Magma flow in sub-aqueous rhyolitic dikes inferred from magnetic fabric analysis (Ponza Island, W. Italy)

Abstract

We have studied seven rhyolitic dikes from the ∼8 km 2 Ponza island (Italy) in order to retrieve the sense of magma flow by using magnetic fabric. These late Pliocene dikes were part of a sub-aqueous dome complex and were emplaced in wet and unconsolidated rocks. Some dikes are inherited from normal faults. Dike thickness ranges from 3 to 100 m, where they are reintruded. We sampled paired margins of four dikes and one margin in three dikes. In addition, we performed a profile in a 3 m thick dike. The magnetic mineralogy of the rhyolite consists of magnetite and likely maghemite. Biotites also participate marginally to the magnetic fabric. The magnetic fabric is generally well defined and flow-related as the magnetic foliation is close to the dike plane. The magnetic foliation is generally imbricated respect to the dike plane. Conversely, in the center of the dike, the magnetic foliation is poorly defined whereas the magnetic lineation is well defined. Optical inspection and textural analysis of seven representative thin sections of rhyolite from the dikes demonstrates a good parallelism between opaque grains and magnetic lineation. We determine the sense of flow by using the geometry of both magnetic lineation and foliation. The magnetic lineation in six out of seven dikes is sub-vertical, suggesting vertical flow. However, a main result of our study is that the lineation by itself is not sufficient to image the sense of flow, being the lineation also related to the intersection of magnetic foliations. The analysis of the imbrication of magnetic foliations is instead a much better tool to image the sense of flow. The sense of flow inferred with imbrication of magnetic foliation shows a good agreement with the geology. The dikes feeding small cryptodomes have shown downward flow that is interpreted as related to deflation after emplacement. Horizontal flow has been imaged in the thickest of the sampled dikes suggesting the presence of domes at shallow depth feeding an array of concentric dikes.