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Petrology, geochemistry, fluid inclusions and U Pb dating of a wide (30 cm) calcite vein in Upper Ordovician (Sandbian) limestone characterize a hydrochemical microcosm of changing fluid sources and carbon pathways related to burial, then uplift during the Phanerozoic along the Ottawa-Bonnechere graben, central Canada. U Pb dating of the host limestone produces an age younger than its chronostratigraphic age suggesting the impact of diagenesis, otherwise well-defined petographically. An interred calcite fragment dated as Cambrian may highlight a platform source in this part of the graben hitherto unrecognized stratigraphically. The vein was initiated with extension across a high-angle narrow (2 cm) fault gouge allowing incursion of hot (80–125 °C) basin-derived (Mg, Ca)-Cl 2 brine recorded by a saddle dolomite–dedolomite–calcite succession, not isotopically datable. Microbial methanogenesis occurred in initial narrow structural recesses giving way to recycled sedimentary bicarbonate with fracture widening. The oldest datable vein calcite (122 ± 5 Ma) consists of microcrystalline geopetal cumulates of vadose meteoric origin demarcating renewed carbonate formation following a period of near-surface alteration, dissolution, and microfracturing of the prior vein calcite during uplift. Initial 207Pb/206Pb ratios are consistent with average crust (0.819 ± 0.003), and δ18O signatures identify a meteoric signature similar to the host rock. Subsequent interlayering of isopachous calcite and pyrite, with an age of 105 ± 3 Ma for the calcite, defines onset of a meteoric phreatic environment. The role of microbial sulphate reduction is characterized isotopically. Initial 207Pb/206Pb ratios scatter below the crustal average (0.84–0.76) that, along with a meteoric δ18O signature, suggest incursion of a new fluid reservoir. A subsequent Sr, Mn, Ba-bearing–magnesium calcite marks the beginning of sustained phreatic meteoric dilution for the remaining vein history. Stable (C, O) isotopes document varying temperature and bicarbonate mixtures of sedimentary and bacterial (sulphate reduction) origins associated with a meteoric fluid. This calcite has a mean age of 94 ± 2 Ma and displays the widest range of initial 207Pb/206Pb ratios, which scatter down to 0.65. Stable and radiogenic isotopic compositions suggest a meteoric setting largely influenced by a crustal fluid reservoir of long residence time and-or transport resulting in leaching of radiogenic minerals. The final vein calcite, also of meteoric origin, has an 87Sr/86Sr ratio similar to that of the initial basin brine, yet with 13C signatures that characterize a source of oxidized methane. U Th series analysis delimits an age older than 0.5 Ma. This stage of vein calcite likely arises through meteoric fracking of Ordovician source rocks during continued basin inversion. In summary, initial brine influx followed by a shallow-crustal meteoric realm document structurally focused fluid migration with and following early Mesozoic graben exhumation. The phreatic meteoric regime of Early Cretaceous age spans a ~ 20 Ma period coincident with transient magmatism arising from near passage of the Great Meteor Hotspot. Regional paleohydrology appears to have been influenced for ~10 Ma after plume passage. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic–Mesoproterozoic boundary has been developed in the last 30 years with different scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obstacle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wernecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its possible conterminous at 1600Ma. The six youngest U–Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640Ma. Lu–Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wernecke Supergroup) give a bimodal population of ages of approximately 1600Ma and 1370Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplacement of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wernecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab volcanics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimentary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present. [Copyright &y& Elsevier]

In the Yukon, the oldest known supracrustal succession, the Wernecke Supergroup, was deposited in a marine basin before 1.71 Ga. The earliest orogenic event to disturb these strata was the Racklan orogeny, which produced folds and fabrics at peak temperatures of 450–550 °C. These features and those of the correlative Forward orogeny are recognized at the surface and in the subsurface throughout much of northwestern Canada. Zones of Wernecke Breccia (hydrothermal breccias, 1.60 Ga) were emplaced into the Wernecke Supergroup after Racklan deformation and metamorphism. Two main types of breccia are recognized: grey sodic breccias and colourful potassic breccias. In the Slab Mountain area, a belt of grey breccias contains abundant megaclasts of country rock including blocks of a subaerial lava succession, the Slab volcanics. These grey breccias are interpreted as a vent facies of Wernecke Breccia, and their emplacement into the stratigraphically lowest unit of the Wernecke Supergroup infers that at least 9 km of exhumation occurred in the core of a major Racklan anticline prior to brecciation. The Slab volcanics are preserved only as clasts in Wernecke Breccia and are interpreted as fragments of a former valley-filling basalt succession which overlay deformed and deeply incised strata of the Wernecke Supergroup. [ABSTRACT FROM AUTHOR]