S. A. Pisarevsky, I. V. Ivanova, and A. N. Khramov
All-Russian Petroleum Research Geological Exploration Institute, St. Petersburg, Russia
E. G. Gooskova
St. Petersburg Branch of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, St. Petersburg, Russia
A. F. Krasnova, N. A. Arestova, and S. B. Lobach-Zhuchenko
Institute of Geology and Geochronology of Precambrian, St. Petersburg, Russia
The research efforts aimed at construction of the apparent polar wander path (APWP) for the Fennoscandian Precambrian started more that 30 years ago [Katseblin, 1968; Neuvonen, 1965]. These and more recent works resulted in several versions of the APWP. To analyze the history of the Fennoscandian Shield drift in the Early Proterozoic, the most recent version of the APWP is typically used [Elming et al., 1993]. Not all fragments of the Early Proterozoic track of this curve are well justified; for instance, the fragment between the "key" poles with the ages of 1.88 Ga and 1.76 Ga is built on the basis of a sufficiently large number of reliable paleomagnetic determinations. There is also a group of reliably dated poles with the ages of about 2.4 Ga. At the same time, the part of the curve between 2.4 Ga and 1.88 Ga is poorly justified.
In addition, greatly differing amounts of the paleomagnetic data are available for different parts of the shield. There are more than 200 paleomagnetic determinations for its western part (the territory of Finland and Sweden), while for the Karelian-Kola region, there are less than 40 determinations, and until recently, only 14 of them have been for the early Karelian (2.5-1.9 Ga). The latest paleomagnetic studies [Damm et al., 1997; Khramov et al., 1997] have improved the situation only a little. Specifically, the southern White Sea region still remains poorly studied from the paleomagnetic point of view. The goal of this paper is to fill the gap to some extent.
The southern White Sea region is a part of the White-Sea mobile belt separating the Karelian granite-greenschist and Kola granulite-gneiss regions. The determining role in the structure of the southern White Sea region is played by the Archean complex of homogeneous rocks of the tonalite-plagiomicrocline granite composition which has experienced multiple structural and metamorphic transformations. A characteristic feature of the southern White Sea region distinguishing it from the western White Sea region is a small amount of mafic rocks. The studies of this region, including the structural-metamorphic analysis have revealed three groups of mafic rocks, the most ancient among which (2.8 Ga) are analogs of volcanic rocks of the Archean greenschist belts of Karelia. According to the isotope-geochemical data the remaining groups of mafic rocks (to which gabbronorite intrusions belong (Figure 1)) are considered to be of one age and formed in the interval 2.5-2.45 Ga [Chekulaev et al., 1994].
The structural scheme of the region of the southern White Sea is completely determined by late Svecofennian deformations. Relicts of the earlier structures are present now only in some lens-like areas. A detailed mapping of these areas made it possible to reveal three long stages of the endogenic development: two Archean and one Proterozoic-Svecofennian.
Oriented hand samples of gabbronorites were collected from (1) a 1-m-thin dike B sampled at the distance of 50 m (13 hand samples, including 4 samples of gneisses cut by the dike); (2) about 250-m-thick intrusion V (31 hand samples), (3) intrusion D with the visible thickness of 70 m and sampled length of 145 m (27 hand samples) (these rocks had different granularities (from small-grain rocks at the intrusion margins to large-grain and gigantic-grain rocks at its center)), and (4) intrusion I with the visible thickness of about 50 m (23 hand samples).
From the body of granitoids near the village of Yukovo, 18 samples were collected along the White Sea coast at a distance of about 1.5 km. In spite of metamorphic Svecofennian transformations, both mafic rocks and granites had regions of rocks with the well-preserved magmatic structures. The samples were oriented by a magnetic compass.
Oriented hand samples were cut into cubes with the rib of 2 cm. From each hand sample, from two to eight cubic specimens were prepared.
The remanent magnetization of the specimens was measured by JR4 "Geophysica" spinner magnetometers (Brno, Czech Republic) at the paleomagnetic laboratory of the All-Russian Petroleum Research Geological Exploration Institute at St. Petersburg. A stepwise thermal demagnetization was performed by a setup developed at the same Institute. The local geomagnetic field was screened in the setup by a three-layer m metal screen. A part of the specimens (intrusion D ) was studied at the Laboratory of Magnetic Properties of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation at St. Petersburg. The experiments involving the stepwise demagnetization with an alternating magnetic field were carried out at the laboratories of the National Geological Institute and the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation at St. Petersburg. The differential thermomagnetic analysis of the specimens was performed at Kazan State University.
