Wiek bezwzględny i pozycja stratygraficzna sjenitów oraz zasadowo-alkalicznych skał północno-wschodniej Polski

Abstract

ABSOLUTE AGE AND STRATIGRAPHICAL POSITION OF SYENITES AND OF BASIC-ALKALI ROCKS IN THE NORTH-EASTERN AREA OF POLAND

Summary

In the north-eastern area of Poland syenites have been ascertained at five sites arranged along a west-east trending line from Tajno to Olsztyn (Fig. 1). At Ełk they constitute a separate intrusion of considerable size, incorporated into the crystalline basement. At Wejsuny, the syenites cut, in the form of some veins, a gabbro intrusion. This consists of augite-hornblende-biotite gabbros. At Tajno, the microsyenite veins transsect an intrusion of basic alkali rocks represented here by augite pyroxenites, as a rule with a constant admixture of hornblende, and by foyaites. Both at Wejsuny and at Tajno the basic intrusions occur within the crystalline basement. In bore holes Olsztyn 1 and 2 the microsyenite veins cut the Cambrian and Silurian formations of the sedimentary cover. Syenites and basic intrusions that occur within the crystalline basement have so far been thought to be of Precambrian age, related to the final stage of the crystalline basement consolidation (~1350.10⁶ years). The determinations of absolute age K-Ar of both the syenites found to occur within the crystalline basement, and those intersecting the Palaeozoic sedimentary cover, point to a similar age amounting to about 295.10⁶ years. Similar age has been obtained (295.10⁶ years) for the Ełk intrusion by isochronous method Rb-Sr on samples of solid rocks, considering the initial relationship Sr⁸⁷/Sr⁸⁶ = 0,707. The oldest age equal 349.10⁶ years has been obtained by K-Ar method for the gabbro from Wejsuny. The gabbros and syenites from Wejsuny, the basic-alkali rocks and microsyenites from Tajno, the syenites from Ełk and the microsyenites from Olsztyn markedly differ from the rocks of the crystalline basement, showing, however, distinct petrogenetic relationships to each other, and this fact proves their comagmatism. They can make members of alkaline differentiation series as follows: gabbro-pyroxenite-essexite-foyaite-syenite. So far only essexite have not been encountered. The Palaeozoic age of these formations, the petrogenetical relationships and their characteristic linear arrangement speak for the existence of a deep fracture in this area, along which platform magmatism developed at tectonic nodes. The development of this magmatism, at least its final stage, took place about 350-265.10⁶ years ago. This is also proved by the structural-tectonic analysis which demonstrates that after the Upper Silurian, and before the Zechstein, the crystalline basement along with the Palaeozoic cover underwent intense vertical differentiation movements.