GEOCHEMISTRY AND PETROGENESIS OF CAPE ANN, MA, MESOZOIC DIKES

Mesozoic intraplate magmatism in the eastern Massachusetts Avalon terrane is represented by over 100 northeast trending dolerite dikes that intrude the Silurian Cape Ann Plutonic Complex (CAPC) and Neoproterozoic sedimentary, volcanic, and calc-alkaline granitoid rocks. Geochronological data are sparse, but the dikes are inferred to be of Triassic age. Olivine and plagioclase are the common phenocryst phases, and the groundmass consists of plagioclase, pyroxene, and opaque minerals. Deuteric alteration of plagioclase and pyroxene is common. Major and trace element data were obtained for 28 dikes using energy dispersive X-ray fluorescence (EDXRF), wavelength dispersive X-ray fluorescence (WDXRF), and Instrumental Neutron Activation Analysis (INAA).

Chemically the dikes can be divided into two groups: silica undersaturated and silica saturated to silica oversaturated. Thermodynamic modeling using PELE shows that the dike temperatures at time of intrusion were on the order of 1200^oC and that olivine and/or plagioclase were the liquidus phases. During fractional crystallization the undersaturated magmas would ultimately crystallize nepheline while the saturated magmas would crystallize quartz. Trace element data demonstrates that olivine and pyroxene fractionation was important and that minor plagioclase fractionation or accumulation occurred.

On standard elemental discriminant diagrams the Cape Ann dikes show characteristics of both OIB and EMORB sources and plot in within plate fields on various tectonic discriminant diagrams. The dike chemistry is distinctly different from that of other Mesozoic dikes and intrusions in New England. This suggests that the dikes were derived from a distinct subcontinental lithospheric source. Reverse modeling indicates that the Cape Ann dike magmas formed over a wide range of temperatures (1450 to 1775 °C) and pressures (2.1 GPa to 5.7 GPa) which are typical of intraplate magmatism.