Paleomagnetism and Geochemistry of similar to 1144-Ma Lamprophyre Dikes, Northwestern Ontario : Implications for the North American Polar Wander and Plate Velocities

We present new paleomagnetic and geochemical data from a suite of the similar to 1144-Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4 degrees, I = 72.1 degrees, alpha(95) = 5.5 degrees, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4 degrees, I = 65.5 degrees, alpha(95) = 8.3 degrees, N = 8) previously reported for the nearly coeval similar to 1142-Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt-type mantle sources, consistent with the mantle plume hypothesis for the formation of the similar to 1.1-Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of rifting. The combined mean direction (D = 303.1 degrees, I = 70.2 degrees, alpha(95) = 4.5 degrees, N = 27) corresponds to a paleomagnetic pole at P-lat = 55.8 degrees N, P-long = 220.0 degrees E (A(95) = 7.3 degrees). The new pole merits the highest classification on the Q-scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with high-quality data from the similar to 1108-Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 +/- 1.4 cm/year, comparable with the present-day velocities of continental plates, before switching to extremely rapid motion between similar to 1108 and similar to 1099 Ma. Plain Language Summary Similar to a magnetic tape, rocks can retain the direction of ancient Earth's magnetic field. Scientists use this record (known as paleomagnetism) to reconstruct past positions of continents and to decipher the geological history of our planet. We investigated paleomagnetism and chemical composition of the similar to 1.14 Ga-old intrusive rocks called lamprophyres exposed in Northwestern Ontario (Canada). We found that the paleomagnetic field directions recorded in lamprophyres are indistinguishable from those recorded by another similar age suite of basaltic intrusions called the Abitibi dikes, from the same area. The combined data from these rocks allowed us to constrain the position of an ancient supercontinent called Laurentia at similar to 1.14 billions of years ago more accurately than it was possible before. Our results convincingly show that, during that time, Laurentia moved with a velocity comparable to present-day plate velocities, before switching to an extremely rapid motion approximately 35 millions of years later. The lamprophyre and Abitibi rocks also share similar chemical signatures, close to those observed for ocean island basalts (e.g., Hawaii). These observations support the hypothesis that a failed ocean opening attempt called the North American Midcontinent Rift was instigated by the arrival of a hot mantle material upwelling to the Earth surface.Peer reviewe

Mineralogy and geochemistry of silicatedyke rocksassociated with carbonatites from the Khibina complex (Kola, Russia) - isotope constraints on genesis and small-scale mantle sources

International audienceThe eastern part of the agpaitic Khibina complex is characterized by the occurrence of dykes of various alkali silicate rocks and carbonatites. Of these, picrite, monchiquite, nephelinite and phonolite have been studied here. Whole rock and mineral geochemical data indicate that monchiquites evolved from a picritic primary magma by olivineþ magnetite fractionation and subsequent steps involving magma mixing at crustal levels. None of these processes or assimilation=magma mixing of wall rocks or other plutonic rocks within the complex can entirely explain the geochemical and Nd-Sr-isotopiccharacteristics of the monchiquites (i.e. a covariant alignment between (87Sr/86Sr)370 0.70367, (143Nd/144Nd)3700.51237 and (87Sr/86Sr)3700.70400, (143Nd/144Nd)370 0.51225 representing the end points of the array). This signature points to isotopic heterogeneities of the mantle source of the dyke-producing magma. The four mantle components (i.e. depleted mantle, lower mantle plume component, EMI-like component and EMII-like component) must occur in different proportions on a small scale in order to explain the isotopic variations of the dyke rocks. The EMII-like component might be incorporated into the source area of the primary magma by carbonatitic fluids involving subducted crustal material. The most likely model to explain the small-scale isotopic heterogeneity is plume activity. The results of this study do not provide any support to a cogenetic origin (e.g. fractionation or liquid immiscibility) for carbonatite and monchiquite or other alkali-silicate dyke rocks occurring in spatial proximity. Instead, we propose that both, carbonatite and picrite=monchiquite, originated by low-degree partial melting of peridotite. Textural observations, mineralogical data, and C and O isotopic compositions suggest incorporation of calcite from carbonatite in monchiquite and the occurrence of late-stage carbothermal fluids