Felsic magma-water interaction in shallow intrusive environments: Timing between fluidal peperites and intrusive hyaloclastites in a jurassic cryptodome from the eastern Deseado Massif (patagonia, Argentina)

Abstract

This work describes the facies architecture, morphology and textures of a Jurassic rhyodacitic cryptodome outcropping in the Don Nicolás Mine, an important Au–Ag producing mine located in the Deseado Massif, Argentinean Patagonia. The access to 70 m deep surface mining operation and to a large number of diamond drill cores offer a unique chance to explore the volcanic rock assemblage of this body, allowing to study intrusion and growth mechanisms of a shallow subvolcanic body, and also providing insights into the timing between different brecciation processes occurring during magma intrusion into a poorly consolidated and water saturated pyroclastic sequence. Facies analysis allowed to infer this body as the result of a rhyodacitic magma intruding in a single and steady endogenous inflation pulse, resulting in the development of a flow banded core and a fluidal peperite envelope due to the combination of relatively low magma viscosities (for a rhyodacitic melt), the development of stable vapor films insulating the flowing magma and also allowing the ductile deformation of the rhyodacite clasts, and the fluidization of the unconsolidated and water saturated host rocks. After the main inflation stage stopped, the collapse of the vapor films resulted in quench fragmentation modifying the originally fluidal shape of many clast at the peperites, specially at the outer portion of the breccia envelope where fluidal clast shapes were almost completely erased due to this process. At the same time water invaded the inner portions of the intrusion, causing widespread quenching and the development of intrusive hyaloclastites at the edges of the coherent facies.