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Origin of coarse lithic breccias near ignimbrite source vents

, : Origin of coarse lithic breccias near ignimbrite source vents. Journal of Volcanology and Geothermal Research 25(1-2): 157-171

Coarse proximal lithic lag breccias associated with ignimbrites are here interpreted to have accumulated within the "deflation zone", this being the zone around a collapsing eruption column within which deflation (by escape of gas) from a dilute and turbulent gas/particle system to a pyroclastic flow is proceeding. They are thus co-ignimbrite deposits, and are the lateral equivalent of ignimbrite, but not the deposits of pyroclastic flows; lateral equivalence is established by matching a compositional zonation in pumice of the lag breccia with that in associated ignimbrite. They are crudely stratified and accumulate by layer upon layer deposition, but the thickening into valleys and lack of impact structures imply that lateral transportation of the material also took place in the turbulent system. The ground layer found at the base of some ignimbrites, which formed by the segregation of heavy material from the pyroclastic flow, differs in being generally thinner and less coarse and lacking internal stratification. Lag breccias and ground layers are both notably fines-depleted, unlike mudflows, caldera collapse breccias, and lithic concentration zone [layer 2b(L)] breccias such as are found near the base of some ignimbrites and formed by the settling of lithics through the pyroclastic flow. Lag breccias are commonly interstratified with ignimbrite, and a new interpretation is proposed that this reflects temporal variations in the width of the deflation zone, and thus records details of the sequence of events in ignimbrite eruptions. An update, to include lag breccias, is also proposed of the "normal sequence" of deposits formed in ignimbrite eruptions. Lag breccia is interpreted to form at an eruption climax, particularly at the onset of caldera collapse.


DOI: 10.1016/0377-0273(85)90010-1

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