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.

Recent investigations in geochronology and tectonics provide important new insights into the evolution of the Grenville Orogen in North America. Here, we summarize results of this research in the USA and focus upon ca. 1.4–0.98 Ga occurrences extending from the Adirondack Mountains to the southern Appalachians and Texas. Recent geochronology (mainly by U/Pb SHRIMP) establishes that these widely separated regions experienced similar tectonomagmatic events, i.e., the Elzevirian (ca. 1.25–1.22 Ga), Shawinigan (ca. 1.2–1.14 Ga), and Grenvillian (ca. 1.09–0.98 Ga) orogenies and associated plate interactions. Notwithstanding these commonalities, Nd model ages and Pb isotopic mapping has revealed important differences that are best explained by the existence of contrasting compositions of deep crustal reservoirs beneath the Adirondacks and the southern Appalachians. The isotopic compositions for the Adirondacks lie on the same Pb–Pb array as those for the Grenville Province, the Granite-Rhyolite Province and the Grenvillian inliers of Texas suggesting that they all developed on Laurentian crust. On the other hand, data from the southern Appalachians are similar to those of the Sunsas Terrane in Brazil and suggest that Amazonian crust with these Pb–Pb characteristics was thrust onto eastern Laurentia during its Grenvillian collision with Amazonia and subsequently transferred to the latter during the late Neoproterozoic breakup of the supercontinent, Rodinia, and the formation of the Iapetus Ocean. The ca. 1.3–1.0 Ga Grenville Orogen is also exposed in the Llano Uplift of Texas and in small inliers in west Texas and northeast Mexico. The Llano Uplift contains evidence for a major collision with a southern continent at ca. 1.15–1.12 Ga (Kalahari Craton?), magmatic arcs, and back-arc and foreland basins, all of which are reviewed.     The Grenvillian Orogeny is considered to be the culminating tectonic event that terminated approximately 500 m.y. of continental margin growth along southeastern Laurentia by accretion, continental margin arc magmatism, and metamorphism. Accordingly, we briefly review the tectonic and magmatic histories of these Paleoproterozoic and Mesoproterozoic pre-Grenvillian orogens, i.e., Penokean, Yavapai, and Mazatzal as well as the Granite-Rhyolite Province and discuss their ~5000 km transcontinental span.SOMMAIREDes recherches récentes en géochronologie et en tectonique révèlent d’importants faits nouveaux sur l’évolution de l’orogénie de Grenville en Amérique du Nord.  Nous présentons ici un sommaire des résultats de cet effort de recherche aux USA en mettant l’accent sur les indices datés entre env. 1,4 et 0,98 Ga, à partir des monts Adirondack jusqu’au sud des Appalaches et au Texas.  Des données géochronologiques récentes (par microsonde SHRIMP principalement) indiquent que les roches de ces régions très éloignées les unes des autres ont subies l’effet d’épisodes tectonomagmatiques similaires, par exemple, aux orogenèses de l’Elzévirien (env. 1.25–1.22 Ga), de Shawinigan (env. 1.2–1.14 Ga), et du Grenvillien (env. 1.09–0.98 Ga), ainsi que des interactions des plaques associées.  Malgré ces points communs, la chronologie Nd et la cartographie isotopique Pb a révélé des différences importantes qui s’expliquent plus aisément par des compositions contrastées des réservoirs profonds de croûte sous les Adirondacks et le sud des Appalaches.  Les compositions isotopiques des Adirondacks sont de la même gamme Pb-Pb que ceux de la Province de Grenville, de la Province Granite-rhyolite et des boutonnières grenvilliennes du Texas, suggérant qu'ils se sont tous développées sur la croûte des Laurentides.  Par ailleurs, les données des Appalaches du sud sont semblables à celles du terrane de Sunsas au Brésil, ce qui incite à penser que la croûte amazonienne, avec de telles caractéristiques Pb-Pb, a été poussée sur la portion est de Laurentia lors de sa collision grenvillienne avec l’Amazonie puis laissée à cette dernière au cours de la rupture du supercontinent Rodinia vers la fin du Néoprotérozoïque, avec la formation de l'océan Iapetus.  L’orogène de Grenville (1,3 à 1,0 Ga env.) est également exposé dans le soulèvement de Llano au Texas et dans de petites boutonnières dans l'ouest du Texas et le nord du Mexique.  Le soulèvement de Llano montre des indices d'une collision majeure avec un continent au sud, entre env. 1,15 et 1,12 Ga (craton de Kalahari?), des zones d’arcs magmatiques, d'arrière-arc et de bassin d'avant-pays, chacun étant présenté ci-dessous.    L'orogenèse de Grenville est considéré comme l'événement tectonique culminant qui marqué la fin d’une période d’environ 500 ma d’accroissement de la marge continentale le long de la bordure sud-est de la Laurentie, par accrétion, magmatisme d’arc de marge continentale, et métamorphisme.  C’est pourquoi, nous passons brièvement en revue l'histoire tectonique et magmatique de ces orogènes pré-grenvilliennes paléoprotérozoïques et mésoprotérozoïques, pénokéenne, de Yavapai, et de Mazatzal ainsi que la Province de Granite-rhyolite, et discutons de son étendue sur env. 5 000 km.

