Origin of the Ayer granodiorite in the Lowell area, Massachusetts

Abstract

The elongate stock of Ayer granodiorite exposed north and west of Lowell, Massachusetts, is reasonably typical of the many bodies of granitic rocks in the central and north‐central parts of the State. It lies within a terrane composed predominantly of steeply tilted, thinly inter bedded quartzite and biotite schist, and many of its structural relations superficially suggest for it a metasomatic origin, involving the quiet replacement of preexisting bedded and foliated rocks by invading fluids of magmatic derivation. The elongation of the stock and a distinct foliation characteristic of the granodiorite itself are both essentially conformable with country‐rock structure, and the contacts, wherever seen, also appear to be concordant. This general parallelism is further emphasized by a consistent orientation of numerous tabular inclusions, the edges of most of which have been obscured by reaction with the igneous material. Moreover, apparent transitions from country‐rock into granodiorite in a direction parallel to their structural trends occur at several places. Dikes and lenses of pegmatite are abundant within and adjacent to the granodiorite body.