TWO DISTINCT OCEANIC TRACTS PRESERVED BETWEEN PERI-LAURENTIAN AND PERI-GONDWANAN TERRANES IN SOUTH CENTRAL CONNECTICUT

The Orange Milford Belt (OMB) in south central Connecticut is a complex of meta-argillites and metabasalts wedged between peri-Laurentian and peri-Gondwanan terranes. Its easternmost extent in New Haven quadrangle is host to the best exposure of two temporally distinct bodies of mafic rocks. The older Maltby Lakes complex (O?m) includes gneissic amphibolite enveloped by epidote-amphibole mylonite. Increasing concentrations of Al and Ti from core to rim and into beards and necks of broken grains indicate the fine-grained fabric developed from the gneissic fabric under prograde metamorphic conditions.

Both the gneissic fabric and the mylonite are intruded by 0.04-10.0 m dykes of the Allingtown basalt porphyry (S?a). The magmatic assemblage of Sa has been wholly replaced by amphibole + clinozoisite-epidote + sodic plagioclase ± chlorite. In most localities the unit has developed a moderate to strong foliation, and contact with the country rock is commonly transposed to near parallel with the regional foliation. Nevertheless, primary structures are commonly preserved. These include massive textures with ≤7 mm plagioclase phenocrysts as much as 75 volume % of the rock, and ≤5 cm uralitized oikocrysts. In some few outcrops, dykes of Sa are observed to intrude earlier Sa dykes, with chill margins increasing in phenocryst density with distance from the contact before being truncated again by another dyke. These structures are characteristic of sheeted dyke complexes.

Tectonomagmatic discrimination based on high field strength elements (Zr, Ti, V) reveals fractionation trends in both bodies that are consistent with origination at mid-ocean ridges. That the Maltby Lakes complex preserves a complete clockwise P-T path that predates the intrusion of the Allingtown porphyry requires that the units originated in different mid-ocean ridges.

The Allingtown porphyry may correlate with the Comerford dykes in New Hampshire (Rankin et al., 2007), which are associated with a brief period of Silurian extension. The earlier gneissic and prograde mylonitic fabrics in the Maltby Lakes package are too far east to have been formed during the Taconic orogeny, compelling the interpretation that their presence is evidence of Salinic deformation in southern New England.