ISOTOPIC, GEOCHEMICAL, AND PETROGRAPHIC CONSTRAINTS ON CORONITIC METAGABBROS OF THE ADIRONDACK HIGHLANDS

Isotopic, geochemical, and petrographic analysis of coronitic gabbros constrain their primary igneous signature and provide insight into the ca.1150 Ma AMCG magmatism in the Grenville Province. The suite forms dikes (e.g., Colden trap dike), lenses, or chimney-like bodies within, or in close proximity to, massif anorthosite. They yield coeval U-Pb zircon ages and likely represent samples of the unfractionated source of massif anorthosite. Medium to coarse grained with ophitic texture, these bodies have corona growth around primary olivine, Fe-Ti oxides, and commonly exhibit spinel-clouded, zoned plagioclase. Coronas consist of concentric garnet, biotite, hornblende, and pyroxene rims. Whole rock chemical analysis constrain the gabbro’s source and yields a fairly flat incompatible trend when normalized to primitive mantle, lacking the subduction-related signature seen in slightly older (1180-1200 Ma) Shawinigan arc plutonic suites. Silica ranges from 41.74 to 48.1 % and MgO ranges from 5.15 to 20.27%. Coronites show a tholeiitic to slightly alkaline, within-plate, and enriched composition, while REE’s are 50-100 times chondritic values. Chrondritic normalized La/Sm ranges from 1.42 to 1.98 and coronites have higher Sm/Nd ratios than that of the anorthosite. E_Nd(T) range from +3.13 to +3.69 (anorthosite = +2.19), plotting well below depleted mantle at 1150 Ma, indicating that coronites are not derived solely from a depleted mantle source. T_DM range from 1548-1662 Ma, significantly older than ~1400 Ma typically of other AMCG rocks. Strontium ratios preserve a metamorphic signature providing a poorly constrained Ottawan isochron age (ca. 1050 Ma), and are highly variable in comparison to the Nd data. This evidence suggests the source of coronitic gabbros is melting of enriched, primitive asthenospheric mantle which ponded under the crust during lithospheric delamination following Shawinigan orogenesis. Spatially and temporally related granitic rocks formed as delamination swept eastward and gabbroic magma ponded beneath Laurentia, melting the base of the Laurentian crust. Anorthosite formed by large-scale fractional crystallization and the rise of buoyant plagioclase crystal mush incorporating crustal components and hybrid isotopic characteristics not observed in the coronitic metagabbros.