Place of birth of the Siberian PlatformDoklady Earth Sciences

About

Authors
V. E. Pavlov, A. V. Shatsillo
Year
2015
DOI
10.1134/S1028334X15050177
Subject
Earth and Planetary Sciences (miscellaneous) / Earth and Planetary Sciences (all)

Text

ISSN 1028334X, Doklady Earth Sciences, 2015, Vol. 462, Part 1, pp. 444–448. © Pleiades Publishing, Ltd., 2015.

Original Russian Text © V.E. Pavlov, A.V. Shatsillo, 2015, published in Doklady Akademii Nauk, 2015, Vol. 462, No. 3, pp. 319–324. 444

According to modern concepts [1, 2], the Siberian

Craton, as a whole, formed in the Late Paleoprotero zoic, approximately 1.9–1.8 Ga ago, as a result of accretion–collision processes which led to amalgam ation of ancient Archean and Early Proterozoic conti nental blocks (terranes and/or superterranes). The accretion of superterranes of the Siberian Craton in modern coordinates was followed by formation of a huge Himalayantype collision orogen. Formation of collusion zones was accompanied by synchronous areal granulite metamorphism in the adjacent terranes, which led to the formation of a fairly uniform field of high temperatures and pressures, which is typical of a middle–lower continental crust thickened during a col lision. A significant part of the Siberian Platform, if not all, was subjected to regional metamorphism.

After termination of the collision processes and uplifting, the erosion of the mountain structure began.

As a result, the middle crustal levels were relatively quickly exhumed forming the peneplain, which was later overlapped by Riphean–Phanerozoic sedimen tary cover of the Siberian platform. During exhuma tion, the temperature of metamorphic rocks dropped below the Curie points of magnetic minerals. Here, rocks acquired remanent magnetization.

If the aboveproposed pattern is correct, then it should be logical to assume that the paleomagnetic record in metamorphosed rocks of the crystalline basement of the Siberian Platform should reflect the direction of the geomagnetic field at the termination of regional metamorphism, that is, approximately, at the formation time of the Siberian Platform. In this case, regardless of the age of rocks of the Siberian Plat form basement and their geographical location, most of the rocks must have paleomagnetic directions cor responding to closely grouped paleomagnetic poles located in the same area of the Earth. It is reasonable that we are talking only about those rocks that pre served ancient magnetization. Due to this, when com paring the poles it is necessary to take into account subsequent tectonic movements within the Siberian

Platform [3].

Thus, the paleomagnetic method is the simplest and the most illustrative way to clarify the existing concepts, describing the process of the formation of the Siberian Platform. If paleomagnetic poles recorded in metamorphic rocks of different parts of the Siberian Platform (for example, the Aldan and

Anabar shields) are close to each other or they are jux taposed, then, the above hypothesis receives strong support. Otherwise, it should be corrected or neglected.

In order to apply the paleomagnetic method pro posed, it is necessary to observe three conditions: (1) Exhumation of rocks (or, more precisely, cool ing in the temperature range of 700–350°C when the greater part of the paleomagnetic signal is formed) must occur during a time period not exceeding 10–20 Ma.

Otherwise, as a result of the movement of the Siberian

Platform, the paleomagnetic pole could be shifted and the rocks could have recorded some of its average posi tion. This, however, does not have a fundamental importance for solving the task. (2) Exhumation of rocks could occur over a time period when the magnetic field rarely changed its polarity. Otherwise, the signal recorded in metamor phic rocks as a result of the imposition of numerous antipodal components could be impossible to inter pret. The results of analysis of the data obtained for

Paleoproterozoic rocks [4] evidences that this condi tion is quite likely. (3) After the formation of magnetization, the study areas did not experience significant declinations and turns. This condition can be monitored by conver gence of regional paleomagnetic data.

We carried out a paleomagnetic study with the pur pose to clarify this hypothesis. As a result, metamor phic complexes of different composition of different age series, exposed in the Uchur and Malyi Aim river valleys (Aldan Shield), were studied. Most outcrops

Place of Birth of the Siberian Platform

V. E. Pavlova, b and A. V. Shatsilloa

Presented by Academician M.A. Fedonkin November 29, 2013

Received December 5, 2013

DOI: 10.1134/S1028334X15050177 a Institute of Physics of the Earth, Russian Academy of Sciences,

Bolshaya Gruzinskaya ul. 10, Moscow, 123995 Russia b Kazan (Volga region) Federal University,

Kremlevskaya ul. 18, Kazan, 420008 Russia email: pavlovhome@rambler.ru, shatsillo@gmail.com

GEOLOGY

DOKLADY EARTH SCIENCES Vol. 462 Part 1 2015

PLACE OF BIRTH OF THE SIBERIAN PLATFORM 445                 studied during our field works are located in the mid dle reaches of the Uchur River near the mouth of

Suklan Creek and a few kilometers below Factoriya

Island (Fig. 1). The strongly folded rocks sampled, referred to the Upper Sunnagin and Kyurikan forma tions, are represented mainly by biotite–hypersthene and biotite–amphibole gneisses, which are interbed ded with garnet–biotite and garnet–sillimanite gneisses, sometimes marbles and amphibole—plagio clase or phlogopite–diopside–plagioclase schists. In the Malyi Aim River valley (the western slope of the

Omnya uplift), about 20 km above the Lata River mouth, small outcrops of biotite–amphibole gneisses of the Batomga Group were investigated.

The studied metamorphic complexes are overlain by Riphean subhorizontal deposits of the Uchur 1 2 3 4 5 6 57°40′

N 57°30′

E131°45′

Fig. 1. Geological scheme of the study area in the Uchur River valley with location of the outcrops studied. 1—Mesozoic granites; 2—Vendian–Lower Cambrian deposits; 3—Lower Riphean deposits; 4—Early Precambrian basement complexes, 5—faults, 6—sampling sites. 446