R.St.J. Lambert

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.

Mineral isotopic age relationships in the polymetamorphic Amîtsoq gneisses, Godthaab district, West Greenland

Abstract U-Th-Pb, Pb-Pb, Rb-Sr and K-Ar radiometric relationships in the minerals from six selected Amitsoq gneiss samples reveal a complicated history of variable mineral response to polymetamorphism. K-Ar dates on biotite range from 2170 to 3220 m.y. (excess argon present), on hornblende from 2340 to 2510, and on a single muscovite at 1670 m.y. Rb-Sr whole rock results give an apparent isochron of at least 4065 m.y., but this result is likely fortuitous from a small sample selection since Pb-Pb whole rock analyses give ~ 3600 m.y. and the zircons in these rocks yield a concordia-discordia intersection at 3600 m.y. Rb-Sr mineral analyses generally give a confusing and variable pattern of isotopic relationships; but hornblende, K-feldspar, apatite, allanite and sphene appear to have last responded to metamorphism at 2200–2600 m.y. Rb-Sr in biotite, epidote and, in part, plagioclase have been affected by an event at ~ 1550 m.y. U-Th-Pb data from sphene, apatite and allanite give almost concordant dates at 2500–2600 m.y. soul 207 Pb 204 Pb vs soul 206 Pb 204 Pb plots yield two separate lines for apatite (slope age 2435 m.y.) and for sphene + allanite (slope age 2530 m.y.), indicating apatite to have a different (less-radiogenic) ‘initial’ Pb than that for sphene and allanite. A similar pattern is found for the soul 208 Pb 204 Pb vs soul 207 Pb 204 Pb plot for sphene and apatite. The Pb-isotopic composition of the feldspars is very homogeneous and the least-radiogenic of all components, pointing towards a homogeneous parent material for the now lithologically diverse Amitsoq gneisses. Using a 0 = 9.307, b 0 = 10.294, C 0 = 29.476, t 0 = 4.56 b.y., ω = 6.9 and soul 232 Th 204 Pb = 27.1 ; the feldspars give a model Pb age of 3500–3600 m.y. by either U-derived or Th-derived Pb. The segregation of the present Amitsoq gneisses from the homogeneous parent material was apparently accompanied by a U and Th loss with preservation or enrichment of Pb at ~ 3600 m.y. ago. No consistent treatment of the present U-Th-Pb data will produce viable data indicating an age > 3600 m.y. for the parent materials of the Amitsoq gneiss. Petrographie observations generally concur with radiometric results and permit the postulation of the reaction: Hbl + K-feld→ biotite + epidote + sodic plag, to account for some of the effects of the latest metamorphism. The total internal radiometric evidence indicates three major metamorphic events affected the Amitsoq gneisses close to 3600, 2500 and 1550 m.y.

Major element chemical composition of shields and the continental crust

Abstract Chemical data from the Canadian Shield and the Precambrian of Scotland indicates that there is a progressive enrichment towards the surface in potassium and possibly in titanium, but observed variations in titanium do not form a simple pattern. The averages for the surface of the shield areas are more siliceous and sodic, and poorer in potassium, titanium and the ferro-magnesian elements than are most published continental crustal averages. Assuming that granulite facies rocks form a substantial part of the shields, the total shield analysis becomes richer in iron, magnesium and calcium, and poorer in silicon and potassium than published estimates of the composition of the shields, and because the shields form the dominant fraction of the continental crust, substantial differences in the average for the continental crust emerge compared with estimates based on igneous rocks. In particular, a total continental crust rich in granulite facies rocks is likely to be richer in silicon and sodium, poorer in iron, magnesium and potassium and very much poorer in titanium than one based on igneous compositions.

