Unveiling the complexity of monogenetic volcanic fields: a petrological exploration of the Puig Jordà (Garrotxa Volcanic Field, Spain).

Abstract

The magmas erupted in the Garrotxa Volcanic Field (GVF) in northeastern Spain have been traditionally attributed to direct ascent of magma from its source in the mantle, a theory supported by the frequent presence of mantle xenoliths in some eruptive products. However, recent petrological and geochemical studies of monogenetic eruptions in other volcanic fields have revealed the common existence of shallow magma pockets that are intercepted by new intruding magma before eruption. Consequently, different magma ascent timescales (direct vs. arrested) would have significant implications for the duration of potential pre-eruptive unrest in the GVF and, hence, for the interpretation of monitoring data. Here we report, for the first time, a detailed study of the mineral phases hosted in the magmas from the GVF. We have focused on the Puig Jordà monogenetic eruption (17 ka), located 3.5 km away from the city of Olot, and previously linked with a significant lava flow (Bosc de Tosca). We have conducted volcano-stratigraphic fieldwork followed by an extensive examination of the mineral phases to elucidate the magma plumbing system architecture. The eruptive sequence was characterised by Strombolian activity, with minor sporadic phreatomagmatic phases. The petrological and geochemical analyses of pyroxene, olivine, and spinel have revealed the occurrence of arrested magma intrusions preceding the eruption. Furthermore, our study has revealed that either the Bosc de Tosca lava flow is not sourced from this volcano or the eruption involved the emission of two distinct magmas: one led to the formation of the pyroclastic deposits, while the other produced the lavas. By comparison with other monogenetic volcanoes from the GVF, the first hypothesis seems more reliable. Thermobarometric modelling of pyroclasts suggests rapid magma ascent from a deep zone at approximately 900 to 1200 MPa and 1200°C to 1250°C, followed by the incorporation of previously emplaced magma batches located at 600 to 900 MPa and 1175 ± 15°C, and a final stage occurring at shallow crustal levels with lower temperatures (~120 MPa and 1110 ± 30°C). Our results show a complex ascent history in a multi-level plumbing system and have direct implications for the interpretation of future unrest episodes in this and other active monogenetic volcanic fields.