New 40Ar/39Ar age constraints on the timing of magmatic events in the Panagyurishte region, Bulgaria

Abstract

Amphibole and mica 40Ar/39Ar ages have been determined from Late Cretaceous plutonic and volcanic successions from the Panagyurishte region (Bulgaria), which is part of the calc-alkaline to alkaline Banatite belt spreading from the Apuseni Mountains to the Black Sea and hosting world-class Cu-Au-deposits. The post-collisional magmatism is younger than early Late Cretaceous orogenic events and contemporaneous with formation of the collapse-type Srednogorie sedimentary basin. Three volcanic intercalations within the oldest sedimentary succession of the Srednogorie zone exposed along the Tolponitsa river show 40Ar/39Ar ages between 93 and 89 Ma and at ca. 86 Ma. Biotites from the Elshitsa granodiorite give an age of 86 Ma. Muscovite from an alteration zone at Elshitsa yield an age of 88 Ma. Amphiboles from a dacite porphyry from Vlaykov Vruh yield an age at 80 Ma. Biotites from the Velichkovo granodiorite give ages at 78–80 Ma. Amphibole ages in part date hydrothermal alteration as the ages are clearly similar to U-Pb zircon ages from the same plutons, i.e. subvolcanic intrusions. Together with previous 40Ar/39Ar amphibole and mica and numerous U-Pb zircon ages from mainly Late Cretaceous plutonic rocks of the Srednogorie area, the new ages display the following major trends: Magmatism lasted over a long period ranging from ca. 93 to 78 Ma. Both the plutonic and volcanic successions yield similar age groups, which are: 93–89 Ma, 86–84 Ma and 80–78 Ma. The age groups and their distribution show that magmatism was discontinuous and shifted southwards. Older subvolcanic rocks (93–89 Ma) developed in a regime of ca. N–S extension. They show that the southward prograding magmatism is older than dextral shearing, which developed between 86–78 Ma within ca. N–S compressional tectonic conditions. The geodynamic setting of the late Cretaceous magmatism in the Srednogorie zone is still controversial. The relationships within the Panagyurishte region, specifically the early stage of extension combined with calc-alkaline magmatism, could be explained by either retreat of a subduction zone or by slab break-off. The subsequent compressional phase is likely related to intra-orogenic shortening following collision.