Timing of Iapetus Ocean Rifting in the St. Lawrence Rift System of Southern Quebec, and Fault and Cleavage Dating along the Champlain Thrust of Vermont, Northern Appalachians

Abstract

Timing of Iapetus Ocean rifting from Ar geochronology of pseudotachylytes in the St. Lawrence rift system of southern Quebec. Abstract: Laser ablation 40Ar/39Ar step-heating analyses for encapsulated and unencapsulated pseudotachylytes from a Neoproterozoic normal fault belonging to the St. Lawrence rift system (Canada) preserve the absolute timing of rifting and initial opening of the Iapetus Ocean. The total gas and retention ages for encapsulated pseudotachylytes from the Montmorency fault (Quebec City) are 610.3 ± 4.6 Ma and 619.0 ± 2.5 Ma. Ten unencapsulated analyses from two pseudotachylyte veins with varying matrix/clast ratios yield total gas ages of 634.7 ± 1.6–663.9 ± 1.8 Ma. These ages show an excellent linear relationship with the proportion of clast inclusions, resulting in lower intercept ages (i.e., no host rock) of 613.3 and 614.2 Ma. These statistically indistinguishable ages constrain major seismic faulting along the St. Lawrence rift system and signifi cantly improve prior estimates for late Neoproterozoic rifting of Iapetus. The upper intercepts, refl ecting host-rock ages, match cooling ages of Grenville basement in the area. We conclude that the time of major continental rifting along the northern Laurentian margin and initiation of the Iapetus Ocean occurred at 613–614 Ma, coeval with emplacement of the 615 Ma Long Range dikes of Labrador. This study also demonstrates that Ar geochronology of pseudotachylytes using varying clast/matrix ratios is a robust method to date ancient faulting.

Crystallization and resetting of low-temperature muscovite; 1 Fault and cleavage dating along the Champlain Thrust (VT), Northern Appalachians Abstract: 40Ar/39Ar step-heating analyses were performed on fine-grained muscovite separates from low-grade (anchizonal) mylonites and slates to test a new approch in interpreting 40Ar/39Ar results for fine-grained clays in fault gouge. The Champlain fault in northwestern Vermont is an ideal site to study 40Ar/39Ar geochronology and thermochronology of fine-grained illite in phyllonites and slates. Being among the first descriptions of a thrust fault, the temporal history and geochronologic age of the Champlain thrust has never been determinedremained enigmatic. We present an illite characterization and 40Ar/39Ar step-heating study of phyllonites from the Champlain thrust and slates from the footwall Ordovician Iberville Formation. Laser ablation 40Ar/39Ar step-heating analyses were conducted on two samples from a 2-3 m thick phyllonite (center and contact with hanging wall) and a thinly cleaved slate containing micron to submicron illite grains. Illite crystallinity and microstructural observations show that the phyllonites and slates experienced anchizonal metamorphism (~200-300 °C). X-ray polytype quantification and TEM imaging show that the samples contain a mixture of detrital and authigenic 2M1 and digenetic 1Md illite. Sample collected at the center of the phyllonite, 40Ar/39Ar analyses of the two finer grain-size fractions yield retention ages of 338 Ma, while a third coarser fraction produces a retention age of 376 Ma. For the hanging-wall contact sample, all three grain-size fractions yield 40Ar/39Ar retention ages around 260 Ma. Linear mixing modeling of total gas and retention 40Ar/39Ar ages from illite separates from the Iberville Formation yielded an age of 350.6 ± 1.2 Ma for the timing of slaty cleavage formation. These results suggest that, following initial Middle to Late Ordovician emplacement, the Champlain thrust experienced two pulses of late Paleozoic metamorphism and deformation. Silurian – E. Mississippian, Acadian-Neoacadian tectonism caused clay mineral transformations and isotopic loss of 40Ar by diffusion in illite in the Champlain Thrust and Iberville Formation. The second pulse caused thermal resetting and overprinting of Acadian-Neoacadian illite that is localized along the boundary with the hanging wall during the L. Mississippian – Permian Alleghanian Orogeny. Regionally, our results show that thermal effects associated with the Acadian- Neoacadian and Alleghanian orogenies caused low-grade clay transformations as far away as northwestern Vermont.