The Rhodope Massif (RM) is regarded as an eastern extension of the Alpine-Himalayan orogenic belt, whose tectonic evolution in SE Europe resulted, among other events, in an extensive magmatism. The RM in Greece and surrounding areas was the loci of an intensive Tertiary (Late Eocene -Oligocene -Miocene) intrusive and extrusive magmatic activity, which was younging southward. This magmatism was related with an extensional event that started with block faulting and depressions and the formation of E-W to NNW-SSE aligned volcano-sedimentary basins. It has been interpreted as post-collisional magmatism as a consequence of the underplating of the African plate beneath the Eurasian one. The magmatism is represented by volcano-plutonic associations with rocks of calc-alkaline, high-K calc-alkaline, and shoshonitic affinity that differ significantly in composition, demonstrating a strong dependence on the present thickness of the crust and decreasing crustal input from NW to SE. Moreover, the mantle wedge underlying the area underwent variable degrees of metasomatism induced by fluids or sediment melts released from the subducting and dehydrating oceanic lithosphere. Its melting generated mafic melts with variable enrichment in incompatible elements, having a large range of geochemical signatures from calc- alkaline to lamprophyric.Among the above volcano-plutonic associations, the Evros Volcanic Rocks (EVR) belong to one of the two largest Tertiary volcanic districts in northern Greece. Most EVR show either calc-alkaline or high-K calk-alkaline characters. The less evolved rocks have mantle normalised trace element patterns with LILE enrichments relative to HFSE and negative spikes of Ta, Nb, and Ti. Their initial Sr and Nd isotope ratios range between 0.70545-0.70773 and 0.51260-0.51242, respectively. These characteristics denote a derivation from a mantle source enriched in incompatible elements and radiogenic isotopes. The EVR show an increase of K2O and hygromagmaphile elements, and a decrease of compatible elements, as the silica content increase. Despite some scattering in the Harker's diagrams, particularly in the most mafic rocks, the latter show trends with different slopes when compared to those of intermediate-acidic rocks. Besides, mafic rocks have large variations of both Sr and Nd isotope ratios for a restricted range of silica. Vice versa, intermediate-acidic rocks have large variation of silica with smaller variations of isotope ratios. In the (87Sr/86Sr)i versus (143Nd/144Nd)i diagram the EVR arrange along a negative correlation that points to crustal rocks, e.g. gneisses from Southern Rhodope. However, the EVR do not fit a single correlation curve and their distribution is consistent with an evolution by fractional crystallization plus mixing with heterogeneous crustal melts. Some trachydacites, having shoshonitic affinity, show peculiar characteristics, not compatible with a derivation from the basic rocks. They seem to be derived from a geochemically distinct original magma, more strongly enriched inK2O and incompatible elements.
Mantle heterogeneity and crustal contamination in Tertiary extensive magmatism in Southern Rhodopes: geochemical and isotope evidence from Evros volcanic rocks (Greece)
Pinarelli L;
2015
Abstract
The Rhodope Massif (RM) is regarded as an eastern extension of the Alpine-Himalayan orogenic belt, whose tectonic evolution in SE Europe resulted, among other events, in an extensive magmatism. The RM in Greece and surrounding areas was the loci of an intensive Tertiary (Late Eocene -Oligocene -Miocene) intrusive and extrusive magmatic activity, which was younging southward. This magmatism was related with an extensional event that started with block faulting and depressions and the formation of E-W to NNW-SSE aligned volcano-sedimentary basins. It has been interpreted as post-collisional magmatism as a consequence of the underplating of the African plate beneath the Eurasian one. The magmatism is represented by volcano-plutonic associations with rocks of calc-alkaline, high-K calc-alkaline, and shoshonitic affinity that differ significantly in composition, demonstrating a strong dependence on the present thickness of the crust and decreasing crustal input from NW to SE. Moreover, the mantle wedge underlying the area underwent variable degrees of metasomatism induced by fluids or sediment melts released from the subducting and dehydrating oceanic lithosphere. Its melting generated mafic melts with variable enrichment in incompatible elements, having a large range of geochemical signatures from calc- alkaline to lamprophyric.Among the above volcano-plutonic associations, the Evros Volcanic Rocks (EVR) belong to one of the two largest Tertiary volcanic districts in northern Greece. Most EVR show either calc-alkaline or high-K calk-alkaline characters. The less evolved rocks have mantle normalised trace element patterns with LILE enrichments relative to HFSE and negative spikes of Ta, Nb, and Ti. Their initial Sr and Nd isotope ratios range between 0.70545-0.70773 and 0.51260-0.51242, respectively. These characteristics denote a derivation from a mantle source enriched in incompatible elements and radiogenic isotopes. The EVR show an increase of K2O and hygromagmaphile elements, and a decrease of compatible elements, as the silica content increase. Despite some scattering in the Harker's diagrams, particularly in the most mafic rocks, the latter show trends with different slopes when compared to those of intermediate-acidic rocks. Besides, mafic rocks have large variations of both Sr and Nd isotope ratios for a restricted range of silica. Vice versa, intermediate-acidic rocks have large variation of silica with smaller variations of isotope ratios. In the (87Sr/86Sr)i versus (143Nd/144Nd)i diagram the EVR arrange along a negative correlation that points to crustal rocks, e.g. gneisses from Southern Rhodope. However, the EVR do not fit a single correlation curve and their distribution is consistent with an evolution by fractional crystallization plus mixing with heterogeneous crustal melts. Some trachydacites, having shoshonitic affinity, show peculiar characteristics, not compatible with a derivation from the basic rocks. They seem to be derived from a geochemically distinct original magma, more strongly enriched inK2O and incompatible elements.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
