Związek właściwości magnetycznych ze składem skał suwalskiego intruzywu anortrozytowego

Krystyna Mizeracka, Henryk Kurbiel




Detailed magnetic survey has been made for the rocks of the Suwałki anorthosite massif. The research comprised the measurements of magnetic susceptibility, magnitude and direction of natural magnetic remanence in rock samples, as well as the research of magnetic stability.

The measurements of the magnetic susceptibility of drill cores, and the measurements of magnetic susceptibility and natural magnetic remanence of rock samples taken from drill cores allowed the magnetic differentiation of the rocks in study to be determined (Fig. 1), and the dependence of magnetic susceptibility upon the ore mineral concentration in these rocks to be established (Fig. 4). Moreover, they yielded quantitative data for the interpretation of the magnetic anomalies. The reverse direction of the natural magnetic remanence of the ilmenite-magnetite ores explains the existence of a small magnetic anomaly on the surface.
The research of the magnetic stability, the thermomagnetic analysis, and the microscope examinations gave detailed data as concerns magnetic properties of the rocks under examination, as well as allowed the relation of the magnetic properties of anorthosites, norites and ilmenite-magnetite ores with their mineral composition to be determined.
The research is a basis to ascertain that apart from a low percentage of ore minerals, observed under the microscope as single grains, the type anorthosite of the Suwałki anorthosite massif reveals a considerable thermal magnetic remanence characterized by the positive inclination found to occur between 21° and 40°, the magnitude of wihich depends upon the amount of thin blades of ore minerals that are found in the plagioclase crystals (Fig. 2, Table 1).
There appears also another kind of anorthosites, distinguishing itself by the greater amount of ore minerals. The magnetic properties of these anorthosites are approximate to some kinds of norites (Fig. 3, Fig. 5).
Magnetite or titaniferous magnetite, characterized by Curie's point amounting to 450°­–500°C, is in the norites the main ferromagnetic component deciding upon their magnetic properties. There occur here also two phases of the haematite – ilmenite series with the Curie's point 60°–130°C and approximately 300°C (Fig. 5).
Due to magnetic stability, composition and amount of ore minerals, the ilmenite-magnetite ores have been subdivided into two kinds. The magnetic properties of the first kind are determined by the occurrence of only one ferromagnetic component, i.e. magnetite, whereas the second kind of ores reveals, in addition to the magnetite, also another main ferromagnetic component, i.e. ilmenite (Figs. 6 and 7). The ilmenite-magnetite ores, particularly, however, ores of the first kind, are as a rule magnetized contrary to the present-day Earth's magnetic field.
Examples of reverse magnetization of rocks are presented, and this is a basis to suggest that the reverse magnetization of the ilmenite-magnetite ores here examined can be due to the self-reversal of the magnetic remanence.

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