PALEOMAGNETISM OF PALAEOPROTEROZOIC POSTCOLLISIONAL GRANITOIDS IN THE SOUTH OF THE SIBERIAN CRATON

Didenko A.N.

Yu. A.Kosygin Institute of Tectonics and Geophysics FEB RAS, Khabarovsk, Russia

Geological Institute Russian Academy of Sciences, Moscow, Russia, alexei_didenko@mail.ru

Selection criteria for paleomagnetically reliable objects developed for Phanerozoic rocks (for example, Didenko, Pechersky, 1993; Van der Voo, 1993), are not often workable when studying Precambrian rocks. Precise age determination of rocks influences significantly paleomagnetic data interpretation. It was just a few years ago when palaeomagnetically reliable, on the one hand, and precisely-dated paleomagnetic data, on the other hand, were practically absent for the Proterozoic Siberian objects; for 5 from 6 age determinations available the rock dating accuracy and age of ChRM, respectively, attains to 100 Ma and more.

It is possible, to some extent, to solve the problem of obtaining reliably-dated palaeomagnetic directions for Proterozoic rocks by studying granites and related rocks. First, granitoids are well-dated by modern isotopic-geochemical methods, and, second, their formation is associated with specific stages in the development of cratons based on existing genetic classifications. In 2005 there were available 9259 paleomagnetic age determinations in the Global Paleomagnetic Database (GPMDB) for rocks of all the continents, of these granitoids were used in 222. As for the Siberian Craton, granitoids were never used, though during recent years there have been carried out a significant number of precision isotopic-geochronological studies for Palaeozoic and Proterozoic granitoids of Siberia.

The results of investigations performed by numerous researchers in Southwest and Northwest Zabaikalye allow us to assert that folding and metamorphic processes occurring in Palaeoproterozoic strata have been accomplished in the region prior to the intrusion of granites of the Kadar, Tarak, Sayan and Shumikhin complexes. According to the isochronous data determination the age of these postmetamorphic manifestations varies from 1.9 to 1.7 Ga. We attempted to meet a lack of reliable palaeomagnetic data for the Palaeoproterozoic in Siberia and investigated the granitoids in the southwestern framing of the craton, recording one of the main borderlines in the geological development of the continental crust in the Early Precambrian – the formation of the super continent at the end of the Early Proterozoic. This required, first, conducting paleomagnetic and geochronological studies, as they say “sample-to-sample”, and, second, using petromagnetic methods for determining the genesis of granite magnetization, magnetic structure and texture of magnetization carriers, the thermal history of rocks. The positive paleomagnetic result was obtained only for two granite complexes of the Angara-Kan and Sharyzhalgai basement protrusion of the Siberia Craton.

The granites of the Shumikhin complex (the Sharyzhalgai block) were selected from two massifs: 1) the middle flow of the Toisuk River; 2) the Bolshaya Olkha River in the “Orlyenok” open pit area. The isotopic-geochronological investigation (zircon U-Pb system – 1855 Ma, amphibole and biotite K-Ar systems) allowed us to determine the time interval of the granite reaching a temperature of 450-470є C as 1850 Ma. The time interval for the granites to acquire high-temperature (580-450C) natural remanent magnetization component (NRM) can be estimated as 1855-1850 Ma. The NRM average inclinations and declinations for the two objects of the Sharyzhalgai protrusion, removed from one another for a distance of 70 km, practically coincide. Based on these grounds we suggest that the massifs did not undergo sufficient rotations around the horizontal axis, and a high-temperature NRM inclination reflects the paleolatitude of the granitoid formation, which is close to the true one. It should be mentioned also the positive reversal test for high-temperature NRM components in granites. We suppose that the paleomagnetic pole obtained (Plat=-23.9, Plong=110.0, dp=3.4, dm=6.4) can be used for the calculation of the Siberian Craton location at approximately 1850 Ma (Didenko et al., 2005).

Small-sized massifs and small thickness dykes of the porphyry-like hypersthene granites, granodiorites and granosyenites – kuzeevites (Kuznetsov, 1988) are localized among the gneisses of the kuzeevite stratum (the Angara-Kan block). For the kuzeevites the age obtained is 1734±4 Ма (Bibikova et al., 2001), which testifies to their intrusion after accomplishing of metamorphism of the granulite facies (1900±20 Ма) and folding. Geological data allow us to assume that when the main stage of metamorphism accomplished the Angara-Kan block was rigidly coupled with the body of the Siberian Craton and did not undergo rotations relative to the latter neither in the horizontal, nor in the vertical planes.

Proceeding from the analysis of paleomagnetic and geochronological data we can assume the following: 1) after intrusion of kuzeevites the total remagnetization did not occur (baked contact test); 2) direction of high-temperature NRM component of kuzeevites corresponds to the time of their intrusion to the gneisses of the kuzeevite stratum. The volume of the intruded kuzeevites does not suggest their stepwise cooling and, correspondingly, a large time gap between closure of zircon U-Pb isotopic system (750-850°С) and recording of high-temperature NRM component of kuzeevites (blocking temperatures being 450-580°С). The paleomagnetic pole obtained (Plat=-42.9, Plong=109.6, dp=3.8, dm=7.3) can be used for the calculation of the location of the Siberian Craton approximately at 1730 Ma (Didenko et al., in press).

Let us advance one more argument in favor of the ancient age of paleomagnetic directions for the granites of Angara-Kan and Sharyzhalgai blocks. In recent years we have obtained paleomagnetic poles for the volcanogenic-sedimentary rocks of the Akitkan series in the Lower Proterozoic of the Siberian Craton (Vodovozov et al., 2007). Among these, there are several poles possessing high index of paleomagnetic stability, which form a regular sequence on the sphere. The poles of the granites can be included integrally in the same sequence.

This research was supported by the Russian Academy of Sciences (Integration Program for Basic Research No. 10, Branch of the Earth Sciences, Siberian Branch of RAS), and the Russian Foundation for Basic Research (Project 06-05-64352).

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