Origin and evolution of the Rocky Pond slice, an enigmatic tectonic block in eastern Massachusetts

Abstract

The Nashoba terrane (NT) is a fault-bounded lithotectonic belt in southeastern New England, with the Merrimack Belt (MB) to its northwest and the Avalon terrane to its southeast. It is an early Paleozoic peri-Gondwanan arc-back arc complex and mainly consists of Cambrian, Ordovician and Silurian metasedimentary and metavolcanic rocks that were metamorphosed to amphibolite facies during the latest Silurian and Early Devonian. The MB is composed of latest Silurian or earliest Devonian metasedimentary units and Silurian to Devonian plutonic rocks. The metamorphic grade increases from greenschist facies in the southeast, adjacent to the NT, to amphibolite facies in the northwest. Both the MB and NT show strong Ganderian affinity. The Rocky Pond Slice (RPS) is located in between the MB and NT, bounded by the Clinton-Newbury Fault (CNF; a shear zone at the latitude of the RPS; ‘fault’ name adopted from previous work) to the west and the Ball Hill Shear Zone (BHSZ) to the east. It is composed of four metasedimentary units. From west to east, they are the greenschist-facies Boylston Phyllite, amphibolite-facies Boylston Schist, amphibolite-facies Sewall Hill Fm. and greenschist-facies southern Vaughn Hill Fm. They are intruded in the north by the Rocky Pond Granite. Detailed field mapping, petrographic investigation, detrital zircon U-Pb laser ablation inductively coupled mass spectrometry (LA-ICPMS) and U-Pb chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) geochronology analysis was carried out in order to unravel the origin and geological history of the RPS. The maximum depositional age of the four metasedimentary units, based on U-Pb detrital zircon LA-ICPMS analysis is interpreted as ca. 490-470 Ma. Detrital zircon populations from the Boylston Phyllite, Boylston Schist and Sewall Hill Fm. are interpreted as having been derived from a combination of Laurentia, peri-Laurentian arcs, Ganderia (Amazonia) and peri-Ganderian arcs. These three metasedimentary units are interpreted as having been deposited at the same time as, and as having had similar sources as the northern Vaughn Hill Fm. and the Tower Hill Fm., which exist along the SE margin of the MB. Zircon of the southern Vaughn Hill Fm. can be interpreted as sourced exclusively from Laurentia and peri-Laurentia arcs. It is not the same unit as the northern Vaughn Hill Fm., and its origin and tectonic history are not clear. The Rocky Pond Granite has a 395.67 ± 0.16 Ma crystallization age, based on U-Pb zircon CA-TIMS analysis. It is concluded that the RPS is not part of the NT or the MB, but a separate terrane. The Boylston Schist and Sewall Hill Fm. underwent sillimanite-zone metamorphism and partial melting, while the Boylston Phyllite and southern Vaughn Hill Fm. underwent greenschist-facies metamorphism only. The RPS was deformed by isoclinal folds overprinted by NW-plunging regional folds. The CNF and BHSZ display sinistral shear overprinted by normal shear. Deformation and metamorphism was a result of the latest Silurian to earliest Carboniferous Acadian (accretion of the Avalon terrane to the southeast) and/or the Pennsylvanian to Permian Alleghanian (formation of Pangea) orogenies. High-grade metamorphism in the Boylston Schist and Sewall Hill Fm. and possibly early greenschist facies metamorphism in the Boylston Phyllite and southern Vaughn Hill Fm., isoclinal folding in the RPS and sinistral shear along shear zones are all most likely to be a result of the Acadian orogeny, based on (1) similarities with structures, metamorphic assemblages, and ages of structures and metamorphism in the NT, (2) rare Devonian metamorphic zircon overgrowths in the RPS, and (3) intrusion of the 395.67 ± 0.16 Ma Rocky Pond Granite, which is not deformed by those structures. The ages of late greenschist facies metamorphism, NW-plunging regional folds, and normal shear are unclear.