DOMAINS WITH GANDERIAN AND LAURENTIAN DETRITAL ZIRCON SOURCES IN SOUTHEASTERN NEW ENGLAND

The southeasternmost Ganderian terrane in southeastern New England is the Putnam-Nashoba terrane. It is a Cambrian-Ordovician arc-backarc complex. To the northwest, basement rocks are overlain by Silurian or possibly earliest Devonian sedimentary rocks of the Merrimack belt. These consist of sandstone, calcareous sandstone, pelitic rocks and turbidite sequences. The youngest detrital zircon U-Pb populations in three western units are ~438 Ma while those in the northeast are ~426 Ma. Detrital zircon U-Pb signatures show sedimentary input from both Ganderian and Laurentian sources and suggest that Laurentian input increases as the maximum depositional age decreases. The belt is interpreted as a thick cover sequence on Ganderian basement.

The Rocky Pond Slice is a ~10 km long NE-trending block along the margin between the Nashoba terrane and Merrimack belt, between splays of the Clinton-Newbury fault. It consists of the generally high-grade Sewall Hill Formation and Boylston Schist in the central and western parts, adjacent to low-grade rocks of the Merrimack belt, and the low-grade Vaughn Hill Formation, adjacent to high-grade rocks of the Nashoba terrane. The metamorphic grade of the Boylston Schist decreases from upper amphibolite to greenschist facies over ~500 m along the western margin of the slice. The three units are intruded by the ~396 Ma Rocky Pond Granite. Detrital zircon populations from two Boylston Schist samples and one Sewall Hill Formation sample suggest Laurentian and Ganderian sources. The youngest zircon age population for the three samples together is ~486 Ma. The fact that no evidence for younger populations exists suggests that these rocks may be Ordovician. The Tower Hill Formation immediately west of the Rocky Pond Slice and the northern Vaughn Hill Formation ~ 8 km north of it yielded ~530 Ma and ~463 Ma youngest zircon populations, respectively, and similar sources as the Boylston Schist and Sewall Hill Formation. Zircon of the southern Vaughn Hill Formation yielded a ~473 Ma youngest zircon population, but its detrital zircon population suggests sources from the Laurentian margin only, and is not consistent with the mixed Laurentian-Ganderian sources of the other units. The origins of any of the above units are not clear, but they may have been emplaced along the Clinton-Newbury fault.