IMPLICATIONS OF COMPOSITIONAL AND TEXTURAL VARIETY IN THE CAPE ANN PLUTON

The Silurian (Thompson and Ramezani, 2008) Cape Ann Plutonic Suite of northeastern Massachusetts appears to represent a layered magma chamber with less silicic magma near Salem, MA and more silicic magma toward the northeast. Abundant mafic pillows hosted by the Beverly Syenite on Salem Neck are consistent with mingling of the two magmas (Fendrock and Thompson, 2015) and with injection of mafic magma at the base of a mafic and silicic layered intrusion (MASLI of Wiebe and Collins, 1998). Granite- and syenite-dominated outcrops exposed on Cape Ann are consistent with the upper reaches of such complexes. This study focuses on mingling textures in granite- and syenite-dominated parts of the pluton. At Lane’s Cove numerous contacts between granite and K-feldspar quartz syenite are exposed. Schlieren appear to represent mafic magmatic bodies that were partially disaggregated into the host granite due to magmatic flow. Several larger mafic bodies consist of a plagioclase-phyric groundmass with large purple plagioclase crystals (to 39 cm long) and glomerocryst “rafts” scattered throughout. These crystal accumulations are likely to be disrupted rafts of cumulates, surrounded by their host magma from a deeper level of the pluton. Rafe’s Chasm hosts a 2.5 m wide composite dike (described by Ross, 2014), consisting of aphyric basaltic margins and an intermediate interior rich in mafic enclaves and K-feldspar phenocrysts. Field relations are consistent with assimilation of felsic magma by basaltic magma, with resorbed K-feldspar phenocrysts providing evidence of the assimilation event. Due east of this dike is a previously undescribed rock unit that is either a very shallow intrusive unit or a roof pendant of rhyolite. The rhyolite (~20 m wide) is porphyritic, with euhedral resorbed melt inclusion-bearing quartz phenocrysts and large (up to 0.5 cm) K-feldspar phenocrysts in a very fine grained matrix interpreted as devitrified glass. The genetic relationship between this apparently volcanic outcrop and the Cape Ann pluton is under investigation. These magma interaction features indicate that the pluton might be the result of repeated influxes of mafic magma into a dominantly felsic chamber, consistent with the MASLI model of Wiebe and Collins (1998), and similar to contemporaneous plutonic complexes on the coast of Maine.