Age and origin of the Cortlandt intrusive complex, New York State: A detailed look at the Stony Point and Rosetown plutons

The Cortlandt Complex is a classic alkaline intrusive complex exposed along the Hudson River in southeastern New York. Early work by James Dana in the 1880s set the stage for over 100 additional studies over the next century. Studies in the past few decades have focused on constraining the genetic relationships between phases of the intrusive complex and the timing of emplacement relative to peak metamorphism and deformation during the Taconic orogeny.

LA-MC-ICP-MS U/Pb zircon emplacement ages were obtained on five samples from the Cortlandt Complex. The five samples yielded dates ranging from 443 to 454 Ma, with an overall weighted mean emplacement age of 447.1 +/- 4.5 Ma. Three of the five samples contained inherited zircons, nearly all of Mesoproterozoic age. A sample of the adjacent Annsville Phyllite contained a diverse population of detrital zircons. The most common ages are between 1.25 and 1.85 Ga, although zircons of Neoarchean age are also present.

The westernmost and relatively small Rosetown and Stony Point plutons are lithologically and compositionally diverse. Major and trace element trends indicate a complex petrogenesis, which is supported by variable initial isotope ratios. The geochemical data indicate that the mafic magmas were derived primarily from an asthenospheric mantle source, while the inherited zircons and more radiogenic isotope ratios of the silicic units imply a significant crustal component was involved in their formation.

The timing and composition of intrusions in the Cortlandt Complex indicate that they are not the result of subduction related to the Taconic Orogeny. We suggest that the mafic units are the result of post-collisional, slab-failure induced asthenospheric upwelling, while the silicic units are primarily partial melts of Grenville-age, garnet amphibolites in the lower crust.