Complex metamorphic evolution in metamorphic carbonates: application of cathodoluminescence to impure UHP calcite-dolomite marbles from the Dora-Maira massif (Western Alps).

The ultra-high pressure (UHP) Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif consists of a Variscan metamorphic basement intruded by Permian granitoids, that experienced Alpine subduction at UHP conditions (ca. 730 °C and 4.0–4.5 GPa). In the BIU, pure and impure calcite marbles, impure calcite-dolomite marbles, and carbonate-silicate rocks occur, which experienced polymetamorphic (Variscan and Alpine) evolution. Evidence for this complex evolution has been mainly recognized from silicate minerals, whereas microstructures for different metamorphic stages have been not described for carbonates. Cathodoluminescence (CL) - coupled with observations under petrographic microscope, and with SEM-EDS and micro-Raman analyses - have been performed on impure calcite-dolomite marbles to detect the local variability in microstructure and mineral assemblage of carbonates. Under the petrographic microscope, the studied 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 epidote s.l., Ca-amphibole, phlogopite, chlorite, serpentine, talc and titanite. Rare quartz and accessory zircon are locally 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, these data revealed that the carbonates from the BIU marbles did record a metamorphic evolution as complex as that shown by the associated silicate minerals. Because in 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. CL observations, then, open new opportunities for the investigations of mineral reactions in metacarbonate rocks and for the determination of equilibrium assemblages, suitable for a more detailed evaluation of the PTX metamorphic conditions.