Abstract

Thin marine limestones from the Dinantian of the St Monans syncline, East Fife, are extensively dolomitized. They occur within a deltaic sequence cut by Upper Carboniferous volcanic rocks. The volcanic rocks have been extensively modified by CO2-enriched hydrothermal fluids. The dolomite is ferroan and nonluminescent. Stable isotope compositions for groundmass dolomite range from δ18O = −4.76‰ to −0.47‰ and δ13C = −3.29‰ to 0.60‰. Later, void-filling dolomite falls into two groups; the first is 18O depleted, with δ18O as low as −10.6‰, while the second is 18O-enriched, δ18O = −2.8‰ to −1.2‰. Despite almost homogeneous staining characteristics, backscattered SEM imaging shows the dolomites to be chemically extremely heterogeneous. Dolomitization was a multistage process in which an early ‘mixed water’ component was variably remobilized to form the ‘matrix dolomite’. Biomolds and other vuggy post-early-dolomite porosity are filled by dolomite precipitated within a volcanically driven hydrothermal system. The chemistry of the later dolomite reflects the mixing of upward moving hydrothermal fluids, modified by interaction with volcanic material, with downward moving seawater feeding the hydrothermal system. In addition, material released during normal processes of siliciclastic diagenesis added a further component to the late stage dolomitization. Only two of the limestones escaped complete dolomitization and appear to have undergone early post-depositional diagenesis within meteoric porefluids. They contain early meteoric calcite cements and later post-compaction calcite cements, dolomite and siderite. One of the implications of this study is that the interpretation of dolomite geochemical data can be vastly enhanced by the use of back scattered electron microscopy to reveal otherwise hidden compositional heterogeneity.

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