SCANNED CATHODOLUMINESCENCE PETROGRAPHY OF QUARTZ IN THE CAMBRIDGE ARGILLITE

The Cambridge Argillite is a thick assemblage of carbonaceous silty shale with silt/fine sandstone interbeds situated within the Boston Bay Group of Massachusetts. The origin of the Boston Bay Group is still being debated, with most of the controversy centered on a potentially glacial origin for Squantum Formation which underlies the Cambridge Argillite. In contrast, comparatively little effort has been expended to understand the origin of the Cambridge Argillite. Scanned Cathodoluminescence (SEM-CL) imaging of samples from the Cambridge Argillite at Hewitt's Cove shows that not all the quartz in these samples are detrital in nature. In addition to detrital quartz, quartz forms diagenetic overgrowths on detrital grains, as well as being redistributed during low grade metamorphism.

Detrital quartz are angular and range in size from 10's to 100's of microns. They are dominated by blue CL colors that suggest derivation from an igneous source. Some grains appear mildly zoned and euhedral and may be of volcanic origin, whereas others have features like healed low temperature veins, suggestive of an intrusive igneous source. A subset of quartz grains shows reddish-orange and dull CL colors that are suggestive of derivation from a low grade metamorphic source. In context, these observations are consistent with sourcing the Cambridge Argillite from a volcanic arc.