Ronald Doig

Eclogite nodules from kimberlite pipes of the Colorado plateau—samples of subducted Franciscan-type oceanic lithosphere

Abstract Remarkable similarities between lawsonite-bearing eclogite xenoliths from kimberlite pipes on the Colorado plateau and Franciscan eclogitic rocks suggest a common origin for both groups of rocks. Petrological and structural evidence as well as RbSr isotopic studies are consistent with interpreting the xenoliths as fragments of Mesozoic oceanic crust subducted beneath the North American continent and returned to the Earths surface via the kimberlite pipes. This radically different model may have important implications for the tectonic evolution of the south-western United States. It may constitute direct proof that recently proposed large-scale underflow of oceanic crust and mantle beneath the North American continent has indeed taken place.

Discrete metamorphic events in the Limpopo belt, southern Africa: Implications for the application of P-T paths in complex metamorphic terrains

Essentially all models for the tectonic evolution of the high-grade Limpopo belt of southern Africa invoke a single orogenic event at 2.7 Ga involving the collision of the Zimbabwe and Kaapvaal cratons with a distinct central zone. By implication, rocks within the belt must have followed parallel pressure- temperature-time ( P - T - t ) paths. The main reasons for this situation are the belief that syn- to postdeformational granitic magmatism in the belt was coeval and the fact that P - T paths are similar. New geochronological data indicate, however, that this magmatism occurred at distinct times in different parts of the belt over a span of at least 70 m.y. In addition, the data indicate that high-grade metamorphism and associated tectonism in the central and southern marginal zones occurred at about 2.0 and 3.15 Ga, respectively. Therefore, P - T - t paths of these zones were independent until their juxtaposition more recently than 2.0 Ga.

The Rb-Sr geochronological evolution of the Ras Gharib segment of the northern Nubian Shield

Rb-Sr whole-rock isochron ages have been determined for six igneous rock suites from the Ras Gharib. These represent all major crustal components of the northern Nubian Shield and are, from oldest to youngest: diorite-tonalite, extrusive rocks, granodiorite-adamellite and leucogranite, muscovite trondhjemite (orogenic Pan-African suites), dyke swarms and anorogenic peralkaline granites. The ages obtained are: 881 ± 58 Ma, 620 ± 16 Ma, 552 ± 7 Ma, 516 ± 7 Ma, 493 ± 7 Ma and 476 ± 2 Ma. The initial 87Sr/86Sr ratios of the Pan-African suites and the dykes are in the range 0.7039 to 0.7047. The initial ratio of the anorogenic peralkaline granites is 0.7110 ±0.0012, higher than any previously reported for igneous rocks from the Egyptian belt. The 493 ± 7 Ma age of the dyke swarms represents a younger limit for the Pan-African compression–accretion event and an upper (older) limit for an extensional tectonic regime. Linked with this is the anorogenic peralkaline granite magmatism (476 ±2 Ma), which is the last manifestation of igneous activity in this region. The low initial ratios of the Pan-African suites provide no support for the existence of an older (Archaen) sialic continental crust in the region. The anorogenic peralkaline granite with the high initial ratio was produced by partial melting of earlier Pan-African suites. The age data are used to construct a tentative correlation scheme for shield rocks of Egypt, Sudan and Saudi Arabia.

2300 yr history of seismicity from silting events in Lake Tadoussac, Charlevoix, Quebec

Silt layers in organic-rich lake sediments are interpreted to represent the historic earthquakes of 1638, 1663, 1791, 1870, and 1925 of the Charlevoix region of eastern Canada. This interpretation is based on the relative spacing of the layers and the accelerator-mass spectrometer {sup 14}C dating of a twig in one of the cores. The silt layers were presumably caused by landslides on tributary streams and by resuspension of the sediment. The cores contain up to 22 layers, 1 to 45 mm thick, over a depth equivalent to about 2300 yr. If there is a correlation between thickness and magnitude, there would have been four events of Richter magnitude 7.5 or greater in A.D. 1663, 1320, 60, and 320 B.C. The 18 other layers could correspond to earthquakes of magnitude 6 to 7. The recurrence interval from 300 B.C. to A.D. 800 is 120 yr, followed until A.D. 1500 by a quiet period, when there was a single large event. The recurrence interval from A.D. 1500 to the present is 75 yr.

Dating Archean metamorphic minerals southeast of the Grenville front, western Quebec, using Pb isotopes

Pb-isotope ratios of high-grade metamorphic minerals such as K-feldspar, biotite, magnetite, apatite, sillimanite, and garnet define collinear arrays on {sup 207}Pb/{sup 204}Pb vs. {sup 206}Pb/{sup 204}Pb diagrams. The samples are from a zone extending 35 km southeast of the Grenville front; the dates are interpreted to reflect peak granulite conditions during an Archean metamorphic event having a minimum age of 2.65 Ga. The gneiss was derived from preexisting crustal precursors and was barely affected by the Grenvillian event, a milder perturbation followed by rapid uplift.

