INTEGRATED BEDROCK MAPPING AND GEOCHRONOLOGY IN THE TERRANES OF SOUTHEASTERN NEW ENGLAND

Mapping supported by SHRIMP U-Pb geochronology constrains the Ediacaran to Paleozoic tectonic evolution of the three successively thrust-stacked belts in southeastern New England, including from highest to lowest, the Merrimack, Putnam-Nashoba (PN), and Avalon terranes.

In the Merrimack terrane of MA and NH, Silurian metasedimentary rocks were deformed, metamorphosed , and intruded by Silurian to Middle Devonian phases of the Ayer Granodiorite (424 ± 3, 407 ± 4 Ma, and 386 ± 3 Ma). The 424 Ma age, from a granodiorite at Eddy Pond, provides a minimum age for the Worcester Formation, negating an earlier correlation of the Worcester with the Devonian Littleton Formation of the Central Maine terrane. Late Devonian ages for the Chelmsford Granite from MA and NH (375 ± 3 and 372 ± 3 Ma) confirm the extent of this syn-tectonic D2 Acadian granite.

Detrital zircons from the Nashoba Formation of the PN indicate a depositional age between about 450 – 400 Ma with a source dominated by Cambrian to Ediacaran ages. Ages of metamorphic zircon, zircon overgrowths, and metamorphic sphene range from about 410 – 305 Ma, with peaks at about 400, 350, and 325 Ma, corresponding to Acadian, Neoacadian, and Alleghanian crystallization events. Intrusion of syn-tectonic tonalite (323 ± 3 Ma) and pegmatite (326 ± 3 Ma) as foliation-parallel sills in the Nashoba Formation indicate a significant Late Mississippian tectonothermal event.

In MA and CT, the Quinebaug-Marlboro belt (QMB) of the PN was the trailing edge of amalgamated peri-Gondwanan fragments on the Ganderian margin. Mafic rock geochemistry indicates derivation from arc and E-MORB sources. Volcanic and intrusive rocks yield Ediacaran to Cambrian ages of about 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 Avalon terrane.

The Avalon terrane in MA and RI includes metasedimentary rocks intruded by arc-related plutonic rocks including the Ironstone quartz diorite (613 ± 7 Ma), Ponagansett Gneiss (612 ± 5 Ma), Northbridge Granite Gneiss (607 ± 5 Ma), and Hope Valley Alaskite Gneiss (606 ± 5 Ma). Mapping, chemistry, and geochronology indicate that the Hope Valley shear zone is not a terrane boundary as previously proposed, but rather is an Alleghanian high strain zone that terminates in the Milford antiform.