LATE CAMBRIAN TO EARLY ORDOVICIAN (PENOBSCOT) PERI-GONDWANAN ARC VOLCANISM IN SOUTH-CENTRAL MAINE: EVIDENCE FROM U-PB AGES AND WHOLE ROCK GEOCHEMISTRY FROM THE ST. CROIX BELT

The Penobscot Bay region of mid-coastal Maine is comprised of several lithotectonic terranes that were accreted onto the continental margin of Laurentia during the Paleozoic. One of these terranes is the St. Croix Belt, which contains metamorphosed Cambro-Ordovician volcanic rocks including the Gushee Member of the Penobscot Formation and the Unnamed Volcanic unit of the Jam Brook Sequence (SZuv). Previous studies, confirmed by field work associated with this research, reveal that both rock units display structures and textures consistent with volcanic formation, including relict interpreted pillow forms, scoria bombs, vesicular textures, and lapilli clasts.

Bulk-rock geochemistry reveals a range of compositions classifying from subalkaline basalt to rhyolite. Meta-basalts show flat REE patterns, while extended-REE diagrams show prominent Nb depletion relative to Th and La, indicative of a source region influenced by subduction processes. Meta-andesites/rhyolites display negative Eu anomalies and overall enriched REE patterns, while extended-REE diagrams also show prominent Nb depletion. Trace element plots vs. zirconium reveal differing trends for mafic and felsic groups. Strong positive correlations for mafic rocks suggest that basalts from both formations can be related by fractional crystallization of spinel peridotite partial melt, while differing trends for intermediate rocks indicates formation from a compositionally unique magma source. Overall, bulk-rock geochemistry for these rocks is consistent with formation in an island arc system produced from ocean-ocean subduction at a convergent plate boundary.

U-Pb zircon geochronology for two felsic rock samples (one from the Gushee Member and one from the Unnamed Volcanic unit) provide new ages of 489.8 ± 1.2 Ma and 487.1 ± 1.2 Ma respectively, providing further indication that these two units were erupted during similar periods in Late Cambrian to Early Ordovician time. Mafic volcanic rocks of this study display comparable geochemistry to the similarly aged mafic rocks of the Annidale Belt of southern New Brunswick, and likely represent the southern portion of the Late Cambrian to Early Ordovician Penobscot arc-back-arc system preserved within the northern Appalachians.