Canadian Journal of Earth Sciences

A system is presented whereby volcanic rocks may be classified chemically as follows:I. Subalkaline Rocks:A. Tholeiitic basalt series:Tholeiitic picrite-basalt; tholeiite; tholeiitic andesite.B. Calc-alkali series:High-alumina basalt; andesite; dacite; rhyolite.II. Alkaline Rocks:A. Alkali olivine basalt series:(1) Alkalic picrite–basalt; ankaramite; alkali basalt; hawaiite; mugearite; benmorite; trachyte.(2) Alkalic picrite–basalt; ankaramite; alkali basalt; trachybasalt; tristanite; trachyte.B. Nephelinic, leucitic, and analcitic rocks.III. Peralkaline Rocks:pantellerite, commendite, etc.

Maps of the paleography of Iran are presented to summarize and review the geological evolution of the Iranian region since late Precambrian time. On the basis of the data presented in this way reconstructions of the region have been prepared that take account of the known major movements of continental masses. These reconstructions, which appear at the beginning of the paper, show some striking features, many of which were poorly appreciated previously in the evolution of the region. They include the closing of the 'Hercynian Ocean' by the northward motion of the Central Iranian continental fragment(s), the apparently simultaneous opening of a new ocean ('the High-Zagros Alpine Ocean') south of Iran, and the formation of 'small rift zones of oceanic character' together with the attenuation of continental crust in Central Iran.With the disappearance of the Hercynian Ocean, the floor of the High-Zagros Alpine Ocean started to subduct beneath southern Central Iran and apparently disappeared by Late Cretaceous – Early Paleocene time (65 Ma). From this time the compressional motion between Arabia and Eurasia has been accommodated in Iran by shortening and thickening of the continental crust. This crustal thickening is accompanied by a progressive, though eventful, transition from marine to continental conditions over the whole region.A striking feature highlighted in this study is the existence of extensive alkaline and calc-alkaline volcanics, which appear to be unrelated to subduction. The intrusion of these rocks started in Middle Eocene time (45 Ma) and extended to the present. It is clear that some major fault systems have played a continuous but varied role from the Precambrian until the present, and whatever controlled the original fold orientation at the onset of continental compression (65 Ma) apparently still controls the orientation of contemporary folding.

In rocks deformed by natural orogenic processes it is usual to find that the finite strain state varies from locality to locality. In some deformed rocks high strain states are localized within approximately planar zones commonly known as "shear belts".The general relationships that exist between variable displacement and variable strain state are established, and these general equations are solved for particular types of strain within shear zones. Only a limited number of types of solution are possible. Using these solutions the geometric forms of the structures found in shear zones in several regions are analyzed. Methods for computing the finite strain through these zones are described, and these finite strains are integrated to determine the total displacements across these zones. Schistosity is developed in some of the shear zones described. It is not parallel to the walls of the shear zone and is therefore not parallel to the dominant displacement (shear) directions. The schistosity appears to be formed perpendicular to the principal finite shortening (i.e. perpendicular to the shortest axis of the finite strain ellipsoid). Variations of the schistosity planes represent variations in the finite strain trajectories of XY planes in the strain states ([Formula: see text] ellipsoid axes). The intensity of development of the schistosity is correlated with the values of the principal finite strains.

Post-depositional concentric deformation produces no significant change in rock volume. Since bed thickness remains constant in concentric deformation, the surface area of a bed and its length in a cross-sectional plane must also remain constant. Under these conditions, a simple test of the geometric validity of a cross section is to measure bed lengths at several horizons between reference lines located on the axial planes of major synclines or other areas of no interbed slip. These bed lengths must be consistent unless a discontinuity, like a décollement, intervenes. Consistency of bed length also requires consistency of shortening, whether by folding and (or) faulting, within one cross section and between adjacent cross sections.The number of possible cross-sectional explanations of a set of data is reduced by the fact that, in a specific geological environment, there is only a limited suite of structures which can exist. This imposes a set of local "ground rules" on interpretation. When these local restrictions are coupled with the geometric restrictions which follow from the law of conservation of volume, it is often possible to produce structural cross sections that have a better-than-normal chance of being right.The concept of consistency of shortening can be extrapolated to a mountain belt as a whole, thereby indicating the necessity for some kind of transfer mechanism wherein waning faults or folds are compensated by waxing en echelon features. These concepts are illustrated diagrammatically and by examples from the Alberta Foothills.

Anomalous-fading rates were measured in K-feldspars separated from 49 sediment samples, mainly from North America. The intensity of the optically stimulated luminescence was found to decrease linearly with the logarithm of time since irradiation between 2 days and ~1 year of storage at room temperature. Anomalous-fading rates ranged from 2% to 10% per decade, a decade being a factor of 10 in time since irradiation. The sample provenances were sufficiently varied that anomalous fading appears to be ubiquitous. We have experimented with correction of optical ages for anomalous fading on the assumption that the observed fading can be extrapolated a further four decades in time. The corrected ages are in satisfactory agreement with independent ages. These results are restricted to the low-dose region of the dose response and are not expected to be applicable to samples older than ~20–50 ka.

