Deformation-induced development of kyanite and fibrolitic sillimanite in monzodiorite orthogneiss, southwest Connecticut

An orthogneiss in Newtown, Connecticut, is unusual because contains abundant aluminosilicates, namely kyanite and lesser sillimanite. The average mineralogy of the gneiss, determined by Rietveld refinement with X-ray powder diffraction data is ~5% quartz, 80% albitic plagioclase, and 15% K-feldspar. This monzodioritic composition and the rare occurrence of sillimanite suggest the protolith was probably not a metasediment as mapped by Scott (1974). Euhedral kyanite blades ≤ 8 cm long produce a strong lineation plunging 20˚ toward N55W. Kyanite crystals ≥ 2 mm long are subhedral with inclusions of rutile, biotite, and quartz and commonly form sharp contacts against smooth grain boundaries with biotite and plagioclase. Kyanite grains ≤ 2 mm long rarely contain inclusions and share interlobate boundaries with plagioclase and biotite. Sillimanite is fibrous and anastomoses through biotite and anhedral ilmenite. Biotite is subhedral and forms a weak foliation. Textures suggest several open-system metasomatic reactions. Subhedral kyanite truncating anhedral biotite and plagioclase suggest the reaction: Bt + Plag = Ky (balancing on Al yields a DeltaVrxn= -83%). Interlobate grain boundaries between plagioclase and kyanite suggest the reaction: 2Plag = Ky (DeltaVrxn= -78%). Optical continuity of biotite inclusions in kyanite with biotite in the matrix suggests replacement of biotite by kyanite. The expected volume of rutile in kyanite created by replacing biotite is 3%, comparable to the ~5% seen in thin section and supporting replacement of biotite by kyanite and rutile: Bt = Ky + Ru (DeltaVrxn= -68%). Similarly, biotite interfingered with sillimanite and ilmenite suggests a replacement reaction: Bt = Sill + Ilm (DeltaVrxn= -47%). The interfaces of these minerals and the low solubility of Al3+ in plagioclase and biotite and Ti4+ biotite suggest local replacement of biotite and plagioclase by specific Al- and Ti-phases. Kyanite and rutile formed in high-P conditions, whereas sillimanite and ilmenite formed in lower-P, high-T conditions (Ernst and Liu, 1998). The large negative DeltaVrxn, local growth of aluminosilicates, and the lack of veins and fractures suggest that reactions were driven by locally high normal stress (pressure solution). The resulting preferred orientation of kyanite and sillimanite led to textural softening and thus strain localization. These replacement reactions require an open system with the intrusion of at least mildly acidic solutions and the loss of alkalis. The scale of open-system metasomatism must have exceeded the 30m size of this outcrop.