Stratigraphy and Petrology of the Mulcahy Lake Layered Gabbro: An Archean Intrusion in the Wabigoon Subprovince, Ontario

The Mulcahy Lake gabbro is an Archean layered intrusion of tholeiiticbulk composition located in the Wabigoon subprovince. The intrusionis 6 km thick at the thickest and is exposed over an area of63 km². It intrudes basaltic to siliceous volcanics of the CrowLake-Savant Lake greenstone belt and is intruded by the Atikwabatholith. Zircon U-Pb data indicate crystallization at 27322+1·0/–0·9m·y. Principal phases are plagioclase, orthopyroxene, augite (andpigeonite in iron-rich rocks), olivine, hornblende and magnetite.Olivine is confined to several horizons. Apatite and then zirconare prominent accessory phases at advanced stages of fractionation.Plagioclase, pyroxenes and olivine are cumulate phases. Hornblendeis invariably an intercumulus phase. Magnetite is ubiquitousthroughout the intrusion, generally as a cumulate phase, andforms centimeter thick layers in fractionated rocks. Fractionationfollowed a tholeiitic trend with iron enrichment in the liquid. The intrusion is divided into lower, mixed, middle, upper andmarginal zones. The lower and middle zones are 2·0 and2·5 km thick respectively. The upper zone is approximately1 km thick, and the marginal zone is measured in hundreds ofmeters. A 200 m thick mixed zone is interposed between the lowerand middle zones. The base of the lower zone consists of ultramaficunits containing olivine of Fo₈₂. The top of the zone has olivineof Fo₂₈. Fractionation of the lower zone, from the floor up,was interrupted by the introduction of pristine liquid whichmixed with more dense and cooler residual liquid in the chamberto form the mixed zone. Further introduction of several minorpulses of liquid constructed the lower part of the middle zone.The upper part of the middle zone was constructed from a majorpulse of liquid plus several minor pulses each of which is representedby reversals in cryptic layering. The upper zone consists ofultramafic to iron-rich gabbro cumulates formed by cooling throughthe roof plus horizons formed by influx of pristine liquid.Marginal zone rocks represent cooling through the walls of theintrusion. Rhythmic layering is well developed in lower and middle zonecumulates. Petrofabric data show that orthopyroxene has a lineationin the plane of layering and parallel to structures suggestiveof flow. Plagioclase laths also have a preferred orientationin many cumulates and in unlayered gabbros as well. Flow, possiblylaminar, of liquid-crystal material is suggested and may belinked to the ultimate development of layering. Pressure during the course of crystallization probably was greaterthan 2 and less than 5 kb. Temperatures estimated from pyroxenesvaried from approximately 1200 to 1000 °C.f_o², is not wellconstrained but was sufficient to allow the formation of thinlocal magnetite cumulates late in the crystallization. The primarymelt was hydrous as indicated by the presence of hornblende.It is very unlikely that the melt was saturated with water duringcrystallization of the cumulate phases.     

THE INFLUENCE OF PERMEABILITY ON COMPRESSIONAL WAVE VELOCITY IN MARINE SEDIMENTS*

To investigate the effect of permeability on the propagation of seismo-acoustic waves through marine sediments, a theoretical model based on Biot's equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability. The model is examined experimentally in a standard soil mechanics consolidation test (itself dependent, among other things, on sediment porosity and permeability) which has been modified to include measurements of compressional wave velocity at 1 MHz and shear-wave velocity at 5 kHz. This test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations. From a number of tests on different samples, five samples are chosen to typify the range of sediment sizes. The results show that the difference between the measured velocity at 1 MHz and the model-derived velocity at zero frequency increases with increasing particle size (from clays to fine sand), with decreasing porosity, and with increasing permeability. For sediments coarser than fine sand the simple model breaks down, possibly because of the dominance of scattering/diffraction effects at the high frequency of the experiment. Within this limitation the model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies; the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements.     

