Petrological and isotopic implications of some contrasting Late Precambrian carbonates, NE Spitsbergen

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Abstract
The dominantly shallow‐marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal‐marine glacial‐outwash deposits, and consisting of micrite, microspar and fascicular‐optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite to cold seawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater.There is good textural preservation of relatively 18O‐rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post‐glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ18O values, closer to the mean of Late Precambrian dolostones.The heaviest oxygen isotope values constrain seawater δ18O to no more negative than — 2 to — 4SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could be global in extent.