PETROGRAPHY, FIELD RELATIONS AND GEOCHEMICAL ANALYSIS OF ORDIVICIAN METAMORPHOSED MAFIC IGNEOUS ROCKS OF THE CAUCOMGOMOC LAKE INLIER, NORTHERN MAINE

The Hurd Mountain Formation occurs in the Caucomgomoc Lake region of northwestern Maine, part of the Cambro-Ordovician Caucomgomoc Lake inlier. It is primarily a mélange of probable Ordovician age consisting of deformed slates, phyllites, metasandstones and metawackes that contains knockers of metagabbros and metabasalts. The knockers exhibit lower greenschist grade metamorphism (chlorite, actinolite, calcite, opaques and epidote). Remnant, partially metamorphosed pyroxenes and plagioclases are present in some samples and display igneous fabrics.

Whole rock geochemical analyses (XRF and ICP-MS) were made of the metabasalt and metagabbro knockers for trace and rare earth element concentrations. A relatively flat pattern on the chondrite-normalized spidergram suggests a depleted mantle source for the knockers. The MORB-normalized spidergram shows a slight negative slope and Th and Ce are slightly enriched between 1 and 10 times N-MORB with a distinctive Ta-Nb negative anomaly suggesting a volcanic arc tectonic setting. On some tectonic discrimination plots (Zr-Ti-Y, Th-Hf-Nb, Zr-Y-Nb), samples plot in both MORB and arc overlap fields, or plot across the MORB and arc field boundary. On the Th/Yb-Ta/Yb diagram samples plot exclusively in the arc fields, but in contrast, plot entirely outside of the arc field on the Ti-V diagram with a V/Ti ratio of greater than 20. On some diagrams (Zr-Ti-Y, Ti-V, Zr-Nb-Y) samples also plot in fields that overlap with within-plate continental tholeiites. However, this is not consistent with observed field relations. Overall, these data are transdiscriminant, displaying characteristics of both MORB and volcanic arc tectonic settings.

The geochemical data obtained from the Hurd Mountain Formation show a strong similarity with the data from the Chesuncook Dome area (Bean Brook Gabbro and Dry Way Volcanics), interpreted to be the result of ridge subduction. A similar conclusion applied to the knockers of the Hurd Mountain Formation is consistent with both their field relations and geochemical signature.