Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Inyo Domes magmatic system erupted 650 years ago in eastern California. It is a series of three lava domes and associated pyroclastic rocks, made up of phenocryst-poor (Fp) and phenocryst-rich compositions (Cp). We examined the chemical composition, quantitative petrographic texture and geology of the ensemble. The Fp unit has considerable compositional variation that was produced by fractional crystallisation of feldspars and pyroxenes in a plexus of conduits and magma storage areas under or close to the Inyo Domes magmatic system. There is no magmatic link with the synchronous North Mono silicic eruption 20 km to the north. The Cp unit is compositionally more uniform and originated from a mushy feldspar cumulate formed in an earlier rhyolitic magma chamber. The eruption started with the reactivation of a major fault which ruptured the surface. Initially, the fault intersected a single hydrous magma source in the plexus and produced a pyroclastic eruption in the south. Other regions of the plexus to the north were rapidly co-opted making further pyroclastic eruptions. As the magmas became degassed, lava domes started to form from fissures. Finally the Cp magma source became reactivated by the injection of new magma, felsic or mafic, into the chamber. The Cp magma was forced out into a conduit of the Fp unit, where it formed a mixed unit (Im). The quantity of the Cp component increased until it dominated. At this point the Fp units were solid and the Cp unit piled up to make the central knobs of the southern domes.