RECOGNITION OF OCEANIC CRUST IN THE ADIRONDACK LOWLANDS

Ultramafic and related gabbroic rocks, retaining some primary textural and compositional igneous features, have been identified at Pyrites in the Adirondack Lowlands within a low strain enclave in a regional extensive amphibolite belt paralleling the Carthage Colton Mylonite Zone. The high Mg (up to 32%), high Cr (up to 0.5%), and low Ti (up to 1.67%) ultramafic rocks preserve relict compositional and textural layering as defined by augitic pyroxene, completely serpentinized olivine, and calcic plagioclase layers. These rocks occur along a regional, linear gravity high extending from Pyrites to the southwest. Zircons recovered from hydrothermally altered peridotite (normative 46% olivine) cut by undeformed, but strongly altered minette dykes (normative 43% olivine) yield ages of 1140+/-5 and 1202+/-10 million years. These ages are thought to represent the timing of metamorphic zircon growth due to heating of the Lowlands during a thermal pulse related to AMCG plutonic emplacement in the underlying plate (now the Adirondack Highlands) and the minimum age of crystallization, respectively. Rubidium-strontium analyses from the suite yield an isochron age of 1231 Ma and an initial ratio of 0.703. Sm-Nd model ages are complex and variable but cluster at ca. 1500 Ma. The low 147Sm/144Nd and the initial 87Sr/86Sr ratios are consistent with OIB (oceanic plateau?) setting or suprasubduction zone-generated magmas. The plutonic rocks are spatially associated with sulphitic, “rusty”, pelitic gneisses hosting widespread pyrite deposits and metamorphosed chemogenic? siliceous layers. Geochemical, isotopic, and field relations suggest this package of rocks was structurally imbricated within a shallow water carbonate sequence (marbles) during the contractional phase of the Shawinigan Orogeny (ca. 1160-1180 Ma).