Tektonika podłoża krystalicznego prekambryjskiej platformy w Polsce

Abstract

TECTONICS OF THE CRYSTALLINE BASEMENT OF THE PRECAMBRIAN PLATFORM IN POLANDSummaryAs a result of the differentiated mobility of the crystalline basement, expressed in the form of a general deformation, i.e. bending or curvature of some of its parts, an erosional plan of the surface distribution of the Gothian, Sub-Jotnian and Jotnian complexes have developed. Primarily they were more wide-spread. An “undulation” of the crystalline basement, still before the sedimentation of the Wendian cover, led to a complete or almost complete removal of the Gothian, Sub-Jotnian and Jotnian complexes from the elevated portions of the basement. They persisted only in the depression areas of the basement.The Mazowsze, Dobrzyń and Pomerania granitoid massifs are, as far as their area is concerned, the largest tectonic units of the crystalline basement. They are surrounded with the Svecofenno-Karelian systems. The older Precambrian complexes made here the substratum of the granitoid massifs. Within the Svecofenno-Karelian geosyncline these massifs most probably played a part of the central massifs, and in their marginal portions were deformed. In the Gothian cycle they underwent regeneration, expressed as granitization and migmatization of the older substratum, as well as local mobilization of the rheomorphic and anatectic granitoids.On these granitoids the Gothian formations accumulated, later on metamorphosed and then eroded almost completely, except for small isolated patches only.The activation, expressed in the form of disjunctive tectonics, was responsible for the formation of grabens and depressions on the massifs, where the Jotnian molasse accumulated due to the lowering and uplifting movements. The deep rupture zones were used by the anorogenic, alkalic-gabbroidal, alkalic-ultrabasic and alkalic intrusions, as well as by the Jotnian subvulcanites and vulcanites. The differentiated vertical mobility of the basement led also to the wide deformations of the sedimentary cover and to the use of the disjunctive zone by the Wendian and Lower Palaeozoic vulcanites and subvulcanites.Svecofenno-Karelian metamorphic complexes. The Svecofenno-Karelian branches, represented by the Podlasie, Ciechanów and Kaszuby complexes, which link the granitoid massifs, represent the well-directed systems of the crystalline basement.In the synclinorial units the Svecofenno-Karelian complexes are represented by gneisses and migmatites, and in the anticlinal (or anticlinorial) ones - by primorogenic, metamorphic granitoids, locally accompanied with the Gothian palingenetic granitoids. Within the more eroded areas, the gneisses are not too thick, and occur as patches in granitoids, disclosing numerous post-orogenic vein granites, pegmatites and palingenetic granitoids.The middle part of the Podlasie complex is most eroded. It consists of several synclinoriums and anticlinoriums of lower order. The petrological analysis demonstrates that the synclinorial intra structures and the separating anticlinorial ones are built of the formations related to the deeper, almost axial geosynclinal zones.This is proved by a considerable percentage of pyroxene granulites, anderbites, charnockites, gabbro-amphibolites, leucogabbro-anorthosites in the metamorphic series.They represent altered, pre-inversion volcanogenic-sedimentary formations, and are basal formation of a large metamorphic complex.The next, deeply eroded part of the Podlasie complex is the Sejny-Augustów . structure. It is disturbed by a zone of tectonic fractures, thrown, and almost completely hidden under the Gothian macrostructure. Thus, in the substratum of the porphyroblastic, hornblende-biotite granitogneisses pyroxene granulites and enderbites can be expected to occur.The rocks of the Sławatycze elevation are here the third deeply eroded part of the Svecofenno-Karelides of the Podlasie complex. Here are found strongly granitized products of preorogenic volcanism.The Svecofenno-Karelian system is characterized by the intense folding of rocks which dip under an angle of about 70–90°, A high anisotropism of the structures, a complicated folding, and an abrupt inclination of the rocks were favourable for the multiple rejuvenation of the mylonitization zones, cataclasis, and brecciation showing directions concordant with the general structural plan, and those of transverse character. These processes were particularly common within the boundary zones of various objects, e.g. isotropic granitoid massifs and anisotropic units of the Svecofenno-Karelian system, as well as between the gneissic intrastructures and primorogenic granitoids, also within the gneissic complexes.A tectonic animation took place in the Gothian cycle, when the Svecofenno-KareIian basement was being covered with the supracrustal rocks subjected then to metamorphism. Such a specific tectonics in the basement was posthumously favourable also for the Wendian activation and basalt outflows.The Ciechanów branch of the Svecofenno-Karelides has so far insufficiently been penetrated by drillings.The recently recognized Kaszuby branch of the Svecofenno-Karelides occurs in the East Pomerania area. It has so far been established that the rock complexes of this branch come from a deeper part of the geosyncline, and are built up of pyroxene granulites, charnockites and pyroxene gneisses.Gothian metamorphic complexes. The reconstruction of the crystal ..line basement, begun in the Svecofenno-Karelian cycle, continued in the Gothian cycle. The Gothian rocks overlying the Pre-Svecofenno-Karelian substratum, are representedby metamorphic schists and gneisses.In the basement of the Peri-Baltic depression, metamorphic schists and gneisses are found sporadically. Predominant are here porphyroblastic granite gneisses, rapakivi-like granitoids, and anorthosite-norite intrusions. In the granitoid massifs – regenerated during the Gothian cyde – land within the marginal zones of the Svecofenno-Karelian system, are found palyngenetic and anatectic granitoids surrounded with metasomatic granites and migmatites.The supracrustal Gothian formations were developed in one metamorphic cycle only. Their dip changes, ranging for the most part from 40 to 70°. Their deformation structures are less complicated than in the polymetamorphic Svecofenno-Karelian formations.On the Svecofenno-Karelian basement these rocks are more disturbed, their granitization and mylonitization being more strongly expressed.The contact zone of the Mazury complex of the Gothides with the Svecofenno-Karelian system and its central granite massifs was in the Sub-Jotnian and Jotnian times an area of both tectonic and magmatic activation. To this zone are related anorogenic alkalic-gabroidal, alkalic-ultrabasic and alkalic intrusions.The Karelian-Gothian cycle ends with the molasse sedimentation in grabens and depressions, accompanied with the alkalic magmatism and subvolcanism. The Jotnian rocks are found in patches showing a meridional elongation related to the system of the Gothian dislocations.The analysis of thicknesses, ranges, and formational and facial development of the individual structural complexes in the sedimentary cover proves the differentiated vertical movements of the crystalline basement of the Precambrian platform, from the Precambrian time to the present day.The vertical movements of the crystalline basement are, due to the displacement of the masses in its substratum, caused by the equalization of their density or thermal differentiation. After the crystallization period of the crystalline basement these movements were intensified by the erosional processes, which removed huge masses of the folded rock massif and leveled its surface. The displacement of the material and its accumulation in the form of molasse at other sites were also the effect of the differentiated vertical movement of the basement.