The bedrock geology of western New Haven quadrangle, Connecticut

Abstract

The Paleozoic rocks in the western third of the New Haven Quadrangle, Connecticut, were mapped at a scale of 1:12,000. This previously unmapped area was targeted as it contains some of the best exposures of the Orange-Milford Belt (OMB), a sliver of argillites and mafic rocks of anomalously low metamorphic grade--and of unknown age and tectonic affinity--wedged between kyanite/sillimanite-grade peri-Laurentian to the west and anatectic peri-Ganderian rocks to the east.

This mapping identifies greenschist facies muscovite-chlorite schists in the map area: the Wepawaug Formation to the northwest and the Savin schist to the southeast. Newly identified in the Maltby Lakes Complex (MLC) are mafic and ultramafic rocks with trace element geochemical signatures consistent with formation in a juvenile ocean-ocean arc system. These rocks preserve a metamorphic history that predates the regionally dominant Acadian fabrics. Lozenges of gneissic amphibolite are enveloped by epidote-rich amphibole mylonites in the eastern exposures of the complex. Higher Al and Ti contents in the mylonitic amphiboles relative to the protolith suggest the mylonites developed under prograde metamorphic conditions. Pods of coarse-grained, MORB-affinity metagabbros are wrapped by an (Acadian?) anastomosing chlorite fabric in the western and northern exposures of the MLC. Both the mafic mylonites and the southern Savin schist are intruded by a swarm of MORB-affinity stitching basalt dikes, the Silurian (?) Allingtown porphyry, which is itself commonly schistose, and locally mylonitic.

These results strongly contradict earlier interpretations (Fritts, 1965; Rodgers, 1985) that interpreted the Orange-Milford Belt as a normal stratigraphic sequence topping to the NW. Rather, these entirely lithodemic units are low-grade early to middle Paleozoic forearc sediments enclosing high-grade, subduction-related oceanic crustal rocks, repeatedly intruded by mid-ocean ridge basalts. Existing thermochronology indicates a Devonian age of the regional greenschist facies metamorphism and a Permian age of fabric overprinting, ductile faulting, and terrane assembly.