Chromium-rich kyanite, magnesiostaurolite and corundum in ultrahigh-pressure eclogites (examples from Pohorje Mountains, Slovenia and Tromsø Nappe, Norway)

Abstract

Chromium-rich kyanite, magnesiostaurolite and corundum (ruby) were found in ultrahigh-pressure (UHP) eclogites associated with garnet peridotite and serpentinite from Pohorje Mountains (Eastern Alps) and the Tromso Nappe (Northern Scandinavian Caledonides). The concentrations of Cr 2 O 3 in kyanite and magnesiostaurolite from the Pohorje Mts. (17.0 and 15.0 wt%, respectively) are the highest Cr values ever reported for these minerals, and the Cr content of corundum from this area (≤12.2 wt% Cr 2 O 3 ) also belong to the highest reported from natural samples. The Cr-rich kyanite of turquoise-blue colour contains inclusions of chromite (Cr* = 0.74–0.78) and Cr-rich magnesiostaurolite (6 to 15 wt% Cr 2 O 3 ; X Mg = 0.7–0.8). Kyanite is commonly surrounded by a corona composed of the assemblage Cr-bearing sapphirine (up to 5.5 wt% Cr 2 O 3 ) + corundum + Al-rich spinel + plagioclase. Blue-green, Cr-rich magnesiostaurolite (5.4–8.5 wt% Cr 2 O 3 ; X Mg = 0.76–0.82) forming ≤500 μm large crystals is associated with Cr-rich kyanite, Cr-rich spinel and Cr-rich corundum which are surrounded by pargasitic amphibole and zoisite. Chromium enrichment in kyanite, magnesiostaurolite and corundum was controlled by substitution for Al in the octahedral sites (Cr 3+ Al −1 ). Magnesiostaurolite compositional variations indicate at least two additional substitutions: Fe 2+ Mg −1 and M 2+ Si(Al,Cr) −2 , where M 2+ are the divalent cations Mg, Fe, Mn, Ni and Zn. The highest Si content and near absence of tetrahedrally coordinated Al was observed in the most Cr-rich magnesiostaurolite. Chromian kyanite, magnesiostaurolite and ruby formed by reactions involving primary chromite derived from a magmatic protolith characterized by high Cr, Mg, Al and low Si, during prograde metamorphism and formation of eclogite at HP-UHP conditions.