Le Velay. Son volcanisme et les formations associées. Notice de la carte à 1/100 000

Abstract

The original aspects of Velay volcanism. Volcanoes making up the volcanic province of the southwestern part of the French Massif central, are spatially associated and grouped into well individualised entities To the east, a chain of Mio-Pliocene basaltic to phonolitic volcanoes occurs, divide up into two distinct sub-groupings where the succession of magmatic and eruptive events are contrasting and may even be inverted. To the west, a Plio-Quaternary basaltic plateau occurs, characterised by a significantly uniform composition, All geological arguments, i.e., mapping of the formations, chronostratigraphy, the structural context, both deep and superficial, magmatology and vulcanology, together confirm the distinct nature of these entities. The groupings. To the east, the phonolitic chain stretches over a distance of 55 km with more than 180 points of extrusions, and at over 100 of which phonolitic lavas formed " domes " or plugs, giving rise to the characteristic regional landscape. The sequence of lava extrusion, the geochemical evolution of the magmas and the volumes of the various lavas extruded, are different in two collapsed zones which contain the major part of the volcanoes. These zones are l'Emblavés to the northwest and Boutières to the southeast. The northwestern graben became distinct during the Palaeogene and filled with salt water lake to lacustrine sediments. The southeasterly located graben received no sedimentary fill and probably developed at a later date, consecutive with the principal phonolitic eruptions about 7 Ma. Slivers of a plateau composed of basalt to trachy-andesite flows, provide evidence of lavas attesting to strong magmatic differentiation that was variably influenced by crustal contamination. The phonolitic lavas which derived from the differentiation process, commonly show a soda hyper-aikaline character which can be explained by gas transfers, Evidence of the important role of gas, especially C02, is abundant, Taking the phonolitic chain as a whole, eruptive events occurred over a relatively extensive period, between 14 and 6 Ma. To the west, the Devès basaltic plateau covers a surface area of 600 km2 between the Loire and the Allier rivers. Vulcanism in this region is fundamentally strombolian. Over 230 scoria cones occur. The lavas were geochemically poorly evoived basalts. The northeastem margin of the plateau covers the sedimentary fill of the Puy-en-Velay graben. The fiII comprises two distinct successions; a lower Palaeogene marly limestone succession, covering an area as large as that covered by Emblavès; an upper, Villafranchian fluvio-lacustrine succession which recorded the beginning of the Devès basaltic volcanic activity. The basalt flows forming the plateau, hide the structure of substratum. However alignments formed by the scoria cones, dyke orientation (N140° to N160°) along with evidence of fissure eruptions, together attest to extension and simultaneous fracturing and volcanism. Numerous radiometric datings across the plateau, in statistical terms, demonstrate two, distinct, eruptive cli-maxes at 1 and 2 Ma. The duration of the overall eruptive phase did not exceed 3 Ma. What context? This volcanic province developed at the end of the Tertiary era on the margins of the Eurasian Plate and more specifically on an ancient Hercynian plinth which was directly confronted by the rising African plate related to Alpine events. Geophysical evidence has shown that the volcanoes are Iocated directly above an asthenosphere rise and an area of crustal thinning. Petrographic and structural studies of ultrabasic xenoliths have reinforced the hypothesis of diapiric mantle rising which recently acquired seismic studies seem to confirm as a first approximation. The diapir would appear to have formed between the Oligocene and the Villafranchian. Seismic data, still undergoing interpretation, reveal that beneath each volcanic massif, zones of low velocity seismic pro-pagation rates continue down to 70 to 180 km into a vast regional anomaly centred in the middle of the Massif central's volcanic region. These low velocity zones linked to higher temperatures in the mantle, could be caused by diapirs though this may not necessarily be the case. The upper mantle is mainly composed of spinel Iherzolites, some of which are impoverished by partial melting and some of which are enriched by metasomatism. The cortege of basaltic xenoliths shows two types of cumulates which originate from two different depths (20-30 km and 30-60 km). In contrast to certain Iherzolites from the upper mantle, these are not deformed. This would suggest that the mantle ,may be zoned where, in its upper part, beneath the crust, the mantle may be rigid, undeformed and heated by swarms of basic veins and sills. Below this, the mantle may be plastic, hot and rising diapirically from deeper zones. On the basis of xenoliths brought up with the lavas, the lower part of the crust is charnockite. The upper gneissic cover has