TACONIAN DELAMINATION IN THE MANHATTAN PRONG? ZIRCON AGES OF MAFIC COMPLEXES WITH ALKALI BASALT AFFINITY, AND THE TIMING OF PEAK GRANULITE- FACIES (OPX-SILL) METAMORPHISM

New LAM-ICPMS zircon ages indicate that mafic bodies with alkali basalt affinity intruded the Manhattan Prong synchronously with the Taconian Orogeny, but before peak metamorphic temperatures were attained. Peak metamorphism was in the granulite facies, at temperatures higher (>850° C) than reached elsewhere in the Appalachians. Even higher temperatures (~950°C) were reached locally where metapelitic cordierite-quartz K-feldspar-garnet-sillimanite-magnetite gneiss was partially replaced by orthopyroxene + sillimanite. Most small (50 - 100 micron) zircons separated from this gneiss show a Grenville component, with a mean inheritance age of 1244 ±110 Ma from detrital zircons, and a lower-intercept age of 434 ±39 Ma. Three near-concordant analyses (considered alone) give a 436 ±0.83 Ma [MSWD=0.14] concordia age, or a 436 ±25 Ma ²⁰⁶Pb/²³⁸U age.

Gabbro from the Stony Point body of the Cortlandt Complex gives a ²⁰⁶Pb/²³⁸U age of 461 ±12 Ma (weighted mean of 5 analyses); metagabbro of the Bedford Complex has a ²⁰⁶Pb/²³⁸U age of 449 ±6.4 Ma (weighted mean of 10 analyses) or a concordia age of 450 ±3.9 Ma [MSWD=0.13]. These two complexes show within-plate, alkali basalt affinities and chemically resemble each other, although the Cortlandt Complex is more fractionated and more crustally contaminated than the Bedford Complex. Other syn-Taconian mafic bodies scattered along the eastern Manhattan Prong (Croton Falls, Hodges) share similar within-plate, alkali basalt affinity. These bodies, and the Bedford Complex, were deformed and partially recrystallized during the later stages of the Taconian Orogeny, indicating that within-plate alkaline igneous activity accompanied (or was interspersed with) Taconian deformation on a regional scale.

We suggest that the alkaline igneous activity and extreme metamorphic temperatures experienced in the Manhattan Prong may have had a common origin: lithospheric delamination during the Taconian orogeny. The stage was set for this event when a mantle plume thinned and underplated the region’s lithosphere during the Late Neoproterozoic.