Le volcanisme cambrien du graben du Maine (est du Massif armoricain) : reconstitution des dynamismes éruptifs

Abstract

The Palaeozoic units of the eastern edge of the Armorican Massif (southern Normandie et Maine Dept.) record the effects of an intense Cambrian volcanic activity localized in a major graben - the Maine Graben. The size of the graben, which trends NNE-SSW, is estimated to be at least 75 km long by 50 km wide. Apart from the Multonne Massif, where the volcanic outpourings took place directly on the Cadomian peneplain, the Maine volcanites are every- where interbedded in Cambrian shallow-water carbonate and clastic sediments and have a maximum accumulated thickness of almost 3 000 m in the more subsident troughs (Coëvrons, Charnie). Volcanic activity in the Maine Graben was characterized by highly explosive subaerial and submarine explosions that occured over a maximum of six cycles (designated V0 to V5) extending throughout the Cambrian from the Atdabanian (V0) to the Cambro-Tremadocian (V5). The main volcanic events within the graben (V1 to V4) were contemporaneous with the deposition of a coastal sandy unit (Sainte-Suzanne Sandstone) containing Lingula from the base up and so have an age that is probably fairly high in the Cambrian succession. Correlations between the major volcanic phases, based mainly of the type and position of the complexes within the regional lithostratigraphic successions, show that the caldera activity was not synchronous throughout the graben. The Ecouves and Perseigne calderas were active early and followed an identical evolution during a single major Vl eruptive cycle, unlike the Assé-le-Boisne - Pail caldera where the activity continued through several peak phases (V2, V3, V4) separated by the desposition of sandy units of the Sainte-Suzanne Sand-stone. The eruptive dynamisms were closely controlled by the different stages in the development of the calderas as well as by interactions between the magmas and the shallow marine environment in which they were erupted. Thus a precaldera stage marked by small eruptions was followed by the main state of caldera formation accompanied by extremely voluminous outpourings of hot welded ignimbrites that, where not intercalated with sediments or volcano-sediments, is several hundred metres thick within the volcanic structure: pyroclastic flows extended outside of the caldera and reached the marine environment to form distal thin cold outflow sheets. In the post-caldera stage, following peak volcanic activity with collapse of the caldera floor, the calderas were progressively invaded by the sea and subjected to erosion. Although interactions between the magma and the marine environment were limited in the Ecouves caldera, they were very well developed in the Assé-le-Boisne - Pail caldera, which lies further south in the Maine Graben and thus closer to the open sea. Abundant hydroclastic events were recorded during the development of this structure, such as phreatoplinian eruptions that spewed enormous quantities of wet ash with accretionary lapilli and which were accompanied by the outpouring of distal outflow sheets that flowed into the outer domain as far as Coëvrons and Charnie. These outflow sheets were trapped in a network of fluvial-marine channels that carried breccia and conglomerate deposits as well as thick lahartype mud flows originating from the mass sliding of the loose powdery pyroclastics. These sudden inflows of considerable volumes of volcanogenic material led to major changes in the geometry of the shallow marine basin, itself undergoing rapid vertical volcano-tectonic movements. In terms of rapidity of deposition and aggradation, the sedimentation that was contemporaneous with the volcanic activity can be described as catastrophic. The structural setting of this Cambrian volcanic activity was a narrow graben in an extensional regime reflected by active subsidence and multiple injections of magma. The graben lies immediately above a major crustal discontinuity that was formed during the Cadomian and that separates the Mancellian back-arc basin from its southern continental margin.