Mylonite to megabreccia: Tracking fault events within a transcurrent terrane boundary in Nova Scotia, Canada

Transcurrent terrane boundaries commonly evolve into long-lived faults that preserve little evidence for early docking events. A remarkable exception is exposed at Clarke Head along the Appalachian Meguma terrane boundary in Nova Scotia, Canada, where a Late Carboniferous fault megabreccia contains Devonian (369 Ma: U-Pb zircon) granulite-grade mylonite fractured by veins filled with Visean amphibole (ca. 335 Ma: Ar-Ar). This fractured mylonite was later mixed with Early Carboniferous sedimentary rocks during megabrecciation (ca. 315–310 Ma). These three fault events are reflected in the tectonostratigraphic record. Devonian (ca. 370–360 Ma) transpressional terrane docking ramped Meguma up against Avalonia and shed clastic detritus across the fault system. The Visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface. The late Namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation. The Clarke Head megabreccia therefore preserves an episodic late Paleozoic fault history spanning some 55 m.y. during convergence between Laurentia and Gondwana and the assembly of the Pangean supercontinent.

Evidence for Silurian sinistral accretion of Avalon composite terrane in Canada

The Kingston dyke complex forms a regionally developed belt of sheeted bimodal dykes within the Avalon composite terrane of southern New Brunswick, Canada. Dyke orientation relative to the belt suggests their injection accompanied sinistral movement, and the belt has previously been attributed to transform activity within the late Precambrian magmatic arc regime typical of the Avalon composite terrane in the Northern Appalachians. However, rhyolite dykes of the sheeted complex yield a U-Pb age of 435.5 ± 1.5 Ma that is interpreted to date their emplacement. The complex therefore provides evidence of Early Silurian sinistral displacement in the Northern Appalachians. Such movement is suggested by palaeomagnetic data and may record the accretion of the Avalon composite terrane to cratonic North America.

The significance of europium anomalies in the REE spectra of granites and pegmatites, Mont Laurier, Quebec

Abstract Negative Eu anomalies in the REE spectra of granitic rocks are usually interpreted as evidence of earlier separation of a mineral phase such as plagioclase. Our study of the behaviour of the REE, and of U and Th, during late stage alteration of granites and the formation of pegmatites, suggests an alternative means for producing Eu anomalies. Albitization of earlier-formed plagioclase has depleted the granites in Eu and enriched the pegmatites in this element. This process is linked to the mobilization of U in the granites which is dependent on the oxidation state of the fluids. A systematic correlation between the ratios Eu Ce and U Th leads us to suggest a similar explanation for irregularities in the abundances of Ce and other rare-earths in the REE spectra of these rocks. We suggest that anomalous behavior of Eu in other environments or processes such as the alteration of basalt and the formation of certain ore deposit types may be caused partly by this mechanism.

U-Pb geochronology of Late Palaeozoic plutons, Cobequid Highlands, Nova Scotia, Canada: evidence for Late Devonian emplacement adjacent to the Meguma-Avalon terrane boundary in the Canadian Appalachians

The Cobequid Highlands in the Canadian Appalachian orogen lie within Avalonia adjacent to the Meguma Terrane. U-Pb (zircon) data show that the age range of voluminous bimodal plutonism in the highlands is from 358 to 363 Ma (late Devonian). This age range is much narrower than that previously suggested by Rb/Sr geochronology and confirms that the Cobequid Highlands preserve the youngest large-scale plutonic event in the Canadian Appalachians. Late Palaeozoic tectonic history of the Appalachian orogen is profoundly influenced by predominantly dextral motion on the Avalon-Meguma terrane boundary. This age of plutonism is coeval with previously published 40Ar/39Ar (muscovite) plateau ages derived from shear zones in the Meguma terrane adjacent to the terrane boundary, which is defined by the Minas fault zone. The NNE trending structural grain of the Appalachian orogen is disturbed in this area by the E-W Minas fault zone and pluton emplacement may have been associated with motion along this terrane boundary.

Sr-isotopic studies of the origin of the Morin anorthosite complex, Quebec, Canada

Rb-Sr isotopic investigations of the rock-types within the Morin anorthosite complex, Quebec, suggest that: 1. both the mangerites and the leuconorites and anorthosites within this complex crystallized with a range of 87Sr/86Sr ratios, and 2. at least some of the mangerites crystallized approximately 100 m.y. after the anorthosites and leuconorites. From this evidence, it is argued that a number of magma sources gave rise to the melts that formed these rocks and that some local hybridization of these melts also took place by assimilation of other rock types. This same pattern is found in the Adirondack and Nain Complexes.