Abstracts:

A paleomagnetic study was made on Middle-Cambrian sedimentary rocks from paleoisland arc system of West Sayan (Siberia, Russia). Forty two samples were collected from five localities. The rock magnetic properties show the dependence only on mineral concentration and suppose the detrital origin of NRM. Stepwise thermal demagnetization allowed to isolate directions of characteristic remanent magnetization (ChRM) in all sites using great circles method. Fold test indicated prefolding age of isolated ChRM. These data in common with previous paleomagnetic studies in the region (Pechersky, Didenko, 1995; Kazansky et. al, 1995) made it possible to propose the first version of geodynamic history of North Sayan paleoisland arc in Cambrian. This island arc was formed on the ocean crust at a considerable distance of Siberian Craton margin and had E-W (in ancient geographic frame) general orientation. During Early Cambrian it was developed in active volcanic regime resulted in formation of thick complex of island arc type. By the Middle Cambrian North Sayan island arc transferred southward (in ancient coordinates) towards Siberian Craton for a distance approximately 1000 km.

The structure of south-western frame of Siberian craton among other comprises the three large linear structures composed of island arc complexes of Cambrian age: Gorny Altai (GA), Kuznetsk Alatau (KA) and West Sayan (WS). In order to reveal the paleoposition of those ancient island arcs 220 oriented samples from 24 localities represented by volcano-sedimentary sections of Early and Middle Cambrian ages were sampled for paleomagnetic study. ChRM directions resulted from stepwise thermal demagnetization successfully passed through the fold test also, whenever possible the baked-contact test.(Early Cambrian: GA - Ds=276, Is=26, a95=7; KA - Ds=225, Is=44, a95=9; WS - Ds=121, Is=27, a95=4; Middle Cambrian: GA - Ds=139, Is=-22, a95=5; KA - Ds=97, Is=-22, a95=10; WS - Ds=144, Is=-14, a95=9).
Taking into account the paleoposition of Siberian Craton [Khramov et.al, 1985] those results leads to following conclusions.
In the Early Cambrian the investigated structures located at latitudes 14-26 N at a considerable distance from the Siberian continent constraining, probably a uniform linear island arc structure of EW orientation (in ancient geographic frame).
Due to collisional processes occurred in the Middle Cambrian this arc was deformed. Thus, WS block was moving southwards (in ancient geographic frame) without essential rotation, whereas GA and KA blocks in addition to southward motion had underwent sufficient rotation towards their modern position.

Cenozoic volcanic pipes of Minusinsk depression rich of mantle xenolites and are probably related to the ‘hot spot’ near the Baikal Lake (Rasskazov.1994, Yarmoluk et. al, 1994). The first paleomagnetic data were obtained from 4 pipes (42 samples) with K-Ar and Sr/Sr ages 65-49 Ma (Zubkov et.al.,1989). All the 4 pipes demonstrate high temperature ChRM component of reversed polarity with average direction D=177.6, I=-62.3, k=237.5, a95=6. However, some samples from Bele pipe demonstrate another direction (D=243.7, I=-56, k=323.2, a95=5.1). Corresponding paleopoles of both directions are statistically differ from the APWP of Eurasia (Besse, Courtillot, 1991). Plaeomagnetic poles from basaltic rocks of near-Baikal region (see Khramov database) are consistent with the first of Minusinsk poles or lie along the great circle between them. This data call into question of implementation of Eurasian APWP to Northern Asia.

Volcanic pipes with K-Ar ages 65-49 Ma (Zubkov et.al.,1989) are widespread in Minusinsk depression. We present the results of the first paleomagnetic investigations on 4 volcanic pipes (42 samples) which were accompanied with 40Ar/39Ar dating. All the 4 pipes demonstrate high temperature ChRM component of reversed polarity with average direction D=178.8, I=-62, k=160.7, a95=7.3. However, some samples from Bele pipe demonstrate another direction (D=244.3, I=-58.2, k=407.4, a95=4.6). 40Ar/39Ar method gave plateau ages of pipes from 74 to 81 Ma. This result have cast some doubt on previous K-Ar dating. Paleomagnetic poles of both directions are statistically differ from the APWP for Eurasia (Besse, Courtillot, 1991). Mezo-Cenozoic paleomagnetic poles from basaltic rocks of near-Baikal region (Khramov, 1984) are consistent with the first of Minusinsk but differ from APWP of Eurasia. This data call into question the implementation of Eurasian APWP to Northern Asia.