Paleomagnetism of Vendian rocks in the southwest of the Siberian Platform

Yenisey Mountain Range

Alesha and Chistyakova Formations

Taseeva R. area.

[40] Most of the rock samples collected in the Chistyakova Formation did not show any regular paleomagnetic signal. Most of the rock material that remained after our magnetic cleaning showed the behavior similar to that of the samples of the Alesha Formation.

Figure 7

[41] The results of the magnetic cleaning allowed us to identify conventionally the low-temperature magnetization which was often destroyed in the temperature range of 350-400^o C and showed a trend similar to the trend of the modern geomagnetic field. In some cases this component was found to be very stable and was destroyed finally only at the Curie point of hematite (Figure 7, Sample TS594). In our opinion, this component is of recent origin and has a chemical nature.

[42] A few tens of the nearly 80 study samples showed a high-temperature component at the temperatures higher than 600-620^o C (Samples TS480, TS483, and TS506 in Figure 7). This component showed a fairly complex distribution pattern which will be discussed somewhat later.

Moshakova and Chistyakova Formations

Outcrop in the vicinity of the Man'zya Settlement.

[43] The samples of the Moshakova and Chistyakova rocks, collected in the vicinity of the Man'zya Settlement, showed a very poor quality of the paleomagnetic record. Many samples showed the irregular, quasichaotic variation of the NRM value and trend in the coarse of cleaning. The magnetization trend of most rock samples varied spasmodically, during the successive heatings from 100^o C to 450-550^o C, around or in the vicinity of the modern geomagnetic field trend, suggesting the presence of some recent magnetic component. At higher temperatures the NRM vector showed a chaotic behavior. In some samples the background was superposed at 300-400^o C by some intermediate component, distinguished by a great scatter, SE, S, and SW declinations and moderate inclination. Averaged for eight samples, this component showed D=180.9^o, I=46.4^o with K=7.6 and a₉₅ =21.1 in the modern system of the coordinates, and D=189.3^o and I=39.8^o with K=7.5 and a₉₅ =21.6 in the old coordinate system. It is obvious that this estimate of the intermediate component is very approximate not only because of the low grouping and high a₉₅ values, but also because of the obvious overlapping of this component spectrum with the spectra of the other magnetic components and because the highly noisy paleomagnetic signal. Here, we mention the presence of this component only for the sake of the complete description and similarity of its trend with the trend of the metachronous component identified earlier while studying the Ordovician reference rock sequence in the middle course of the Angara River in the vicinity of the Rozhkov R. mouth [Shatsillo et al., 4].

[44] Some of our samples showed a component similar to the component described below as a medium-temperature component for the outcrops of the Moshakova and Chistyakova suites in the vicinity of the Greben Cliff. This component decayed in the temperature range of 250-600^o C and showed NW declination and moderate inclination (in the modern and old systems of the coordinates). We failed to identify this component in its uncontaminated form.

Figure 8

[45] Finally, in the temperature range of 500-530^o C to 680^o C we identified a high-temperature magnetization component in several samples (Samples MN194 and MN227 in Figure 8), which showed mainly SSE declination and low inclination. This component was also identified in the rocks of the same age outcropping in the vicinity of the Greben Cliff. For this reason the data obtained for the high-temperature magnetization component of the samples collected from these outcrops will be discussed below together.

Outcrops in the Greben Rock vicinity.

[46] The quality of the paleomagnetic signal recorded in the rocks of the Chistyakova and Moshakova formations in the series of the outcrops located in the vicinity of the Greben Rock is not much better than that of the samples collected from the outcrop in the vicinity of the Man'zya Settlement. Their main difference is the more clearly manifested medium temperature component which decays at the temperatures of 250-550^o C and shows the NW declination and moderate inclination, as well as the presence of the relatively larger number of samples including a high-temperature component, the latter almost always showing the maximum blocking temperatures in the vicinity of the hematite Curie point. The recording interval of this component and its preservation seem to be associated with the development of remagnetizing components in the samples, namely, of the low-temperature modern component and of the medium-temperature of the metachronous one (as will be discussed below). On the whole, the NRM behavior of the samples, collected from these outcrops, in the course of their cleaning, can almost always be explained by the development of these components and by their overlapping deblocking temperature spectra. The examples of the Zijderveld diagrams demonstrating the presence of low-, medium-, and high-temperature components are shown in Figure 8 for the sample numbers of AA381, AA429, AA439, AA441, and AA452.

Figure 9

Redkolesnaya Formation

Outcrops in the Irkineeva R. Valley.

