Petrochemical and 40Ar/39Ar geochronogical evidence of postcollisional Tertiary calc-alkaline volcanism in the Ulubey (Ordu), eastern Pontide, NE Turkey
Résumé
Post-collisional Tertiary volcanism in the Ulubey region at the western edge of the eastern Pontides paleo-arc started with sediments in a shallow marine environment in the Paleocene-Eocene time and then continued extensively with sub-aerial andesitic and rare basaltic volcanism during Eocene and Miocene time. The volcanic rocks studied are mainl yandesite/trachyandesite and rarely basalt/trachybasalt andpyroclastics, and show porphyric, hyalo-microlitic porphyricand rarely glomeroporphyric, intersertal, intergranular, fluidal and sieve textures. The volcanic rocks indicate a magma evolution fromtholeiitic-alkaline transitional to calc-alkaline, and have medium to high-K in character. Petrochemical data of the rocks suggest a dominant fractional crystallization and less crustal contamination processes in their evolution. Most samples have also low Mg#, Cr, and Ni, which indicates that they have undergone significant fractional crystallization from mantle-derived melts. Generally, major and trace element geochemical variations in the rocks can be explained by fractionation of common mineral phases. E-MORB normalized trace element patterns show that Ulubey volcanic rocks have geochemical patterns with enrichment in LILE and to a lesser extent in LREE, but depletion in HFSE. The volcanic rocks have moderate LREE/HREE ratios relative to E-Type MORB, depletion in Nb, Ta and Ti, and high Th/Yb ratios indicate that these patterns probably reflect magma derived from an enriched source formed by mixing of slab and asthenospheric melts previously modified by fluids and sediments from a subduction zone. The chondrite-normalized REE patterns of these volcanics resemble to each other and spoon-shaped with low to medium enrichment, indicating similar source area for the basaltic and andesitic rocks. The most primitive samples have YbN<10, which indicates the presence of garnet as a residual phase in the mantle source. The 40Ar-39Ar dating of the basaltic rocks 15.1±0.6 Ma and the andesitic rocks are between 44.6±0.1 and 49.4±0.1 Ma. The rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks formed clinopyroxene ± titanomagnetite controlled fractionation whereas andesitic rocks developed hornblende + plagioclase + clinopyroxene ± titanomagnetite controlled fractionation and, rare crustal contaminaion in shallow level magma chamber(s).