Brooklyn Injection Gneiss Complex: Geochemical and tectonic synthesis

Middle Proterozoic rocks of the granulite facies Brooklyn Injection Gneiss (BIG) Complex represents a diverse suit of rocks associated with the stepping back of the Grenville subduction zone and subsequent development of an island arc/back-arc basin situated along the Laurentian margin at ca. 1,035 to 980 Ma. A depleted mantle model of partial melting and subsequent fractionation producing LIL element (K, Rb, Ba) enrichments with HFS element (Ta, Nb, Zr, and Ti) depletions, similar to that of modern island arcs, is responsible for generating the Ravenswood bimodal suite which has an intrusive relationship to the Fordham Gneiss units of the Manhattan Prong. Magmas with calc-alkaline signatures were generated during initial arc/back-arc development where the role of the previous New Jersey Highlands calc-alkaline magmatism associated with the sub-continental lithosphere was still influential. As the basin matured and widened transitional N-type MORB and tholeiitic island arc basalts entered the basin via mantle upwelling. The transformation of the Laurentian margin from a passive continental margin to a back-arc depositional setting is reflected in the variability and character of the Fordham gneiss lithologies as well as their associated REE profiles. The establishment of a volcanic arc/back arc ophiolite complex along this section of the Laurentian margin during the Grenvillian orogeny, suggests that the end stage Grenvillian event did not involve a continent-continent collision but an island arc-Laurentia collision at ca. 960 Ma. It is suggested that the New York Recess is a pre-Grenvillian feature centered on a persistent mantle plume associated with repeated cycles of compressional orogenics and continental breakups. Reactivation of the Ramapo - East River border fault system is responsible for the development and collapse of both the Middle Proterozoic Fordham and Paleozoic Manhattan marginal basins. Taconian overthrusting of the Middle Proterozoic BIG Complex is conducted along a low angle detachment in a similar manner to other Appalachian models. This study suggests that eastern seaboard complexes, with volcanic arc/back-arc ophiolitic affinities that have not been carefully dated, may need to be reevaluated for a possible Middle Proterozoic association.