Abstract
Corundum, sapphirine or gedrite, and locally garnet occur in amphibolites derived from leucotroctolites during regional Barrovian metamorphism. The latter reached conditions transitional between amphibolite facies and granulite facies at high pressure, estimated to be Pt ⋍ 9 - 11.5 kbar, T = 750 - 800°C. The occurrence of sapphirine or gedrite depends on the SiO2/(FeO + MgO) ratio and on anorthite content of the primary plagioclase. The hornblende - corundum assemblage precludes gedrite + sapphirine in plagioclase-bearing rocks according to the reaction hb + co ⇌ gd + sa + pl + H2O. An anorthosite vein with corundum, garnet and sapphirine contains Mg-, Cr-rich staurolite (XMg = 52 ; Cr2O3 : 0.8 -2.2 wt. %). Mg-rich staurolite is unusual because restricted to peraluminous, silica-deficient rocks with high Mg/Fe ratio. Moreover the presence of sodium favours gedrite-hornblende-corundum assemblages instead of staurolite-sapphirine-plagioclase. For the temperatures considered, the low pressure stability limit of magnesian staurolite (XMg ~ 50) is close to the kyanite-sillimanite transition.
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