[47] We examined two outcrops of the Redkolesnaya rocks in the Irkineeva R. Valley. One of them, located up the river on its left bank, is composed mainly of brown and cherry-red, medium-grained sandstones. In the course of cleaning, the NRM vector trends of these samples varied irregularly or grouped with a large scatter around the trend of the modern geomagnetic field at the heating temperatures higher than 400-450^o C. At the same time, some samples showed an easily interpreted paleomagnetic signal demonstrating the presence of the low- and high-temperature magnetization components (see Sample IRK145 in Figure 9). The former was almost completely destroyed at 350-500^o C, being close to the trend of the modern geomagnetic field, the latter showed the maximum deblocking temperatures in the vicinity of the hematite Curie point, northern declination, and moderate positive inclination. A few samples from this outcrop showed a high-temperature component of opposite direction.

[48] In the other outcrop, located somewhat lower along the river at its right bank, the study rocks were represented by more fine-grained siltstone varieties of reddish and greenish colors. The paleomagnetic record was much more distinct there. The temperature cleaning showed the undoubted presence of two components: a conventionally low-temperature one with its direction close to that of the modern field and a high-temperature one showing mainly southern declinations and moderately negative inclinations (see Sample IRK-174 in Figure 9). The former component showed a significant destruction in the temperature range of 100-400^o C, although in some cases the spectrum of its deblocking temperatures extended to higher-temperature regions and was overlapped significantly by the spectrum of the high-temperature component. In these cases the latter was difficult or impossible to identify. Nevertheless, a fairly large number of samples showed their high-temperature components at high temperatures (550-600^o C).

The outcrops in the vicinity of the Greben Cliff.

[49] The NRM values of the rock samples collected in several outcrops at the right bank of the Angara River, higher than the Greben Rock, usually show two magnetization components (see Samples ANG327 and ANG330 in Figure 9), namely, a low-temperature one with the single vector trends scattered around the trend of the modern magnetic field and a high-temperature magnetization component with the maximum blocking temperatures of 650-680^o C. In some cases where the effect of the low-temperature component was low, the Zijderveld diagrams showed one high-temperature component with the deblocking temperatures ranging from 200^o C to the Curie point of hematite (see Sample ANG331 in Figure 9). There were also some opposite cases where the "low-temperature" component predominates and even replaces wholly the high-temperature component up to the region of temperatures higher than 600^o C. In most of the cases the high-temperature component shows NNE declination and moderate positive inclination. There are only a few examples where the Zijderveld diagrams showed the presence of the high-temperature component of opposite polarity. Unfortunately, in these cases, with only one exception, the high-temperature component could not be identified because of the overlapping of the blocking temperature spectra.

Ostrovnaya Formation.

[50] The rock samples of this formation usually contain "noisy" paleomagnetic signal which cannot always be interpreted confidently. Some samples showed the directions which were not repeated in any other samples, that is, were not characteristic in the Zijderveld sense [Zijderveld, 1967]. In other samples the spectra of different components overlap in such a manner that the Zijderveld diagrams do not show any rectilinear segments. Nevertheless, even in the case of this generally unfavorable background, there are some samples which show though a nosy, yet, a regular paleomagnetic signal repeated in other samples. The samples of this kind usually show two magnetization components: the low-temperature component (decays toward the temperature of 300-350^o C), scattered in its direction around the modern magnetic field, and an old high-temperature component of N and NNE declination and moderate to low inclination, the maximum blocking temperatures being around 590-600^o C and 680^o C. Some samples showed the opposite polarity of the ancient high-temperature magnetization (see Samples AA269, AA270, AA278, and AA300 in Figure 8). It should be noted that generally the character of the record and the distribution of high-temperature components in the rocks of the island-arc formation are close to those described in the rocks of the Mota, Irkutsk, and Ust-Tagul formations of the Sayan Region, having the same age. Some of the samples showed the medium-temperature, unipolar component of NW inclination and moderate positive declination (Figure 8, Sample AA305), similar to the medium-temperature component of the Moshakova and Chistyakova rock samples collected from the outcrop in the vicinity of the Greben Rock.

Kliminskaya Formation.

[51] The samples collected in the rocks of this formation clearly showed a component with NW declination and moderate to low inclinations (in the modern system of the coordinates), see Samples TS524 and TS603 in Figure 7. Since in many cases this component overlies obviously some higher-temperature component, it is here referred to as the medium-temperature component, even though the spectrum of its blocking temperatures (beginning from 200-250^o C) extends to 600^o C and higher temperatures and has NNE declination, often being the only one stable component. The high-temperature component (see Samples TS533 and TS609 in Figure 7) was identified in 3-4 samples and has NNE declinations and low inclinations (in the old system of the coordinates).