A NEW INTERPRETATION OF THE MALTBY LAKES "VOLCANICS" AND RELATED ROCKS, WESTERN NEW HAVEN QUADRANGLE, CONNECTICUT, USA

New mapping in the Paleozoic rocks in the Orange-Milford belt of south-central Connecticut has led to the identification of fault-bound slices of schists, breccias, mylonites, and polydeformed mafic and ultramafic rocks. Structures previously interpreted as bedding are now understood to be metamorphic foliations, and field relationships indicate fault or intrusive contacts for all map units in the area.

The southern-most unit, the Savin schist, a multiply deformed sandy argillite, extends north to include the muscovite-, chlorite-, and quartz-rich schist east of the Maltby Lakes. Here, mylonites and foliated breccias define a fault contact between the Savin schist and the Maltby Lakes “volcanics.”

Serpentinite outcrops at the eastern extent of the Maltby Lakes unit. Pillow-like structures pervade further north. These open the possibility that the Maltby Lakes unit constitutes a slice of oceanic crust. Existing geochemistry supports N-MORB affinity. The mafic to ultramafic Maltby Lakes schists and mylonites bear a complex metamorphic history. Amphibole porphyroclasts with Si contents greater than 7.2 p.f.u. are enveloped in amphibole mylonites with Si contents as low as 6.5 p.f.u., suggesting prograde deformation prior to juxtaposition with the Savin, and to dissection by the Allingtown dyke swarm.

The Allingtown porphyry synkinematically intrudes both the Maltby Lakes and Savin units. Though in places the rock is deformed only brittly, and retains igneous flow textures, the relic plagioclase + pyroxene assemblage has been almost entirely saussuritized/uralitized. Further, the Allingtown porphyry is pervasively schistose, containing bands of reaction-softened chlorite + amphibole + albite. Where contact with country rock is exposed, the dykes boudinage and are transposed into the dominant schistosity. Tectonic inclusions of the Allingtown can be found throughout the Maltby Lakes package.

Contact between the Maltby Lakes amphibolite and the Wepawaug slate to the north is covered in New Haven quadrangle, but attenuated and folded veins and dextral shear sense indicators increase toward the contact on either side. These structures plus the abrupt drop in metamorphic grade into the Wepawaug Schist show this also to be a fault contact.