Tholeiitic to transitional alkalic basalt and basaltic tuff form widely separated but distinctive units within the Nassau Formation of late Proterozoic or Early Cambrian age, within the Rensselaer Plateau and Chatham slices of the Taconic allochthon in eastern New York State and western Massachusetts. Examination of all known occurrences of these basaltic rocks and detailed mapping of the enclosing strata indicate that these basalts are submarine lava flows and water-transported tuffaceous deposits restricted to the lower third of the stratigraphic section in both slices. In each slice, the basalts thicken to the west along with thickening and coarsening of enclosing graywacke beds (Rensselaer Graywacke Member of the Nassau Formation) and thin eastward into pillow basalt and tuffaceous basalt that is associated with fine-grained graywacke and thinly laminated purple and green turbidites of the Nassau Formation.

These high-TiO2, low-MgO basalts resemble very closely late Proterozoic basalts and feeder dikes of the Catoctin Formation of the Blue Ridge in Virginia and late Proterozoic metadiabase dikes in the northern Reading Prong in New York, both of which intrude pre-Iapetan, eastern North American basement. Major-element and rare-earth chemistry and geology also establish a correlation between the Taconic metabasalts and metabasalts preserved in the Tibbit Hill Volcanic Member of the Pinnacle Formation that unconformably overlies middle Proterozoic basement, in central Vermont. Palinspastic reconstruction places the Chatham slice oceanward of the Rensselaer Plateau slice that contains abundant coarse-grained detritus of probable eastern North American Proterozoic basement These relations suggest a fault-bounded submarine basin (Nassau-Rensselaer basin) located near the continental margin that was fed by submarine fan deposits. Submarine morphology of the distributing fan complex may have channelized the basalts. The association of the tholeiitic to transitional alkalic basalts with interpreted marine-fan deposits suggests that the volcanism occurred after Iapetan rifting and thermal subsidence over tectonically thinned sialic crust, perhaps near the time and place of eventual separation.

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