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Field evidence, as well as binary and ternary variation diagrams, provide evidence that the anorthosite-mangerite-charnockite-granite gneiss (AMCG) suite of the Adirondacks evolved in bimodal magmatic complexes without a continuous line of liquid descent from mafic through acidic igneous members. Major- and trace-element distributions suggest an anorogenic setting for the emplacement of AMCG magmas. Recent U-Pb zircon dating indicates that AMCG complexes were emplaced during the interval 1160 to 1130 Ma. Peak granulite facies conditions appear to have occurred at ~1070 to 1030 Ma, as indicated by metamorphic zircons formed by exsolution of Zr from mafic silicates and Fe, Ti oxides, as well as by monazite and sphene cooling ages. Continued exsolution of Zr from magnetite-ilmenite that reequilibrated to 500° to 600°C resulted in zircon ages of ~ 1000 Ma in oxide-rich rocks.

The AMCG suite of the Adirondacks appears to be part of a protracted anorogenic cycle that traversed North America and northern Europe in mid-Proterozoic time and manifested the break-up of a supercontinent believed to have been assembled from 1900 to 1600 Ma.

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