Pre-Grenvillian history of the Adirondacks as an anorogenic, bimodal caldera complex of mid-Proterozoic age

The Adirondack highlands are characterized by the coexistence of a distinctive trinity of metaigneous rocks indicative of anorogenic or mild rift-related magmatism: (1) anorthosites and anorthositic gabbros; (2) mangerites, charnockites, and hornblende granites; and (3) alaskitic gneisses. This association, which is referred to as the AMCAL suite, is characterized by high FeO/(FeO + MgO) ratios, chemical trends that range from mildly alkaline to subalkaline and peraluminous, and high concentrations of halogens and rare-earth elements (REE) together with anhydrous assemblages. Outcrop patterns show that the least silicic members of the AMCAL suite tend to occur at structurally low positions, such as anticlinal cores, whereas more silicic members occupy structurally higher positions, such as the flanks of anticlines. It is inferred that this configuration is inherited from relatively high-level, zoned, and bimodal magma chambers cored by mafic rocks successively enveloped by mangeritic, chamockitic, and granitic magmas. Alaskitic gneisses are believed to represent metamorphosed rhyolitic ash-flow tuffs and their subvolcanic equivalents. Geochronologic data suggest an emplacement age of about 1300 Ma for the AMCAL suite and support the inference that prior to the Grenville orogeny, the Adirondacks evolved as part of the belt of mid-Proterozoic, anorogenic and rift-related magmatism that traverses North America and extends into northern Europe.