REACTION AND TEXTURAL SOFTENING IN A SHEAR ZONE IN THE NEWTOWN ORTHOGNEISS, SANDY HOOK, CT

Petrographic analysis of a shear zone within the Newtown orthogneiss reveals both prograde and retrograde metamorphic paths preserved as arrested partial replacement reactions. The protolith of the shear zone is a granodioritic intrusive body characterized by microcline phenocrysts ≤ 8 cm in diameter in a hornblende + plagioclase + biotite matrix.

Prograde metamorphism is evidenced by ~1 cm lineated, discontinuous amphibole, a texture best preserved within meter-scale boudins. In the schistose matrix, garnet is embayed and partially to almost completely replaced by plagioclase + biotite + quartz, but can be found in contact with relic hornblende grains. From this we can infer the garnet-forming, prograde, open-system replacement reaction:

Amphibole + 6 H⁺ → Garnet + 4 Quartz + Fe²⁺ + 2Ca²⁺ + 4 H₂O

In the shear zone, the dominant retrograde schistosity strikes SSW and dips moderately to the NW. Where most strongly developed, the fabric is defined by cm-scale bands of contiguous biotite folia and ribbons of K-feldspar + plagioclase + quartz, interpreted to be derived from the amphibole-rich matrix and microcline phenocrysts, respectively. Amphibole is truncated and replaced by quartz and foliated biotite, suggesting deformation by the dissolution-precipitation reaction:

Amphibole + 2 K⁺ + 2 Fe²⁺ + 2 H₂O → 2 Biotite + 2 Quartz + 2 H⁺ + 2 Ca²⁺

Microcline phenocrysts exhibit a range of textures. Where preserved as elongate megacrysts in areas of low shear stress they show plagioclase + quartz myrmekitic mantles. Quartz is richest in felsic ribbons, where it and plagioclase are observed embaying and replacing microcline. Both textures are consistent with the reaction:

Microcline + Ca²⁺ + Na⁺ → Plagioclase + Quartz + K⁺

The overall retrograde reaction is:

K-feldspar + Amphibole + H₂O ± Garnet → Biotite + Plagioclase + Quartz + Sphene

and is a powerful example of reaction softening.

Folding of quartz veins and a biotite lineation trending 330/10 indicate top-northwest displacement. Poles to the planes of pegmatite veins crosscutting the outcrop are approximately parallel to the biotite lineation, indicating similar extension direction. This suggests deformation was continuous during exhumation from amphibolite-facies conditions through the rock’s ductile-brittle transition.