GRANULITE MEMBER OF THE ERVING FORMATION, WEST-CENTRAL MASSACHUSETTS: DETRITAL ZIRCONS SUGGEST A LOCHKOVIAN MAXIMUM DEPOSITIONAL AGE
Field relations alone have not been able to resolve questions of Erving’s correlation with other units and the nature of its basal contact. For example, in the core of the Northfield-Wendell syncline, Erving displays different relationships on each limb: to the E it lies above Littleton and Fitch, which is linked to Bernardston Lochkovian conodonts. To the W it lies locally against Littleton, but elsewhere on Clough, Partridge or even Fourmile Gneiss, suggesting an onlapping unconformity or a thrust fault. Robinson and Rosenfeld (1960) recognized similarities between the Erving and Gile Mountain granulite and Standing Pond amphibolite in the “fish hook” of SE VT, linking the Erving to questions about “VT” and "NH" Silurian-Devonian strata. Published detrital zircon maximum depositional ages from the Littleton and Gile Mtn, as well as zircon ages from metavolcanics in the Littleton and Meetinghouse Slate, all fall within the range from 407 to 412 Ma.
In order to address the depositional age of the Erving, we chose a W-facing section of Clough, Littleton and Erving granulite at Round Mountain, on the E limb of the Northfield Syncline. The collected sample of Erving granulite lies approximately along the axis of the Northfield Syncline at the highest stratigraphic level within a section involving as many as 9 amphibolite layers interpreted as basaltic flows or ash falls. Our results indicate a maximum depositional age of ca. 418 Ma, which suggests these strata are coeval with or younger than the Littleton. They can be no younger than the 380 +/- 5 Ma Belchertown Pluton. Further work by TIMS dating of youngest grains may refine the results. We propose that the apparent onlap of Erving granulite in the Northfield-Wendell syncline is a primary depositional relationship that reflects synorogenic sedimentation, perhaps as a submarine fan complex within the Littleton.