Paper No. 35-0
MESOPROTEROZOIC MEGACRYSTIC AMPHIBOLITES, NEW JERSEY HIGHLANDS: PETROGENESIS AND TECTONIC IMPLICATIONS VOLKERT, Richard A., New Jersey Geol Survey, P.O. Box 427, Trenton, NJ 08625,
[email protected] and GORRING, Matthew L., Department of Earth & Env. Studies, Montclair State Univ, Upper Montclair, NJ 07043 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 (An29-44) up to 13 cm long subparallel to foliation in a groundmass of mediumgrained unzoned amphibole (magnesiohastingsite) and less calcic plagioclase (An18-38). Augite (En3435), 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 750o-800o C based on Al-in-hornblende and hornblendeplagioclase 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/YbN =3-18) and have small negative Eu anomalies (Eu/Eu*=0.760.91). 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