had a complex geological history marked by tectonic thickening. The diapiric rise of the Velay granite at about 280 Ma marked the end of the Hercynian orogeny prior to the quiescence of the Mesozoic era. Nature of the Lavas. Geochemically the lavas constituted a soda alkaline series. The western plateau is exclusively basaltic ; mild differentiation was able to produce practically all the hawaiites from fractionated crystallisation (pyroxene and/or amphibole cumulates), situated at two levels within the upper mantle (see above). Magma generation was probably situated in the upper mantle between 80 and 100 km depth. Isotopic ratios for 87Sr/86Sr in the basic lavas are grouped around 0.7031 which is typical of the mantle. To the east, where most of the volcanoes and phonolitic intrusions are assembled in two, tectonically collapsed zones, the alkaline series underwent very strong differentiation. The plateau made up of basalt and trachyandesite flows shows a progressive geochemical evolution toward benmoreïte compositions. Again the pyroxene and/or amphibole cumulates attest to fractionation. Relative volumes of basaltic lava compared to differentiated alkaline lava for the phonolitic chain alone, are low. A subordinate sub-series, limited to the northwestern Emblavés graben, is marked by potassium enrichment and strong fractionation of the amphibole. This series terminated with biotite-bearing rhyolitic lavas the volumes of which were quite negligible, comprising three minor intrusions. However its location along the main lineament is of interest. Evidence of the major role played by C02 during petrogenesis is particularly present in this sub-ordinate series, with trachytic glass containing C02 bubbles. The 87Sr/86Sr ratios are low (0.7033 to 0.7035) for the main series and higher (0.705) for the sub-ordinate series. This would seem to indicate crustal contamination. The evolved fractions of the principle series were often enriched in Na by gas transfers. They are geochemically hyper-alkaline and show either; (1) mixed mineralogical paragenesis with initial miaskitic and later, agpaitic crystallisation (aegerine and zircono-silicate characteristics) or (2) glomerules or agpaitic veins in a mainly miaskitic mass. Pyroclastites and vulcanology. The morphology and structure well illustrate the essential magmatic contral. Within this, a continental environment basaltic volcanism is typically strombolian whilst trachyte and phonolitic volcanism which has left almost solely structures marked by lava accumulations (a high lava to pyroclastite ratio) and variably built up would thus appear to have been rather more péléian to vulcanian. The paucity of pyroclastics cannot be justified by removal through erosion pyroclastics occur in only minor volumes at sites where it might have been afforded protection by basalt flows such as the area where basalt flows succeeded the phonolitic extrusion which is the case for the northwestern graben in particular The greatest density of extrusion and intrusions occurring in the collapsed zones correlation between fracture and substratum dyke orientations and sub-volcanic intrusions revealed by erosion, together highlight the structural control which regimented the final conditions under which the magmas rose. The absence of major volumes of pumice is indicative that cataclysmic eruptions probably never took place and which would have been related to the emptying of the great superficial magma chambers. It does however reinforce the petrographic studies which lean towards an interpretation whereby multiple magma chambers existed at various levels with small superficial apophyses These apophyses were possibly the location for the evolution of the small volumes of differentiated magmas which correspond to optimal conditions for gas transfer and for the production of agpaitic phonolite which characterises this province The model anvolving surface located apophyses with a more vertical geometry and as sheets inserted between blocks of superficial crust, in addition to the usual propellants for magma rise (instantaneous degassing, density differentiation etc.) enables lateral compression due to relative movements of the strongly fractured blocks of substratum and variably tilted, to be added. The sum of propelling agents explains vulcanism where the lava would appear to have " forced out " rather than have been specifically erupted ". FinaIly an environmental control with palaeotopography, the palaeohydrographic network, and the aquifers, enabled the usual modifications to these main vulcanologic types, creating conditions favourable to phreomagmatic dynamism. Phreomagmatism was particularly present at Devès where 58 crater lakes for a total of 85 within the region as a whole, have been identified. In the areas liowest elevation such as Puy-en-Velay and Langeac, the persistence of a fluvio-lacustrine palaeoenvironment which was largely submerged through the Villafranchian, brought about Surtsian type volcanism. More than 25 rings of palagonite-bearing hyaloclastic tuffs were thus formed.