Scanning electron microscopy of Pleistocene tills in Estonia

Tills from four Pleistocene glaciations were recovered from drill cores in Estonia and subjected to particle size and microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thick continental ice-sheets following the transport of, at most, several hundred kilometers during four Fennoscandian glaciations. The main problem is to determine if the type and range of microtextures present on the grain surfaces are diagnostic of transport in continental ice. The frequency of occurrence of microtextures including fractures, abrasion, and relief features are used to test the ability of continental ice to damage quartz particles emplaced as till. The range of quartz dissolution and presence of coatings on grains are also used to reconstruct the paleoenvironment that existed prior to transport as well as to estimate diagenetic effects that occurred following emplacement. The available data indicate a high degree of reworking of quartz grains from one glaciation to another. While the shapes and microtextures of grains from source rocks are not known, the great range of fracture and abrasion microfeatures, and high frequency of occurrence on grains in all tills, indicate that glaciers are effective crushing agents. An increase in the prevalence of chemically etched grains from older to younger tills suggests that some grains ( c . 50%) escape crushing, either because of preservation in the ice and lack of grain-to-grain contact, or as a result of massive reworking of weathered grains following interglaciations.     

LINE CURRENT FILTERING OF FIXED-LOOP TRANSIENT ELECTROMAGNETIC DATA1

The VLF filtering technique of Karous and Hjelt has been applied to fixed-loop step-response transient electromagnetic data. This allows the data measured in each channel to be converted to an equivalent current-density pseudosection. For a conductive half-space, the maximum value of the equivalent current density starts near the transmitter loop and migrates outwards as a function of delay time. The rate of migration tends to increase as a function of delay time, with the increase being faster for a surficial conductive layer than it is for a half-space. Theoretical and field examples show that the currents tend to be more persistent in the relatively conductive areas, so that a pseudosection which is the average of the current densities at all delay times will highlight the more conductive zones. In resistive ground, it is not so critical to average the pseudosections as a particular delay time may give a better idea of the conductivity structure. For example, the latest possible delay time will reveal the most conductive features.     

Zones of Progressive Regional Burial Metamorphism in Part of the Tasman Geosyncline, Eastern Australia

A succession of metamorphic zones can be recognized along theMolong Geanticline, an ancient rise within the Tasman Geosynclineof Eastern Australia. In order of increasing grade, the zonesin the volcanogenic rocks are: (1) an albite-quartz zone, containingneither prehnite, pumpellyite, actinolite, nor biotite; (2)a prehnite zone; (3) a prehnite-pumpellyite zone; (4) an actinolitezone; and (5) a biotite zone. Zeolite associations are not developed.Zones 4 and 5 lie within the greenschist facies. The rocks alongthis geanticline are generally only slightly deformed, lackingcleavage. The grade of this metamorphism within the Molong Geanticlineincreases partly in accordance with increasing stratigraphicdepth and partly increases eastwards towards the adjacent HillEnd Trough. Within the Trough, rocks show metamorphic assemblagesakin to those of Zones 4 and 5 as defined within the MolongGeanticline, but deformation is more extreme and a regionalcleavage is characteristic. In the region about the Hill EndTrough and the adjacent geanticlines there is an apparent variationfrom essentially static burial metamorphism of the geanticlinesthrough transitional types to tectonic dynamothermal metamorphismof the trough. Both metamorphisms are envisaged as having arisenduring the same general period of metamorphism, but deformationof the geanticlines was much less than deformation in the interveningHill End Trough.     

