UDC 550:382.3:552.163

GEOPHYSICS

V. V. GERNIK

PROSPECTS FOR THE STUDY OF REGIONALLY METAMORPHOSED STRATA BY THE PALEOMAGNETIC METHOD

(Presented by Academician D. S. Korzhinskii, 17 IV 1968)

The objects of paleomagnetic studies are, as a rule, minimally altered rocks. Mute metamorphic strata, whose age determination requires primary attention, are considered poorly suited for paleomagnetic investigations. It is assumed that metamorphic processes, by destroying either the ferromagnetic mineral itself or its initial remanent magnetization, must inevitably change the direction of the latter, if not destroy it altogether. It is also assumed that metamorphism gives rise to a new magnetization and that in this case there is no plane with which the direction could be related (¹). The complex tectonic setting characteristic of metamorphic strata gives rise to doubts concerning the correct reconstruction of the original bedding of the layers. At the same time, a distorting influence of stress effect and magnetic anisotropy is admitted.

However, the results of many years of paleomagnetic studies of metamorphic strata on the western slope of the Polar Urals fundamentally change the established view of the problem in question. These studies were carried out from 1959 to 1966 on the basis of the fully equipped magnetic laboratory of the Pechora Geophysical Expedition.

The stimulus for work that at first glance appeared unpromising was the discovery of a fairly rigid relationship between the directions of \(I_n\) and the bedding planes of metamorphic and metamorphosed pre-Ordovician rocks. This relationship, established initially by a formal calculation of the angles between \(I_n\) and the bed surface for arbitrarily selected specimens, was subsequently confirmed by direct tracing of the direction of \(I_n\) along fold bends and by comparing the clustering of vectors in the modern and ancient coordinate systems. Moreover, \(I_n\) in specimens of coeval green schists collected in different areas showed good agreement of directions.

The very fact of the convergence of the directions of \(I_n\) in the ancient coordinate system over a large area makes it possible to conclude that: a) the hinges of folds at the sampling sites are approximately horizontal; b) the blocks making up the folded region did not undergo appreciable rotational movements; c) the schistosity planes, along which bedding elements of relict stratification or contacts were often measured, correspond in these areas to the primary bedding.

The uniformity of the directions of \(I_n\) in rocks that are folded and schistose to varying degrees excludes any noticeable influence of stress effect and magnetic anisotropy. The difficulty of determining the facing side of an isoclinal fold is easily eliminated by comparing the results of unfolding folds with different strikes.

The spatial relationship of \(I_n\) with the bedding surfaces of paleomagnetically stable rocks can and should be widely used for structural constructions, examples of which are given, in particular, by Irving

(2). On the other hand, proof of paleomagnetic stability specifically by Graham’s fold method also has a direct bearing on the age of the magnetization under study.

Indeed, the basic condition for the applicability of this method—the pre-folding origin of \(I_n\)—for metamorphic rocks means that their \(I_n\), regardless of its nature, must have formed within a definite time interval: from the formation of the rock to the nearest tectonic inversion, i.e., during the period of sediment accumulation. Consequently, any a priori assumptions about a substantial age gap between the metamorphogenic \(I_n\) and the epoch of rock formation are untenable where its stability has been proved by the unfolding (fold) method. Further experiments also cast doubt on the very genetic dependence of stable \(I_n\) on metamorphism.

Table 1

Mean directions of \(I_n\) of paleomagnetically stable metamorphic rocks of Ordovician age
(greenschist facies)

Note. Mean directions of \(I_n\) whose numbers are marked with an asterisk have here been reversed by 180° for ease of comparison.

Table 1 gives the results of paleomagnetic studies in a greenschist belt 350 km long. Mean directions, concentrations, and confidence radii were calculated at the outcrop level. Directions of \(I_n\) for outcrops were obtained from 2–3 individually measured specimens. The sole criterion used in rejecting initial data was the scatter of vectors within an outcrop. All outcrops with within-site concentration \(K_{\mathrm{v}} \geqslant 4\) were included in the calculation. The initial directions of \(I_n\) selected according to this principle agree satisfactorily in the ancient coordinate system without magnetic cleaning and without corrections for partial paleomagnetic instability. The magnitude and direction of \(I_n\) with stability established by the field method do not change appreciably in demagnetizing alternating fields with a maximum amplitude of 400 Oe.

On the other hand, \(I_n\) of specimens from rejected outcrops (\(K_{\mathrm{v}} \leqslant 3\)) changes sharply in both magnitude and direction in an alternating magnetic field. According to the method of comparing stability characteristics developed by G. N. Pet-

rovaya, the isothermal nature of the unstable \(I_n\) was established. Repeated measurements of the rejected specimens after 3–4 months showed a wide occurrence of the viscous component.

In Table 1 and in Fig. 2 it is easy to see that, for the same petrographic varieties, the directions of \(I_n\) form two separate groups of mutually opposite vectors. Such a difference in directions in identical rocks, often observed in the same sections of a monofacial sequence, in the absence of mechanical causes such as tectonic displacements, can be attributed only to the age factor. It follows from this that the \(I_n\) under consideration is not obliged in its origin to the processes of metamorphism, which, having obscured the primary lithological differences of the rocks, could not level out their initial remanent magnetization. A study of its nature by the method of comparing stability characteristics showed that it may be either thermoremanent or chemical (Fig. 1b). In the first case, the syngenetic character of \(I_n\) with the rock is obvious, considering its independence from regional metamorphism. For the same reason, chemical magnetization could have occurred only at the stage of diagenetic changes—again, simultaneously with the formation of the rock.

Fig. 1. Demagnetization in an alternating magnetic field of specimens of albitophyres of the Manya suite.
\(a\)—specimen from a rejected outcrop; \(b\)—paleomagnetically stable specimen; 1—natural magnetization; 2—ideal; 3—thermoremanent.

Fig. 2. Mean directions of \(I_n\) of metamorphic rocks of the green-schist facies.
1—projection of \(I_n\) onto the lower hemisphere; 2—onto the upper. The numbers of the groups and points correspond to Table 1. The group boundaries are drawn taking account of the radii of confidence.

From this point of view, the elongation of the directions of group I along the circumference of the stereographic projection (Fig. 2) is hardly due to statistical scatter alone: here the stratigraphic sequence of the rocks studied is probably also involved, which can be established by resolving the disputed question of the direction of migration of the geomagnetic dipole in pre-Silurian time (3). However, this uncertainty does not prevent the solution of problems of a correlational nature. In particular, from the distribution of the mean directions of \(I_n\) (Fig. 2, Table 1) it may be concluded that the Nyargovei and Kokpel suites are of the same age and that each is characterized by two time intervals (groups I and II), one of which (I) corresponds to the period of accumulation of the Manya suite.

The observed resistance of primary ferromagnetic minerals to metamorphic processes is rather difficult to explain on the basis of mineralogical and geochemical data, since, as M. I. Abdulla acknowledges, we have very scant data on the nature and distribution of iron and titanium oxide phases in metamorphic rocks (^4). The preservation of the initial remanent magnetization may be explained as follows. Under the microscope, in effusive rocks and schists, the coexistence of two ore fractions is often noted: adiagnostic ore dust impregnating the groundmass, and comparatively large grains of titanomagnetite, usually developed along microfractures. It must be assumed that the carriers of the primary \(I_n\) are the finest grains of the finely dispersed fraction, which possess a large coercive force, whereas unstable components are inherent in large grains, often clearly of secondary origin. The quantitative ratio of these two phases in the rock apparently determines the stability of its \(I_n\).

The resistance of the initial magnetization to heating at the greenschist stage of metamorphism—if, even following V. S. Sobolev, its lower limit is taken to be 400–450° (^5)—is readily explained by the experiments of M. A. Grabovskii and G. N. Petrova, which showed that repeated heating of a rock to 450–500° cannot substantially alter its thermostable magnetization (^6).

Important indications of the reliability of the results presented here are the similarity of the directions of \(I_n\) in rocks of different genesis, as well as the presence of normally and reversely magnetized varieties.

All-Union Scientific Research Institute
of Exploration Geophysics

Received
14 IV 1968

CITED LITERATURE

^1 D. I. Kollinson, A. E. M. Nairn, Paleomagnetism, Moscow, 1962. ^2 E. Irving, Paleomagnetism and its Application to Geological and Geophysical Problems, N. Y., 1964. ^3 V. Bucha, Geomagnetism and Geoelectricity, 17, Nos. 3–4, 435 (1965). ^4 M. I. Abdulla, The Nature of Metamorphism, Moscow, 1967. ^5 V. S. Sobolev, Geol. i geofiz., No. 1, 7 (1964). ^6 M. A. Grabovskii, G. N. Petrova, Izv. AN SSSR, ser. geofiz., No. 5, 524 (1956).