Crustal thickening by tectonic wedging of the Ganderian rocks, southern New England, USA: Evidence from cataclastic zircon microstructures and U–Pb ages

Zircon grains in granites from Avalonian and Ganderian rocks in southern New England host brittle internal structures that are interpreted here to show the host rocks to be allochthonous. Significant among these structures are inherited cores that are commonly fractured and even dismembered. This intense fracturing is interpreted as reflecting an important cataclastic event. The maximum age of fracturing is constrained to be younger than the age of the youngest broken magmatic core: 360 ± 4 Ma. The minimum age must be older than the oldest zircon rim that heals these fractures: that is, Early Permian. The occurrence of cataclastic zircon requires that these rocks were relatively cold (i.e. in the upper crust) during the Carboniferous and therefore were not involved in the pervasive high-grade Acadian metamorphism. The present map pattern of inter-tonguing high-grade and low-grade slices of peri-Gondwanan rocks shows that they were assembled by crustal-scale tectonic wedging. The consequent thickening ultimately led to anatexis in the lower slices during the Permian. Thus, evidence for cataclasis of zircon grains from the Avalon and Gander terranes of southern New England suggests that these peri-Gondwanan terranes remained outboard of Laurentia during the well-documented Early Devonian Acadian orogeny and were first fully accreted to North America during the Alleghanian (Permian) orogeny.