NEW DISCOVERY OF MESOPROTEROZOIC HIGH-MAGNESIUM ORTHOAMPHIBOLE GNEISS IN THE GRENVILLE OF THE NEW JERSEY HIGHLANDS

Rare Mesoproterozoic orthoamphibole gneiss (OAG) forms conformable, lens-shaped, pavement outcrops <4 m thick and 25 m long at two locations 1700 m apart in the western NJ Highlands. The northern body occurs within marble and the southern one within quartzofeldspathic gneiss. Contacts are sharp with enclosing country rocks at both locations. The mineral assemblage overlaps between locations and includes amphibole (gedrite) + oligoclase (An₁₃-An₂₀) + biotite ± fluorapatite, titanomagnetite, zircon, and rutile. Whole-rock samples have MORB-like trace element abundances and plot as tholeiitic basalt on classification diagrams. OAG are characterized by 48±2.5 wt.% SiO₂, high TiO₂ (1.9±0.8%), Al₂O₃ (18±1.2%), and MgO (12±2.3%), and very low CaO (1.9±1.3 %). Chondrite-normalized rare earth patterns are moderately enriched (La_N/Sm_N=1.6±0.7, Gd_N/Yb_N=1.4±0.5) and all samples have negative Eu anomalies (Eu/Eu*=0.68±0.1). High MgO is due to alteration as low Cr (10-200 ppm) and Ni (12-80 ppm) imply magma fractionation prior to emplacement. Alteration indices point to modification of the mafic protolith through pre-metamorphic chloritization and plagioclase dissolution. Origin of the OAG is consistent with seafloor hydrothermal alteration of a mafic volcanic, producing a Al- and Mg-rich and Ca-poor protolith that was metamorphosed to the present assemblage by 1030 Ma during Ottawan orogenesis. Lack of a common lithologic association suggests OAG were emplaced as thin dikes or sills into a sequence of marine supracrustal rocks in a back arc basin that was undergoing rifting.