V. E. Chamberlain

The Ordovician, Silurian and Devonian time scales

A reassessment of published Ordovician, Silurian and Devonian isotopic age determinations available has necessitated the correction of several calculated dates and revision of several stratigraphic conclusions in the light of later work. The revised dates plot close to a single time scale line, except for Rb Sr dates on acid volcanic rocks. We conclude that the base of the Devonian is at 410 Ma, the base of the Silurian at 437 Ma, and the base of the Ordovician (Tremadoc Series) at 519 Ma. These dates are within the range of previously published dates for the base of the three periods. We place more reliance on our estimates for the Upper Ordovician, Silurian and Devonian than for the Lower Ordovician. A comparison with other time scales shows that our new scale is similar to many previously published, except for those which rely heavily on acid volcanics.

Cordilleria Revisited, with a Three-Dimensional Model for Cretaceous Tectonics in British Columbia

The Cordilleria microcontinent theory for the tectonics of British Columbia in the Cretaceous and Early Tertiary and for the origin of the Canadian Rocky Mountains and related features is reviewed. New isotopic, geological, and paleomagnetic data support the hypothesis that the amalgamated superterranes of British Columbia and Alaska were not attached to the mainland of North America in the early Cretaceous and were probably well south of their present relative position. Consideration of that evidence, together with sedimentological evidence from the Albian, leads to a slightly earlier best estimate (115 Ma) for the time of initial collision in the Yukon. New geophysical data for southeastern British Columbia and Idaho lead to a comparison with the Iapetus Suture in northern England: it is suggested that the Cordilleria Suture dips west from the present Purcell Thrust and related structures and should cut the Moho. A new model for regional deformation in SE British Columbia is proposed, involving a flake structure based on regional thrusts, which originally dipped northwest. These were contemporaneous with the first phases of thrusting to the east in the cover above the A-type subduction zone of the Rockies, from which they were separated by a major zone of weakness at the surface of the subduction zone.

CO2 permafrost and Martian topography

Abstract The role of CO 2 permafrost as an erosive agent on Mars is considered. In the CO 2 H 2 O system, with a CO 2 triple point at 217°K and 5.1-bar pressure, carbon dioxide solid, liquid, or gas, CO 2 clathrate, and ice are possible stable phases in the range of temperatures and pressures likely to be encountered in the Martian regolith. It is argued that conditions may exist in which CO 2 permafrost is extensive on Mars, provided that adequate CO 2 is available: the maximum ratio of H 2 O:CO 2 required in the subsurface pore space system is 17:3. Erosional processes likely to result from such permafrost are block slumping, leading to canyon development; pit chains along faults; chaotic terrain where massive permafrost destruction has occured; large-scale flows of slurry; and perhaps even the flash floods which create channels.

Lead isotopes and the sources of the Columbia River Basalt Group

A detailed study of 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios suggests that the number of Pb isotopic reservoirs required for the Columbia River Basalt Group (CRBG) (as currently defined) must be increased from the presently accepted four to at least six. The identities of the six reservoirs are two of mid-ocean ridge basalt (MORB) slightly contaminated with sediment (Rl and R2, Rl being “Cascades” Pb); one a probable plume component (R3); two of unspecified mantle material but probably of Rl type, contaminated with local crustal Pb (R4 and R5); and the sixth a complex enriched reservoir of 2150 Ma age (R6). Rl is an end-member for all CRBG, except the Saddle Mountains Basalts. R2 is a second end-member for the Picture Gorge Basalt and appears to be unique to it. Both Rl and R2 are related to the Juan de Fuca subduction zone and the back arc spreading associated with it. R3 is the principal source for most of the Wanapum Basalt, as well as for most of the chemically evolved portions of the Grande Ronde Basalt. Volumetric and tectonic considerations require that one source of the CRBG be a plume or, at least, a nonlocal crustal or uppermost mantle source, and R3 is the ideal candidate for that role. R4 is the second source for the Imnaha Basalt, and R5 is the source for the 206Pb rich varieties of the Grande Ronde Basalt. R4 and R5 are identified with contamination by local Phanerozoic crust because of their similarity to local crustal Pb and because the rocks which contain them also have Cu contents correlated with their Pb isotopic compositions. R6 is the parent for Pb in all the Saddle Mountain Basalts. This last source appears to have been homogeneous 2150 m.y. ago but has since split into a number of separate, discrete pockets, each with its own characteristic Pb isotopic signature. Some of these appear in individual Saddle Mountain Basalt flows, while other sources have mixed with R4 to produce individual Saddle Mountain Basalt flows of the Ice Harbor set. R6 is similar in all its isotopic properties to the Precambrian basement that is exposed to the immediate north of the Columbia River Basalt Group. The overall model for Columbia River Basalt Group genesis is termed “plume reinforced back arc magmatism”.

