PETROLOGY AND GEOCHEMISTRY OF METAMORPHOSED CAMBRIAN-ORDOVICIAN VOLCANIC ROCKS OF THE ST. CROIX BELT, WESTERN PENOBSCOT BAY, MAINE

The northern Appalachian orogen contains several peri-Gondwanan terranes that were accreted onto the continental margin of Laurentia during the Paleozoic. The Penobscot Bay region of mid-coastal Maine is composed of a number of these lithotectonic terranes, including the Cambro-Ordovician St. Croix and Ellsworth Belts, and the enigmatic Jam Brook complex of uncertain age. Preserved within all of these belts are metamorphosed volcanic rocks that contain information important in determining tectonic environment of formation. The purpose of this contribution is to present new petrologic and whole rock geochemical data from meta-volcanic rocks in the St. Croix and Jam Brook belts.

Within the St. Croix Belt, metamorphosed volcanic rocks are found at the base of the Penobscot Formation (Gushee Member). These volcanics are best exposed in a 30km long belt up to 1.5km wide extending from Union to Belfast, ME. In the field, rocks display features such as relict pillows and amygdaloidal scoria consistent with formation in an eruptive setting. Thin sections reveal a range in mineralogies and textures, including amphibolite with relict vesicles, and fine-grained felsic rocks with relict quartz and feldspar phenocrysts. Bulk rock geochemistry of the Gushee meta-volcanics (n=25) yields a range of compositions from 44 - 76 wt. % SiO₂. Preliminary trace element geochemistry displays flat REE patterns with a distinct niobium anomaly suggestive of a source region influenced by subduction.

Immediately west of the St. Croix Belt are metamorphosed volcanic and sedimentary rocks of the Jam Brook complex. Within this unit, metamorphosed volcanic rocks are exposed along a 0.5km wide horizon extending 5km north of Sennebec Pond. In the field and in thin section, the rocks contain textures and structures consistent with a volcanic origin. Limited bulk rock geochemistry from these rocks (n=5) yields a bimodal distribution of chemical compositions with mafic samples ranging from 49 - 52 wt. % SiO₂ and a felsic sample containing 76 wt. % SiO₂. Trace element geochemistry displays flat REE patterns, also with distinct niobium anomalies.

Analysis of these rocks is on-going, and the data collected will be compared with volcanic rocks of similar age along strike (e.g., Ellsworth and Annidale belts) and integrated into tectonic models of the area.