Zeolitization during cooling or heating? Evidence from Late-Cretaceous Coastal Ecuador

Abstract:

Traditionally zeolitisation of thick marine pyroclastic sequences has been attributed to low grade burialmetamorphism (eg Coombs et al. 1959). When glassy rocks are buried, they are exposed to higher temperatures with increasing burial depth which results in the formation of progressively higher grade and less hydrous minerals. However, in many areas mineralogical patterns can not be explained by burial depth only. This is the case when high grade alteration is found in unburied rocks, when high grade assemblages overly lower grade assemblages (metamorphic breaks) or when lateral instead of vertical zonations are observed (eg Levi, 1970). In some cases this could be explained by differences in composition of the rocks or the pore water or differences in permeability and fluid pressure in the rocks, with high grade assemblages forming in more permeable beds (Coombs et al., 1959). An alternative is that authigenic phases seal rocks and prevent further alteration (eg Levi, 1970). Another model states that alteration occurs early after deposition, because a high amount of hot fluids circulates through pyroclasts in active volcanic regions, which would result in the formation of different assemblages depending on rock permeability, composition and fluid chemistry (eg Hall, 2000). A last possibility is that the volcanic heat present in the pyroclasts at deposition causes alteration processes, or causes fluid circulation which leads to alteration. In welded pyroclasts this could lead to the formation of “geo autoclaves” where high vapour pressures contribute to the formation of authigenic phases (eg Aleksiev and Djourova, 1975).

Año de publicación:

2010

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