NEGSA Field Trip 2007: Stop 10

Stop 10: Sharpners Pond quartz diorite

The Sharpners Pond diorite is a large, calc-alkaline, intermediate pluton in the Nashoba Terrane. It is early Silurian in age, and is composed mostly of hornblende diorite and a variety of tonalites. Although broadly inhomogeneous, this outcrop is exceptional with deformed breccia and possibly pillows of mafic, probably dioritic rock in pinkish granodiorite or granite.


These are the two end member rock types: pinkish-gray granite and dark-gray mafic rock. Here the mafic rock appears to have some kind of foliation, which may be truncated by the granite. If you stare long enough you may be able to convince yourself that the foliation in the mafic rock extends into the granite.

On this outcrop face there appear to be mafic breccia blocks enveloped in the pinkish granitic rock. Careful examination shows that the mafic rocks are not all alike; some are coarser, some are darker.


This image was taken around the corner from the one immediately above (corner is to the right). On this face the breccia does not look particularly deformed, and many of the mafic enclaves (non-genetic term) look remarkably like the pillows seen at Stop 3. In this image the inhomogeneity of the mafic and felsic rocks can be seen. Some of the felsic rock is pink or white, and some grades into an intermediate gray, suggestive of contamination of the felsic rock by the mafic.


These joint surfaces nicely expose a deformed mass of breccia blocks and/or pillows. Notice again the diversity of shades of gray of the mafic blocks.


This image looks like an intrusion breccia, with coarse granitic or dioritic rock in between fine-grained mafic blocks.