Timing of Igralishte pluton in Ograzhden Mountain, SW Bulgaria: implications for the tectono-magmatic evolution of the region

Abstract

U-Pb-zircon and monazite ID-TIMS and in-situ LA-ICP-MS and EPMA analyses are combined to define the magmatic age of the Igralishte granite, which is exposed over 32 km2 in the high-metamorphic rocks of the Serbo-Macedonian Massif (Ograzhden Mountain, SW Bulgaria). The granite is dated at 243.28 ± 0.84 Ma by concordant zircons (ID-TIMS data) and this intrusion age is confirmed also by in-situ LA-ICP-MS and EPMA monazite analyses. The intrusion of the Igralishte granite at middle Triassic time defines the age of the high-grade metamorphism in Ograzhden Mountain as Pre-Triassic. A tectonic and hydrothermal overprint is dated at 36.36 ± 0.56 Ma by the Rb-Sr whole-rock-biotite reference line. This age is considered as the youngest thermal event in the region of the Igralishte pluton that reached the 300 ± 50 °C closing temperature of the Rb-Sr isotope system in the biotite, but could not fully reset the Rb-Sr isotope system of the muscovite. The initial strontium ratio of 87Sr/86Sri = 0.7078 and the inheritance of old lead components in the zircons suppose crustal-dominated source of the magma. Preferred tectonic scenario suggests dehydration melting and granite formation in the middle/upper crust at the Permian-Triassic boundary and the Lower-Middle Triassic caused by mafic intrusions at the base of the crust.