RbSr and zircon study of ~2800 Ma Lewisian silicic gneisses from the Torridon Inlier of NW Scotland: Dyke intrusion and an open system

The Torridon Inlier of the Lewisian of Scotland contains ~30 sq. km of granodioritic gneisses. A coordinated study of this inlier, including detailed mapping, geochemical analyses and geochronological work, has indicated that Rb-Sr whole rock analyses yield “ages” that are functions of the local structural setting and of the chemistry and mineralogy of the rock, as well as of the overall geological history of the region. This Archean region has been invaded by a swarm of northwest striking dykes of Inverian (2400 to 2240 Ma) age, accompanied by new foliations and major structures. However, several structurally distinct areas, termed “pips”, 1–2 km2 in extent, are free of Inverian foliation and largely free of Inverian dykes. The 38 samples analysed for Rb and Sr scatter widely about a 2.66 Ma isochron; but give meaningful results only when subdivided according to their structural setting and lithology. A subset of silicic gneiss samples from the area most strongly overprinted by Inverian structures yields an age of 2240 ± 70 Ma with an initial 87Sr86Sr of 0.7098 ± 18. A subset of samples from “pips” further north yields an age of2790± 100 Ma (initial 87Sr86Sr = 0.7020± 10), identical to the 2780± 70 Ma U-Pb age obtained from zircon fractions separated from two of the samples. Samples from this northern district, located outside the “pips” in areas showing weak Inverian foliation, or within 50 m of Inverian dykes, or both, have grossly disturbed Rb-Sr systematics. The apparent ages of the felsic gneisses have been increased, while those of their mafic enclaves have been decreased (to as low as 1380 Ma). Field and geochemical evidence relate the increase directly to Rb loss and/or radiogenic Sr gain in the immediate vicinity of major dykes, which themselves have gained Rb and K by comparison with dykes elsewhere in the Lewisian. In terms of Rb-Sr work on complex terranes, these results must caution against direct interpretation of such data, especially in the absence of detailed field knowledge.

Petrology and geochemistry of the archean rocks of the malton gneiss complex, British Columbia

Abstract Hornblendic and felsic groups of Archean age and each of broadly similar geochemical composition across the Malton Gneiss Complex southeast of Valemount, Bristish Columbia, show detailed mineralogical and chemical variations from one geographical district to another. The complex appears to be composed of meta-igneous rocks which were originally developed as petrologically discrete units on a roughly 10-km scale, but which all belonged to one similar petrographic province. Overall they are characterized by rather high immobile trace and alkali element abundances. Their nearest geochemical equivalent appear to be found in the grey gneiss complexes of the North Atlantic Craton rather than in the granite-greenstone complexes of the Canadian Shield, but no other Archean complex possesses their overall characteristics. Some similarities in A-F-M and Q-Ab-Or content of these gneisses and those of the Laramie Range, Wyoming, are noted.

The Malton Gneiss: Archean gneisses in British Columbia☆

Abstract Petrological and geochemical analyses of gneisses from the Malton Gneiss Complex (199°W 52°30′N, south of Valemount, B.C. and west of the Rocky Mountain trench) reveal four distinct mineralogical and geochemical groups, based on their petrology, Y and Nb contents and Fe Mg ratios. The Complex appears to be an Archean meta-igneous complex, which has been chemically modified, with alkali enrichment, during an Upper Proterozoic igneous event. Group 1, the most common rock type in the Complex, consists of mafic biotite—hornblende gneisses and amphibolites. They have low Y contents and the lowest Fe Mg ratios in the Complex. (Na2O + K2O) averages about 5%, K Rb ratios are between 200 and 400, K/Ba between 200 and 100 and Sr Ba ratios vary widely. The group 2 rocks are granitic and tonalitic gneisses characteristically garnetiferous and preserving their Archean age. They have Y Nb ratios of about 2 and low Y and Nb contents. Rb values are also low, but not exceptionally so, the range being 70–180 ppm. K Rb ratios lie between 20 and 400, K/Ba between 20 and 50 and Sr/Ba between 0.1 and 0.5. Groups 3 and 4 are alkali granitic gneisses generally occurring as intrusive sheets. Group 3 gneisses have Y Nb ratios of approximately 0.5 and have higher Nb contents than groups 1 or 2. Group 4 gneisses have Y Nb ratios of apporoximately 1 and both Y and Nb contents are higher than in groups 1 and 2. Parameters indicating an increase in differentiation are higher in groups 3 and 4 than in groups 1 and 2, but group 3 gneisses include some rocks with very high K Rb ratios, the range being 200–800 and the average above 400. The Malton Gneiss has no obvious analogues in the nearest surface Canadian Shield. It has geochemical similarities with the Grey Gneiss of the North Atlantic Craton and may be similar to the Archean of Western Montana.