Calcite pseudomorphs after ikaite (glendonite) are associated with coldwater depositional systems, including glaciomarine and deepwater settings, as dictated by the limited stability field of ikaite. Ikaite precipitation is favored by elevated alkalinity and dissolved phosphate, conditions encountered commonly in association with organic-rich marine sediments where methane oxidation is occurring. The rapid recrystallization of ikaite to calcite during slight warming or pressure release results in considerable solid volume loss, producing a highly porous crystal mesh. Preservation of the original ikaite crystal form requires precipitation of diagenetic calcite cement during early burial to prevent compaction and collapse of pseudomorph structures. During later burial diagenesis remaining pore space may be filled with deeper burial calcite cement. Glendonites from the Permian of the Sydney Basin occur in subtidal shelf facies containing glacial dropstones and a normal marine fauna. Stable oxygen isotope signatures of modern ikaite suggest carbonate precipitation in equilibrium with ambient seawater; carbon signatures are usually strongly negative relative to normal marine carbonate, consistent with derivation of carbonate from methane oxidation. Review of published data suggests that while Holocene glendonite may provide reliable isotopic records of the conditions of ikaite precipitation, precipitation of later calcite cement within the glendonite structure reduces the significance of the isotopic signature as an indicator of primary depositional conditions. Bulk glendonite samples from the Permian Sydney Basin, Australia, have a broad range of d 18 O and d 13 C (d 18 O PDB = 25 to 215%; d 13 C = 28 to 216%), in contrast to the narrow range for brachiopod carbonate (d 18 O PDB = +1 to 25%; d 13 C = +5 to +7%) from the same strata. Handpicked separates of ''primary'' glendonite and secondary calcite also have a wide range of stable isotope values. The data from Sydney Basin glendonites indicate that diagenetic precipitation of calcite has blurred the isotopic signature of primary ikaite replacement calcite at the scale of micosampling done in this study. Australia) during his visit to the area in 1840 as part of the US Exploring Expedition. During the expedition he also collected specimens from the Astoria area, Oregon, and recognized the similarity between these samples and those from Glendon (Dana 1849). He concluded that the samples were pseudomorphs because of their granular interior, but he was unable to determine the identity of the precursor. These pseudomorphs were subsequently named glendonites by David et al. (1905) after the original locality, but many other names, including barley corns, chrysanthemum stones, fundylite, gennoishi, hedgehogs, jarrowite, opal pineapples, pseudogaylussite, thinolite, and White Sea hornlets, have also

U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from migmatitic metapelites in the easternAdirondack Highlands demonstrate that substantial anatexis took place ca. 1050 Ma during the late, but still high-grade, ca. 1090-1050 Ma Ottawan orogeny. These results contrast with data from metapelites of the southwestern Adirondack Highlands and Adirondack Lowlands, which indicate that anatexis occurred ca. 1200-1160 Ma, during the Shawinigan orogeny and subsequent (ca. 1155 Ma) anorthosite-mangerite-charnockite-granite (AMCG) magmatism. Ca. 1180-1150 Ma zircons from this western regime do not contain ca. 1050 Ma (Ottawan) metamorphic overgrowths. The absence of ca. 1050 Ma Ottawan anatexis and overgrowths in the Adirondack Lowlands is explained by the observation that, consistent with titanite cooling ages, post-1150 Ma temperatures did not exceed ~700 °C. In the southwestern Adirondack Highlands, the absence of ca. 1050 Ma anatexis can be accounted for by earlier dehydration of metapelites during ca. 1180-1150 Ma Shawinigan-AMCG metamorphism. The occurrence of ca. 1050 Ma anatexis and formation of metamorphic zircons in the eastern Adirondacks cannot be explained by higher temperatures, because geothermometry indicates that all of the Adirondack Highlands reached a nearly uniform 750-800 °C during this period. Accordingly, the occurrence of ca. 1050 Ma Ottawan anatexis in the eastern regime is interpreted to be the result of: (1) infl uxes of hydrous fl uids at ca. 1050 Ma, or (2) decompression melting during late extensional exhumation. A recently recognized low-angle late Ottawan (ca. 1050 Ma) fault system may have provided both fl uid conduits and footwall decompression. The sense of displacement along the shear zonehas not yet been unequivocally determined, but preliminary investigations of kinematic indicators demonstrate normal displacement. Thus, this structure may be an analogue of the ca. 1050 Ma northwest-dipping Carthage-Colton zone normal fault system that juxtaposed the Adirondack Lowlands against the Adirondack Highlands. In this context, these fault zones form a quasi-symmetrical core complex or gneiss dome, within which the high-grade core of the Adirondack Highlands was tectonically exhumed. A similar east-dipping, along-strike normal fault in Quebec (Tawachiche shear zone) exhumed the eastern Morin and Lac Taureau terranes at ca. 1050 Ma and may merge with the eastern Adirondack shear zone described here.