K20 < 2 % up to 2.5 %, and FeOt/MgO = 2-3 %; they plot at the limit between low-K and medium-K fields (Fig. 10c) in the Pecerillo and Taylor (1976) nomenclature. The Fe0t/Mg0 ratio is low to medium and Si02, content is high: the samples plot in the calc alkaline field in thc SiO2 vs. Fe0tlMg0 diagram (Fig. 9a). In the (Nb/Zr)N vs. Zr diagram (Fig.9c), the samples of Cap Blanc trondhjemite fall in the field of rocks produced in subduction zones. Pink aplite dykes that cut the massif have a high-K character (Fig. 9c) and represent a later magmatic phase. Setting of the metamorphic rocks of the Cap de Miquelon Group. As a fïrst conclusion on the Cap de Miquelon Group, data from geological mapping do not support the hypothesis of a crystalline basement with a cover, later intruded by a dioritic pluton (Cap Blanc trondhjemite, Anse à la Vierge diorite). This is demonstrated by the existence of the same metagreywacke that is implied in prograde metamorphism from epizone to amphibolite facies, and was affected by partial melting around the diorite plutons. A more coherent hypothesis is that of a volcanic-arc root that variably transformed the surrounding sedimentary rock. Ages proposed for the Cap Blanc intrusion are consistent with the existence of the root of a volcanic arc that worked between 631-606 Ma; its effusive equivalent might be the volcanic rocks of the Harbour Main Croup in the Avalon Peninsula. The migration of the arc during subduction would have led to a new arc in the Burin Peninsula, beginning at 6O8 Ma with the Love Cove- Marystown volcanism and ending around 580 Ma with that of St. Pierre. The St. Pierre Croup The St. Pierre Group (Fig. 11) is mainly composed of felsic rocks, the base of which is regionally unknown. The group is divided in five superposed or juxtaposed formations (Fig. 12): the Pointe du Diamant Fm., the Cap aux Basques Fm., the Vigie Fm., the Cap Rouge Fm., and the Trépied Fm. In the group, mafic terms are poorly developed and restricted to the lower part (Pointe du Diamant Fm. and locally in the Cap aux Basques Fm.). In this case, they are submarine pillow basalt, breccia, and tuff intruded by dolerite dykes. Some andesite flows are also present. The mafic rocks of St. Pierre show an arc/back-arc geochemical signature (Rabu et al., 1992). The other formations of the group consist of felsic rocks, which represent at least 80 % of the suite, and are widely exposed on St. Pierre Island (Fig. 11) and in the central part of Miquelon. They are composed of pyroclastic nappes (ignimbrite, breccia, tuff, ash) and rare interbedded lava flows, mainly in the northeastern part of St. Pierre Island (Le Frigorifique, Cap Rouge). Dacitic to rhyolitic in composition (Rabu et al., 1992), the felsic sequence was part of an arc environment. Locally, interbedded volcaniclastic and sedimentary rocks occur within volcanic deposits (northeast of Cap aux Basques, Cap à I'Aigle, Anse à Henry). The St. Pierre Group shows the same lithological associations as the Marystown Group in the Avalon Peninsula (Strong et al., 1978), but geochronological data obtained on lava (Rabu et al., 1993c) show the rocks of the St. Pierre Group (581 ± 12 Ma) to be clearly younger than those of the Marystown Group (608 + 20 - 7 Ma (Krogh et al., 1987). They could be compared to rhyolite dykes cutting the Harbour Main Group in the Bonavista Peninsula (New- foundland) and dated at 585 Ma (Krogh et al., 1987). The Pointe du Diamant Formation. The Pointe du Diamant Formation is the local base of the St. Pierre Group, in which mafic lava, pyroclastic tuff and breccia, and dykes are dominant. The felsic component is represented of tuff, or by intrusive dykes. Basaltic lava flows commonly are massive, but pillow structures are well-exposed at Pointe du Diamant (stop 1.1). Chlorite and calcite vugs are present. The rocks have a microlitic texture and a mineralogy of spilite: albite, chlorite, pistacite, leucoxene, pyroxene and chloritized olivine(?). In the SiO2 vs. Nb/Y diagram (Fig. 13), basalt of the Pointe du Diamant Fm. plots in the subalkaline field. In the SiO2 vs. FeOt/MgO diagram (Fig. 14a), it plots in the tholeiitic field (poor Fe-enrichment); the low-TiO2 content (Fig. 14b) allows a comporison with arc tholeiites. The strong Ta anomaly observed in these rocks (Fig. 15a) is consistent with a subduction environnent (Joron and Treuil, 1977; Gill, 1981). A similar environment is deduced from other diagrams such as Th-Hf-Ta 'Fig. 16a) or Th-Tb-Ta (Fig. 16b), classifying rocks according to their geotectonic environment. In these two diagrams, basalt of the Pointe du Diamant Fm. plots in the same area as that from the Fourchu Group in Nova Scotia (Stirling Block (Dostal et al., 1990), and from Paimpol (Armorican Massif, Auvray, 1979; Cabanis et al.,1986). Pyroclastic tuff and breccia with a mafic to intermediate matrix, are interbedded with lava flows. The breccia are poorly sorted (clast size 1-50 cm) and polygenic, but all fragments are volcanic rocks (rhyolite, tuff, ignimbrite, basalt, etc.). They have a chaotic aspect, and present a lack of internal organization of the consistently angular clasts. Numerous dolerite and gabbro dykes cross-cut the Pointe du Diamant and Cap aux Basques formations. The paragenesis is plagioclase, clinopyroxene, chlorite, epidote, pumpellyite, opaque minerals, and minor quartz and pseudomorphosed olivine. Dykes are basaltic in composition (SiO2 < 46.5%). In the SiO2 vs. FeOt/iMgO and TiO2 vs. FeOt/lMgO fields (Fig. 14a, b), they show strong Fe and Ti enrichment consistent with a tholeiitic affinity deriving from a depleted mantle (Fig. 15b), which is different from the Pointe du Diamant basalt. In the Th-Hf-Ta diagram (Fig. 16a), dolerite and gabbro plot in the field of subduction zone lavas. In the Th-Tb-Ta diagram (Fig. 16b), which is used to distinguish arc and back-arc lavas, they obviously plot in the field of back-arc basalt, and fit very well with that drawn by the basalt of the Marianas back-arc basin (Wood et al., 1981; Bougault et al., 1981). The Cap aux Basques Formation. The Cap aux Basques Fm. overlies the Pointe du Diamant Fm. It is made up of pyroclastic rock (stops 1.4 to 1.10): ignimbrite with fiamme, ash, tuff, lapilli tuff and breccia; lava flows are very scarce. Mafic terms are exceptional (stop 1.5); some dolerite dykes cross-cut pyroclastic rock (stop 1.11). Two parageneses are observed: - Volcanic rock with phenocrysts of plagioclase and opaques, but without quartz and K-feldspar; piemontite is scarce. This facies is restricted to the base of the formation (see type 1 in geochemistry). - Volcanic rock, generally with a porphyritic texture, characterized by the presence of phenocrysts of quartz and biotite; minor K-feldspar can be present. This type of rock is ubiquitous in the formation (see type 2 in geochemistry). Intermediate basaltic andesite is present in very small volumes (lava jlows and dykes). The Vigie Formation. The Vigie Fm. is composed of the two superposed Galantry and Pain de Sucre members. The Galantry Member is characterized by the presence of a very coarse breccia including clasts that can be up to several cubic meters in size. Their origin is composite but mainly volcanic (mafic and felsic). Some elements are angulur and were lithified before their inclusion in the breccia, but others have lobate edges and were included in a plastic stage as uncooled lava. Moreover, true rhyolite Iava flows can be mapped in the breccia. Pyroclastic falls with bombs are associated with these facies. The Galantry Member is assumed to have been formed during the collapse of a volcanic caldeira, and its petrographic character is that of the Cap Rouge and Trépied volcanic rocks (see below), although the relationships between those formations are as yet unclear. The Pain de Sucre Member forms the top of the formation, consisting of ignimbrite with felsic and intermediate lapilli. The paragenesis is quartz-plagioclase-K-feldspar-biotite-green hornblende. The Cap Rouge and Trépied formations. The Cap Rouge Formation is only exposed in the northeastern part of St. Pierre island. It is overlain by the Trépied Formation, but its base is unknown. It consists of three superposed lithological units, from the base to the top: - Pyroclastic tuff and breccia, with interbedded volcaniclastic epiclastic breccia and red sandstone. - White rhyolite flow with a nodular parting, laterally passing to banded ignimbrite. - Red, banded rhyolite. The Trépied Formation is composed of bedded tuff, ignimbrite, vesicular lava, breccia with angular felsic and mafic clasts, and lapilli tuff. The rocks are characterized by the constant presence of quartz, albite and K-feldspar; mafic minerals are subordinated (rhyolite sensu stricto - see type 3 in geochemistry). Piemontite is ubiquitous. Geochemical characters of felsic rocks of Cap aux Basques, Vigie, Cap Rouge and Trépied formations. Three geochemical types of felsic volcanic rock are recognized in the St. Pierre Group: - Type 1: volcanic rock from the base of the Cap aux Basques Fm. is defined as dacite and rhyolite (SiO2 = 67-78%), but even in more felsic samples no quartz phenocrysts are seen. - Types 2 and 3, respectively from the Cap aux Basques Formation, and the Cap Rouge and Trepied formations, are mainly rhyolite with SiO2 > 72%. The presence of K-feldspar is constant In the type 3 volcanic rock. A good discrimination of such rocks is obtained in the SiO2 vs. CaO + MgO + Fe203t diagram (Fig. I7a). In the