Jurassic and Cretaceous tectonic evolution of the Sakar and Srednogorie zones, Bulgaria: ⁴⁰Ar/³⁹Ar mineral ages and structures

We present new ⁴⁰Ar/³⁹Ar mineral ages from metamorphic, plutonic and volcanic rocks of the principally E-W striking the Sakar and Srednogorie zones, Bulgaria and use these ages together with structural observations to constrain the Jurassic and Cretaceous tectonic history of these zones along northern margins of the Rhodope massif. The Srednogorie zone seems to be particularly important because of its richness of magmatic rocks and associated ore deposits.
   The Sakar zone SW of Topolovgrad exposes the Palaeozoic Sakar granite mantled by the metamorphic Laslovo Formation. Along the contacts between granite and their volcanosedimentary metamorphic cover, often shear zones developed with a mylonitic fabrics formed within upper greenschist to amphibolite facies-grade metamorphic conditions giving the structure an appearance similar to a mantled gneiss dome. ⁴⁰Ar/³⁹Ar amphibole and white mica dating yield ages ranging between 144 and 136 Ma constraining the age of the main tectonic event of ductile deformation within a deep crustal level at ca. the Jurassic/Cretaceous boundary. Further to the southeast, towards the Rhodope massive, younger white mica ages gradually decreasing to ca. 124 Ma were found in metamorphic rocks. A further, secondary thermal overprint was found in staircase Ar release patterns with a maximum age of a secondary thermal overprint younger than 69 Ma. Together, these ages indicate the principal age of the Sakar zone at the Jurassic/Cretaceous boundary, which predates the formation of the Srednogorie basin, and two stages of thermal overprint.
   The Srednogorie zone comprises an Upper Cretaceous siliclastic marine infill of a volcanosedimentary basin and abundant volcanics, subvolcanics and shallow plutons. In the eastern Srednogorie zone, alkalic rocks are abundant. A new ⁴⁰Ar/³⁹Ar amphibole age of 82 Ma from a hornblende andesite from the Fakjijska river S of Sredec indicate an important stage of effusive volcanic activity. In contrast, the following ages from plutonic rocks are interpreted to date cooling through the Ar retention temperature, ca. 500–550°C for amphibole and ca. 300°C for biotite after magma crystallization. They represent, therefore, the minimum ages in respect to their crystallization. These ages include: biotite ages of 83, 79 and 79 Ma from the Granitovo granodiorite, Izgrev diorite and Rosen syenite (at Černomorez), respectively; and amphibole ages of ca. 86, 85 and 76 Ma from microgabbro within the Granitovo granodiorite, the Oman-Fakia gabbro (near Danica) and a gabbrodiorite of ENE Samokov. Together, these ages proof the wide range of magmatism between 86 and 76 Ma and a particularly important cooling event in the Srednogorie zone at ca. 80 Ma. In the case of the Izgrev diorite in the eastern Srednogorie zone, cooling at ca. 79 Ma is associated with chalcopyrite mineralization along conjugate shear-extension veins. Their structural assessment proofs NW–SE extension, and this event is in agreement with many similar but poorly dated observations in the eastern Srednogorie zone. This indicates that regional extension was of transtensive type at ca. 80 Ma in respect to the strike of the Srednogorie
zone. Inversion of the Srednogorie basin comprises regional N-S shortening and a subsequent stage of NE-SW transpressive motion as abundant sets of conjugate strike-slip faults indicate.