EVIDENCE FOR COEVAL, NON-COMAGMATIC, RELATIONSHIPS WITHIN THE ADIRONDACK ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE (AMCG) SUITE, NEW YORK STATE

Field, chemical, and recent SHRIMP U-Pb zircon geochronological evidence indicate a bimodal, not comagmatic, origin for the ca. 1155 Ma Adirondack AMCG suite. Field evidence includes the small volume of intermediate rocks between A and MCG, all of which are restricted to 100m-scale transition zones consisting of commingled magmas (Keene Gneiss). Commingling is manifested by pillowing, chilling, linear chemical variation trends, interlayering of end members, and by reaction rims on feldspars. Perthite xenocrysts in anorthositic-ferrodioritc magma show corroded alkali-feldspar cores surrounded by plagioclase, whereas andesine xenocrysts in granitoids now exist as corroded cores within perthite. Chemical evidence includes the divergent trends of some suite members in variation diagrams, i.e., plots of mole% Mg vs. %volume-of-mafics in MCG show smooth trends, whereas the same plots for Keene Gneiss show wide scatter indicating non-equilibrium. A similar plot for A vs. MCG members documents opposing differentiation trends. This is also true for plots of the same rocks in the system K20-MgO-FeO. These results reflect the fact that the MCG granitoids move towards silica enrichment, whereas anorthositic members differentiate along Fenner-trends and culminate in ferrodiorites. Identical results are obtained when Adirondack AMCG compositions are plotted in the model system Di-Ol-Qtz [Pl, Ilm] (J. S. Scoates, pers. comm., 2004). The opposed differentiation trends of A vs. MCG also emerge in plots of delta18O(zircon) vs. wt. % SiO2, further documenting bimodality. SHRIMP zircon geochronology indicates: a) some early (ca. 1170 Ma) granitoids preceded anorthositic rocks by ~15 Ma making comagmatism unlikely, b) granitoid zircons commonly contain cores of older (ca 1200-1300 Ma) zircon that could not have survived in mafic magmas of the anorthosite suite, especially the jotunitic varieties recently cited by some as parental magmas. We conclude that the anorthosites represent differentiates from gabbros ponded near the crust-mantle interface, whereas the granitoids are somewhat earlier to coeval crustal melts generated by heat derived from gabbroic differentiation. AFC interaction between these sources resulted in small, but significant, isotopic exchanges, e.g., elevated Sr⁸⁷/Sr⁸⁶ in A and delta18O in AMCG.