Zircon U-Pb geochronology of the Ottawan Orogeny, Adirondack Highlands, New York: regional and tectonic implications

The origin of the Antarctic continent can be traced to a relatively small late Archaean cratonic nucleus centred on the Terre Adélie regions of East Antarctica and the Gawler Craton region of South Australia. From the late Archaean to the present, the evolution of the proto-Antarctic continent was remarkably dynamic with quasi-continuous growth driven by accretionary or collisional events, episodically punctuated by periods of crustal extension and rifting. The evolution of the continent can be broken into seven main steps: (1) late Palaeoproterozoic to middle Mesoproterozoic accretion and collision added crust first to the Antarctic nucleus's eastern margin, then to its western margin. These events resulted in the incorporation of the Antarctic nucleus within a single large continent that included all of Proterozoic Australia, a more cryptic Curnamona–Beardsmore Craton and most probably Laurentia. (2) Rifting in the middle to late Mesoproterozoic separated a block of continental crust of unknown dimensions to form an ocean-facing margin, the western edge of which was defined by the ancestral Darling Fault in Western Australia and its unnamed continuation in Antarctica. (3) Inversion of this margin followed shortly and led to the Grenville aged collision and juxtaposition of proto-Antarctica with the Crohn Craton, a continental block of inferred Archaean and Palaeoproterozoic age that now underlies much of central East Antarctica. The Pinjarra Orogen, exposed along the coast of Western Australia, defines the orogenic belt marking this collision. In Antarctica the continuation of this belt has been imaged in sub-ice geophysical datasets and can be inferred from sparse outcrop data and via the widespread dispersal of syn-tectonic zircons. (4) Tectonic quiescence from the latest Mesoproterozoic to the Cryogenian was the forerunner to Ediacaran rifting that separated Laurentia and the majority of the Curnamona–Beardsmore craton from the amalgam of East Antarctica and Australia. The result was the formation of the ancestral Pacific Ocean. (5) The rifting of Laurentia was mirrored by convergence along the opposing margin of the continent. Convergence ultimately sutured material with Indian and African affinities during a series of Ediacaran and Cambrian events related to the formation of Gondwana. These events added much of the crust that today defines the East Antarctic coastline between longitudes 30°W and 100°E. (6) The amalgamation of Gondwana marked a shift in the locus of subduction from between the pre-Gondwana cratons to Gondwana's previously passive Pacific margin. The result was the establishment of the accretionary Terra Australis and Gondwanide orogenies. These were to last from the late Cambrian to the Cretaceous, and together accreted vast sequences of Gondwana derived sediment as well as fragments of older and allochthonous or para-allochthonous continental crust to Gondwana's Pacific margin. (7) The final phases of accretion overlapped with the initiation of extension and somewhat later rifting within Gondwana. Extension started in the late Carboniferous, although continental separation did not begin until the middle Jurassic. Gondwana then fragmented sequentially with Africa–South America, India, Australia and the finally the blocks of New Zealand separating between the middle Jurassic and the late Cretaceous. The late Cretaceous separation of Antarctica and Australia split the original Antarctic nucleus, terminating more than 2.4 billion years of shared evolution. The slightly younger separation of New Zealand formed the modern Antarctic continent.

It is argued that the Grenville Province is a large hot long-duration orogen with a plateau in the hinterland, remnants of which are preserved in the hangingwall of the Allochthon Boundary Thrust and characterised by metamorphism from ca. 1090 to 1020 Ma (Ottawan phase of the Grenvillian Orogeny). Hinterland rocks are grouped into three tectonic units on the basis of their Ottawan metamorphic signatures, the allochthonous High Pressure Belt, the allochthonous Medium–Low Pressure Belt, and an orogenic lid lacking evidence for penetrative metamorphism. P–T and geochronological data indicate Ottawan metamorphism developed under a relatively high geothermal gradient and was followed by slow cooling, compatible with some form of channel flow. Metamorphic rocks in the Parautochthonous Belt in the footwall of the Allochthon Boundary Thrust, also divided into medium and high-pressure units, were metamorphosed from ca. 1000 to 980 Ma (Rigolet phase) under a lower geothermal gradient and underwent rapid cooling. Their evolution is interpreted to record advance of the orogen into its former foreland after channel flow had ceased. The Allochthon Boundary Thrust is thus a material focal plane separating high-grade rocks derived from opposite sides of the orogen metamorphosed at different times under different P–T–t gradients. Preservation of the Orogenic Lid and low pressure segments of the allochthonous Medium–Low Pressure Belt is a result of gravitational collapse of the orogenic plateau, initiated in late Ottawan time, and the formation of a crustal-scale horst-and-graben architecture. This study emphasises the importance of gravitational collapse during the prolonged compressional phase, a feature not presently accommodated in numerical models of large hot long-duration orogens.

Granitic intrusions emplaced within Laurentia during the Grenville Orogeny (1.15–1.05 Ga) are exceptionally Zr-rich, primarily a function of high modal zircon, relative to Paleozoic granitoids emplaced within eastern Laurentia during phases of Appalachian orogenesis. Erosion of Grenville source rocks would generate disproportionately large volumes of detrital zircon compared to less zircon-fertile source regions. The latter sources are difficult to detect by standard in situ U–Pb dating methods of detrital zircon (SHRIMP or LA-ICP-MS U–Pb analysis of centers of > 100 μm grains), or could be overlooked during sampling. Grenvillian zircon fertility biased the Neoproterozoic to Recent sedimentary record as a result of two factors: (1) zircon durability and insolubility led to recycling during repeated orogenesis; (2) inertness of zircon below the onset of anatexis means dominantly metamorphosed sedimentary terranes failed to generate significant new zircon corresponding in age to the time of accretion of those terranes to Laurentia. Zircon growth under anatectic conditions generates new zircon, commonly as overgrowths on preexisting zircon, which may be too narrow to easily analyze by laser or ion beam techniques. Accretion of metasedimentary terranes therefore could be rendered all but undetectable via standard U–Pb detrital zircon dating. Grenville age modes dominate detrital zircon age distributions for Laurentian Neoproterozoic rift basins, Appalachian Paleozoic synorogenic clastic sequences, Appalachian metasedimentary terranes, and modern rivers draining these terranes. This is an artifact of the Grenville zircon signal that echoed throughout all Paleozoic orogenies on the eastern Laurentian margin. The natural Grenville bias in the detrital zircon record is further amplified by standard sampling biases (such that large (e.g. 100–300 μm) crystals are preferentially chosen) and analytical biases (e.g. cores are much more frequently analyzed).