Magma emplacement into the Lemptégy scoria cone (Chaîne Des Puys, France) explored with structural, anisotropy of magnetic susceptibility, and Paleomagnetic data

Abstract

The Lemptegy volcano is a small monogenetic scoria cone located in the Chaine des Puys, Auvergne, France, which erupted about 32,000 years ago. A first edifice (Lemptegy 1) formed during a trachybasalt eruption as a group of satellite vents of the Puy de Gouttes scoria cone. A second trachyandesitic edifice (Lemptegy 2) formed soon after and completely covered Lemptegy 1 with an 80-m-high breached cone. Since 1946, the Lemptegy volcano has been quarried for scoria and today offers unprecedented three-dimensional exposure of the subvolcanic plumbing system. To map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of Lemptegy 2, structural mapping, petrographic observations, anisotropy of magnetic susceptibility (AMS), rock magnetic, and paleomagnetic data were collected. Field structural mapping and thin section study of tension gashes, Riedel shears, striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined. Twenty AMS sites were established in ten dikes (one to four sites in each dike) with 504 specimens analyzed and 479 specimens used to infer magma flow patterns. Structural data, the maximum susceptibility axis (K 1), and the imbrication of the magnetic foliation (K 1–K 2) planes indicate both upward and downward sense of flow, as well as flow toward and away from the central vent. Rock magnetic experiments reveal that a cubic Fe–Ti oxide phase, likely low-Ti titanomagnetite, is the principal magnetic phase carrying both the remanence and anisotropy. Paleomagnetic data from some sites yield statistically distinct, at the 95 % confidence level, remanence directions while at other sites the data are indistinguishable at the 95 % confidence level. The paleomagnetic results, observed steeply tilted scoria layers, internal unconformities, and faults show that as each dike was emplaced, it displaced earlier dikes evidencing subvolcanic deformation. The Lemptegy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones, such as Cerro Negro (Nicaragua), and thus provides an excellent field laboratory to investigate active scoria cones world-wide.