TRACE ELEMENT SIGNATURES IN CARBONATES FROM CALC-SILICATE ROCKS IN THE THERMAL AUREOLE OF THE BALLACHULISH IGNEOUS COMPLEX

The role of fluids in metamorphic processes is well documented, and their impact on accessory mineral texture development and trace element distribution, particularly in silicates, is increasingly well understood. In this study textures in metamorphic carbonates from the aureole of the Ballachulish Igneous Complex are correlated with CL response and trace element abundances to establish textural and compositional relationships in complexly zoned carbonate minerals.

Carbonate in samples consist of dolomite and calcite, while silicate portions are dominated by feldspars, and a mixture of neso- and ino-silicate minerals. Low temperature carbonate assemblages are finer grained, with calcite forming very fine-grained aggregates around dolomite. The dolomite grains may be larger with calcite veins, or small grains with ternary junction grain boundaries. Both silicate and calcareous sub-domains contain apatite, zircon, rutile, sphene, pyrite, and monazite. The low temperature calcite has orange CL, while dolomite is red. Dolomite grains are compositionally zoned with Mg-rich cores and more calcic rims, which have brighter orange luminescence. The calcite veins are identified by their strong, contrasting CL response compared to host dolomite.

High temperature samples are coarser grained, have significantly fewer accessory minerals, and are characterized by silicate-mineral veins and inter-growths. Grain boundaries between calcite and dolomite are straight, except near veins of silicate minerals where calcite occurs as very fine-grained aggregates. Silicate minerals inter-grown with calcite have curved grain boundaries and no preferred orientation. Calcite and dolomite grains have orange and red CL respectively, but both minerals show a brightening of CL response at their grain boundaries, which weakly correlate with trace element abundances.

It is proposed that CL zoning in dolomite and calcite represent recrystallization textures that grew during emplacement of the igneous complex, and their increased CL response reflects the accommodation of changing abundances of trace elements, some of which may have been released into the bulk-rock system upon the breakdown of accessory minerals, which become increasingly rarer at higher temperatures.