La caldeira du Monte Cinto : établissement du log détaillé des formations volcaniques permo-triasiques de la vallée de I'Asco (Haute Corse)

Abstract

Western Corsica constitutes the northern part (150 km x 50 km) of the 400 km-long, 50 km-wide Corsican-Sardinian batholith (Ghezzo et al., 1989). It is made up of two contrasting Carboniferous magma suites (Orsini, 1980 ; Rossi and Cocherie, 1991). The earlier Lower to Middle Carboniferous suite (Marre et al., 1982; Cocherie et al., 1992), is of Mg-K calc-alkaline (MKCA) composition (Ferré, 1989), whereas the later, Middle to Upper Carboniferous suite (Cocherie, 1984), is dominantly calc-alkaline (CASS) (Bralia et al., 1980; Poli et al., 1989). The plutonic rocks intruded Lower Palaeozoic metamorphic units, now preserved as enclaves and rafts (Palagi et al., 1985; Ménot and Orsini, 1990). The batholith was unroofed by ero- sion and then intruded and overlain by volcanic deposits. In northwestern Corsica, two separate episodes of late to post-Variscan volcanic activity have been defined (Vellutini, 1977; Cozzupoli et al., 1990). An Early Permian episode is represented by rocks of the andesite-dacite-rhyolite high-K calc-alkaline suite. The age of the second episode is as yet poorly constrained, palaeontological determinations have yielded Late Permian age. Volcanic formations preserved within Scandalo-Senino and Monte Cinto cauldrons belong to the basalt-trachyte-comenditic rhyolite alkaline silica-(over) saturated suite. Gabbro, monzonite and alkali-feldspar granite constitute the plutonic equivalents emplaced at 2 000 m depths (Vellutini, 1977; Bonin, 1980,1988; Bonin et al., 1987; Johansen, 1988; Platevoet, 1990; Egeberg et al., 1993). The Monte Cinto massif is a caldera-bearing alkaline complex, where more than 80% of the total area is occupied by volcanic rocks . As the caldera is now largely eroded the term "cauldron" is preferred (Smith and Bailey, 1968). Tertiary tectonic movements created relief which has been highly dissected by Quaternary glaciers. The upper Asco valley is a 10 km-long NE- trending natural cross-section within the interior of the caldera along which relief reaches 1 400 m between Asco vil- lage and the summit of Muvrella and 2 000 m between Asco and Monte Cinto. Three major units are very well exposed and the complete stratigraphie sequence cun be observed through a thickness of as much as 2 000 m (figure 1). The aim of this paper is to detail the stratigraphie succession on a trail along the Stranciacone valley, from the border fault marked by the Santonaccia, Casanovaccia and Corbica valleys, which join the Stranciacone valley at 660 m elevation, up To the summit of Muvrella (2 148 m) (Mercury et al., 1992). The lower series The lower series is exposed from the caldera border fault (660 m elevation) to beyond the Manica bridge (995 m elevation). This marks the onset of the alkaline volcanic activity, and is composed of eight units forming two subseries which have been distinguished on the basis of the 40° angular unconformity which seperates the two subseries and a difference in general colour, the lower subseries being greyish-green, and the upper one dark green to violet (figure 1). The lower subseries. Along the D147 road from the Osso di Pino Spring to beyond the Roggia bridge (788 m elevation), three subvertical eruptive units (numbered 1 to 3) are exposed near the border fault. They are heavily intruded by late rhyolitic dykes 1 to 8 m-thick and a large border dome, as much as 1 km in diameter from Casanovaccia valley to Osso di Pino Spring. Intrusive rocks everywhere show lobate contacts, suggesting emplacement within somewhat ductile and not entirely consolidated host rocks. Each unit is about 110 m-thick and composed of three sequences (figure 2): 1) A volcano-sedimentary sequence, O.5-15 m-thick and averaging 10 m-thick, composed of epiclastites and/or pelites deposited in shallow lacustrine environments; 2) A sequence, 5-7 m-thick, of ash-fall tuffs and cinerites, essentially in units 2 and 3, in fine-grained beds 10 cm to 1 m-thick. 