PETROLOGIC MINERALOGY IN THE SMALLS FALLS FORMATION OF NEW ENGLAND: A PROGRESS REPORT

A collaborative effort between the author and the late Prof. Charles V. Guidotti was originally undertaken to characterize a body of geologically significant rocks of distinctive bulk composition, and to learn what geochemical signatures are preserved through metamorphism. Our study was intended to be a long term effort to study mineral and whole rock chemistry of samples from all parts of a Silurian depositional basin now represented by the Smalls Falls Formation, using a variety of techniques. What has been accomplished so far?

The Smalls Falls Formation today consists largely of sulfidic (Po ± Py ± Apy) and graphitic schists, quartzites and granular calc-silicate rocks that are the metamorphosed equivalents of black shales. The iron-rich nature of the formation has produced unusually Mg-rich silicate mineral assemblages as Fe has partitioned into the sulfide fraction of the rock. This formation crops out for several hundred kilometers along strike in the Northern Appalachians, from Northern Maine to Massachusetts. Viewed from northeast to southwest, New England rocks now exposed on the surface represent successively deeper crustal levels, as manifest by the regional disappearance of andalusite in favor of sillimanite and kyanite along strike.

A working hypothesis at the beginning of our study was that the protolith was generally homogeneous when deposited in its basin. However, trace element patterns in the Smalls Falls have shown significant variations in oxicity of bottom waters, a signal that is preserved through metamorphism. For example, high whole-rock ratios of V/(V+Ni) and V-rich micas show probable anoxic conditions in bottom waters in the southwestern portion of the basin, but the basin was not everywhere anoxic. Continuing work with δ¹⁵N studies of ammonium-bearing micas continues to suggest that ¹⁵N is not significantly fractionated during prograde metamorphism of the Smalls Falls Formation.