The prograde history of three Mn-rich garnets from the UHP Lago di Cignana Unit (Italy)

Extensive rock recrystallization and element redistribution during retrogression often hampers our understanding of the early stages of metamorphism. Garnet is the mineral that best preserves information about its growth during the prograde history of the rock as compositional zoning. In most metamorphic rocks, garnet zoning varies between almandine, grossular, and pyrope end-members with minor spessartine content. This variability and the diffuse presence of mineral inclusions in garnet enables the coupling of thermodynamic tools (e.g., pseudosections) with classical element exchange and elastic geothermobarometry to gather information on their pressures and temperatures of equilibration. Such studies give their best results when applied to metapelites due to their relatively large mineral variability over the typical PT range of metamorphic rocks. However, monomineralic lithotypes, such as impure quartzite or marble, consist of minerals stable over a wide PT range and therefore lack mineralogic change. Furthermore, currently available solution models are not calibrated for use on unconventional bulk rock compositions and therefore do not guarantee reliable geothermobarometric results.

In this contribution, we use elastic geobarometry to track prograde garnet growth from low- to ultrahigh-pressure conditions in three Mn-rich garnets (up to 50% sps) from an impure marble from the Lago di Cignana Unit (Italy). The rock consists of mainly quartz and calcite with garnet porphyroblasts. The three garnets show a very large core-to-rim compositional zoning with Mn-rich cores, Fe-rich mantles, and rims with a slight increase in Mg. Mineral inclusions in garnet cores and mantles are mainly quartz, with minor titanite, calcite, and apatite. Coesite, aragonite, zircon, and rutile are instead present within garnet rims. The three investigated garnets vary in shape, zonation, inclusions type and size while having a comparable core-to-rim composition. In two garnets, quartz inclusions are tiny (20-30 μm) and spread evenly within the garnets. The third garnet has larger quartz inclusions (50-100 μm) in the core and smaller in the mantle, decreasing progressively in size from the inner to the outer mantle (50-10 μm). Elastic geobarometry on these quartz inclusions in garnet allowed the tracking of the pressures at which garnet cores and mantles formed. We can show that these garnets formed during multiple distinct growth stages along the prograde path from 1.2 GPa and 430°C to 1.8 GPa and 500°C and finally at UHP conditions, as testified by the coesite-bearing garnet rims. This difference in pressure and temperature of garnet growth might be due to local (cm-to-mm-sized) changes in chemical composition at the scale of the thin section and/or to reaction overstepping.