Evidence for an enriched asthenospheric source for coronitic metagabbros in the Adirondack Highlands

Abstract

Coronitic metagabbros (CMGs) in the Adirondack Highlands display chemical and isotopic features consistent with derivation from an enriched asthenospheric mantle source and are samples of the parental magma of Adirondack anorthosite. Primary ophitic textures in the CMGs are overprinted by mineral coronas developed during granulite-facies metamorphism of anhydrous olivine gabbronorites during the Ottawan orogeny. Restricted in silica content (45–48 wt%) and olivine normative, the CMGs are predominantly tholeiitic in composition, although a minority display some calc-alkaline features. Unlike older Adirondack mafic and felsic suites, the CMG rocks lack or have greatly reduced, incompatible element patterns (N PM ) generally associated with subduction processes. Rare-earth elements (N CH ) have minor light rare-earth element (LREE) enrichment with La/Sm values from 1.42 to 1.98, compatible with a transitional to enriched mantle source. When CMGs are plotted on various major (TiO 2 versus P 2 O 5 ) and trace-element (Sm versus Cr) diagrams, the CMGs form a continuous field between that of oxide- and apatite-rich gabbros (OGNs and OAGNs) and anorthosites and leucogabbros within the Adirondacks. Initial epsilon Nd (e Nd ) values are +3.13 to +3.69, generally higher than Adirondack anorthosite values, but significantly less than contemporaneous depleted mantle. Neodymium T DM model ages that are ∼400 million years older than their crystallization age and enriched compositional trends indicate that they were not derived from depleted mantle. These data indicate Adirondack CMGs were derived from a previously untapped and enriched asthenospheric source. Asthenospheric upwelling was triggered by lithospheric delamination following Shawinigan orogenesis at ca. 1160 Ma and provides a link between tectonism, mantle geodynamics, and massif anorthosite petrogenesis in the Grenville allochthonous monocyclic belt.