Paleomagnetism of Vendian rocks in the southwest of the Siberian Platform

Analysis of the Magnetization Components

Biryusa Area of the Sayan Region

Aisa Formation.

Figure 5

[30] The large number of the study samples contain a noisy paleomagnetic signal, difficult to interpret, and characterized by the presence of several magnetization components with the overlapping spectra of the blocking temperatures. The pattern of the paleomagnetic record is somewhat different from one outcrop to another. Yet, each outcrop shows a confident magnetization component, which is referred to as a medium-temperature one in the text that follows. This component is a monopolar one, has a NW magnetic declination and intermediate inclination and decays over a wide temperature range, varying sometimes from 100^o C to the Curie point of hematite. Its typical destruction interval varies form 250^o to 500-560^o C. In the Zijderveld diagrams (ZD) (samples B109 and B119 in Figure 5), its mean temperature component usually does not "move" to the origin of the coordinates, suggesting that the samples may often contain some more stable, high-temperature component. In some samples, in spite of the obvious presence of the high-temperature component, the latter could not be identified because of the beginning chemical alterations recorded by the notable growth of the magnetic susceptibility of the samples and resulting in the chaotic or quasichaotic behavior of the NRM vector at the high cleaning temperatures. Nevertheless, we managed to identify, with a variable accuracy, the most stable high-temperature magnetization component in 40 out of the almost 100 samples collected from the Aisa Formation. This component showed its bimodal distribution in the stereogram, its low inclinations, and SSW (NNE) declinations. It was most often recorded in a fairly narrow temperature range from 600^o C to 680^o C (see Samples 243 and B148 in Figure 5), although there were some scarce exceptions.

[31] One of the outcrops studied in the Tagul R. area showed another stable component (see Sample 243 in Figure 5), which is referred to below as an intermediate one. This component disintegrated in the temperature range of 540-640^o C and showed a trend close to that of the high-temperature component with southern declination and low inclination. In fact, the more or less confident recording of this component in the outcrop discussed was possible only because of the fact that the high-temperature component has a different polarity here. The problem of the time of this component formation relative to the high-temperature component will be discussed here somewhat later. The traces of the presence of the intermediate component have also been discovered in analyzing the Zijderveld diagrams of another Tagul outcrop. However, the trend of the intermediate in this outcrop could not been determined because of the high effect of the blocking spectra of the magnetization components.

[32] It is worth mentioning that the study samples did not show any modern component, which is usually widespread in all rock types as a low-temperature and poorly stable one. The magnetization vectors, destroyed in the low-temperature region of 100-250^o C are distributed irregularly (laboratory viscous magnetization?) possibly with some potential very poor grouping around the trend of the medium-temperature component.

Ust-Tagul Formation.

[33] The magnetization of the rocks of this formation is distinguished usually by the presence of several magnetization components, often with the significant overlapping of their blocking temperature spectra. This is aggravated by the chemical alteration of the rocks, which often begins where the heating temperature is higher than 550-600^o C. Thus, the identification and trend calculation of the magnetic components (especially, of the high-temperature component) in the study rocks of the Ust-Tagul Formation is not a simple and sometimes an unsolvable problem. Nevertheless, considering the general fairly unfavorable background, some of the study samples allow one to identify and classify their magnetization components.

[34] The low-temperature range (20-250^o C) showed a poorly stable component, often with high inclination and variable declination, which can be interpreted as some irregular mixture of laboratory viscous, modern, and partially middle-temperature components.

[35] Similar to the rocks of the Aisa Formation, the Ust-Tagul samples showed the most distinct medium-temperature component, usually not extending to the origin of the coordinates and destructible in the temperature range of 250^o to (500-600)^o C (see Samples 173 and 286 in Figure 5). This is a monopolar component with NW declination and moderate inclination. In some cases (see Sample 173 in Figure 5) this component can be the only magnetization component in the study sample.

[36] The high-temperature component (see Samples 298, 274, and 166 in Figure 5) is usually distinguished in the temperature range higher than 600^o C, although it often begins to decay significantly earlier in the temperature range, where the contribution of the medium-temperature contribution is fairly significant. The overlapping of the medium- and high-temperature components often leads to the fairly complex behavior of the paleomagnetic signal in the temperature range of 500-600^o C. There are individual samples, where the contribution of the medium-temperature component is insignificant compared to that of the high-temperature component. In such cases, the behavior of the NRM vector is controlled in the course of cleaning mainly by the presence of the high-temperature component (see Sample 298 in Figure 5). Most of the identified vectors of the high-temperature component showed NNE declination, moderate and low (up to negative) inclination, and also SSE declination and low inclination. In the text that follows we attempt to demonstrate that the distribution of the high-temperature component vectors can be explained by the superposition of two highly temperature-stable components.

Central Sayan Region

Urik R. outcrops.

[37] The samples of our collection, representing the Mota and Irkutsk formations, have similar paleomagnetic properties and, hence, are discussed here together.

Figure 6

[38] Similar in age and lithology to the rocks of the Ust-Tagul Formation, the samples collected in the outcrops of the Urik R. Valley, demonstrate the presence of several magnetization components often with the overlapping blocking temperature spectra. The low-temperature component, which is removed toward the temperature of 200-250^o C tends to follow the modern field trend and seems to be largely a mixture of the modern and laboratory viscous components. The mean temperature component (Samples 151 and 25 in Figure 6) was identified in a number of samples over a wide temperature range, from 200-250^o C to 550-620^o C, showing NNW declinations and moderate inclinations. In some samples the spectrum of the deblocking temperatures of the mean-temperature component was found to extend as far as the Curie point of hematite, involving problems with its identification, because its maximum deblocking temperatures are similar to those of the component, which is referred to below as the high-temperature component. The identification criterium of this component was chosen in this study to be the width of the unblocking temperature spectrum, which is usually significantly lower in the high-temperature component.

[39] The high-temperature magnetization component (see Samples 16, 21, 59, and 182 in Figure 6) was identified confidently only in relatively few samples (usually at temperatures higher than 600^o C), although some traces of its presence could be found in much more samples. The maximum deblocking temperatures of this component are close to 680^o C, which proves that the carrier of this component is hematite. Generally speaking, the spectra of the deblocking temperatures suggest that the carrier of informative components, discussed in this paper, is hematite. Our thermomagnetic analysis of individual samples confirmed this conclusion. Similar to the Ust-Tagul Formation, the high-temperature magnetization component of the samples collected from the Urik outcrops shows either NNE declinations and moderate to low (up to negative) inclinations, or SSW declinations and low inclinations.