Magnetic anisotropy of cenozoic igneous rocks from the vardar zone (kopaonik area, serbia)

Abstract

The Vardar zone is the suture between colliding Adriatic and Eurasian plates, comprising profoundly different tectonostratigraphic units which came into contact during Early Palaeogene. The zone was subsequently intruded by igneous rocks, concentrated at certain places, like the Kopaonik area,where plutonic and extrusive igneous rocks are in abundance. The largest I-type plutons (Kopaonik, Drenje and Željin) form the central part of an N-S running anticline, with an undulating and northward sinking axis. The anticline is conceived as having been formed during compression (D3 phase of deformation of post-Cretaceous age), prior to the intrusion of I-type granitoids or alternatively, the updoming of the regionally metamorphosed rocks was caused by the intrusion of the plutons. The granitoids, forming the core of the anticline and the satellite bodies intruded during Oligocene (at the depth of 10 km), but were exhumed during a mid-Miocene extensional D4 phase of deformation, simultaneously with the intrusion of S-type granite which crops out NW of the anticline. Dacitoandesites, mostly situated west of the anticline, are considered as of Oligocene age, but older than the plutonic rocks. The Miocene granite has visible foliation, the I-type granodiorites and the dacitoandesites occasionally exhibit visibly oriented mineral fabric. In such cases, the fast and inexpensive magnetic susceptibility measurements can provide information about the degree and type of the orientation of the fabric. Our aim was to find out if the loosely dated D3 and the welldated D4 deformations left imprint in the magnetic fabrics of the above-mentioned rocks by studying the magnetic susceptibility anisotropy of nearly 300 independently oriented samples from 25 localities. We found that the granodiorites often have high or extremely high degree of magnetic anisotropy, suggesting that it was acquired under the influence of stress in the last stage of solidification. In the main body of the Kopaonik and Željin plutons and in the satellite body of Dubovo anisotropy of magnetic susceptibility (AMS) ellipsoids can be related to the N-S running Kopaonik- Željin anticline. In the northern part of the Kopaonik pluton, and in the Drenje massif the pattern of the schistosity planes do not fit to the anticlinorium, neither seem to satisfy an onion-shell model put forward earlier for the Kopaonik pluton. On the contrary, the orientations of the AMS ellipsoids are consistent with those for the Drenje mass (not included in the 'onion-shell' structure) and they are probably related to regional extension. AMS fabric of the Kremíci satellite body (W of the Kopaonik pluton) and that of the Miocene S-type granite is also interpreted as to reflect Miocene extension. Thus, we suggest that at certain places the magnetic fabric of the I-type intrusions was 'frozen in' during compressional deformation (D3). The magnetic fabric of the northern part of the Kopaonik pluton, the Drenje mass, the Kremíci body must have been still deformable during D4 (ductile) phase, when the Polumir granite was emplaced. The magnetic fabrics of the dacitoandesites did not reflect deformation. The reason may be that they cooled completely before the intrusion of the I-type granodiorites. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.