The Cape Ann is an iron-rich Late Ordovician alkali granite, cut by aplites, pegmatites, and alkali basalts. The granite contains the minerals annite, fayalite, and hastingsite, and this is the type locality for annite, the iron end member biotite.
This is an overview of a flat pavement on Cape Ann. Alkali basalt dikes that cut the granite probably represent some of the mantle-derived liquids that heated the crust and permitted large-scale melting and production of the Cape Ann Granite and related rocks in the area.
This is the granite itself. It is a white to gray hypersolvus granite with one feldspar (perthite). Here the granite contains ~30% quartz, and <10% mafic minerals that include annite, hastingsite, and fayalite.
Here there are large expanses of aplite. This is one side of an aplite dike with graded layering, the grading being differentiated by grain size. The layering is parallel to the dike wall, with the interior of the dike toward the top. The grading seems to show precipitation of fine-grained quartz, feldspar, and mafic minerals, followed by progressive coarsening of the grain size inward to the beginning of the next coarse layer. This may represent rapid crystallization during sudden drops in PH2O, perhaps during venting events, followed by repressurization and inward coarsening of growing crystals. Alternatively, this may represent successive stages in dike widening.
This aplite body is partly made up of small pegmatite segregations, possibly from unmixing of an aqueous fluid phase during crystallization. The fluids, immiscible with the silicate liquid at low pressure, separated out and allowed crystallization of the coarser pegmatitic bits.
Larger bodies of pegmatite cut the granite and aplite. This pegmatite dike segment is especially rich in blue quartz, along with coarse perthite and traces of mafic minerals.
This pegmatite has large, hexagonal quartz crystals that obviously grew into void space. The blue color of the quartz is quite vivid. The quartz crystals are surrounded by coarse alkali feldspar. The dark crystals at the bottom left are the remains of cigar-size fayalite crystals.
Edge of a pegmatite body showing coarsening-inward textures (center of the pegmatite is up). Blue quartz, perthite, and the remains of fayalite crystals can be seen.
This is a closeup of a pegmatite, showing blue quartz, perthite, and fayalite crystals. The fayalite makes up the black, coarsely fractured cores to the dark grains. The fine-grained rims are made of subsolidus reaction products that include magnetite, hastingsite, and biotite.
Well exposed on this coast are sparse, medium-gray mylonite zones. They look like gray dikes, but close examination will reveal intense deformational fabrics, most obvious along the mylonite margins. This mylonite contains relict pink alkali feldspars. The motion sense on this nearly horizontal surface is dextral.