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