RIPPLE MARK INDICES AND THEIR USES

The following dimensionless parameters (two of them well-known and five of them new) are defined for determination of ripple mark geometry: ripple index (RI), ripple symmetry index (RSI), continuity index (CI), bifurcation index (BI), straightness index (SI), and two different parallelism indices (PI₁ and PI₂). In general, RI = 15 or less indicates wave or water current origin; RI = 17 or more indicates wind or swash origin. RSI = 1.5 or less indicates wave or swash types; RSI = 3 or more indicates wind or water current types. CI = 15 or more suggests wind or wave origin; CI = 10 or less suggests water current origin. BI = 10 or more suggests wave varieties; BI = 1 or less suggests wind varieties. SI = 10² or more indicates wind or deep-water wave types; SI = 15 to 10² indicates wind or wave types; SI =4 or less indicates current types. PI1 = 7 or more suggests wave origin; PI1 = 1 or less suggests water current origin. PI2 = 0.4 or more is probably the result of swash or water current action. PI2 = 0.2 or less is probably the result of wind or wave action. Longitudinal ripple marks (such as rib-and-furrow) and deformed or modified varieties (such as flat-topped tidal-flat ripple marks and nearly- flat-topped intermittent creek ripple marks) have been excluded, inasmuch as (1)they are commonly easy to identify from their appearance, and (2)they are difficult to measure with ordinary methods. Plots of two indices against each other on coordinate paper can be particularly useful; the best combinations are RI vs. RSI, and RI vs. PI1, although several other pairs are almost as good. Where all seven parameters can be obtained, the confidence one can have in the interpretation is close to 98%. The effects of current bias, or depth bias, on wave-type ripple marks, extend to both the symmetry (RSI) and to sediment-transport interpretations. Unless the investigator is reasonably sure that no such bias is present (i.e., RSI = 1.0 instead of some significantly higher value such as 1.5), wave-type ripple marks cannot be used to determine direction of either wave approach or sediment transport. If no such bias is present, wave-type ripple marks still cannot be used to determine precise sediment transport direction. If RSI = 1.0 precisely, it is not even necessary that the ripple crests parallel the waves that formed them. The same restrictions apply to the interpretation of micro-crossbedding (that is, ripple mark internal structure). Despite these seemingly severe limitations, general geometry commonly permits a reliable interpretation, and hence ripple marks can provide a great deal of useful data for paleogeographic interpretations. The swash-zone variety of ripple marks includes two sub-types: those modified by a small but unmistakeable hydraulic jump, and those not so modified. The RI can be used to distinguish between these two, even when they were not observed to form.     

Geochemistry of till in perennially frozen terrain of the Canadian shield— application to prospecting

Shilts, W. W. 1977 06 01: Geochemistry of till in perennially frozen terrain of the Canadian shield - application to prospecting.
Geomorphological (drumlins, ribbed moraine) and geochemical features associated with till in the District of Keewatin are arranged in belts or dispersal trains paralleling the main direction of ice flow. Both types of features can be related to chemical and physical characteristics of specific types of source rocks.
For dispersal studies in perennially frozen terrain, till samples were collected from mudboils at spacings of approximately 1.6 km. The texture of till samples varies significantly from sample site to sample site because of varying source-rock lithologies and periglacial processes. Thus, because fractions coarser than clay are mostly quartz and feldspar and contain very little metal after weathering in the active layer, they were removed by centrifugation so as not to mask the 'true' relative metal contents of samples. The clay-sized fraction was separated from till samples and analyzed on the assumption that it contained scavenging phases that adsorb cations in proportion to those released by weathering of mineralized particles that were originally physically dispersed by glacial action.
Dispersal patterns of copper, zinc, nickel, and uranium were derived for approximately 1000 samples evenly distributed over a 2500 km² grid. From these maps, large dispersal trains of copper and nickel were found, and known areas of high potential for uranium and Cu-Zn mineralization were clearly indicated.     

The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H

Phase relations for the bulk compositions 3CaO·2FeO_x·3SiO₂+excessH₂O and CaO·FeO_x·2SiO₂+excess H₂O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO₂. Andradite, Ca₃Fe³⁺₂Si₃O₁₂, synthesized above 550 °C hasan average unit cell edge, a_o, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, a_o increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P_fluid andradite is stable above an f_O2of 10¹⁵ bar at 800 °C and 10-³² bar at 400 °C. At lowerf_O2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe²⁺SiO₄)+ wollastonite and kirschsteinite+xonotlite (Ca₆Si₆O₁₇(OH)₂). Synthetic hedenbergite, CaFe²⁺Si₂O₆, has average unit cell dimensionsof a_o = 9.857± 0.004 Å, b_o = 9.033±0.002Å, c_o = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P_fiuid, hedenbergite is stable below an f_O2 of10-¹³ bar at 800 °C and 10-²⁸ bar at 400 °C. Above thesef_O2 values, hedenbergite+O₂ breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the f_O2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H₂O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences.