Stratigraphy and geochemistry of the Ammonoosuc Volcanics, central Massachusetts and southwestern New Hampshire

Abstract

The Middle Ordovician Ammonoosuc Volcanics are interpreted to be Ordovician island arc volcanics, deposited during the Taconian Orogeny and later metamorphosed during the Devonian Acadian Orogeny at amphibolite facies conditions. Mafic rocks from the Lower Member believed to have original compositions are chemically like low-K basalts through low-K andesites (48-58 wt percent SiO2, <0.9 wt percent K2O). In the Lower Member, felsic rocks believed to have original compositions are chemically like low-K dacites and low-K rhyolites (65-75 wt percent SiO2, <0.9 wt percent K2O), while those from the Upper Member are similar to normal dacites and rhyolites (67-77 wt percent SiO2, 1.51 to 3.65 wt percent K2O). Minor and trace elements in the mafic Ammonoosuc rocks are comparable with modern arc basalts. Minor and trace elements in felsic rocks (↔63 wt percent SiO2) from the Upper Member and most from the Lower Member are comparable with modern volcanic arc rocks and could have been generated by partial melting of amphibolites with trace element abundances like those of island arc basalts. Higher K, Rb, and Ba distinguish Upper Member felsic rocks from those found in the Lower Member. -from Author