PRESCOTT INTRUSIVE COMPLEX, CENTRAL MASSACHUSETTS, GEOCHEMISTRY AND STRUCTURAL REINTERPRETATION

The Prescott Complex of metamorphosed igneous intrusions in the central Bronson Hill Anticlinorium, is dominated by Cooleyville Gneiss (32 samples; 449 Ma, Tucker and Robinson, 1990), with lesser Prescott Gabbro, in one large mass and smaller bodies enclosed in the Gneiss (29 samples, 407 Ma). The Gneiss is weakly to strongly foliated and lineated, with ~1 to 4 mm grain size and 62-75% SiO₂. The Gabbro is massive to weakly foliated, fine-grained (<0.5 mm) to very coarse (~15 mm) with 47-53% SiO₂. The wide composition gap implies no genetic relationship, consistent with the ~42 m.y. age difference. The rocks are calc-alkaline and metaluminous to weakly peraluminous, and all are LREE-enriched with La_n/Yb_n ratios of 2.3-17 for gneisses, 1.5-4.6 for gabbros. Multi-element diagrams show strong positive anomalies for Pb, Li, and other LILEs, and negative anomalies for Nb, Ta, and, to a lesser extent, Zr and Hf, consistent with arc-related sources. Compositions are like those of the nearby Late Ordovician Monson and Fourmile Gneisses, plutonic roots to the Bronson Hill Arc, but differ from other local igneous rocks: Early Devonian Erving Formation amphibolite (REE like N-MORB), Middle Devonian Belchertown Intrusion (380 Ma, strange chemistry), Late Devonian Hardwick Tonalite (360 Ma, mildly alkaline). The complex is exposed within the Late Devonian Prescott Syncline, on the east limb apparently intruding the Late Ordovician Partridge Formation, based on near-contact rock types contaminated with schist (including gabbro with 5 cm garnets), and inclusions of Partridge in Gneiss and Gabbro. On the west limb it is in apparent intrusive contact with Partridge and the Ammonoosuc Volcanics, and fault contact with Fourmile Gneiss of the Kempfield Anticline. The north end was originally interpreted as intruding Silurian Clough Quartzite and Devonian Littleton Formation, but the 449 Ma radiometric age contradicts this. The north contact is now interpreted as a southeast-dipping Mesozoic normal fault, connecting the previously mapped Ludlow-Lighthouse Hill fault on the west with the New Salem faults to the northeast. We correlate the Cooleyville geochemically with the plutonic Monson Gneiss, but likely a hypabyssal intrusion into Partridge at a high level in the Taconic arc. The much younger Gabbro origin remains enigmatic.