The obtained data were statistically processed using a standard procedure [Fisher, 1953; Kirschvink, 1980; Zijderveld, 1967] by the IAPD computer program [Torsvik, 1986].
The increase of Q values from the center ( Q = 1.7 ) to the contact zone ( Q = 2.4 ) manifests initially a magmatic origin of the minerals, which are a bearer of the magnetization. In the near-contact region these minerals might have saved the initial magnetization, whereas the less magnetically tough minerals of the central part of the intrusion had more chances to obtain a new magnetization in the Svecofennian activization period. Actually, the most ancient component is present exclusively in the endocontact zone, and the rocks of the central part of the intrusion keep the Svecofennian magnetization (Figures 6c and 6d).
The magnetization of the most metamorphosed parts of intrusion I (Belomorsk) proved to be rather low: NRM = (0.04-0.96) mA m-1. However, several specimens from the weakly altered parts of the body were found to be strongly magnetic: NRM = (1.2-3.4) mA m -1. The magnetic susceptibility of the rocks of intrusion I proved to be in approximately the same limits as that of other bodies studied: k = (4.5-5.5) 10-4 . The thermal demagnetization of these rocks revealed a lower stability of the characteristic magnetization compared with those of intrusions B and V. It falls sharply even at the first temperature steps (Figure 5) and sometimes becomes comparable with the noise level after heating up to 300o-400o C. The reliable (statistically significant) directions are presented in Figure 6b. Of particular interest is the presence of two polarities (though one of them is indicated in Figure 6b by only one point). The mean direction of the characteristic magnetization of intrusion I is shown in Table 1.
A considerable part of the specimens studied was found to have a low-temperature component ( TUB = 200o-300 oC)
and a low-coercive component with greatly "scattered" directions (Figure 6e). The characteristic magnetization of granitoids of the Yukovo massif also proved to have greatly differing directions (Figure 6f). Because of the poor statistics of the directions, these magnetizations are not given in Table 1.
The history of rock metamorphic transformations in this region manifests that the most probable epochs of remagnetization might have been metamorphism episodes with the age of 2.45, 1.9, and 1.8 milliard years.
As can be seen from Figure 7, the most reliably defined pole derived from intrusions B and V (pole 1, Figure 7) is in a good agreement with the Svecofennian fragment of the APWP. This leads to the conclusion that the gabbronorites of intrusions B and V were fully remagnetized in the Svecofennian epoch, to be more exact, about 1.8 Ga. This magnetization is characterized by a rather narrow interval of unblocking temperatures (450o-530o C). It can be supposed that the secondary heating, which caused remagnetization, was characterized by the temperatures not exceeding the above indicated one. Since the temperature of the Svecofennian metamorphism is evaluated by petrologists within the 650o-550o C interval, the age of this magnetization cannot be more than 1.8 milliard years. However, additional studies are needed to confirm this conclusion.
The paleomagnetic pole deduced from intrusion I (pole 4) and both poles determined from intrusion D are also consistent with the APWP. However, because of a lower quality of the determinations and the necessity to revise the 2.45-1.88 Ga fragment of the APWP, it is impossible to date exactly these magnetization components. Our tentative estimates of their ages are listed in Table 1. It is clear that only pole 3 can correspond to the rock formation time (2.5-2.45 Ga) because it is derived from the contact region of intrusion D. Intrusion I and the central part of intrusion D were also remagnetized in the Svecofennian. Because of the insufficient reliability of the 2.45-1.88 Ga fragment of the APWP, only the lower limit of the age of this remagnetization can be estimated (1.88 Ga).
The paleomagnetic studies of gabbronorites of the southern White Sea region have revealed that their major part was remagnetized in the Svecofennian epoch. The paleomagnetic pole inferred from intrusions B and V on the Borshevets Island has satisfactory statistical characteristics and is fairly reliable. By comparing it with the APWP for Fennoscandia, its age is estimated to be 1.8 Ga. The paleomagnetic age inferred from the contact part of intrusion D on the Emestrov Island probably corresponds to the rock formation time (2.5-2.45 Ga); however, it is poorly justified statistically and requires verification. Intrusion I (Belomorsk) and the central part of intrusion D were also remagnetized in the Svecofennian epoch; however, the age of this remagnetization is not less than 1.88 Ga.
The narrow spectrum of unblocking temperatures of the characteristic component of magnetization of intrusions B and V suggests that the temperatures of secondary heating, which led to remagnetization of rocks in the Svecofennian, were between 450o C and 530o C.
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