Application of cathodoluminescence to the study of metamorphic textures in UHP calcite-dolomite marbles from the Dora-Maira massif.

Impure calcite-dolomite marbles from the ultra-high pressure (UHP) Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif (Western Alps) are studied under cathodoluminescence (CL), petrographic microscope, and SEM-EDS. Most of the polymetamorphic (Variscan and Alpine) evolution of these marbles has been already reconstructed mainly by silicate minerals, whereas the local variability in microstructure and mineral assemblages of carbonate minerals are still unconstrained. Under petrographic microscope, the studied impure calcite-dolomite marbles are medium- to coarse-grained, locally with a weak foliation defined by: i) phengite or Mg-chlorite wrapping around both porphyroclastic dolomite and clinopyroxene; ii) alignment of neoblastic clinopyroxene and garnet; iii) silicate-rich layers. Different relative amounts, kinds and compositions of silicate minerals are present. Early mineral assemblages include garnet, clinopyroxene, phengite, olivine, talc, and rutile, and late mineral assemblages include epidote s.l., Ca-amphibole, phlogopite, chlorite, serpentine, talc and titanite. Rare quartz and accessory zircon are also observed. Dolomite is strongly zoned under CL and SEM, mainly because of variations in Ca, Mg and Fe contents. The locally zoned dolomite core is most likely a Variscan relic which may include garnet, diopside and rare ilmenite. The locally zoned dolomite rim, which partly corrodes the pre-Alpine core, is interpreted as a product of the Alpine metamorphism. In some samples, the outermost rim locally includes Ca-amphibole, phlogopite and titanite. In one sample, late dolomite, in equilibrium with serpentine, partly overgrows olivine. The rock matrix is constituted by weakly-deformed medium-grained Mg-calcite that partly corrodes dolomite. In rare samples, exsolved dolomite is present within Mg-calcite. CL observations reveal that Mg-calcite is usually zoned with a rim composition variable from sample to sample. Rare, undeformed and fine-grained (Mg-)calcite is locally present at the grain boundaries. In conclusion, microstructures in carbonates from the BIU marbles are complex such as that shown in the associated silicate minerals. Because in impure marbles both carbonate and silicate minerals record the same growth history under CL, this is a simple but powerful tool to correlate the evolution of the two groups of minerals. Therefore, the use of CL in studies on metacarbonate rocks allows to better constrain their PTX metamorphic conditions because it is suitable for a more detailed evaluation of the equilibrium mineral assemblages.