GEOCHEMICAL AND ISOTOPIC ENRICHED MANTLE SIGNATURE OF ADIRONDACK NEOPROTEROZOIC DIKES

A swarm of E-NE-trending undeformed and unmetamorphosed basaltic dikes crosscutting Grenville rocks across the Adirondack Region have isotopic and chemical trends indicative of derivation from enriched mantle. Dikes and flows suspected to be of similar age (ca. 600 Ma) have been noted from Labrador to Virginia and are believed to represent extension of the crust and rifting of Rodinia prior to opening of the Iapetus Ocean. Across the entire Adirondack region the dikes are alkaline, enriched in incompatible elements, average 46+/-1.86 wt% SiO₂. They lie in within-plate and ocean island basalt fields on tectonic discrimination diagrams. CIPW normative calculations indicate that some are slightly quartz normative while the vast majority are olivine normative. Magnesium numbers average ca. 0.50. A subset of dikes from the Champlain Valley, High Peaks Region, Northern Adirondacks, and the Adirondack Lowlands were selected for isotopic analysis based on minimal alteration. The dikes have positive epsilon Nd values (0.3-5.31) and Nd T_DM model ages ranging from 804-1638 Ma. Strontium (⁸⁷Sr/⁸⁶Sr) initial ratios vary between 0.7046-0.7091. The variation in Nd and Sr isotopes indicates an enriched mantle (EM1) component; however, some samples have more radiogenic initial Sr ratios. Chemical similarities and overlapping range of epsilon Nd ratios with the Catoctin basalt flows of Virginia (ca. 570 Ma) indicate similar processes and origin; however, a slightly more radiogenic ¹⁴³Nd/¹⁴⁴Nd signature, indicate differences in the mantle source or age. Comparison with geochemical trends and Nd and Sr isotopic signatures of 1150 Ma coronitic metagabbros from throughout the Adirondack Highlands, suggest derivation from the same or similar enriched mantle source. A trachyte dike (59.3 wt. % SiO2; 10.5 wt. % Na₂O+K₂O; ε_Nd – 1.72; T_DM – 1066 Ma) associated with the swarm contains zircon and is a target for U-Pb geochronology.