Pirin metamorphic and igneous evolution revisited in a geochronological frame based on U-Pb zircon studies

Abstract

New isotopic data obtained from zircons by the U-Pb method during the last decade show the necessity of a revision of the tectonometamorphic and igneous evolution of the Pirin–Pangaion Unit of the Alpine Morava–Rhodope tectonic zone. The northern (Pirin) subunit of this unit consists of four areas built up of metamorphic rocks (amphibolite facies) of the Rhodopian supercomplex. These areas are separated by Late Alpine granite plutons of batholithic dimensions. The northernmost (Krupnik) area is built up of rocks of the Rupchos Group (complex) of varied composition and intense multiphase deformation and metamorphism. The oldest metamorphic phase is dated as Cadomian (528–517 Ma), and its products also suffered Hercynian superimposed amphibolite-facies metamorphism and anatexis. A septum of granitised metamorphics preserved between the Krupnik granite pluton (Triassic, ca 240 Ma) and the early Oligocene North Pirin (Kresna) pluton is characterised by Hercynian (330–300 Ma) anatexis. The principal metamorphism within the southern area around the early Oligocene South Pirin (Teshovo) granite pluton is dated both in the Bachkovo quartzo-feldspathic gneisses (named also Kriva-reka metagranites or orthogneisses) and in the Lukovitsa schists and paragneisses as Hercynian (ca 300–290 Ma). Therefore, the polymetamorphic evolution of the Pirin Subunit consists of a Cadomian phase and a strong Hercynian overprint followed by Triassic granitoids. We confirm the Thanetian–early Eocene age (58–55 Ma) of the Spanchevo granite, partially formed by in situ anatexis. Its intrusion was followed by a compression phase during the mid-Eocene thrusting of the Ograzhden Unit over the Pirin–Pangaion Unit, along the Strimon Thrust. The last magmatic phase (ca 34–31 Ma) was represented by a granitoid association formed during the latest Eocene–early Oligocene in extensional conditions and as a result of mixing of anatectic crust-derived magma with magma of mantle origin. It was part of a volcano-plutonic association related to the Macedonian–Rhodope magmatotectonic province. A high-temperature metamorphic overprint of the same age is recorded in the metamorphics as formation of andalusite in the metapelites, and skarns at the contacts of some of the granitoid bodies.