No abstract

Anatectic leucogranites are common in metapelites within both the highlands and lowlands terranes of the Adirondack Mountains of northern New York State. The formation of these igneous bodies, which are folded in the lowlands and commonly mylonitized in the highlands, has been widely considered an event accompanying the ca. 1050 Ma Ottawan orogeny, during which metamorphic grade reached granulite facies in the highlands, while the lowlands experienced amphibolite facies metamorphism. Sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons separated from leucosomes and melanosomes in both the southern highlands and the lowlands indicate that primary anatexis occurred ca. 1180-1160 Ma, and is thus a manifestation of heating during the earlier Shawinigan orogeny (ca. 1210-1160 Ma) and associated anorthosite-mangerite-charnockite-granite (AMCG) magmatism (ca. 1165-1150 Ma). The absence of Ottawan overgrowths on Shawinigan zircons in these leucosomes suggests that by Ottawan time the rocks were too dry for further melting or zircon growth to occur. However, electron microprobe analyses of monazites from the southern highlands reveal multiple age zones, including cores with ages of ca. 1170-1180 Ma, consistent with primary growth during Shawinigan orogenesis, complex zones formed ca. 1140-1155 Ma during AMCG magmatism, and ca. 1050-1020 Ma formed during Ottawan orogenesis and high-grade metamorphism. Throughout the Adirondacks, leucosomes and melanosomes contain older, ca. 1320 Ma, zircons that are considered to be remnant detrital zircons derived from arc rocks of the Elzevirian terrane. The apparent absence of Archean detrital zircons suggests that the protoliths of the metapelites were deposited in restricted basins that did not receive detritus from the Superior craton.

Sensitive high-resolution ion microprobe U-Pb zircon ages for late to posttectonic leucogranites fix the timing of extensional collapse of a portion of the Mesoproterozoic Grenville orogen of eastern North America. Plutons of Lyon Mountain Granite (LMG) were emplaced within the Carthage Colton shear zone synchronously with formation of extensional mylonite at 1045-1037 Ma. Leucogranite melts were generated in the hot granulite facies core of the Adirondack Highlands-Central Granulite terrane that served as the lower plate for down-to-the-northwest extension. The LMG suite is associated with hightemperature hydrothermal magnetite deposits in the Adirondack Highlands, and widespread Cl ؉ CO 2 hydrothermal alteration of upper-plate rocks is localized along the Carthage Colton shear zone where LMG granites are present. The relationships between melt generation, granite intrusion, high strain rates, extensional collapse, and hightemperature hydrothermal activity provide a framework for understanding midcrustal processes in modern and ancient orogenic belts.

The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200-1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fi elds. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (T DM , depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613-1634) and smallest epsilon Nd (ε Nd ) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss-Rockport Granite suites (1155-1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangeritecharnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.

Recent zircon-based geochronological investigations in the Adirondack Mountains, in the state of New York, demonstrate that 1155 ± 6 Ma massif anorthosite was emplaced during the post-contractional phase (1160-1140 Ma) of the ca. 1200-1140 Ma Shawinigan orogeny. Emplacement of many other anorthosite-mangerite-charnockite-granite (AMCG) suites also correlates with the waning stage of orogeny and includes the ca. 1155 Ma Morin and Lac-St-Jean complexes, the ca. 1650 Ma Mealy Mountain complex, and the ca. 1450 Ma complexes of eastern Labrador. The correlation also applies to the ca. 930 Ma Rogaland anorthosite complex of Norway and the ca. 1060-1020 Ma late-to post-Ottawan anorthosites of central Quebec and the Appalachians of Virginia and southeastern Pennsylvania. These correlations suggest models involving delamination of overthickened orogenic lithosphere by foundering or convective removal, followed by athenospheric ascent, and ponding of gabbroic melt at the crust-mantle interface. Orogen rebound following delamination results in stable, dynamically balanced settings in which gabbroic magma evolves slowly at high pressure to produce high-aluminum pyroxene and coarse, intermediate plagioclase characteristic of massif anorthosite. Related melting of the lower crust produces mangeritic and charnockitic magma. Ultimately, both anorthositic and granitic magmas ascend, and lower-pressure fractionation yields the classic AMCG suites. Transtensional reactivation of lithospheric-scale shear zones and old sutures also correlates with important AMCG magmatism and provides conduits for gabbroic magma that ponds at the crust-mantle interface or in the deep crust to produce AMCG suites. Flat-slab subduction, back-arc extension, slab breakoff, and hotspots represent alternative settings that can account for gabbroic underplating and fractionation into AMCG suites, if consistent with geochronological constraints.