UNRAVELLING THE PROLONGED HISTORY OF THE GANDERIAN MARGIN OF SOUTHEASTERN NEW ENGLAND

Geologic mapping and U-Pb SHRIMP and ⁴⁰Ar/³⁹Ar geochronology in southeastern New England constrain the tectonic evolution of the fault-stacked belts of the eastern Ganderian margin. From highest to lowest, the faulted Ganderian margin rocks of the Merrimack and Putnam-Nashoba (PN) terranes overly the Avalon terrane.

In the Merrimack terrane in MA and NH, Silurian sedimentary rocks were deformed, metamorphosed, and intruded by phases of the Ayer Granodiorite (424 ± 3, 407 ± 4 Ma, and 386 ± 3 Ma). The ~424 Ma granodiorite at Eddy Pond provides a minimum age for the Worcester Formation, so it does not correlate with the Devonian Littleton Formation in the Central Maine terrane. Late Devonian ages for the Chelmsford Granite (375 ± 3 and 372 ± 3 Ma) help establish the spatial extent of this syn-tectonic D2 Acadian granite. ⁴⁰Ar/³⁹Ar plateau ages of 249-246 Ma from Ms and Bt indicate rapid exhumation/cooling during Early Triassic uplift of the PN terrane along the Clinton-Newbury fault.

Depositional age constraints from detrital zircon from the Nashoba and Tatnic Hill formations (PN) include a maximum age of ~500 Ma (cores) and a minimum age of ~430 Ma (oldest metamorphic rims). Zircon populations include 1250-860 and 700-520 Ma. Metamorphic zircon and titanite, and zircon overgrowths have peaks at about ~400, 350, and 325 Ma, corresponding to Acadian, Neoacadian, and Alleghanian events. Intrusion of granite (407 ± 4 Ma), tonalite (323 ± 3 Ma), and pegmatite (326 ± 3 Ma) as foliation-parallel sills in the Nashoba Formation indicate significant Early Devonian and Late Mississippian tectonothermal events.

The Quinebaug-Marlboro belt (QMB) of the PN in MA and CT was the trailing edge of amalgamated peri-Gondwanan fragments on the Ganderian margin. Mafic rock chemistry shows two groups that occur in both the Quinebaug and the Marlboro formations: one overlaps the MORB-OIB field and one is arc-related. Pb isotopic data overlap those of Ganderia magmatic sequences in coastal Maine. Nd isotopic compositions also overlap Ganderia but the data cannot be used to preclude Avalonian contributions. Volcanic and intrusive rocks yield U-Pb zircon ages of 543 – 501 Ma. The QMB defines the eastern limit of the PN on the upper plate of the Lake Char – Bloody Bluff fault, above rocks of the Ediacaran Avalon terrane.