Sr and Nd isotopic compositions, age and petrogenesis of A-type granitoids of the Vernon Supersuite, New Jersey Highlands, USA

Voluminous late Mesoproterozoic monzonite through granite of the Vernon Supersuite underlies an area of approximately 1300 km(2) in the Highlands of northern New Jersey. The Vernon Supersuite consists of hastingsite +/- biotite-bearing granitoids of the Byram intrusive Suite (BIS) and hedenbergite-bearing granitoids of the Lake Hopatcong Intrusive Suite (LHIS). These rocks have similar major and trace element abundances over a range of SiO2 from 58 to 75 wt.%, are metaluminous to weakly peraluminous, and have a distinctive A-type chemistry characterized by high contents of Y, Nb, Zr, LREE, and Ga/Al ratios,and low MgO, CaO, Sr and HREE. Whole-rock Rb-Sr isochrons of BIS granite yield an age of 1116 +/- 41 Ma and initial Sr-87/Sr-86 ratio of 0.70389, and of LHIS granite an age of 1095 +/- 9 Ma and initial Sr-87/Sr-86 ratio of 0.70520. Both suites have similar initial Nd-143/Nd-144 ratios of 0.511267 to 0.511345 (BIS) and 0.511359 to 0.511395 (LHIS). Values of epsilon(Nd) are moderately high and range from + 1.21 to + 2.74 in the BIS and + 2.24 to + 2.95 in the LHIS. Petrographic evidence, field relationships, geochemistry, and isotopic data support an interpretation of comagmatism and the derivation of both suites from a mantle-derived or a juvenile lower crustal parent with little crustal assimilation. Both suites crystallized under overlapping conditions controlled by P-T-f(H2O). Lake Hopatcong magma crystallized at a liquidus temperature that approached 900 degrees C and a pressure of about 6 kbar, and remained relatively anhydrous throughout its evolution. Initial P-T conditions of the Byram magma were greater than or equal to 850 degrees C and about 5.5 kbar. BIS magma was emplaced contemporaneous with, or slightly preceding LHIS magma, and both magmas were emplaced during a compressional tectonic event prior to granulite facies metamorphism that occurred in the Highlands between 1080 and 1030 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.