3) A thick pyroclastic sequence, averaging 95 m-thick, of mas- sive tuff in dark, metre-thick, clast-bearing beds. The tuff is generally porphyritic, with phenocrysts of alkali feldspar, skeletal quartz and oxides, and contains lithic clasts and devitrified pumice in a groundmass of alkali feldspar, quartz and dark minerals converted to chlorite. Dykelefs of black retinite (completely devitrified obsidian) 10-20 cm thick, form "rheodykes" pressed from the compacting pyroclustic deposits. Zircon typology (Pupin, 1976, 1980) on sample Ul (unit 1) is typical of alkaline felsic rocks (figure 5). Chemically. the rocks are persilicic, slightly peraluminous (0.16 to 1.33% CIPW-normative corundum) and show alkaline affinities, with relatively low Na2 0 contents and high K2O contents (Table 5). Their present composition is not magmatic, but the result of high-temperature alteration processes accompanying emplacement, compaction and cooling of the eruptive formations. The Upper subseries. This can be observed along the D147 road from beyond the Roggia bridge (840 m elevation) to beyond the Manica bridge (995 m elevation). Continuous exposures are abondant, especially in the cliffs above the D147 road and on the slopes above the left bank of the Stranciacone. This group comprises units 4 to 8, which have centripetal WSW dips of 20°-50°. Along the track on the left bank of the Stranciacone from the Roggia bridge to the Ventosi valley slope breccias dipping at 50° channel the subvertical unit 3, at the top of the first subseries, and are overlain by the massive pyroclastites of unit 4 dipping at 25°. The 100 m-thick unit 4 and 50 m- thick unit 5 are made up of green to violet pyroclastics exposed from the Roggia bridge up to a stone-built spring at 930 m elevation. From bottom to top, the sequence consists of: 1) 5 to 10 m-thick aphyric rhyolite with levels rich in violet felsitic clasts, 2) porphyritic rhyolite bearing phenocrysts of red alkali feldspar and black quartz, 3) aphyric rhyolite less than 5 m thick. "Rheodykes" of black retenite 1 to 3 cm thick can be seen near the top of unit 4. Along the Giunte track on the left bank of the Stranciacone, near a late rhyolite dome at 1 025 m elevation, a 20 m-thick pelitic fine-grained grey green interval intercalated between units 4 and 5 contains flattened black nodules, which could represent remnants of organic matter (currently under study) and is overlain by a thin conglomerate layer below unit 5. Unit 6, 80 m-thick, is made up of violet ignimbrite bearing 2 x 20 cm fiamme and devitrification nodules up to 5 cm in diameter (pyromeride facies) and, at the top, an antoclastic breccia with angular fragments of aphyric rhyolite. On the slopes of Capu a l'Altore (between 1010 and 1030 m elevations), a 20 to 30 m-thick sandstone-shale sequence overlies unit 6 and passes laterally to a lahar containing huge (several cubic meters) blocks of aphyric violet ignimbrite and pyromeride of unit 6 set within a very fine-grained green matrix. East of the Peralzi Valley, the lahar passes at the top to 6 m of coarse-grained quartzite. Unit 7 of violet ignimbrite, is thicker (about 150 m) and also more diversified. At the Giunte bridge consisting (949 m elevation, the sequence is, from bottom to top: (1) pyroclastic breccia with clasts of porphyritic red rhyolite, aphyric black rhyolite and pink granite-porphyry up to 25 cm across, 2) ignimbrite with white pumices fragments 1 to 2 cm- long and clasts of porphyritic and aphyric violet rhyolite less than 10 cm-long 3) ignimbrite with 2 x 10 cm fiamme. Between the Giunte and Manica bridges is the 100 m-thick unit 8, composed of aphyric green ignimbrite. The main deposit contains numerous red fiamme; the top is fiamme-depleted and the fiamme are smaller. The porphyritic rhyolites of units 4 to 8 contain quartz and alkali feldspar phenocrysts and pumice set in a fine-grained crystalline groundmass. Pyro-merides and aphyric rhyolites are composed only of devitrified glass and pumice. Chemically the rocks are alkaline metaluminous, with less than 1% CIPW-normative corundum and no aegirine (Table 5). The middle series. The widely developed middle series forms most ot the highest summits, such as Monte Cinto (2707 m). It consists of a 740 m-thick sequence of subhorizontal layers of clastic and porphyritic rhyolite (figure 1), emplaced as surge flows of very variable thickness. The series begins with units 9 and 10 35 m-thick, composed of green to black porphyritic rhyolite, with phenocrysts of white to red alkali felspar, black quartz, calcic-sodic amphibole + biotite. Rhyolitic clasts are rare and 1 to 3 cm in diameter. Large enclaves of MKCA monzonite are present in unit 10. This is followed by about 300, 1-3 m-thick, layers of red