CHARACTERISTICS, AGE, GEOCHEMISTRY, AND MINERALIZATION OF A POSTOROGENIC FELSIC MAGMATIC SUITE, NEW JERSEY HIGHLANDS

A suite of largely pegmatitic granitic to monzogranitic postorogenic rocks is widespread in the New Jersey Highlands. These rocks occur as tabular, unzoned intrusions that are undeformed, contain xenoliths of country rock, and crosscut synorogenic structures formed during 1090-1030 Ma Ottawan metamorphism. U-Pb zircon (TIMS) ages of 1005 to 965 Ma indicate that this postorogenic magmatic activity extended for at least 65 Ma past the regional metamorphic thermal peak. A ⁴⁰Ar/³⁹Ar hornblende plateau age suggests temperatures in the Highlands remained above about 500^o C until 919 ± 5 Ma during regional uplift and unroofing. The postorogenic rocks are metaluminous to mildly peraluminous (ASI=0.84-1.07), have high Si, K, Na, and low Al, Ca, and Mg. Preliminary geochemical data suggest an affinity to 1.1 Ga A-type granites that occur in the Highlands. Postorogenic rocks correspond to the NYF (Nb-Y-F) class and the allanite-monazite subclass of rare earth pegmatites of Cerny (1992). Postorogenic magmatism likely formed in response to extension and delamination following collisional overthickening, through partial melting of lower crustal rocks similar to the source for the A-type granites. Postorogenic rocks <990 Ma contain magnetite±U±Th±Y±REE mineralization, produced from coeval magmatic and slightly younger hydrothermal fluids. Early hydrothermal activity occurred before 900 Ma based on the scarcity of low-T mineral assemblages; later low-T, F-rich fluids reflect a Neoproterozoic event. Post-Ottawan mineralization documents a metallogenic event in the Highlands at ~990 to 940 Ma, coeval with U±Th±REE mineralization at 975 to 950 Ma in the NY Hudson Highlands and 980 to 937 Ma in the Canadian Grenville Province.