The thermal history of the earth in the Archean

It is argued that the earth is in thermal equilibrium, with heat production equal to surface heat flow. To produce an average flux of 60 mW m−2 a mantle with about 245 ppm K, 0.028 ppm U and Th at 0.080 ppm is required if there is zero heat production in the core. Such a mantle could comfortably produce tholeiites of the type found in Leg 37 of DSDP, drilling into the flank of the Mid-Atlantic Ridge. The thermal equilibrium argument can be extended back to the Archean—Proterozoic boundary, but no further, for thick Archean continents could only stabilize if the mantle—crust heat flux is limited to about twice todays value. The paradox of lack of extensive pre-3000 Ma continents may be resolved if a model with a heat-flow maximum is adopted; the double convection systems of McKenzie and Weiss come close to providing such a solution. Arguing further that high heat flows today are invariably associated with igneous activity, it is reasoned that all the Archean continents could be produced in only 60 Ma if the surface heat flow ever averaged 180 mW m−2, three times todays average or about equal to Iceland. Concluding that such an event has never occurred, together with other analogies, leads to a heat flux model in which a maximum of 125 mW m−2 is reached at 2800 Ma ago, preceded by a subsidiary maximum at 3500 Ma. Rapid continental growth occurred from 3000 to 2600 Ma, leading to stabilization as the heat-producing elements were transferred from mantle to crust. After consolidation of the scattered sial, the sub-continental mantle began to develop a lithosphere, leading to a period of “thin-skin tectonics” (the Proterozoic). Finally, when some of the oceanic lithosphere reached a thickness comparable with that of today, the plate-tectonic regime began.

Structural regimes and metamorphic facies

Abstract This paper is an attempt to bring together the physical and chemical aspects of metamorphism by relating rheology to metamorphic facies in the orogenic environment. Five regimes are considered and defined within an idealised orogenic crust. The first regime extends from below the level of diagenesis to the point at which metamorphic reactions commence. Experimental studies suggest that at this level deformation by creep has a logarithmic relationship with time, but it is known that other factors such as fluid-pressure will profoundly influence actual strains. The onset of metamorphic reactions defines the upper level of the second regime, which is characterised by the formation of micas and amphiboles at the expense of pre-existing sedimentary minerals. These reactions are all associated with the release of volatiles, which will bring about a marked increase in the creep-rate at this level. These reactions are accompanied by new structural styles, similar folds, widespread schistosities and recumbent isoclinal folds on all scales. The third regime occupies the levels approximately represented by the epidote-amphibolite and amphibolite facies, characterised by a restricted mineralogy and similar Theologies for all the common rock types. Deformation will be primarily by grain boundary or intra-lattice diffusion following the β-creep law (ϵ = βtn; n Regime four is equated with the higher pressure-temperature amphibolite facies and is distinguished from regime three by two groups of reactions, the first, the dehydration of the remaining common hydrous minerals, and the second, partial melting with the release of a granitic fluid. Both reactions effect an increase in the creep-rate, accompanied by the formation of gneissose structures, extensive flow folding and possible diapiric movements: rates of strain will vary widely with bulk chemistry of the system. Regime five corresponds to the granulite facies, extending into the upper mantle. Deformation is by prolonged laminar flow, causing simple structural styles, with all rock types possessing similar rheology.

Reflectance spectra of glass-bearing mafic silicate mixtures and spectral deconvolution procedures

Abstract The reflectance spectra of lunar, and perhaps asteroidal and Mercurian, soil analogs can be used to ascertain the effectiveness of continuum-removal procedures designed to isolate specific mineral absorption features. Spectra differences between natural and laboratory-produced glasses preclude the direct application of the latter to the analysis of remote sensing data. Nevertheless, it appears that currently used straight line continuum-removal procedures yield generally reasonable reconstructions of mafic silicate absorption bands in terms of band center wavelength positions. Better results would be achieved by removing a more accurately derived glass continuum from observational spectra.

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.

Model calculations of the thermal fields of subducting lithospheric slabs and partial melting

Abstract A new numerical model that simulates a downgoing slab is used to study the conditions required to produce melting on its upper surface. Models with dip angles of 26.6° and 45°, rates of subduction of 0.7 and 5.6 cm y−1, varying heat sources and rising material from the top of the slab are included. The results indicate that melting will not be greatly affected by dip angle, though the rate of subduction and the amount of shearstrain heating are important. When melting occurs, material rising from the top of the slab may produce high heat flow values at the surface of the earth on the continental side of the ocean trench, if the process continues sufficiently long. The sinking slab produces a positive gravity anomaly on the continental side of subduction, which is reduced in amplitude when rising material is present.

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.