GEOCHEMICAL ANALYSIS OF THE MEDFORD DIABASE DIKE, MIDDLESEX FELLS, MASSACHUSETTS: IMPLICATIONS FOR THE TIMING AND TECTONIC ENVIRONMENT OF EMPLACEMENT IN THE EASTERN AVALON TERRANE

The Medford diabase dike (MDD) within the Middlesex Fells Reservation has typically been mapped and considered to be a Mesozoic-aged intrusion based on its NE-trend, typical of Jurassic diabase dikes, and its cross-cutting relationships with adjacent country rocks. The MDD is a ~140 m thick, medium-grained, diabase dike intruding Late Proterozoic felsic volcanic rocks and the Spot Pond granodiorite pluton. Recent ⁴⁰Ar/³⁹Ar dating of a biotite separate by Ross (2020) yielded a slightly hump-shaped age spectrum with a minor plateau of 304.4 ± 0.6 Ma suggesting a Late Paleozoic age for the MDD. This study investigates the geochemical nature of the MDD and compares it with the chemistry of known Jurassic-aged diabase intrusions and basalt lava flows within Massachusetts, Connecticut, and New York and a Late Proterozoic diabase dike near the Middlesex Fells Reservation.

Preliminary geochemical results indicate the MDD does not share a similar signature with well-established Jurassic-aged igneous bodies related to the Mesozoic breakup of the supercontinent Pangea. The MDD exhibits slightly lower SiO₂ values and elevated total alkalis compared to Mesozoic igneous rocks falling within the alkali field versus tholeiitic for Mesozoic igneous rocks. The Mesozoic rocks have TiO₂ values that tightly cluster between 1.04-1.15 wt.% compared to MDD which is significantly higher at 2.9 wt.%. The Mesozoic rocks have Al₂O₃ contents between 13.3 and 14.4 wt. % compared to >16 wt. % for the MDD. Trace element concentrations of Cr fall within 77-300 ppm for the Mesozoic rocks and <32 ppm for the MDD. Further, Mesozoic basalts and diabase display higher Mg# (42-55) compared to the MDD (Mg# 32) and MORB chemistry. The MDD exhibits a chemical signature more closely associated with ocean-island basalts.

Based on the geochemical results, we support the idea that the Medford diabase dike is a Late Paleozoic intrusion unrelated to the crustal extension and magmatism associated with the breakup of Pangea in Triassic-Jurassic time. Whereas known Mesozoic igneous rocks display tholeiitic, MORB chemistry, typical of rift-related magmatism, the MDD does not. An alternative model for the emplacement of the MDD is thus required to explain the presence of Late Paleozoic intrusive activity in eastern Massachusetts.