GEOCHEMISTRY OF SILLS AND DIKES OF THE BOSTON HARBOR ISLANDS AND RHEOMORPHIC MELTING OF THE CAMBRIDGE ARGILLITE COUNTRY ROCK

Major and trace element XRF analyses were completed for 13 sills and 6 dikes from seven of the Boston Harbor Islands and for two samples of andesitic to dacitic matrix mingled with dolerite within two of the sills. REE analyses were also completed for one of the dikes and five of the sills. The rocks were classified as follows using the scheme of Irvine and Baragar (1971): ten tholeiitic dolerite sills; two picro–basalt sills; one potassic ankaramite sill; four tholeiitic dolerite dikes; two andesite dikes.

Compared to the sills and dike swarm at East Point Nahant, the Boston Harbor Island dolerites are more tholeiitic as a group and are lower in Nb and Zr with most also lower in TiO2 and Zr/Y. Chemically, the Boston Harbor Island dolerites more closely resemble the Paleozoic dolerites of eastern Massachusetts (other than Nahant) than they do Mesozoic dolerites of the Coastal New England Province. Plots on tectonic discriminant diagrams show that, in general, the Boston Harbor Island dolerites have a plate margin/oceanic signature whereas Paleozoic dikes and sills along the Massachusetts coast have island arc signatures.

Substantial mingling of dolerite magma with andesitic to dacitic liquids occurred within the margins of several of the sills to create irregular to ovoid, mafic, magma pillow-like structures within the felsic matrix. Locally felsic veins and dikes up to 0.6 m thick extend into the sill from the adjacent argillite. At the base of one sill, a 1.5 m thick dismembered lobe of dolerite invaded over 2 meters into felsic melt. At another locality a thin layer of argillite is absent beneath a portion of a sill showing extensive mingling but reappears beneath adjacent portions of the sill where no mingling occurred. This suggests its absence is due to melting by the sill magma. The andesitic to dacitic chemical compositions of matrix samples are compatible with rheomorphic melting of argillite in contact with sill magmas.