EDMAP BEDROCK GEOLOGIC MAP OF ACCRETED TERRANES, WESTERN NEW HAVEN QUADRANGLE, CT, USA

The crystalline Paleozoic rocks in the western third of the New Haven quadrangle, CT, were mapped at a scale of 1:24,000 and compiled in ArcGIS. 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 intrusives of anomalously low metamorphic grade—and of unknown age and tectonic affinity—wedged between sillimanite-grade peri-Laurentian and anactectic peri-Ganderian rocks to the west and east, respectively.

Our mapping confirms the presence of muscovite-chlorite schists in the N and S extents of the map area. Newly identified in the centrally-located Maltby Lakes greenstones are lozenges of coarse-grained metagabbros and strongly-lineated, clinozoisite-rich amphibolites enveloped within epidote-rich amphibole mylonites. High Al and Ti contents of the amphiboles show the mylonites developed under amphibolite facies conditions. The mylonitic fabric is increasingly folded and/or crenulated toward its ductile fault contact with the polydeformed Ordovician (?) schist to the SE. Along the greenstones’ northern boundary, chlorite-rich mylonites and pinstriped quartz veins indicate a ductile fault contact with the Devonian (?) phyllite.

Both the mafic mylonites and the southern schist are intruded by a swarm of stitching basalt dykes, the Silurian (?) Allingtown porphyry, which is itself commonly schistose, and locally mylonitic. Tectonomagmatic discrimination diagrams based on high field strength elements (Ti, Zr, Y) strongly support an N-MORB affinity for the basalt porphyry. All of these rocks are nonconformably overlain by Triassic arkoses. A Jurassic diabasic dyke intrudes the greenstones, and late brittle Mesozoic faults marked by breccias and fabric offsets cut the entire map area.

Our results strongly contradict earlier interpretations (Rodgers, 1985) that called for the OMB to constitute 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 asthenospheric melts. Existing thermochronology indicates a Devonian or older age of the regional metamorphism, and a Permian age of ductile faulting and terrane assembly.