Mesoproterozoic syenite in the New Jersey Highlands, USA: Petrogenesis and implications for the post-subduction transition from calc-alkaline to alkaline magmatism along the eastern Laurentian margin

Voluminous intrusions of hastingsite and/or hedenbergite syenite and quartz syenite form linear bodies up to 40 km long in the western New Jersey Highlands. Hastingsite syenite yielded a U[sbnd]Pb zircon crystallization age of 1187 ± 8 Ma, indistinguishable from previous U[sbnd]Pb zircon ages of 1182–1188 Ma obtained from spatially associated granites. Syenite and quartz syenite contain normative quartz over a range of SiO₂ from 58 to 66 wt% and are metaluminous (aluminum saturation index = 0.82–0.96), ferroan, (Fe/Fe + Mg = 0.82–0.97), alkaline, and have an A₂-type geochemical affinity. Light rare earth elements are moderately enriched [(La/Sm)_N = 1.7–3.5] relative to heavy rare earth elements [(Gd/Yb)_N = 1.4–1.9], and most syenites have positive Eu anomalies of 1.03–2.89 indicating feldspar accumulation. Crystallization temperatures calculated from zircon saturation thermometry are 934 ± 28 °C for hastingsite syenite and 925 ± 24 °C for hedenbergite syenite. Quartz syenite has a εNd(t) of +3.5 and depleted mantle model age (T_DM) of 1.34 Ga similar to granites (εNd(t) of +2.1 to +3.4; T_DM = 1.34–1.44 Ga), suggesting a comagmatic relationship. Whole-rock δ¹⁸O values of syenite and quartz syenite calculated from mineral separates are 8.5–9.3 ‰ that are higher than mantle values and consistent with a δ¹⁸O-enriched source modified by a subduction-related component. Detachment of the subducting slab led to a thermal perturbation by upwelling asthenosphere that partially melted 1.25 Ga juvenile source rocks. Our results are compatible with syenite melt production through ∼15–30 % fractional crystallization of a source comparable to ferrodiorite. Syenite records the regional transition from calc-alkaline arc-derived magmatism to potassic alkaline magmatism <40 m.y. following the cessation of subduction.