MESOPROTEROZOIC MEGACRYSTIC AMPHIBOLITES, NEW JERSEY HIGHLANDS: PETROGENESIS AND TECTONIC IMPLICATIONS

Megacrystic amphibolites are recognized from widespread locations that cross structural domains in the Highlands along an east-west trend. They form conformable layers 0.5 to 2.0 m thick of limited strike length that lack a common lithologic association. Amphibolites contain weakly zoned subhedral plagioclase megacrysts (An_29-44) up to 13 cm long subparallel to foliation in a groundmass of medium-grained unzoned amphibole (magnesiohastingsite) and less calcic plagioclase (An_18-38). Augite (En_34-35), biotite, Fe-Ti oxides, and apatite occur locally. Mineral chemistries are fairly uniform between locations. P-T estimates are 4.8-5.5 kb and 750^o-800^o C based on Al-in-hornblende and hornblende- plagioclase thermobarometry, indicative of granulite facies metamorphism. Whole-rock analyses of the groundmass are consistent with an alkaline to tholeiitic basalt protolith (TiO₂=1.5-3.3%; Zr/Ti=0.01-0.02; Y/Nb>2.0). Low, uniform MgO (~4-6%), Mg# (40-51), and Cr and Ni (both 10-70 ppm), imply moderate amounts of crystal fractionation involving mainly olivine and plagioclase. REE patterns are LREE-enriched (La/Yb_N =3-18) and have small negative Eu anomalies (Eu/Eu*=0.76-0.91). Relatively high HREE (~8-14x chondrite), Y (20-60 ppm), and low Sc (20-35 ppm) suggest partial melting of a garnet-free mantle source at depths <65 km. Samples plot mainly in the within-plate basalt field or are transitional to MORB on tectonic discrimination diagrams. Geochemical data support magma generation through partial melting of an enriched, subduction- modified continental lithospheric mantle source. Magma was emplaced as dikes during an extensional tectonic event sometime between 1.2 and 1.1 Ga related to continental rifting or synorogenic lithospheric delamination.