Ferrar Basaltic Tuff-Breccias Formed by Direct Eruption: Evaluating an Hypothesis

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

0087919 Elliot This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the initial phase of magmatism associated with the break-up of Gondwanaland. Mesozoic break-up of Gondwanaland was marked by a major magmatic event during which voluminous basaltic magmas were erupted at the surface and intrusive sills and dikes were emplaced at depth within the underlying sedimentary sequence. In Antarctica, the extrusive rocks include thick tuff-breccias (coarse pyroclastic rocks) hypothesized to have formed by subsurface explosive interaction of basaltic magma and water in aquifers. Volcanic fields where rising magmas interact explosively with water in aquifers, or at the surface, are widespread in modern rift settings. In terms of areal extent and thickness of deposits, depth of magma/water interaction, and dominance of basaltic tuff-breccia, in comparison with other well documented examples these basaltic pyroclastic rocks in Antarctica constitute a unique volcanic field. Study of the paleovolcanology of these rocks will yield important new information on the origins and emplacement mechanisms of tuff-breccia deposits and on the evolution of volcanic fields in which tuff-breccias form a significant component. The goal of this research project is to understand the processes involved in the formation of these exceptionally thick tuff-breccias. The objectives of the research are to: (1) document the three-dimensional architecture of the basaltic pyroclastic rocks; (2) establish the depth of magma/water interaction; (3) evaluate aquifer recharge; (4) establish the nature and extent of the volcanic field and its paleovolcanologic setting; and (5) evaluate the hypothesis that these tuff-breccias are the result of direct eruption from volcanic vents. The large-scale three-dimensional relationships of the pyroclastic rocks will be established by field examination of lateral and vertical changes in facies (rock associations), and relationships to adjacent rock units. Rock samples will be collected for examination of microscopic structures and textures that provide clues to the modes of interaction, eruption and emplacement. Sedimentary rock fragments in the tuff-breccias will be systematically sampled for comparison of the lithology and palynology with that of the underlying rock sequence and thus establish the time-dependent changes in depth of magma/water interaction. Building on reconnaissance work, the results of this study are expected to have broad implications for understanding how tuff-breccias are formed by phreatomagmatic processes (the explosive interaction between magma and water) and the tectonic settings in which it occurs. Results are also expected to develop the paleovolcanologic setting of the Transantarctic Mountains during the Jurassic.