SiO2 vs. Zr diagram (Fig. 17b), the three types are also well identified by the variation of Zr content that decreases from Type 1 to Type 3. The REE contents provide some indications on the origin of the three types of volcanic rock (Th vs. Ta ; La vs. Ta diagram - Fig. 18a, b). In the Type 1 rocks, the Nb/Ta ratio is near the chondritic value taken as the average value for mantle rocks. In the Type 2 and 3 rocks, this ratio is similar to that of crustal alu minous leucogranite (average value from upper continental crust or greywacke). Differences between the three types of rocks are summarized in the spider diagrams normalized to the Primary Mantle (Fig. 19). In spite of these differences, most felsic samples plot in the field of magmatic rocks produced in subduction zones (see Zr vs. (Nb/Zr)N diagram - Fig. 20). According to their low Nb/Ta ratio, the Type 3 rhyolites plot near the field of crustal leucogranites associated with continent-continent collision zones. Intermediate rocks of St. Pierre are restricted in volume. In the Th-Hf-Ta (Fig. 15a) and Th-Tb-Ta diagrams (Fig. 15b), they plot in the field of rocks produced in subduction zones, which is confirmed by a low-Ti content (Ti02 < 1.2 %) and a strong Ta anomaly. Nevertheless, they differ from the Pointe du Diamant basalt by lower Hf/Ta and Tb/Ta ratios. Clasts taken from the Galantry Member are either felsic or mafic, but rarely intermediate in composition. In the Si02 vs. FeOtlMgO diagram (Fig. 14a), mafic clasts plot in the tholeiitic field; the low- Ti02 content is similar to that of Pointe du Diamant basalt. Setting of St. Pierre volcanism. The geochemical character of felsic and mafic volcanic rocks of St. Pierre indicates a setting in a subduction environment. Dolerite and gabbro of St. Pierre are similar to basalt associated with present-day back-arc basins. Nevertheless, a second set of dolerite dykes cut across all formations, including the Paleozoic ones, on Langlade. Their geochemical character resembles that of continental tholeiites. The Fortune Group (Fig. 23). The lower part of the group is not observed in St Pierre and Miquelon, and the first deposits attributed to the Fortune Group belong to Member 3 of the Chapel Island Formation. l The Chapel Island Formation is composed of greenish, argillaceous, bedded siltstone with sandstone beds and calcareous lenses. Three members have been identified: - Member 3 is a massive grey-green siltstone in 1-m-thick beds that show internal lamination. This member is characterized by the presence of diagenetic calcareous nodules. - Member 4 starts with pinkish, nodular limestone in discontinuous layers wrapped in reddish and grey-green siltstone. They have provided a characteristic association of the base of the Aldanella attleborensis Zone (Watsonella crosbyi Zone, sensu Landing, 1989) (Rabu et al., 1993b), with Aldanella attleborensis (Shaler and Foerste, 1888), Aldanella sp., Watsonella crosbyi Grabau 1900, "Ladatheca" cylindrica (Grabau, 1900) and Circotheca?. Only one specimen has been determined as Aldanella attleborensis. On the basis of this faunal association, a Tommotian (earliest Cambrian) age is proposed for Member 4. - Member 5 starts with well-layered, green siltstone including abundant trace fossils. Fine-grained detrital clasts are present in reddish and greenish argillaceous sandstone with muscovite flakes. The first lenses of microconglomerate are present 15 m below the top of the member; reworked pebbles of quartz and volcanic rock arc well-rounded and sorted. In the Chapel Island Formation, detrital feldspar and lithic fragments mark the inheritance of a composite sediment supply with plutonic, volcanic, metamorphic and sedimentary rocks. Epidote is common at the base and decreases towards the top, where it is replaced by glauconite. The mineralogical association of argillite and siltstone is quite uniform with quartz + feldspar + chlorite + illite, and is consistent with the relatively high-K2O (2-3.4%) and high-Na2O (1.7-2.7%) contents, and by a variable CaO content (0.5 to 12%). The Random Formation (around 130 m) conformably overlies the Chapel Island Fm. It is composed of quartzite and is divided in two superposed members: - Lower member contains well-bedded feldspthic sandstone and quartzite; near the base, discontinuons Ienses of microconglomerate occur. Sedimentary features (herring-bone structure, ripples) are numerous and underlined by heavy mineral concentrations. - Upper member consists of white quartzite with heavy mineral layers. Glauconite, already present at the top of the Chapel Island Fm., is still common in the lower part of the Random Fm. The volcanic inheritance is also found at the base of the formation, but this type of fragment progressively