Geochemical and isotopic evidence for the origin of rhyolites from Petrota (Northern Thrace, Greece) and geodynamic significance

Major- and trace-element contents, Sr- and Nd-isotope ratios and K/Ar radiometric ages of rhyolite samples from Petrota, eastern Rhodope, Greece, have been determined. These rocks were emplaced during an extensional regime after the collision between Eurasia and Africa, that occurred in the Palaeocene. The rocks are peraluminous rhyolites characterized by K2O/Na2O > 1.3, and can be distinguished into two groups on the basis of the different contents of elements especially of mafic affiliation. The measured Sr- and Nd-isotope ratios range from 0.708540 to 0.735720 and 0.512419 to 0.512487, respectively. A half of the samples has high Sr-isotope ratios (0.714358-0.735720), that result by hydrothermal alteration operated by supergenic fluids circulating through the metamorphic basement. These fluids were neutral or mildly alkaline, as the rocks retained their bulk contents in alkaline elements. Moreover, the fluids were nor enriched in complexing agents, as Nd was not mobilized. The narrow range of the Nd-isotope ratios indicates that the parental magmas of the two groups of rocks derived from a common source. Therefore, the different chemical compositions of the two groups likely reflect distinct degrees of partial melting. The magma source was represented by lower to middle crust, as indicated by the chemical features and the relatively low Nd-isotope ratios of the rocks. The Sr-87/Sr-86 ratios of the least altered rocks are consistent with this suggestion. The comparison of the studied rhyolites with granites of similar geodynamic significance from the Pieria complex in northern Thessaly provides support to the genetic hypothesis. The K/Ar radiometric ages of the rocks range from 20.7 +/- 1.6 to 24.5 +/- 2.5 Ma. There is a correlation between the K/Ar radiometric ages and the Sr-isotope ratios of the rocks, indicating that the least altered rocks with respect to Sr-isotope ratios are those showing older radiometric ages. Lastly, the least altered rhyolites show chemical and Sr-isotopic characteristics similar to those of other Oligocene rhyolites from the central and eastern Rhodope. Therefore, all these volcanics represent the product of meltings of similar levels of continental crust after the Eurasia-Africa collision.