GEOCHEMISTRY OF THE CAMBRIAN ELLSWORTH SCHIST AND CASTINE VOLCANICS, PENOBSCOT BAY AREA, MAINE: IMPLICATIONS FOR PALEOTECTONIC SETTING

Cambrian rocks in the Penobscot Bay area of central coastal Maine dominantly consist of deformed and lower greenschist facies volcanic rocks of the Castine Volcanics (~502 Ma), Ellsworth Schist (~509 Ma), and the correlated North Haven Greenstone. The tectonic setting represented by these units has remained controversial with past interpretations including continental rift, back-arc basin, and subduction-related magmatic arc chiefly based on lithologic association and limited geochemistry. For this study, samples of the Ellsworth Schist and Castine Volcanics have been analyzed in detail including high-precision INAA analyses for Th, Ta, Hf, and rare-earth elements (REEs).

Results show that the Ellsworth and Castine consist of bimodal basalt and rhyolite sequences with similar compositional characteristics. Basalts from both sequences typically show: (1) tholeiitic trends of FeO_t (8-12 wt%) and TiO₂ (1-3 wt%) enrichment; (2) flat REE patterns at ~10-30 x chondrite with slight depletion to enrichment of light REEs ([La/Sm]_n=0.6-1.6); and (3) no negative Ta anomalies on primitive mantle-normalized plots (PMNP). Two rhyolite compositional types are present in both sequences. One type, dominant in the Castine, is characterized by flat REE patterns ([La/Yb]_n=1-2.6), negative Eu anomalies (Eu/Eu*=0.3-0.5), and positive Th but small negative Ta anomalies on PMNP. The second rhyolite type, dominant in the Ellsworth, has lower middle and heavy REEs, Zr, and Hf contents than the first type with steeper REE patterns ([La/Yb]_n ~ 3-6), and smaller negative Eu and Ta anomalies. Also present locally in the Castine are rare calc-alkaline andesites with light REE-enriched patterns ([La/Yb]_n=4.6-9.5), and negative Ta and Ti anomalies on PMNP.

Data for tholeiitic basalts from the Ellsworth and Castine consistently plot in the fields of N-MORB and E-MORB on tectonic discrimination diagrams and are most similar compositionally to oceanic plateau basalts. The rhyolites lack the prominent negative Ta anomalies of continental and subduction-related rhyolites, also suggesting an oceanic extensional setting. An oceanic setting for the Ellsworth and Castine is further supported by the presence in these volcanic sequences of Pb-poor massive sulfide deposits with relatively unradiogenic Pb-isotope compositions.