CONTACT RELATIONS OF THE NAHANT GABBRO AND WEYMOUTH FORMATION, NAHANT, MASSACHUSETTS

The Ordovician Nahant Gabbro pluton has been described as a thick sheet intruded into a syncline in the CambrianWeymouth formation with which it interfingers to form a "cedar tree" or "Christmas tree" laccolith (Lane, 1888, Clapp, 1921, Kay, 1965). However, the interfingering sills differ markedly from the Nahant Gabbro in both petrographic and chemical characteristics and are unlikely to be co-magmatic with it (Ross and Bailey, 2001). The only dated sill has a 40Ar-39Ar whole-rock age of 445 + 6Ma (Ross and Baily, 2001) compared to Late Cambrian to Early Ordovician Rb/Sr and K-Ar ages for the gabbro (Zartman and Marvin, 1971). This casts doubt on the previously assumed timing of folding and intrusion as well as the form of the intrusion and nature of its contact with the Weymouth Formation.

The contact between the gabbro and the Weymouth formation is not exposed but field relations require that it strikes nearly north-south. Detailed mapping and new geochemical data has allowed the correlation of selected, northwest-trending mafic dikes exposed on both sides of the contact. These dikes show no significant offset that can be related to the contact, indicating it to be intrusive rather than a fault.

The field and petrologic evidence suggests that, rather than a laccolith intruded into a syncline and interfingering with the Weymouth Formation, the gabbro is the erosional remnant of a lopolith. Rather than predating intrusion, the syncline represents local downwarping accompanying loading of the crust by the gabbro as magma was withdrawn from depth.