TECTONIC SETTING AND REGIONAL CORRELATION OF ORDOVICIAN METAVOLCANIC ROCKS OF THE CASCO BAY GROUP, SOUTH-CENTRAL MAINE: EVIDENCE FROM TRACE ELEMENT AND ISOTOPE GEOCHEMISTRY

Late Cambrian (?) to Ordovician rocks of the Casco Bay Group are extensively exposed in coastal and south-central Maine and represent a major lithotectonic belt in this region. Despite their critical position in time and space in the northern Appalachians, these rocks are frequently ignored in tectonic models of the region because little is known about their tectonic setting and their relationship to rocks of similar age in other parts of the orogen. Here we report and interpret new trace element and isotope geochemical data from the Spring Point Formation (469 +/- 3 Ma), a prominent metavolcanic unit in the upper part of the Casco Bay Group.

Field mapping and major element geochemistry indicate rocks of the Spring Point Formation are bimodal (SiO2=51-55% and 70-77%). Rare earth element patterns are flat to slightly LREE-enriched (La/Ybn=0.8 to 2.0) in the mafic samples and LREE-enriched (La/Ybn=2.5 to 4.5) with negative Eu anomalies in the felsic rocks. On element normalized diagrams, mafic rocks have near-MORB values in HFSE but are enriched in LIL elements. Slight to pronounced negative Nb and pronounced negative Sr and Ti anomalies occur in both mafic and felsic samples. In most tectonic discriminant diagrams, mafic samples plot in ocean ridge or back-arc basin fields whereas felsic samples plot in within-plate fields. Th/Yb enrichment and negative Nb anomalies suggest the mafic samples are not purely mantle melts but have a subduction or crustal component.

Mafic samples have initial 87Sr/86Sr ratios in the range of 0.7069 to 0.7082 and epsilon Nd values from –1.3 to +0.6. These compositions are not consistent with extraction from convecting upper mantle and are best linked to derivation from a more evolved source. The Sr and Nd initial ratios are more evolved than typical for oceanic arcs and argue for an enriched source, as might be manifest in a back-arc environment, although significant juvenile crustal contribution cannot be ruled out.

Collectively, the trace element and isotopic data from the Spring Point Formation are consistent with an episode of ensialic back arc spreading in Late to Middle Ordovician time. The age, structural position and lithologic and geochemical characteristics of these rocks argue strongly for correlations with portions of the Bathurst Supergroup in the Miramichi Highlands of New Brunswick.