Evidence for pre‐regional metamorphic fluid infiltration of the Lower Calcsilicate Unit, Reynolds Range Group (central Australia)

Grandite garnet‐rich calcsilicate rocks from the Lower Calcsilicate Unit of the regionally metamorphosed Reynolds Range Group (central Australia) crop out along a strike‐parallel section in which a transition zone from M2₂ amphibolite to granulite facies rocks is exposed. Across this transition the grandite‐rich layers do not show systematic changes in mineral assemblages, compositions and modes, or stable isotope compositions. These layers are deformed by F2₂ folds that are associated with the peak of regional low‐pressure/high‐temperature metamorphism. Therefore, the grandite‐rich layers appear to pre‐date regional metamorphism and to have acted as closed chemical systems during prograde M2₂ metamorphism. Mineral assemblages in the grandite‐rich layers are consistent with their formation through the infiltration of oxidized, water‐rich fluids (X_co2 < 0.1–0.3; log f_o2 ‐16 to ‐14). The stable isotope values of calcite (Δ¹³C=‐4.2 to ‐0.8%₀ PDB; Δ¹⁸O = 10.5–14.0%⁰ V‐SMOW) and bulk‐silicate fractions (Δ¹⁸O = 6.1 to 10.8%) of the grandite‐rich layers are most consistent with the infiltrating fluid being from a magmatic source. It is most likely that fluid infiltration occurred during the pre‐M2₂ contact metamorphism (M2₁) that affected much of the Reynolds Range Group. The preservation of these assemblages is probably due to their high variance and little pervasive fluid‐rock interaction having occurred during M2₂. The clinopyroxene‐ and feldspar‐rich calcsilicate rocks that host the grandite‐rich layers contain poikiloblastic grandite garnet that formed during prograde M2₂ metamorphism. Thin marbles that locally occur with the grandite‐rich layers contain a third garnet generation that is post‐ or late M2₂. This grossular‐rich garnet occurs in coronas around calcite, plagioclase, clinopyroxene, wollastonite and scapolite. These coronas are consistent with cooling and/or compression. However, because the marble assemblages are themselves overprinted by M2₁ grandite‐rich layers the development of coronal garnet does not reflect a continuous P‐T‐t path. Rather, it more probably reflects the partial re‐equilibration of M2₁ contact metamorphic assemblages to post‐M2₂ conditions.