GEOCHEMICAL AFFINITIES OF INTERMEDIATE AND MAFIC LAYERS IN METAMORPHOSED LATE PROTEROZOIC SEDIMENTARY AND RHYOLITIC STRATA, PELHAM DOME, MASSACHUSETTS

In the late 1960’s Robinson and D. D. Ashenden collected samples during construction of the Northfield Mtn. Pumped Storage Hydroelectric Project. One sample set was of metamorphosed mafic and intermediate igneous rocks, mostly hornblende-rich boudins, from Late Proterozoic strata deepest in the core of the Pelham gneiss dome. Most samples are from Poplar Mountain Quartzite and stratigraphically overlying Poplar Mountain Gneiss of sedimentary derivation. A few are from underlying Dry Hill Gneiss having the right chemistry and interpreted as metamorphosed alkali rhyolite (Hodgkins 1985), and yielding an igneous U-Pb zircon age (Tucker & Robinson 1991) of 613 ±3 Ma (Vendian). These mafic rocks appear to have been emplaced penecontemporaneously with the Dry Hill Gneiss and are conspicuously different from mafic rocks in the overlying Late Ordovician Fourmile Gneiss (Bull 1996). These were metamorphosed one or more times to high grade with partial melting of gneisses and pegmatite emplacement. They have reasonably coherent igneous characteristics for immobile elements, but alkali and alkaline earth metals have, not surprisingly, undergone substantial redistribution. Preliminary data on 22 analyzed samples show 7 are intermediate rocks, 13 are mafic resembling basalts, and two are ultramafic rocks, apparently contaminated pyroxene-rich cumulates. The mafic rocks are all high in TiO₂ (up to 4%), high in Nb (up to 111 ppm), and LREE-enriched. In western New England, rocks most closely resembling these are the Eo-Cambrian Tibbit Hill Formation (Coish 1985), interpreted as volcanics associated with rifting of the Laurentian margin, though Nb concentrations are higher in some Northfield samples. We interpret most mafic samples to have been basalts derived from enriched mantle, emplaced as dikes or flows during a Late Proterozoic rifting event. The plate setting was most likely on a peri-Gondwanan margin of Iapetus, though the Laurentian margin cannot yet be excluded. Work is proceeding to complete the data sets and expand comparisons with data sets of known provenance.