MAPPING AND GEOCHRONOLOGY OF CONTACT ZONE OF PALEOZOIC CAPE ANN PLUTON, SALEM NECK, MA

The Cape Ann pluton is a Paleozoic alkalic intrusion post-dating 630-585 Ma Avalonian magmatism in SE New England. Such alkalic compositions are associated with intra-plate magmatic systems produced when hot, mantle-derived magmas invade and melt continental crust. This study investigates whether apparently co-genetic mafic and felsic rocks on Salem Neck, MA in the contact zone of the Cape Ann pluton could represent this type of relationship.

The contact zone has previously been mapped only at 1:24,000 and is a ~400 m wide EW-trending band separating the Beverley Syenite on the northern tip of Salem Neck and the Salem Gabbro-diorite to the south. The dominant rock type in the contact zone has been described as camptonite, basaltic porphyritic ellipsoids in syenite that are cut by nepheline-sodalite syenite dikes. Dikes of hybrid, intermediate rock also are present, as is coarse grained gabbro-diorite. Detailed mapping for this project (that can be viewed at scales up to 1:800 using ArcMap) now reveals that Salem Gabbro-diorite extends the length of the northwestern coast of Salem Neck, while the southeastern half of the peninsula with its associated basalt and syenite, proves to be a pillowed complex representing mingling of mafic and felsic magmas. All units are cut by syenite, mafic, and hybrid dikes, and the syenite unit contains blocks of gabbro-diorite. The gabbro-diorite/pillowed complex contact trends NE-SW at nearly right angles to the previously mapped contact zone. Flattened pillows strike parallel to the contact and dip to the southeast indicating that the pillowed complex overlies the gabbro-diorite.

To test the texturally implied co-genetic relationship of mafic and felsic units on Salem Neck, syenite has been sampled for chemical abrasion-thermal ionization mass spectrometry at MIT’s Radiogenic Isotope Lab. The sample is a fine grained, bluish-grey syenite that weathers white, and separates basaltic pillows along crenulated contacts. CA-TIMS analyses from zircons separated from this sample will give weighted mean ²⁰⁶Pb/²³⁸U dates that can be compared with precisely dated granites farther north in the pluton. Uncertainties of ±100 Ka on all of these dates are orders of magnitude more precise than the available upper concordia intercept date of 450 ±25 Ma obtained more than 40 years ago.