A thick, lithologically varied sequence of high-grade metamorphic rocks, pre-Acadian in age, has been delineated in the Southbridge, Wales, Westford, Eastford, Stafford Springs, and Monson quadrangles of south-central Massachusetts and adjacent Connecticut. The sequence consists largely of metamorphosed aluminous mudstone and siliceous siltstone. Metamorphosed limy mudstone, euxinic mudstone, fine-grained volcaniclastic debris, and lava form marker units that may extend tens of kilometres laterally. The sequence is divided into four formations, each containing many lenticular units. Three formations in the southeastern part of the area are separated by thrust faults and apparently top westward in an imbricate homoclinal sequence. The fourth and youngest formation is in isoclinal synclines in a faulted block that parallels the eastern border of the Monson Gneiss. The observed rock sequences contain rocks similar in gross lithology to sequences present in east-central Massachusetts and southern New Hampshire.

Major-element chemical analyses of 130 samples of high-grade plagioclase gneiss, together with geologic data, suggest that metadacitic gneisses of the late Proterozoic Avalon and Putnam-Nashoba terranes include four extrusive and two intrusive units formerly mapped as Monson Gneiss and Middletown Formation. Evidence for an igneous rather than sedimentary protolith for these gneisses comes from the mineralogic and chemical homogeneity, and the consistency of the chemical variations within individual bodies of gneiss with fractional crystallization processes. Evidence for the extrusive origin of some units comes from interlayered contacts with metasediments and metarhyolitic (alaskitic) and metatholeiitic (amphibolitic) volcanics, and from a 20- to 50-m-scale chemical cyclicity, typical of modern ash-flow tuffs. Evidence for an intrusive origin of two bodies comes from abundant amphibolite and calc-silicate xenoliths, from a massive structure lacking compositional layering, and from kilometer-scale chemical zoning or homogeneity. The identification of some units as lithodemic orthogneisses indicates that intrusion as well as extrusion led to the lithologic sequences in the Avalon terrane of southeastern Connecticut. It also precludes the lithostratigraphic correlation of these orthogneisses with paragneisses. Given this geochemical and geologic evidence, we believe that previously advanced stratigraphic arguments that major recumbent folds relate gneiss bodies now interpreted to be intrusive and extrusive should be reexamined. We believe that the part of the Avalon terrane consisting of metavolcanic rocks may be a right-side-up volcanic pile, and that major recumbent folding may not be present.

Abstract During the 1970s, geologists considered that the Upper Ordovician Taconic Orogeny represented the collision of Laurentia with the Ammonoosuc arc, now largely exposed on the Bronson Hill anticlinorium. Subsequently, several researchers noted that magmatic rocks which intrude and overlie the Ammonoosuc arc are younger than the c. 455–451 Ma Taconic Orogeny. This led them to hypothesize that a Middle Ordovician collision was followed by westward-dipping subduction beneath the amalgamated Laurentian–Ammonoosuc zone to produce the younger arc rocks. In this model, the Taconic allochthons and foredeep were produced later in a retro-arc setting above westward-dipping subduction. However, those models prove inadequate due to the lack of ash beds, foredeep sedimentation and deformation on the Laurentian platform prior to the Upper Ordovician Taconic Orogeny. Here, we resolve the dilemma by recognizing that the magmatic rocks, which post-date the 455–451 Ma Taconic Orogeny, are not arc rocks but, instead, typical post-collisional slab-failure rocks as old as 450 Ma, with Sr/Y > 10, Sm/Yb > 2.5, Nb/Y > 0.4 and La/Yb > 10. Thus, in New England and western New York, the Upper Ordovician Taconic Orogeny represents the collision of the Ammonoosuc arc with Laurentia followed by slab failure of the descending plate.

ABSTRACT Studies of impact structures in Sweden date back almost 60 years. They have so far resulted in the confirmation and understanding of eight impact structures and one impact-derived breccia layer, including the largest confirmed impact structure in the western part of Europe, the Siljan impact structure. Several additional structures have been proposed as impact derived, but they have to date not been confirmed. In this contribution, I summarize the current state of knowledge about the impact cratering record of Sweden. This is an up-to-date, comprehensive review of the features of known impact structures (and impact-related deposits) in Sweden. The described impact structures formed over a time period spanning from the Cambrian to the Cretaceous, and the preservation of several small (~1–2 km in diameter) Paleozoic impact structures indicates that the conditions securing their protection were close to optimal, with formation in a shallow epicontinental sea and rapid cover by protective sediments followed by a regional geologic evolution permitting their preservation. The generally well-preserved state of some of these crater structures contradicts the general assumption that such small impact structures can only be preserved for approximately a couple of thousand to a few million years. The Lockne-Målingen, Tvären, Granby, and Hummeln impact structures all have ages that place their formation in a period of proposed increased cratering rate on Earth following the breakup event of the L-chondrite parent body in the asteroid belt. However, to date, evidence other than a temporal correlation is missing for all of these structures except for Lockne (and Målingen), which has been shown to have formed by the impact of an L-chondritic body.