Kornerupine Breakdown Reactions in Paragneisses from Southern Madagascar

Dietrich Ackermand, Brian F. Windley and Andriantefison H. Razafiniparany
Mineralogisch-Petrographisches Institut, Universität, Olshausenstr. 40, 2300 Kiel, Germany
Department of Geology, The University, Leicester LE1 7RH, U.K.
Service de Géologie, Université, Antananarivo, Madagascar

Abstract: Kornerupine-rich layers up to several centimetres thick with minor sillimanite, spinel, Fe oxide and ilmenite occur in a diopsidite in sillimanite-cordierite gneiss south of Beraketa (24°27′S, 46°48′E), southern Madagascar. Kornerupine, sillimanite, spinel and hematite grains up to 1 mm across have mutual polygonal boundaries indicating textural equilibrium at their crystallisation. Kornerupine has XMg 0.67–0.80 and 0.9 to 2.6 wt.% B2O3. Sillimanite contains up to 2.0 wt.% Fe2O3. Spinel is essentially (Mg,Fe2+) Al2O4 with an XMg range of 0.29–0.40 and exsolution lamellae of Fe oxide. Textural relations demonstrate two limited reactions, each confined to areas less than 500 µm across: (1) Kornerupine and spinel reacted along grain contacts to form very fine-grained tourmaline, corundum and chlorite. The replacing phases are symmetrically zoned with a central tourmaline and hematite, bordered by an aggregate of chlorite, tourmaline and corundum, followed outwards by a rim of chlorite against the kornerupine and spinel. (2) Within kornerupine grains, zoned, round aggregates consist of very fine-grained chlorite, tourmaline and corundum of different composition than in (1). They define the terminal reaction of kornerupine breakdown.

Geothermobarometry indicates that the early kornerupine-bearing assemblage was stable at 7.0 kbar and 700 °C. This P-T point lies close to the retrograde, nearly isothermal trajectory defined independently by nearby sapphirine-bearing assemblages. The fine-grained aggregates formed most likely during further cooling, or by increasing water fugacity.

Keywords: kornerupine • breakdown products • granulite facies • Madagascar

Mineralogical Magazine; March 1991 v. 55; no. 378; p. 71-80; DOI: 10.1180/minmag.1991.055.378.06
© 1991, The Mineralogical Society
Mineralogical Society (www.minersoc.org)