AGE AND TECTONIC SETTING OF THE QUINEBAUG-MARLBORO BELT, SOUTHEASTERN NEW ENGLAND

The Quinebaug-Marlboro belt (QMB) represents the trailing edge of the peri-Gondwanan Ganderian margin exposed in the Putnam-Nashoba terrane of southeastern New England. New geologic mapping, SHRIMP U-Pb geochronology, and geochemistry of the QMB indicate the presence of Cambrian-aged volcanic and intrusive rocks. Igneous zircon ages from a boudin of mafic gneiss in the Quinebaug Fm. are 540 ± 6 and 539 ± 7 Ma. The granitic Grafton Gneiss, dated at 515 ± 6 Ma, locally intruded mafic gneiss of the Marlboro Fm. Volcaniclastic granofels in the Marlboro Fm., structurally above the Grafton Gneiss, yielded igneous zircon dated at 501 ± 4 Ma, showing that part of the Marlboro Fm. is younger than the Grafton Gneiss. Detrital zircon from migmatitic metapelites in the overlying Nashoba Fm. and correlative Tatnic Hill Fm. yield detrital age populations as young as 540-500 Ma, constraining the maximum age of deposition. The oldest metamorphic rims on detrital zircon formed at about 435 Ma, constraining the minimum age of deposition. Provenance ages of the metapelites are similar to rocks of Ganderian and Avalonian affinity, with abundant grains yielding ages between about 500 and 650 Ma suggesting dominant Ediacaran - Cambrian sources, most likely from the underlying QMB. The Quinebaug Fm. must be older than about 418 ± 3 Ma, based on a new age from diorite of the cross-cutting Preston Gabbro.

Mafic rocks from the QMB are subalkaline tholeiites with trace elements that resemble those derived from arc and E-MORB sources. Some trace element data show significant negative Nb-Ta anomalies, similar to arc-like rocks, whereas others show no negative Nb-Ta anomaly and are more similar to rocks from E-MORB to OIB or back-arc settings. Geochemical signatures point to a mixture of sources that include both mantle and continental crust. All sampled rocks contain Late Silurian to Carboniferous metamorphic zircon overgrowths. Metamorphic zircon and sphene ages range from about 410-305 Ma with peaks at 400, 350 and 325 Ma. Age and chemistry support regional correlations with parts of the Penobscot arc/back-arc system in Maine, New Brunswick, Nova Scotia, and Newfoundland. Rocks in the Ellsworth terrane in coastal Maine offer the best regional correlation for the QMB.