POSSIBLE AMCG-RELATED MEGACRYSTIC AMPHIBOLITES AND META-ANORTHOSITIC SHEETS IN THE NEW JERSEY HIGHLANDS

Megacrystic amphibolite dikes from five locations and meta-anorthositic sheets from two locations in the western New Jersey Highlands (Young and Icenhower, 1989; Volkert and Gorring, 2001) appear to be related based on field relations and geochemistry. Both rock types are intrusive into the New Jersey Highland supracrustal package and are themselves intruded by 1100 Ma granitoids of the Vernon Supersuite and are penetratively foliated. Megacrystic amphibolites appear to be analogous to mafic rocks associated with anorthositic rocks in the Adirondack Highlands (Olsen, 1992). Both have high TiO₂ (2.5-3.5%), Al₂O₃ (14-19%), FeO_T (10-16%) and comparable trace and REE abundances that form a distinct Al-Fe mafic magma type characteristic of anorthosite associations. New Jersey Highland megacrystic amphibolites are also geochemically similar to anorthositic hornblende gabbro dikes from the Honey Brook Upland (Crawford and Hoersch, 1984). Meta-anorthositic sheets (60-240 m thick) are mainly medium-grained, well-foliated gneisses and folded into upright to slightly overturned synforms. The rocks are extremely leucocratic with abundant plagioclase (60-80%) and subordinate hornblende and/or biotite. Whole-rock geochemistry of the meta-anorthositic sheets are characterized by high Al₂O₃ (20-25%) and CaO (8-11%); low MgO (1-3%) and K₂O (<1.5%); SiO₂ between 50-55%, and significant positive Eu anomalies (Eu/Eu* up to 3.8). Megacrystic amphibolites are tentatively interpreted to have formed coeval with meta-anorthosite in the New Jersey Highlands through plagioclase separation from a common mafic magma ponded at the base of the crust. Premature tapping of this magma by extensional fractures may have interrupted the extensive fractionation and plagioclase separation necessary to form voluminous anorthosite intrusions. Instead, this process resulted in emplacement of meager amounts of anorthosite as thin sills and megacrystic amphibolites as thin dikes. Based on similar field relations, geochemistry, and spatial associations of anorthositic rocks in the New Jersey and the Adirondacks, we interpret the former to be coeval with AMCG magmatism in the Adirondacks Highlands dated (ca. 1145; McLelland et al., 2001).