WEST AVALONIAN AFFINITY OF PLUTONS IN SOUTHEASTERN-MOST MASSACHUSETTS, USA

Meta-sedimentary rocks in the vicinity of Newport, Rhode Island contain minor Mesoproterozoic detrital zircon compared to basement units in westerly and northerly portions of the Southeastern New England Avalon zone. Tonian and Cryogenian detrital suites dominating the Newport samples have lately been used to distinguish a belt of possible West African affinity extending northeast from Newport to Chatham, Massachusetts. However, characteristics of plutonic rocks within this proposed Newport block show similarities with long accepted West Avalonian intrusives.

For decades the only age constraint on Newport-area granites has been a 595 ± 12 Rb-Sr isochron obtained mainly from samples on the south side of Newport Neck. Also contributing to this isochron was a sample from Newport's Cliff Walk located to the east. Recent re-sampling of the the Cliff Walk granite now establishes a high precision CA-TIMS date that falls within errors of three ~609 Ma dates previously obtained from widespread Dedham Granite west and south of Boston, MA. Sm-Nd and Lu-Hf isotopic ratios from the dated Dedham samples give rise to similar Mesoproterozoic model ages: Nd-T_DM of 1.154, 1.121 and 1.102 Ga and Hf--T_DM of 1.2-1.1 Ga. A 1220 ± 44 Ma LA-ICPMS date from an inherited grain in the Cliff Walk granite provides direct evidence of Mesoproterozoic source rocks. Further insight about crustal contributions to Avalonian plutons comes from Fall River Granite underlying the proposed Newport block SE of the Narragansett Basin. The Fall River sample (604.39 ± 0.23 Ma via CA-TIMS) yields a Sm-Nd isotopic composition (e_Nd(600) = -0.4) modelled by mixing melted mantle with 10-15% Paleoproterozoic crust of Baltic origin. The similar signature from 606.25 ± 0.20 Ma Milford Granite bordering the western margin of Avalonia in Massachusetts argues for a single Ediacaran magmatic arc in southeastern New England.

Granite from a 1987 USGS borehole in Chatham Harbor, MA may reveal additional linkages. Two air abraded zircon fractions yielded ²⁰⁷Pb/²⁰⁶Pb dates of 601.8 ± 3.3 Ma (n = 3) and 596.4 ± 3.9 Ma (n = 1), considerably older than the reported upper intercept date of 584.0 +8.6/-7.7 Ma (obtained from batch zircon fractions not pre-treated for Pb loss). Possible correlatives to the north could include 599 ± 2 Ma Esmond Granite or 599 ± 1 Ma Westwood Granite.