Brookville

At the upper contact of a 500 m thick dolerite sill nepheline and analcime-bearing syenite occur, both cutting the sill rocks and hornfelsed argillite. Poor exposure precludes the accurate determination of field relationships. The syenite contains prisms of kaersutite and brown biotite flakes in a groundmass of albite, perthite, titanaugite zoned to aegirine-augite and patches interstitial to the feldspar of nepheline, analcime, chlorite, calcite and natrolite. Specimens are very variable, most containing analcime but only a few nepheline. Fluorite, pyrite, ilmenite and other accessories are present. Modal and chemical data including Sr isotope determinations are available (Barker and Long, 1969).