CONTRASTING STYLES OF VARISCAN GRANITIC PLUTONISM, CENTRAL BULGARIA: A ZIRCON U-PB GEOCHRONOLOGIC STUDY

Two contrasting styles of granitic magmatism related to the Variscan Orogeny are observed in the Balkan and Sredna Gora terranes of Bulgaria. Biotite-bearing granites (minor sphene) occur in both terranes and dominate the first suite. More mafic intrusions, some with hornblende (granodiorites and minor gabbros), occur as part of this suite but are limited to the Balkan Terrane. A second suite comprised of two-mica, leucocratic granites (minor garnet) is found only in the Sredna Gora. Previous dating by Rb-Sr suggested that the first series constituted an older group of intrusions emplaced at ~340 Ma, whereas the two-mica granites were younger phases emplaced between ~300 Ma and 240 Ma. Zircons from all samples of the first suite show concentric magmatic zoning and completely lack inherited cores. In stark contrast, zircons from the two-mica granites are almost entirely xenocrystic (~80-90%), with very thin magmatic rims around most grains. These striking differences in zircon inheritance and preservation likely indicate differences in zircon saturation in the source region and in the temperature of melt generation. The two suites were probably derived by different melt generating mechanisms related to differences in temperature and fluid influx (Miller et al., 2003). Ages of representative samples from the first suite are 313.8±3.5 Ma and 304.1±3.3 Ma (Balkan), and 303.5±3.3 Ma and 307.7±4.5 Ma (Sredna Gora). The similarity of these granites across this terrane boundary suggests intrusion occurred after the docking of these terranes. Two granites from the second suite are 312±3.8 Ma and 285±5.2 Ma, bracketing the ages of the first series. Xenocrystic cores from both samples yield a clustering of ages ~450 Ma. The new isotopic ages indicate that emplacement of the two suites did not take place in separate intervals, but was roughly coeval. Although likely derived by different melt-generating mechanisms, and possibly from different sources, the similar ages of the two suites suggest formation in the same tectonic setting. The presence of gabbroic-dioritic rocks argues against derivation purely by crustal anatexis during collision, or by rift related melting. We propose a tentative model wherein both suites of granites were derived in a subduction zone setting, the first by influx of heat from the mantle and the second by influx of water-rich fluids into Ordovician rocks.