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Micro-Sized Dolomite Inclusions in Ferroan Calcite Cements Developed During Burial Diagenesis of Kimmeridgian Reefs, Northern Iberian Basin, Spain

¹ Departamento Estratigrafía-U.E.I. de Correlaciones Estratigráficas, Facultad C.C. Geológicas, Universidad Complutense de Madrid-CSIC, 28040 Madrid, Spain; mibenito{at}geo.ucm.es
² Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109-1005, U.S.A.
³ Departamento Estratigrafía-U.E.I. de Correlaciones Estratigráficas, Facultad C.C. Geológicas, Universidad Complutense de Madrid-CSIC, 28040 Madrid, Spain

Burial diagenesis of the Upper Jurassic Torrecilla Reef Complex is recorded by a complex paragenetic sequence initiated with emplacement of ferroan calcite cements and followed by the precipitation of ferroan saddle dolomite. Ferroan calcite cements contain micro-sized dolomite inclusions (MDIs). Elemental and isotopic compositions of MDIs are virtually indistinguishable from those of ferroan saddle dolomite cements. In contrast to other scenarios for the formation of microdolomite inclusions that invoke either a magnesian calcite precursor or incomplete dedolomitization, the paragenetic relations of MDIs to their host ferroan calcite and their geochemical composition implies formation by fine-scale burial replacement of ferroan calcite by fluids associated with the emplacement of ferroan saddle dolomite.

Petrographic observations and mass-balance considerations suggest that it is unlikely that Fe and Mg incorporated into ferroan dolomite could have been derived from the cannibalization of the reefal host rock. Rather, dolomitizing fluids were likely associated with the hydrothermal, low-grade metamorphic event that affected the Cameros Basin during the middle to Late Cretaceous. Interaction of these fluids with preexisting burial cements produced MDIs in the ferroan calcites.