Ar-40/Ar-39 geochronology and Sr-Pb isotopic evidence of post-collisional extensional volcanism of the eastern Pontide paleo-arc, NE Turkey
Résumé
Tertiary aged post-collisional volcanics show stratigraphic, petrographic and age differences in the northern and southern zones of the eastern Pontides. The Eocene sequences in E-W direction are exposed as basin fillings in the southern zone and overlay the Upper Cretaceous and Paleocene aged units. The volcanic rocks in the Gümü..hane and Alucra areas are mainly basalt, basaltic andesite, andesite, trachyandesite and rare rhyolite-obsidien. Petrochemically, the Gümü..hane area volcanics vary from basalt to dacite with low alkali tendency, whereas the Alucra area volcanics from basalt to basaltic-trachyandesite with high alkali tendency. The differences on the element variations of the tholeiitic-alkaline transition and calc-alkaline rocks can be explained by fractionation of clinopyroxene + hornblende + plagioclase ± magnetite ± apatite. The trace element variations show some similarities with enrichment of LILE and negative Nb, Ta and Ti anomalies. Particularly, the enrichments in LILE and a lesser extent in LREE, but the depletions in HFSE indicate that the rocks evolved from a parental magma derived from an enriched source formed by mixing of slab and asthenospheric melts. The chondrite-normalized-REE patterns of these volcanics resemble to each other and spoon-shaped with low to medium enrichment (LaN/LuN=2-35), indicating similar source area for the Gümü..hane and Alucra area volcanics. The 40Ar-39Ar dating of the studied volcanics are between 37.7±0.2 and 44.5±0.2 Ma. The (87Sr/86Sr)i values of the rocks are between 0.70457 and 0.70556, and their 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotopic ratios range 18.59 to 18.73, 15.62 to 15.64 and 38.65 to 38.86, respectively. Pb- and Sr- isotopic ratios may imply that the rocks derived from a source region at the boundary of the mantle-crust, but the samples with low (87Sr/86Sr)i values (<0.705) reflect the mantle source in their origin. The increasing values of SiO2 (wt.%), Sr (ppm) ve (1/Sr)x103 ppm-1 versus (87Sr/86Sr)i values suggest fractionational crystallisation (FC) rather than assimilation (AFC) in their evolution. Based on volcanic variety and distributions together with field and petrological data, volcanic activity in the Eocene time of the eastern Pontide is closely related to extensional tectonic regime with transtansional faults.