Monazite is an underutilized mineral in U–Pb geochronological studies of crustal rocks. It occurs as an accessory mineral in a wide variety of rocks, including granite, pegmatite, felsic volcanic ash, felsic gneiss, pelitic schist and gneiss of medium to high metamorphic grade, and low-grade metasedimentary rocks, and as a detrital mineral in clastic and metaclastic sediments.In geochronological applications, it can be used to date the crystallization of igneous rocks, determine the age of metamorphism in metamorphic rocks of variable metamorphic grade, and determine the age and neodymium isotopic characteristics of source materials of both igneous and sedimentary rocks. It is particularly useful in the dating of peraluminous granitic rocks where zircon inheritance often precludes a precise U–Pb age for magmatic zircon. The U–Pb systematics of the mineral are not without complexity, however. Being a mineral that favors incorporation of Th relative to U, it can contain considerable amounts of excess ²⁰⁶Pb derived from initially incorporated ²³⁰Th, an intermediate decay product of ²³⁸U. Corrections for this effect can be made using the Th/U ratio of the host rock, but these corrections may not always be valid. Monazite is known to be capable of preserving inheritance in a manner similar to that of zircon, and it can lose Pb during episodic or prolonged heating events of uppermost amphibolite and granulite facies metamorphic grades. Monazite is less retentive of Pb than zircon during high-temperature igneous and metamorphic processes, and a few studies of its behavior suggest that its closure temperature is approximately 725 ± 25 °C. Examples of U–Pb systematics from most of the above situations are presented in this paper to illustrate both the utility and complexity of monazite in geochronological studies in an attempt to encourage more widespread application of this dating method.

The Appalachian Orogen is divided into five broad zones based on stratigraphic and structural contrasts between Cambrian–Ordovician and older rocks. From west to east, these are the Humber, Dunnage, Gander, Avalon, and Meguma Zones.The westerly three zones fit present models for the development of the orogen through the generation and destruction of a late Precambrian – Early Paleozoic Iapetus Ocean. Thus, the Humber Zone records the development and destruction on an Atlantic-type continental margin, i.e., the ancient continental margin of Eastern North America that lay to the west of Iapetus; the Dunnage Zone represents vestiges of Iapetus with island arc sequences and mélanges built upon oceanic crust; and the Gander Zone records the development and destruction of a continental margin, at least in places of Andean type, that lay to the east of Iapetus.The Precambrian development of the Avalon Zone relates either to rifting and the initiation of Iapetus or to subduction and a cycle that preceded the opening of Iapetus. During the Cambrian Period, the Avalon Zone was a stable platform or marine shelf.Cambrian–Ordovician rocks of the Meguma Zone represent either a remnant of the continental embankment of ancient Northwest Africa or the marine fill of a graben developed within the Avalon Zone.Silurian and younger rocks of the Appalachian Orogen are mixed marine and terrestrial deposits that are unrelated to the earlier Paleozoic zonation of the system. Silurian and later development of the orogen is viewed as the history of deposition and deformation in successor basins that formed across the already destroyed margins and oceanic tract of Iapetus.

A total of 204 surging glaciers has been identified in western North America. These glaciers surge repeatedly and probably with uniform periods (from about 15 to greater than 100 years). Ice flow rates during the active phase may range from about 150 m/year to > 6 km/year, and horizontal displacements may range from < 1 to > 11 km. Ice reservoir and ice receiving areas can be defined for surging glaciers, and the reservoir area does not necessarily coincide with the accumulation area. Glaciers of all shapes, sizes, and longitudinal profiles can surge, and no unusual "ice dams" or bedrock constrictions are evident. Surges occur in many different climatic, tectonic, and geologic environments, but only in certain limited areas (mainly in the Alaska, eastern Wrangell, and St. Elias mountains). Three types of surging glaciers are defined: (I) large to moderate-sized glaciers with large displacements and very fast flow, (II) large to moderate glaciers with moderate displacements and flow rates, and (III) small glaciers with small displacements and moderate to fast flow rates. All three types involve an inherent instability which is self-triggered at regular intervals, but with Type I surges an additional (unknown) mechanism produces the very high flow rates.

Large igneous provinces (LIPs) are high volume, short duration pulses of intraplate magmatism consisting mainly of flood basalts and their associated plumbing system, but also may include silicic components and carbonatites. Many LIPs have an associated radiating diabase dyke swarm, which typically converges on a cratonic margin, identifies a mantle plume centre, and is linked to breakup or attempted breakup to form that cratonic margin. We hypothesize that every major breakup margin in Canada can be associated with a LIP, and we attempt to identify this LIP. To this end, we focus mainly on high-precision age determinations and the distribution of diabase dyke swarms, which are uniquely valued for preserving the record of magmatic events. The analysis extends from the Phanerozoic to the Neoarchean, but our most complete information is for the Superior craton. There, events at 2.50–2.45, 2.22–2.17, and 2.12–2.08 Ga (LIP and plume) are linked with rifting and breakup or attempted breakup of the south-southeastern, northeastern, and southern margins, respectively. Events at 2.00–1.97 Ga are probably linked with the northern margin (Ungava promontory), while the Circum-Superior event at ca. 1.88 Ga is linked to the north to northwestern margins during a time of Manikewan Ocean closure. Similar linkages for other cratons of North America improve understanding of the breakup history to help identify which blocks were nearest neighbours to Canadian crustal blocks in Precambrian supercontinents. Such interpretations provide a framework for interpreting other geological features of these margins to further test models for the timing and location of breakup.

Hydrogeologists and ground water geochemists have observed a decline in the measured (platinum electrode) potential, E_H, of ground water as it migrates from upland recharge areas to lowland discharge areas under confined conditions. Such variations in E_H and variations in concentrations of elements with variable oxidation states (e.g., oxygen, nitrogen, iron, manganese, sulfur, and carbon) can be accounted for by a sequence of oxidation–reduction reactions occurring in the flow systems, the sequence being based on thermodynamic principles as outlined by Stumm and the microbial catalysis of such reactions. This sequence of reactions results in the identification of three redox zones in ground water flow systems: (1) oxygen–nitrate, (2) iron–manganese, and (3) sulfide. The mobility and concentration of most transition metal and nonmetal ions vary according to zone—a matter of considerable significance in exploration geochemistry and in ground water pollution studies.