INTERPRETATION OF COMPLEX NON-CONCENTRIC ZONING FABRICS IN SPARRY CALCITE CEMENTS - MIDDLE JURASSIC LIMESTONES, CENTRAL ENGLAND

Spary calcite cements in Middle Jurassic limestones of central England contain complex non-concentric compositional zonation. Detailed petrographic study allows identification and characterisation of the most common zonal patterns. Although crystallographic indexing of the zones is hampered by the lack of associated concentric zoning and the irregularity of zonal boundaries, crystallographic interpretation of the zoning can still be made using the full range of regionally consistent petrographic and geometrical relationships. The fabrics can be modelled in terms of a ubiquitous primary sector zonation and a localised and episodic sector-specific, frequently lamellar zonation. Compositional variation indicated by staining and cathodoluminescence show that neither of the crystal faces forming the sectors have compositions which necessarily represent equilibrium precipitation from the "bulk" porefluid. Analogous zoning fabrics have been documented from many other ancient sparry calcite cements but have not previously been interpreted in this manner. Their widespread occurrence suggests that important and fundamental kinetic effects associated with growth processes on the propagating crystal surfaces may have a major influence on their trace element contents.