DETRITAL ZIRCONS AND GANDERIA'S SOUTHERN MARGIN, COASTAL MAINE

In central Newfoundland, a large, Upper Cambrian (~494 Ma) ophiolitic thrust sheet, the Gander River Ultrabasic Belt (GRUB), constitutes one of the most striking features of the northern Appalachian orogen. Gander Zone arenites and shales of Cambro-Tremadoc age occupy the footwall southeast of the GRUB line and the Cormacks window. Arenig strata that unconformably overlie both the ophiolite and Gander Zone sediments, combined with a 474 Ma stitching pluton, demand a Tremadoc/early Arenig age of emplacement (Penobscot orogeny). In the simplest scenario, the ophiolite formed in a back-arc setting southeast of the Cambrian Victoria arc; Gander Zone arenites accumulated along the southeasterly passive margin; and hot ophiolite was obducted shortly after its formation.

In New Brunswick and Maine, a thousand kilometers southeast (length of Java), discontinuous Cambrian marine igneous rocks (509 Ma Ellsworth, 502 Ma Castine, 497 Ma Lawson Brook, 493 Ma East Scotch) of the Ellsworth and Annidale terranes (EAT) comprise bimodal assemblages largely devoid of arc or continental signature. Rare arc-like rocks, however, suggest an ensimatic back-arc setting. The structurally lower St. Croix terrane comprises, in downward order, Caradocian shales and orthoquartzites (Kendall Mountain), quartzofeldspathic wackes (Woodland), and Tremadocian black shales and basalts (Calais-Penobscot). Both assemblages display top-to-NW sense-of-shear in early structures that must be post-Tremadoc and possibly post-Caradoc. A white-weathering, pin-striped arenite from Ellsworth Falls, close to highly sheared rocks at the northwestern limit of the Ellsworth terrane, yielded dominantly 545±4Ma (n=28; 73%) and lesser younger (507±6 Ma) and older Neo- (ca. 630 Ma, 680 Ma), Meso- (1.21 Ma, 1.50 Ma), and Paleoproterozoic (1.97-2.09 Ma) zircons, an age distribution typical of Gander Zone arenites.

The Ellsworth-Annidale terranes and GRUB share similar rock assemblages, ages, and contact relationships with the Gander Zone but their emplacement histories may differ. Paleo-position of EAT and its age of emplacement remain important questions.