NORTHERN VERMONT METADIABASIC INTRUSIVES: GEOCHEMICAL CONNECTION TO THE BOLTON IGNEOUS GROUP

Pre-Silurian metadiabasic intrusives are found in the Northern Vermont Rowe-Hawley Zone in specific lithotectonic packages that lie at the highest structural levels; they occur in metasedimentary rocks previously mapped as Stowe, Moretown, and Cram Hill formations. The metadiabases are characteristically gray, massive, rounded, granular, weakly-foliated, +/- plagioclase phenocryst rocks with distinct buff-colored weathering rinds. The intrusive origin is based on the presence of preserved chilled margins, xenoliths of metasedimentary lithologies, and sharp contacts that cut Pre-Acadian foliations. The metadiabases are frequently boudinaged within the dominant foliation and individual bodies cannot be traced over large distances. The highly altered metadiabases have a plagioclase-epidote-actinolite-chlorite-calcite-quartz mineralogy, but preserve an ophitic texture that may also have included pyroxene.

Major and trace element classification methods show the metadiabases to be sub-alkaline tholeiitic basalts to basaltic andesites. Chondrite-normalized REE patterns are LREE-enriched with flat HREE and have slight negative Eu anomalies. Overall, LREE abundances range from 20-40X chondrite whereas HREE range from 10-20X chondrite. MORB-normalized multi-element spider diagrams show irregular LILE abundances, distinctive Th, Ta, Nb,Ce anomaly patterns, and generally flat P to Cr patterns near unity. Various tectonic discrimination diagrams show the metadiabases to plot in the MORB/BABB field. “Modern” analogues for the metadiabases may be found in ensialic backarc basins.

Many mafic samples from the Bolton Igneous Group (BIG) in Quebec (Melancon et al., 1997) show nearly identical geochemical signatures to the northern Vermont metadiabases; the BIG mafic rocks occur in the St. Daniel Melange as blocks with ambiguous contact relationships. The northern Vermont metadiabases could be the hypabyssal equivalents of the BIG.