ORIGIN OF THE QUINEBAUG-MARLBORO BELT IN SOUTHEASTERN NEW ENGLAND

The Quinebaug-Marlboro belt (QMB) represents the trailing edge of amalgamated peri-Gondwanan fragments on the Ganderian margin of southeastern New England. Mapping, geochronology, and geochemistry of the QMB on the eastern side of the Putnam-Nashoba terrane indicate the presence of Cambrian-age volcanic and intrusive rocks. The Quinebaug Formation in Connecticut and the Marlboro Formation in Massachusetts define the QMB and the eastern limit of the Putnam-Nashoba terrane on the upper plate of the Lake Char – Bloody Bluff fault, above the rocks of the Avalon terrane. Both formations consist of mafic gneisses, mafic to intermediate metavolcanic rocks, and minor felsic gneiss. Metabasalts have a wide geochemical range -- most are subalkaline tholeiites and have trace elements that resemble those derived from arc and E-MORB sources. Metabasalts show wide isotopic variations for Pb (²⁰⁶Pb/²⁰⁴Pb: 18.313-19.461; ²⁰⁷Pb/²⁰⁴Pb: 15.571-15.690; ²⁰⁸Pb/²⁰⁴Pb: 37.936-40.098), εNd (~ +1.0 to +6.2), and ⁸⁷Sr/⁸⁶Sr (~0.70413- 0.70669). Metabasalts from different parts of the QMB overlap in geochemical signatures, and point to a mixed source that included the mantle and continental crust.

SHRIMP analyses of igneous zircons yield Cambrian ages. Zircons from the interior and exterior parts of a mafic boudin in the Quinebaug Formation. yield ages of 543 ± 7 and 540 ± 6 Ma, respectively. The granitic Grafton Gneiss, dated at 515 ± 6 Ma, locally intruded mafic gneiss of the Marlboro Formation. Structurally overlying volcaniclastic rocks yield complex zircons. Oscillatory zoned cores are 501 ± 3 Ma, suggesting that part of the Marlboro Formation is younger than the Grafton Gneiss.

All sampled rocks show evidence of Late Silurian to Carboniferous metamorphism. Metamorphic zircon, zircon overgrowths, and sphene ages range from 410 – 305 Ma with peaks at about 400, 350, and 325 Ma corresponding to Acadian, Neoacadian, and Alleghanian orogenesis. In contrast, the adjacent Avalon terrane lacks Devonian and older Paleozoic metamorphic ages questioning the interpretation that the collision of Avalon led to the Devonian Acadian orogeny in southern New England.