Variscan to Alpine tectonothermal evolution of the Central Srednogorie unit, Bulgaria: constraints from ~(40)Ar/~(39)Ar analysis

摘要

The Panagyurishte region, east of Sofia in Bulgaria, is characterised by amphibolite facies metamorphic basement rocks, overlain by Late Carboniferous to Triassic and Late Cretaceous cover sequences, which together comprise the Central Part of the Srednogorie unit. This succession is intruded by subvolcanic Cretaceous calcalkaline magmatic bodies, which are part of the so-called Banatite Belt and host world-class Cu-Au-deposits. The goal of this study is to constrain the age of tectonometamorphic events recorded in the Srednogorie basement. The amphibolite facies metamorphic Srednogorie basement consists of two-mica gneisses, micaschists, ortho-amphibolites, small serpentinite bodies, and anatexites of pre-Upper Carboniferous age. New ~(40)Ar/~(39)Ar ages of white mica and biotite from southern and northern sectors of the central part of the Srednogorie unit show well-defined Late Palaeozoic plateau ages of ca. 317-305 Ma, which indicate a Variscan age for the postmetamorphic cooling after the last amphibolite facies metamorphic overprint. Subsequent post-Variscan intrusion and slow cooling of the cross-cutting Poibrene diorite is indicated by Late Palaeozoic cooling ages of 302.9 ± 1.4 and 263.00 ± 0.69 Ma reported for hornblende and biotite, respectively. During the Cretaceous, southern sectors of the Srednogorie basement and overlying Late Carboniferous to Triassic cover successions were affected by greenschist facies metamorphic overprint. The latter event is limited to southern sectors near Alpine low-angle normal faults. Muscovite and biotite samples from several subhorizontal ductile shear zones yield consistent Cretaceous ~(40)Ar/~(39)Ar plateau ages of ca. 99-105 Ma. Greenschist facies retrogression affected the basement mainly along basement-cover (Stefanian-Permian) contacts, which are strongly sheared. Subsequently, the steep ductile dextral Maritsa shear zone formed along a belt of granitoid intrusions within Theologically weakened crust along southern margins of the Panagyurishte region, whereas the Central Srednogorie collapse-type basin seals structures formed during early Late Cretaceous.