GEOCHEMISTRY AND ORIGIN OF VOLCANICS IN THE ORDOVICIAN PARTRIDGE FORMATION, BRONSON HILL ANTICLINORIUM, WEST-CENTRAL MASSACHUSETTS

The Partridge Formation is largely composed of aluminous sulfidic mica schist but contains substantial quantities of metamorphosed bimodal mafic and felsic volcanics chemically similar to the underlying Ammonoosuc Volcanics. The volcanics in the Partridge Formation were regionally metamorphosed in the kyanite + muscovite + staurolite through sillimanite + orthoclase zones during the Devonian Acadian Orogeny. The mafic Partridge volcanics, now amphibolites, have compositions similar to low-K tholeiitic basalts. The amphibolites generally have straight REE patterns, are slightly LREE-enriched, have small positive Eu anomalies, and all have negative Nb anomalies. Modeling indicates that the basaltic protoliths to the amphibolites were derived from 3 to 15 percent partial melting of a spinel lherzolite mantle source having flat chondritic abundances for most elements, but depleted by about 1.6x for Hf and Zr for most samples, and depleted in Nb by about 3x for all samples. The primary magmas were modified by low pressure fractional crystallization of olivine and possibly also of pyroxene and plagioclase. The felsic Partridge volcanics, now gneisses, have calc-alkaline compositions ranging from dacite to rhyolite. The felsic gneisses have somewhat LREE-enriched patterns and negative Nb, Sr, Eu, and Ti anomalies. Modeling indicates that the felsic gneisses were derived from the partial melting of a pyroxene + plagioclase + olivine granulite source having a Nb-depleted composition similar to that of the Partridge amphibolites. Melting presumably took place in the dee crust. Although identification of the tectonic environment for eruption of the Partridge volcanics is somewhat problematic, evidence favors a back-arc basin setting. The volcanics in the Partridge Formation, and by analogy the Ammonoosuc Volcanics, record an episode of Taconian igneous activity that is chronologically similar to but in some respects lithologically and chemically different from the presumed main part of the Taconian magmatic island arc exposed as calc-alkaline dome gneisses in the Bronson Hill anticlinorium. A back-arc basin setting for the Ordovician volcanics requires that they be in fault contact with the underlying dome gneisses.