Cambrian magmatism, Variscan high-grade metamorphism and imposed greenschist facies shearing in the Central Sredna Gora basement units (Bulgaria)

Abstract

Gneisses from the deep structural levels of the European Variscan Belt are well exposed in the Central Sredna Gora in Bulgaria. In general, migmatites predominate, but unmigmatized domains (or domains with incipient migmatization) are also documented in this area. This paper presents new structural, petrographic and U-Pb isotope geochronological data from such an unmigmatized part of the Variscan high-grade metamorphic basement (the Koprivshtitsa Unit). A predominant part of this unit represents an alternation of metagranitoids and metabasites. The protolith crystallization age of the metagranitoids is constrained at 491.5 ± 7.6 Ma by U-Pb laser ablation method on zircons. This age coincides with the previously available Late Cambrian protolith ages of metabasic rocks that crop out within the adjacent migmatitic unit. The Korpivshtitsa Unit comprises also lesser orthogneisses with Late Neoproterozoic protoliths. Based on the available local and regional paleogeographic reconstruction schemes, we suggest that the Upper Cambrian magmatic rocks intruded Upper Neoproterozoic crust during the initial opening stages of the Rheic Ocean or a related basin. Subsequently, both were involved in the Variscan high-grade deformation. The contact of the Koprivshitsa Unit with the migmatitic part of the metamorphic complex coincides with a north-vergent greenschist facies thrust zone – the Chuminska Shear Zone. The exact time of the shearing is still not well constrained but it clearly postdates the Variscan high-temperature metamorphism of the gneisses.

Sm-Nd age determination of eclogites from Sredna Gora Mountains – HP metamorphic event or final retrogression

“…Upper Cambrian metabasic protolith ages of 502.8±3.2 Ma and 493.8±9.8 Ma have been determined by Peytcheva and von Quadt (2004) and Antonov et al (2010), respectively. Similar protolith age for metagranite (491.5±7.6 Ma) to the east of the study area is obtained by Lazarova et al (2015), whereas Сarrigan et al (2006) yeld the oldest protolith age of metagranitoids (616-595 Ma). The age of high-temperature amphibolite facies metamorphism is determined in the range 336-305 Ma (Velichkova et al, 2004;Сarrigan et al, 2006).…”

On the Rhodopean protoliths and their Middle Jurassic to Early Cretaceous evolution. A review

“…Early Cretaceous discrete low-grade strike-slip and north-vergent shear zones were reported also in the crystalline basement of the Sredna Gora Zone (e.g., Lazarova et al, 2015;Balkanska et al, 2021). The maximum temperature between ~300 °C and 400 °C, attained in the basement of this unit during the Early Cretaceous (140-138 Ma), was attributed to tectonic burial (Balkanska et al, 2022).…”

“…Mixed protoliths of continental and oceanic affinities have been assumed for the high-grade metamorphic rocks (Zagorchev, 2008). Dating of the metaigneous rocks revealed Ediacaran meta-granitoids (616.9±9.5 Ma and 595±23 Ma; Carrigan et al, 2006) and Late Cambrian metagabbros (493.8±9.8 Ma; Antonov et al, 2010), hornblendebiotite orthogneisses (502.8±3.2 Ма; and metagranitoids (497±12 Ma; Lazarova et al, 2015). The metamorphic evolution of the Sredna Gora complex includes an Early Devonian eclogite facies metamorphism followed by an amphibolite facies re-equilibration (at 398±5.2 Ma; Gaggero et al, 2008) and Early Carboniferous high-temperature metamorphism and anatexis (336.5±5.4 Ma; Carrigan et al, 2006).…”

Assessing the potential of Assarel porphyry copper deposit for critical raw materials: mineral-geochemical data for combination with agile exploration methods and better geo-modeling

“…The basement of the Central Srednogorie is Cadomian to Variscan in age. It is built up by metamorphic rocks (mainly gneisses, gneiss-schists and amphibolites) with Paleozoic and older protoliths and 330-340 Ma high-grade metamorphic overprint (Carrigan et al, 2005(Carrigan et al, , 2006Lazarova et al, 2015). They are intruded by Variscan granitoids and gabbros at 314-305 and at ≈290 Ma (Carrigan et al, 2006;Peytcheva, von Quadt, 2004) and intruded and overlain by calk-alkaline magmatic rocks with Late Cretaceous age (intrusives/shallow intrusives, volcanics, and volcaniclastics/epiclastics) in shallow marine sedimentary basin.…”