Rekonstrukcja paleonaprężeń związanych z intruzją hercyńskich granitoidów masywu Strzelina na podstawie analizy struktur dylatacyjnego odkształcenia odśrodkowego

Stanisław Achramowicz

Abstract


Paleostress in the country rocks to the Strzelin grainitoids from the analysis of dilational centrifugal structures, SW Poland

In metamorphic country rocks to the Strzelin-Žulova granitoids two generations of specific dilational centrifugal microstructures have been found. They are represented by sun-like and broom-like structures, older and younger respectively. The older microstructures developed subsequently to the regional metamorphic peak due to temperature rise timed after the D3 folding but before the formation of the domainal S3 foliation. The younger microstructure set developed after D4 event during another temperature rise recorded well by a late cordierite blastesis. Both the temperature rises, subsequent to the regional metamorphic peak, were caused by an emplacement of the Hercynian granitoid magma. A centrifugal microstructure consists of tension fissures disposed radially around local deformational centre occupied by metainictic allanite, solid solutions of Feand Ti-oxides, and incomplete quartz-chloritc-limonite pseudomorphs after biotite. Such centres arc not inclusions. Exothermic chemical reactions connected with an increase of the miscibility of those oxides, dehydration of Fe-hydrooxides and regeneration of the structure of the metamictic allanite, were all responsible for the initiation of the dilational centrifugal deformation. Temperature rose then up to 300° (350) - 390°C. Radial distribution of the inikrofractures developed under control of the external stress field. It follows from the analysis of effective stresses that the radial fractures intersect along the line coinciding with δ1 axis of the external principal stresses. The method of reconstructing the paleopositions of δ2 δ3 planes and δ2 find δ3 stress axes has been proposed. It allowed to reconstruct the attitudes of the regional paleostress field during both older and younger periods of dilational deformation and to estimate the amount and sense of rotation of the metamorphic rocks from the time of formation of the sun-like microstructures till the cessation of the regional tectonic events. In that time the Strzelin-Doboszowice region was domed as the whole due to intrusion of a granitoid magma, which resulted in the formation of the Ziębice dome. Its eastern portion is hidden beneath Cenozoic deposits, but the western part is well exposed and outlined by the reconstructed orientation of the paleostresses. The most intense dynamic and kinematic influence of the emplacing granitoid upon the country rock came from the geometrical centre of the dome (region E of Ziębice). It is concluded that the recognized dilational microstructures and their comprehensive relation to the external stress field enables deciphering of the principal paleostress orientation. The centrifugal deformation in the country rocks to the Strzelin-Žulova granitoid massif took place under regional uplift and decompression, and was triggered by a two fold temperature rise connected with the emplacement of the granitoid magma in its present setting due to the northwestward migration along a steep path from deeper magma chamber located to the southeast.

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