DEFORMATION-INDUCED DEVELOPMENT OF KYANITE AND FIBROLITIC SILLIMANITE IN MONZODIORITE ORTHOGNEISS, SOUTHWEST CONNECTICUT

An orthogneiss in eastern Newtown, CT, is unusual in that it contains large kyanite (Ky) blades and fibrolite (Sill) needles. The average composition of the gneiss, determined by Rietveld refinement with XRD data, is ~5% quartz, 80% albitic plagioclase (Plag), and 15% K-feldspar. This mineralogy represents a monzodioritic composition, suggesting that the protolith was probably not a metasediment as mapped by Scott (1967).

Euhedral Ky blades ≤8cm long in outcrop, produce a strong lineation trending N55W. Ky crystals ≥2mm in thin section are subhedral with inclusions of rutile (Ru), biotite (Bt), and quartz. Ru inclusions within Ky are ~5% by volume, occurring randomly and in trails.These Ky grains commonly form sharp contacts against Bt and Plag. Ky grains ≤2mm rarely contain inclusions and are interlobate with Plag and Bt. Sill is acicular and radiating when including Plag. However, where intergrown with Bt, Sill is fibrous, anastomosing, and also intergrown with anhedral ilmenite (Ilm). Bt is subhedral and forms a weak foliation. Plag is anhedral, and quartz is present as anhedral grains in the matrix and as ovoid inclusions in Ky.

These textures suggest several reactions. Ky truncation of Bt and Plag supports the reaction Bt + Plag → Ky (ΔV_rxn= -83%). Interlobate contact between Plag and Ky shows 2Plag → Ky (ΔV_rxn= -78%). Optical continuity of Bt inclusions within and surrounding Ky suggests replacement of Bt by Ky. EMP analysis of Bt gave a formula of KFe_0.73Mg_1.7Ti_0.07Al_1.8Si_2.7O₁₀(OH)₂. The expected volume of Ru in Ky created by replacing Bt is thus 3%, comparable to the ~5% seen in thin section and supporting replacement of Bt by Ky and Ru: Bt → Ky + Ru (ΔV_rxn= -68%). Similarly, Bt interfingered with Sill and Ilm suggests a Bt replacement reaction: Bt → Sill + Ilm (ΔV_rxn= -68%).

Overall, the interfaces of these minerals and the immobility of Al³⁺ and Ti⁺⁴ suggest local replacement of Bt and Plag by specific Al- and Ti-phases. Ky and Ru formed in high-P conditions, whereas lower-P, high-T conditions formed Sill and Ilm (Ernst and Liu, 1998). The large -ΔV_rxn and local growth of aluminosilicates in an orthogneiss suggest that reactions are facilitated by local shear stress, causing K⁺, Na⁺, and SiO₂(aq) to leave the reaction site driven by local chemical potential gradients.