ULTRAMAFIC AND SEDIMENTARY DIKES, PENOBSCOT BAY: CAMBRIAN EXTENSION AND ORDOVICIAN ACCRETION ALONG SOUTHEASTERN GANDERIA

The Ellsworth terrane, chiefly Middle-Upper Cambrian bimodal volcanic rocks, occupies a critical position between the dominantly sedimentary St. Croix terrane to the northwest and Avalon platform sensu stricto beneath the Gulf of Maine. Two recent observations support a model of seafloor generation during the Cambrian followed by accretion of the Ellsworth terrane to the St. Croix margin of Ganderia during the Ordovician.

On Deer Isle, previously unrecognized dunite invades tectonized harzburgite in a pattern identical to that observed in ophiolitic mantle elsewhere. These dikes represent reaction zones around melt channels through the upper mantle where pyroxene dissolved and olivine precipitated, thus converting harzburgite to dunite. Spinel compositions (Fe³⁺ <0.1; Cr/(Cr+Al) 0.2-0.5; Fe²⁺/(Mg + Fe²⁺) 0.3-0.6) cluster with ophiolitic rocks. While XRF results from reconnaissance sampling of Ellsworth igneous rocks are scattered, consistent with an accretionary setting, trace element concentrations and ratios in Deer Isle/North Haven mafic rocks are consistent with an arc-related ocean floor setting (Ti/V >40, high (Ti+Cr), intermediate-low Y and Nb, Nb/Y<0.5).

On North Haven Island, 10 km southwest, a northwest-trending vertical sedimentary dike of black shale mélange occurs along an imbricate zone at the base of the North Haven Greenstone. South-younging pillowed basalts are intercalated with black shale that is identical to the nearby Penobscot Formation of presumed Tremadocian age. Limited kinematic data suggest sinistral transpression, and the mélange style suggests a relatively cold accretionary setting. To the northeast, windows of Penobscot Formation and common top-to-northwest structures within Ellsworth rocks strongly suggest that the sedimentary St. Croix terrane underthrust and elevated the Ellsworth marine igneous rocks. Timing was certainly post-Tremadoc and most likely pre-late Llandovery (Ames Knob unconformity). Late Silurian igneous rocks which puncture and metamorphose the Ordovician collision zone may be related to break-off of a northwest-dipping slab following the arrival of Avalonia.