porphyritic rhyolite without fiamme, containing brick-red alkali feldspar, black quartz and scarce white mica phenocrysts. Some 2 cm-thick dykelets of black retinite occur in the higher layers. The top of the series is composed of two, 25 m-thick, black to red ignimbrite units with columnar jointing Phenocrystsare alkali feldspar, quartz and scarce sodic amphibole ± silicic mica. Amphibole compositions (Table 1, figure 3) range from katophorite to richterite in units 9 and 10 and from arfvedsonite to riebeckite in rocks at the top of the middle series. Incomplete Y-site occupancy suggests either actual vacancy (Czamanske and Dillet, 1988), or the presence of Li not detected by electron microprobe (Hawthorne et al., 1993). Compared to Bonifatto granites, which represent the plutonic counterparts (Johansen, 1988; Egeberg et al., 1993), amphibole is Mg-rich and Mn-poor, this could be related to higher fOz at the eruptive stage. Mica composition is highly variable (Tables 2 to 4, figure 4). In unit 10, the mica is biotite partly converted to chlorite (Table 2). At the top of the middle series, the mica is silicic and belongs to the montdorite group (Robert and Maury, 1979; Czamanske and Dillet, 1988) with Si 5.9-6.6. In the middle zone, white mica (Tables 3 and 4) is highly silicic (Si 6.35-7.67) and relatively mafic (Fe + Mn + Mg. 1.1-2.7), which could suggest Li-bearing zinnwaldite-trilithionite solid solution series (Bailey, 1984; Tindle and Webb, 1990), as in the Nigerian younger granites (Kinnaird, 1979). Zircon typology (figure 5) confirms the overall (per)alkaline affinity. Zircon xenocrysts of MKCA affinities have been observed in unit 10. Chemically, the rocks (Table 5) are metaluminous at the base and in the middle of the series, while the units at the top are clearly peralkaline comendites (> 1% CIPW-normative aegirine). The upper series. The horizontal Upper series, 410 m- thick, forms the Muvrella ridge which dominates the Stranciacone valley. The First unit is composed of 110 m of massive purple-red ignimbrite. Spectacular 5 m-diameter columnar jointing can be easily observed in the cliff above the Haut-Asco ski station. The ignimbrite is made up of 60% of porphyritic rhyolite bearing phenocrysts of red alkali feldspar, black quartz and amphibole converted to chlorite, 12-15% of 1-5 cm porphyritic devitrified fiamme and 25- 28% of lithic clasts 1-3 cm. Above six are distinct cooling units each 50 m-thick, and composed of, from bottom to top: 1) 10 m of green cinerites and tuffs, with small 1-2 cm-long clasts, in centrimetre-thick alternating beds, 2) 20 m of porphyritic violet ignimbrite, with quartz and alkali feldspar phenocrysts, fiamme and white pumice as large as 2 m x 20 cm, 3) 20 m of massive green breccia, depleted in fiamme, with alternating layers bearing brownred clasts 10-20 cm-long and small 1-2 cm clasts. At Punta Aculaghia (2034 m} and the summit at 2003 m between Bocca di Stagnu and Bocca Aculaghia, intrusive rhyolttrc domes are associated with 30-50 m thick auto-clastic and rheomorphic breccias of pyromeride, porphyritic rhyolite and cinerite. Chemically the rocks are metaluminous to peraluminous (Table 5), with an Na<< K distribution indicating extensive hydrothermal imprints. Late intrusives. Numerous felsic dykes are made up of vitreous rhyolite, some showing flowage structures, retinite and hydraulic breccias. They were emplaced in the central part of the cauldron and form two radial and concentric sets of subvertical dykes up to 5 m-thick. Subvertical dolerite dykes up to 3 m-thick were emplaced within and outside the cauldron and commonly show extensive alteration. Intrusive domes were emplaced within the cauldron and are composed of porphyritic rhyolite with large alkali feldspar and quartz phenocrysts, such as in Tighiettu Valley. In Casanovaccia and Santonaccia Valleys located in the border zone, a huge rhyolitic dome has intruded the first unit of the lower series. Flowage structures are marked by flow folds and centimetre-thick layers. Modal composition is 1% alkali, feldspar, 3% quartz and 96% devitrified groundmass. Extra-caldera formations. Below Monte Padru (2390 m), alkaline rocks are exposed dipping 10-30° towards the centre and unconformably overlying folded calc-alkaline deposits of the 'Asco green series" (Vellutini, 1977). They are densely intruded by late rhyolite dykes, sills and domes. A preliminary survey of