A NEW LATE PENNSYLVANIAN AGE FOR THE MEDFORD DIKE, MEDFORD, MASSACHUSETTS

The Medford dike has been considered to be Early Jurassic based on its NE-trend and that it cuts all other dikes wherever it has been mapped with one exception, a thin north-south trending dike (Wilson, 1901) now covered by urban development. This assumption was reinforced by a K-Ar biotite age of 190 + 6 Ma (Krueger, in Ross, 1981). However, a new 40Ar-39Ar biotite age of 304.38 + 0.56 Ma pushes its age back to Late Pennsylvanian.

The dike is noteworthy due to its unusual thickness reaching a maximum of 140 m at Pine Hill in Medford. It is also stands out from other dikes in the region with its diabasic texture giving it a characteristic “salt and pepper” appearance in the field. It also contains abundant biotite compared to other dolerite dikes in the region that generally lack biotite.

The most unusual aspect of the dike is its highly variable mineral and chemical compositions with location across its width, unlike any dike the author has encountered. Its west margin “facies” extends 300 cm into the dike and contains up to 38 volume percent biotite, 23.5 percent kaersutite and 50.9 percent fresh olivine and substantially less labradorite than the rest of the dike. The 150 cm thick east marginal facies contains less than 6 volume percent biotite, up to 27 percent olivine, and no kaersutite. The central facies contains up to 5.6 percent biotite, 7.9 percent olivine, up to 6.8 percent quartz + k-feldspar + granophyre and no kaersutite.

These wide variations in mineralogy are also reflected in the major and trace element chemistry. On an alkalies-silica diagram the data plot in three widely separate clusters: western facies in the alkaline field, eastern facies in the subalkaline field and the central facies, richer in SiO2 and alkalies, plots on the discriminant line. The breadth of variation in the dike equals or exceeds that of all the Eastern North American Mesozoic and CNE dikes taken as a group.

Abundant field, chemical and petrographic evidence indicates the central and eastern facies of the dike suffered significant contamination from the granite country rock during its intrusion. It is clear that the Medford dike is unique and does not resemble Mesozoic dikes or any other dikes in the region. These characteristics lend strong support to the Late Pennsylvanian age for the dike rather than it being Early Jurassic.