POST-RIFTING FATE OF THE PERI-GONDWANAN ELLSWORTH SCHIST, NEWBURY NECK, MAINE

In coastal Maine, east of Penobscot Bay, the Cambrian Ellsworth Schist crops out in the Ellsworth-Mascarene terrane between the northwesterly St. Croix terrane and southeasterly Avalonian terranes. On Deer Isle, it partially encloses a serpentinite body, which may be an intrusion (Stewart, 1998) or a slice of mantle. Recent 1:24000-scale mapping in the Newbury Neck quadrangle south of Ellsworth constrains lithologic and structural variations that bear on the evolution of this region.

Ellsworth rock types include quartz-muscovite-chlorite schists, greenstones and metarhyolites, and, at the head of Morgan Bay, impure quartzites and a conglomerate. Chlorite-rich greenschists suggest a reworked volcanogenic protolith for much of the formation. The sheet-form meta-igneous rocks comprise tuffs and locally sills, indicated by gabbroic texture and possible chilled margins. Igneous sheets and the few clearly-bedded strata are flat to southeast-dipping except where steepened on rare late folds. A lineation, best displayed by elongate quartz phenocrysts in metarhyolites, plunges consistently gently southeast. Thin quartz laminae and abundant quartz veins define pervasive cm-scale, strongly asymmetric, tight to isoclinal folds that verge southwest to north. Sigmoid quartz laminae and veins indicate top-to-the-northwest sense-of-shear. The main foliation subparallels the quartz laminae; it appears to be axial planar to the asymmetric folds locally, but may be affected by these folds elsewhere. Crenulation cleavages correspond with the minor and variably oriented late folds. The formation is chlorite-grade except in the aureole of the Blue Hill Pluton, where rare pelitic beds contain andalusite.

Pebbles of Ellsworth schist at the base of the overlying Castine Volcanics (Late Cambrian) indicate an angular unconformity. The serpentinite-Ellsworth contact is sheared, and the serpentinite has a harzburgite protolith with elongate orthopyroxenes.

The strongly bimodal character of the Ellsworth igneous rocks points to extension (Stewart, 1998), which may have exhumed mantle (tectonized harzburgite). The intense top-to-the-northwest deformation and low-grade metamorphism suggest a southeast-dipping convergent boundary active in Late Cambrian time along which rift and mantle rocks were juxtaposed.