CORONITIC METAGABBROS WITHIN THE ADIRONDACKS: A PROBE OF POST SHAWINIGAN ASTHENOSPHERIC MANTLE AND RELATED INSIGHTS INTO AMCG MAGMATISM

Olivine normative coronitic metagabbros (CMG) within the Adirondack Highlands display hallmarks of derivation by partial melting of transitional to enriched asthenospheric mantle. Crystallization ages are consistent with post or late kinematic intrusion with respect to the Shawinigan orogeny (1160-1200 Ma) and synchronous with the widespread Anorthosite-Mangerite-Charnockite-Granite (AMCG). The CMG are exposed as dikes and lozenges along the margins of massif anorthosite or within close proximity. Their trace element chemistry displays flat incompatible element trends (N_PM), and LREE values which are enriched relative to chondrites, notably lacking any anomalies invoking a source compatible with a metasomatized, or subduction altered lithospheric mantle. This is dissimilar to mafic-ultramafic rocks (ca. 1300 Ma) within the Adirondack Lowlands, and many of intrusive related Shawinigan granitoids. Samarium-Nd isotopes display a consistent range of values (ε_Nd(T)= +3.19 - +3.69; n=10), which are below contemporaneous depleted mantle, and yield T_DM ages older than their T_C and other rocks within the region, suggesting that their derivation was not from a partial melt of a reservoir which became depleted at 3.2 Ga, or by melting of an older ultramafic rocks. On major and trace element plots (ie. TiO₂ versus P₂O₅ and Sm versus Cr, respectively), the CMG form a continuous field between that of oxide- and apatite-rich gabbros and anorthosites and leucogabbros within the Adirondacks, making them permissible as the parental magma for the anorthositic rocks. Geochemical and isotopic data indicate that the source of CMG was not the highly metasomatized and subduction altered lithospheric mantle present during the initiation of Shawinigan orogenesis, but mantle with a transitional to enriched signature. These data suggest that the majority of the lithosphere had been removed by delamination, which would have triggered partial melting of a primitive asthenospheric mantle, giving rise to the CMG, and fractionally crystallized (anorthosite), and crustally derived (Mangerite-Charnockite-Granite) members of the AMCG suite. The characteristics of AMCG suites elsewhere will vary due to differences in host rocks, and the nature (depleted vs undepleted) of the mantle reservoir.