Tectonic Evolution of the Adirondack Mountains and Grenville Orogen Inliers within the USA

Abstract

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.

Tectonic Setting and Evolution of the Grenville Orogen: An Assessment of Progress Over the Last 40 Years

“…These results suggested a high degree of continuity of major tectonic features, such as the ages of the pre-Grenvillian accretion and the timing of the Grenvillian Orogeny, along the 5000 km of exposure in North America (e.g. Mosher et al 2004Mosher et al , 2008Tollo et al 2004;McLelland et al 2010McLelland et al , 2013Rivers et al 2012), and hence that it comprised a single orogen.…”

“…For most authors this profound reinterpretation, involving the superposition of two high-strain events under granulite-facies conditions, represented the final demise of stratigraphic reasoning and the basement-cover concept as appropriate paradigms for unravelling gneiss terranes. A modern tectonic interpretation of the map shown in Figure 13 can be seen in McLelland et al (2013). In this context, one of the seminal achievements of the 1980s was the development of new techniques for mapping the high-grade Grenvillian gneiss terranes, such that by the end of the decade, formerly intractable gneiss complexes were being subdivided into mappable units, their structure interpreted in three dimensions, and their value as information repositories exploited.…”

“…Gower 1996), the Paleoproterozoic to Mesoproterozoic (~1650-1400 Ma) age of many granitoid gneiss complexes in the hinterland of the orogen that formed in an arc setting (see later), and the <1400 Ma age of 'accreted terranes' in the southwestern hinterland (e.g. Easton 1992; Corrigan et al 1994;McLelland et al 2010McLelland et al , 2013). Map and cross-sectional views illustrating present understanding of the age distribution of crust in the Grenville Province that resulted from these studies are shown in Figure 18 (from Rivers et al 2012;after Ludden and Hynes 2000).…”

“…back-arc rift and late-to post-orogenic settings; Corrigan and Hanmer 1997;McLelland et al 2010;Rivers et al 2012), the critical common factor inferred to be the arrival of a large volume of asthenospheric magma at the base of the continental crust where it can undergo the deep-seated fractionation envisaged by Emslie (1978). McLelland et al (2013) have emphasized that emplacement of several AMCG massifs followed shortly after crustal thickening and the peak of metamorphism, leading them to suggest that delamination of an overthickened crustal root and associated decompression melting of rising asthenosphere were important mechanisms in the emplacement process.…”

“…The proposed Grenvillian evolution involves collision leading to orogen widening, and possibly delamination of a thick lithospheric root. of the suture, based on the change in Pb isotopic signature, is shown in Figure 1 (from Hynes and Rivers 2010; see also McLelland et al 2010McLelland et al , 2013Rivers et al 2012). This is an important result, providing the first quantitative evidence for a 'two-sided system' and hence that the Grenville Orogen developed in a collisional orogenic setting, a conclusion recently disputed on paleomagnetic grounds (Evans 2009).…”