Age of Variscan magmatism from the Balkan sector of the orogen, central Bulgaria

Abstract

New high-resolution, secondary ion mass spectrometry U–Th–Pb data obtained from zircon and monazite from representative Variscan intrusions in the Balkan sector of the Variscan orogen (Central Bulgaria) constrain the ages of magmatism to the interval ∼315–285 Ma. The samples investigated are granitoids from two contrasting intrusive suites. Calc-alkaline granitoids of the first suite include the San Nikola granite (311.9±4.1 Ma) and the Petrohan granodiorite (304.6±4.0 Ma) from the Balkan Terrane, and the Smilovene (304.1±5.5 Ma) and Hisara (303.5±3.3 Ma) granites from the Sredna Gora Terrane. Zircons from these intrusions predominantly lack inherited cores. The second suite consists of two-mica, leucocratic granites and includes the Koprivshtitsa (312.0±5.4 Ma) and Strelcha (289.5±7.8 Ma) plutons. Zircons from intrusions of the second suite are 80–90% xenocrystic. Apparent ages of inherited cores are the magmatic age and slightly older (∼300–400 Ma), to Ordovician (∼450 Ma) ages, and to Neoproterozoic (∼600–900 Ma) ages. Gabbroic magmas related to the first suite are found only in the more northerly Balkan Terrane, while the leucogranites occur only in the southerly Sredna Gora Terrane. Although coeval, the two suites of intrusions are not genetically related, and furthermore were likely derived from different source regions via different melt generating mechanisms. We propose a tectonic and petrogenetic model, whereby following juxtaposition of the Balkan and Sredna Gora Terranes during Variscan collision, the two suites of magmas were generated by continued subduction under eastern Europe due to the influx of mafic magmas into the base of the crust, and by influx of water-rich fluids into high-grade metapsammitic rocks of the Sredna Gora metamorphic series. The new ages are somewhat younger than ages for post-collisional intrusions in the central European Variscan Massifs (i.e., Bohemian, Black Forest, and Vosges), where ages of intrusions are predominantly ∼340–320 Ma, although some younger intrusions may also occur. Within the intra-Alpine massifs, both older (∼340–320 Ma) and younger (∼310–290 Ma) granitoids are present. Farther west, intrusions in the Iberian Massif appear to be predominantly younger ∼325–290 Ma, similar to the intrusions of the Balkans. This may indicate that collision of adjacent crustal blocks culminated at earlier times in the central and more northerly portions of the orogen, and moved both east, west, and south as the evolution of the orogen progressed.