NEGSA Field Trip 2007: Stop 5

Stop 5: Salem Highland Pluton, a composite mafic intrusion

This is a rather large Late Ordovician mafic body that sits at the center of the highest gravity and magnetic anomalies in northeastern Massachusetts. This pluton is composed of numerous small gabbro bodies and basaltic dikes, many of which have chilled margins. Some dikes, including at this location, are composite as indicated by chilled margins all facing the same way, at least locally.

This is an intrusive contact between a fine-grained gabbro, below, and a dike, above. The dike has an obvious chilled margin, grading from microscopic grain size against the gabbro to ~1 mm crystals at the image top.


These are thin syenite dikes that cut fine-grained gabbro, clearly after the gabbro was solid.


Granitic dike cutting a fine-grained gabbro. Dike to the far left is quite coarse, really a fine-grained pegmatite, with visible quartz.