the formations exposed along the Santonaccia Valley indicates a rhyolite intrusion about 90 m thick and 150 m of subaerial deposits. The intrusion is composed of porphyritic rhyolite passing laterally to autoclastic breccia. Though K-rich (6.5% K2O) and Na-poor (2.4% Na20), suggesting strong alteration, the whole-rock chemistry has retained its primary peralkaline composition (Table 5). The subaerial deposits are from bottom to top: 1) 30 m of green pyroclastic breccias containing violet clasts and blocks, 2) 10 m of bedded epiclastites consisting of alternating 40-50 cm-thick, fine and coarse-grained green layers with violet clasts, 3) 40 m of pyoclastites similar to 1, 4) 10 m of epiclastites, similar to 2, (5) 30 m of violet ignimbrite with 1-10 x 0.1-1 cm fiamme, rare clasts and porphyritic groundmass, and (6) massive violet porphyritic rhyolite crowded with red alkali feldspar phenocrysts, more than 30 m-thick. Units 1 to 4 resemble the 110 m-thick units 1 to 3 of the lower series, but they are on average only 30-40 cm-thick. Again, 30 m-thick units 5 and 6 resemble the ca. 100 m-thick units of the second subseries of the lower series. It is therefore suggested that the volcanic formations observed correspond to falls deposited outside the caldera at the same time as the intra-caldera lower series. The extra-caldera pyroclastites are 30 to 60% thinner than the intra-caldera deposits. Discussion. The reconnaissance of the complete sequence of volcanic ,formations within the upper Asco valley indicates: 1) The Monte Cinto cauldron comprises the eroded remnants of a caldera that was active during a post-Variscan anorogenic magmatic episode. As they cut the Evisa peralkaline complex, which has been dated at the Permian-Triassic boundary (Bonin et al., 1978), the eruptive rocks are less than 245 Ma old. This is substantiated by the K-A isochron age on amphibole of 243 + 15 Ma, obtained on the associated Bonifatto grey-green granite (Maluski, 1977). Other K-A determinations, either by the conventional technique on minerals and whole rocks, or by the whole-rock isochron method have failed to provide reliable ages (Vellutini, 1977; Cozzupoli et al., 1986). 2) The Monte Cinto cauldron constitutes one of the numerous volcanic-plutonic massifs of the Permian-Triassic alkaline province of the Western Mediterranean (Bonin et al., 1987). Two structural levels of emplacement are exposed. About 2000 m of volcanic deposits and more than 1200 m of the granite cupola of Bonifatto (Egenberg et al., 1993). The subvolcanic granite cupola Intrudes red ignimbrites of the middle series at 1460 m elevation within the Ficarella valley, near Spasimata (Vellutini, 1977; Bonin, 1988). The granitic rocks can be closely correlated with volcanic equivalents though the parallelism is not perfect. 3) The volume of volcanic deposits filling the caldera can be estimated. The original diameter of the caldera is 18- 20 km and about 1980 m thickness of volcanic rocks have been observed. The thickness measured is a minimum value because of subsequent erosion. The volume calculated on this basis ranges between 500 and 625 km3 and, if it is assumed that two thirds to three quarters of the total products have been removed by erosion (Christiansen and Blanck, 1972), then the total volume of volcanic products actually emitted could be as much as 1500 to 2500 km3, which is a likely amount to be found filling an "ash-flow caldera" (Williams and Mc Birney, 1979; Wood, 1984). The Bonifatto granites occupy a volume of about 450 km3 which is of the same order of magnitude. Assuming that 10% magma is emitted at the surface while 90% remains deep-seated (Crisp, 1984), 15,000 to 25,000 km3 of silicate materials have been added to the continental crust. 4) The Tertiary volcanoes of the French Massif Central are smaller than the Monte Cinto cauldron, which ressembles more closely the Pliocene-Quaternary caldera system of Yellowstone (Wyoming, USA, Christiansen and Blanck, 1972), where, during three stages of caldera collapse within a 2.2 My period, 60 volcanic eruptions occurred and emitted 6000 km3 of rhyolite and only 100 km3 of basalt. If it is recalled that each eruption can occur within some days, and even some hours, the total activity occured in a remarkably short time, despite the huge volume of products emitted. On Earth, such calderas are active during period of only a few million years (Wood, 1984).