Proterozoic Basic dykes in the Ukrainian Shield: A palaeomagnetic, geochronologic and geochemical study-The accretion of the Ukrainian Shield to Fennoscandia

摘要

A palaeomagnetic, geochronologic and geochemical study of basic dykes in the Ukrainian Shield has been undertaken with the purpose of testing a hypothesis for timing of accretion of the Ukrainian Shield with Fennoscandia. On the basis of geological data it has been suggested that the two shields amalgamated at ca. 1900-1800 Ma. With this study of 11 basic dykes the palaeomagnetic database for the Ukrainian Shield is extended. The palaeomagnetic data indicate three generations of dykes. This is supported by the age data and the difference in geochemical compositions. Ages of two ca. 25 m wide dykes (Susly) in the North-Western Block of the shield have been determined, one for which a positive palaeomagnetic field test is demonstrated. The lower of the two ~(40)Ar/~(39)Ar hornblende ages of this dyke, 2061 ± 20 Ma, is similar to the U-Pb, zircon age of the granitic host rock (ca. 2067 Ma, U-Pb, zircon), which suggests that the dyke intruded shortly after crystallization of the granite. Combined with the magnetization of the baked contact and that of another dyke a new ca 2060 Ma pole is presented (Plat. = 15.7°, Plon. = 182.9°, A_(95) = 13.7°), which may be regarded as a VGP. An U-Pb age of 1722 ± 12 Ma of baddeleyite from the Khmelnik dyke, which yields a well-defined VGP, is similar to the age of nearby anorthosite complexes. A new 1740-1770 Ma key pole for the Ukranian Shield is also presented (Plat. = 26.5°, Plon. = 169.1°, A_(95) = 3.9°). On the basis of palaeomagnetic and geochronological data a 2060-1720 Ma apparent polar wander for the Ukrainian Shield is defined. This apparent polar wander is significantly different from that of Fennoscandia during the same time interval and indicates that the two shields were not in their present relative positions at ca. 1770 Ma. A scenario is here proposed where the Ukrainian Shield (and Sarmatia) collided with Fennoscandia at ca. 1900-1800 Ma and after that event, at ca. 1720-1660 Ma, it rotated approximately 43° into its present position.