disappears towards the top; distribution of plutonic and metamorphic fragments is uniform. The Langlade Group (Fig. 23). The Brigus Formation (around 1.5 m) is surely identified at Cap Percé (Stop 4.4), and probably is stratigraphically missing elsewhere on the island, thus reflecting the major erosional disconformity that is well-known in the Avalon Peninsula at the base of the Adeyton Group (O'Brien et al., 1990). Near the top of the formation, which is composed of green argillaceous sandstone in 30- 50-cm-thick beds, occur thin layers of pale green tuff. Mafic in composition (SiO2 = 49%), these rocks are highly potassic (K2O = 6.98%). The Chamberlain's Brook Formation (around 30 m) shows a uniform distribution on the island. It is composed of red argillite, ochre calcareous sandstone and yellow, bioclastic limestone in thin beds. The limit with the underlying Brigus Fm. is marked by a 10-cm-thick manganese crust. Composition is globally the same as that of the Chapel Island rocks with, however, a strong Na2O (0.30 - 1.16%) and CaO (< 1%) depletion. The Manuels River Formation (> 80 m) conformably overlies the Chamberlain's Brook Fm and starts with a 10-cm-thick layer of bentonite, similar to that well-known in the Avalon Peninsula (O'Brien et al., 1990). The formation is composed of black shale with sandy and calcareous nodules (up to 30 cm in diameter) stuffed with trilobite fragments. Cone-in-cone structures are well-preserved (Bonte, 1946). Composition of the sediment is not significantly different from that of the underlying deposits, except for CaO that is absent here. At Anse aux Soldats, the basal contact of the Manuels River Formation is marked by a thin layer of white bentonite exposed on a structural surface. The first meters of the formation have provided trilobites of the Hydrocephalus hicksii Zone. Above, Paradoxides davidis, Clarella venusta and Bailliella bailey have been found (see photographic plate). The vertical evolution and composition of the Cambrian rocks on St. Pierre and Miquelon are the same as that found on the Burin Peninsula. The Belle Rivière Group (Fig. 23). No palaeontological or geochronological dating is yet available. The Paleozoïc attribution is assumed on the basis of the unconformity between these rocks and those of the Langlade Group (see map of Langlade), and on the lithological similarities between the rocks of the Belle-Rivière Group and the Devonian-carboniferous Terrenceville Fm. in Newfoundland. Along the Anse du Gouvernement section (stop 4.6), the three superposed formations of the group are exposed. From bottom to top, these are the Anse du Gouvernement Formation, the Cap aux Morts Formation and the Cap Sauveur Formation. The Anse du Gouvernement Fm. is composed of fluidal ignimbrite and rare breccia. It unconformably overlies the Manuels River, Random and Chapel Island formations. The Cap aux Morts Formation conformably overlies the previous formation. It is made up of mauvish to green, poorly bedded, vuggy basalt. Small pillow structures have been observed, but are not common. In the massive lava flows, vesicles are numerous and flattened in a rough plane of flowage. Vesicles are filled by calcite, chlorite, epidote und opal. At the Anse du Gouvernement stop, basalt is massive but pillow structures are seen in the last metres of the formation, near the contact with volcaniclastic deposits. The Cap Sauveur Formation conformably overlies the Cap aux Morts basalt. It starts with a 1-m-thick, reddish, coarse conglomerate reworking angular volcanic rock, which is overlain by interbedded coarse volcaniclastic sundstone and red siltstone. Towards the top, graded bedding occurs, the deposits are progressively greener, and sedimentary features become increasingly common, indicating a coastal, marine environment. In the lower part of the formation, clasts are only volcanic; they were strongly weathered before reworking (kaolinization, silicification, enrichment in Ti and Fe, etc.) and are consistent with the presence of a nearby continental area at that time. During this phase the basin was exclusively filled with erosion products from nearby areas. Towards the top, the volcanic clasts were progressively replaced by plutonic metamorphic- and sedimentary rock pebbles. In the exposures of Anse du Gouvernement, the Cap Sauveur Fm. is a sequence, first emplaced in a continental environment directly over lava flows, which progressively passed into a marine environment as shown by: - the increase of graded bedding, - the development of green layers (reducing environment), - the extension of sediment-supply areas from the initial underlying lava flows exposed on the borders of the basin, to the reworking of other formations that were farther away.