Geochronological, isotopic and petrogenetic investigations of Cenozoic Volcanic rocks in the Talysh Massif, NW Iran: Insights for the Eocene magmatic flare-up

Early Cenozoic magmatism is widespread in the Central Tethyan orogenic belt linking the tectonic zones of Iran, South Armenian Block (Lesser Caucasus) and Turkey, but its relationship to the Southern Neotethys Ocean is questioned. This study focusses on the southern part of Talysh massif, NW Iran, where thick geological successions of high potassic magmatic rocks erupted in the Eocene. This paper reports geochronological and geochemical data for this succession, which is mostly comprised of shoshonitic basalt and rare rhyolitic pyroclastic rocks emplaced during an Eocene magmatic flare-up. ⁴⁰Ar/³⁹Ar dating of magmatic hornblende and biotite from basalts and trachy-andesites and U/Pb zircon dating of a dacitic tuff show that the magmatic event is spread on a 10 Myr range, from 50 to 40 Ma. Four main stages are documented at ∼49.8, 45–43, 43.2–41.6 and 39.8 Ma. One zircon yielded a significantly older U/Pb age of 616 ± 16 Ma, which involves recycling of a Neoproterozoic crystalline basement. Mafic lavas are enriched in LILE and depleted in HFSE, with high Th/Yb, Ba/Th and Nb/Zr ratios. In addition, Sr, Nd, Pb and Hf isotopic ratios suggest an enriched mantle source, with assimilation of continental crust during differentiation processes. These data are in agreement with partial melting of a garnet-bearing subduction-modified subcontinental mantle and interactions with a spinel-bearing mantle during magmatic ascent. This magmatic flare-up could have been triggered by an Asthenosphere upwelling. This upwelling could be caused to combined onset of south-dipping subduction of the Transcaucasus basin and verticalization of the north-dipping subduction taking place to the south of Iran. Asthenosphere flow and magmatic ascent were likely facilitated by trans-lithospheric strike-slip faults and block rotations highlighted by paleomagnetic data. A transition from calc-alkaline towards a more alkaline magmatic component with time, from south to north of the Talysh Massif, suggests a slab steepening in response to roll-back in the Late Eocene. Subsequent to this period, volcanism ceased in the South Talysh and markedly declined in the North Talysh massif, where it underwent a transformation into an adakitic-type magmatism during the Late Miocene and Quaternary.