GEOCHEMISTRY OF THE SILURIAN SMALLS FALLS FORMATION IN NORTHERN AND WEST-CENTRAL MAINE: REGIONAL METAMORPHIC EFFECTS AND PROVENANCE

We report here mineralogical and geochemical data for 25 samples of sulfidic black shale and schist of the Silurian Smalls Falls Formation from northern to west-central Maine that span chlorite to staurolite-andalusite metamorphic grades. These data augment Van Baalen (2006) including for major elements, TOC, S, immobile and other trace elements, and rare earth elements (REE).

Principal phases in chlorite-grade samples are quartz, muscovite, chlorite, organic matter, and pyrite accompanied locally by trace amounts of pyrrhotite, chalcopyrite, sphalerite, and ilmenite. Framboidal pyrite (uncommon) is 4 to 8 μm in diameter, implying locally euxinic bottom waters. A similar mineral assemblage occurs at biotite and garnet grades, except for greater proportions of pyrrhotite compared to pyrite (no framboids exist) and traces of arsenopyrite. Amphibolite-facies samples (e.g., type area at Smalls Falls) differ in containing andalusite, phlogopite, cordierite, abundant pyrrhotite, little or no pyrite, and traces of tourmaline and rutile. Regardless of metamorphic grade, TOC contents are uniformly low (0.43-1.74 wt %). Total S is generally high (mostly 3-7 wt %; up to 13.0 wt %); TOC/S ratios are extremely low (<0.7). Contents of redox-sensitive trace elements are anomalously low for such sulfide-rich strata, including for Mo (<2-61 ppm), V (61-217 ppm), and Ni (34-95 ppm). Given the abundance of iron sulfides, these data suggest significant loss of organic matter and some included metals (e.g., V, Ni) at P-T conditions below those of chlorite grade.

PAAS-normalized data for REE display broadly flat patterns with La/Yb ratios of ca. 9-14 except for three of four garnet-grade samples that have comparatively low La/Yb ratios of 4.7-7.7, likely reflecting preferential mobility of LREE. Local small positive Eu anomalies (Eu/Eu* 1.07-1.45) are attributed to primary anoxic depositional conditions and not to a hydrothermal component. No Ce anomalies are present. Relatively high Th/Sc ratios (avg 0.82 ± 0.20) indicate a predominantly felsic source terrane; high Sn contents (up to 29 ppm) suggest influence of an evolved granite source. Bulk compositions on a Th-Sc-Zr/10 discrimination diagram plot mainly in the field for continental arcs, consistent with a source in post-Taconic arcs on the Laurentian margin to the west.