GENERATION OF THE AMMONOOSUC AND HAWLEY ISLAND ARCS VOLCANICS BY FRACTIONAL CRYSTALLIZATION OF A BASALTIC ANDESITE IN THE NORTHERN APPALACHIANS

Compositional characteristics of the Ammonoosuc Volcanics (≈ 49 SiO₂ wt%, < 0.5 K₂O wt%) of the Bronson Hill arc (BHA), of the Hawley volcanics (≈ 55 SiO₂ wt%, < 0.6 K₂O wt%) of the Shelburne Falls arc (SFA), and late Ordovician gneiss (< 62 SiO₂ wt%, < 1.0 K₂O wt%) of the Pelham Dome show a calc-alkaline trend, typical of volcanic rock series occurring within island arcs and active continental margins. Volcanic major element components such as TiO₂, MnO, P₂O₅ shows a negative linear trend correlation with SiO₂, whereas Al₂O₃ correlate positively. The opposite correlation can be found in the gneiss suite, a decrease in Al₂O₃ with the increase in SiO₂. The CaO content of volcanics versus gneiss is very discriminatory, ranging from 5 to 10 percent in the volcanics and 0.5 and 4 for the gneiss and metasediments. In terms of minor elements, V, Cr Ni, correlates negatively with SiO₂ following a linear trend. In the Th/Yb versus Ta/Yb diagram, the Ammonoosuc and Hawley follow a linear correlation and plot in the typical calc-alkaline basalt field, whereas the gneiss and metasediments plot within-plate basalt. Additionally, the Ammonoosuc and Hawley volcanics display subcondritic Nb/Ta and relatively high Sr/Y, ratios that have been interpreted as a common chemical characteristic of island arc magmas, and the implication among other minerals of garnet and amphibole in their formation. Element patterns for chondrite-normalized values of the volcanics, felsic gneiss and metasediments display LREE-enriched patterns, a relatively flat heavy rare earth (HREE) regions and pronounced negative Nb, Pb, Sr, Hf, Zr, Eu and Ti anomalies, common in basalts from island arcs and to a lesser extent in basalts from back-arc basins. Negative Hf and Zr anomalies are also found in some island- arc basalts. Additionally, REE patterns for the Hawley volcanics are essentially the same as the Ammonoosuc volcanics. The above chemical characteristics, simple fractional crystallization modelling and the recent studies of the generation of bi-modal geochemical characteristics or “compositional gaps” solely by basalt crystallization in a deep crustal hot zone, (Melekohva, et al., 2013) favored an island arc tectonic setting over a back-arc formation for the Hawley and Ammonoosuc Volcanics.