CRITICAL EVALUATION OF THE HIGH-PRESSURE STATUS OF CALDERITE, MN3FE2SI3O12

The garnet species calderite (Mn₃Fe₂Si₃O₁₂) occurs albeit only rarely in metamorphosed Mn-Fe metasediments.  Experiments on the stability of end-member calderite appear to indicate that it is an index mineral of high pressure.  An apparent exception to that inference was the report of calderite in amphibolite facies rocks from Newfoundland.  Six samples from Mn- and Fe-rich bands from Proterozoic iron formations from the Grenville Province in NE Quebec and NW Newfoundland near Labrador City were analyzed to reevaluate the compositions of the garnet.  The high calderite content (68-83%) of the garnets is confirmed and remains among the highest yet described. Coexisting minerals include aegirine, hematite, jacobsite/magnetite, kutnahorite, rhodonite and quartz. Assuming ideal molecular mixing on the jacobsite-magnetite join, coexisting jacobsite/magnetite-hematite equilibrated on the displaced hematite-magnetite (HM) sensor at Dlog₁₀fO₂ of HM+0.2 to +5.2. Calderitic garnet is dominated by the reciprocal solid solution calderite–andradite–spessartine–grossular and should be plotted in that composition space.  The Wabush assemblages are buffered on the equilibrium Mn₃Fe₂Si₃O₁₂ = MnSiO₃ + Fe₂O₃.  Calculations show that this reaction is sensitive to solid solutions in garnet, rhodonite and hematite. The rocks of this study were subjected to the upper amphibolite facies with peak P-T estimates of 10±1 kbar and 1000±50 K, so calderite should not be regarded as an index mineral of HP conditions.