Geochemistry, Sr-Nd Isotopes and Zircon U-Pb Dating of Magmatic Rocks from the Talesh Range, Western Alborz: New Insights into Late Cretaceous Evolution of the Southern Eurasian Margin

The eastern flank of the Talesh range of western Alborz, northwestern Iran, exposes volumetrically significant Late Cretaceous volcanic and volcaniclastic associations as well as cross -cutting dykes and minor subvolcanic equivalents. The volcanic units appear as massive to pillowed flows interlayered with volcaniclastic beds and subordinate limestones. We present new field data, zircon U - Pb ages, bulk -rock major and trace element geochemistry, and Sr - Nd isotopes on the igneous rock suite. Geochemical data show that the lava units and dykes are mainly basaltic in composition with minor basaltic andesites and trachytes. They form two contrasting series of arc -related calc-alkaline (Group 1) and OIB-like (Group 2) rocks. The zircon U - Pb age of Group 1 rocks is 95.6 +/- 1.8 Ma (2 sigma ), whereas for Group 2 rocks, cross -cutting relationships and stratigraphic constraints indicate a somewhat younger age. Group 1 rocks have 87 Sr/ 86 Sr (i) varying from 0.7038 to 0.7070 and epsilon Nd (t) values ranging from + 0.5 to + 5.0. Group 2 rocks have 87 Sr/ 86 Sr (i) = 0.7040 - 0.7065 and epsilon Nd (t) from - 3.0 to + 3.7. Trace element and isotopic modelling shows that the primitive Group 1 magmas were derived from a metasomatized mantle source enriched by the addition of -6 - 10 % sediment melt component, while Group 2 rocks are consistent with melts of an asthenospheric mantle source enriched by -1 - 5 % EMII component. Trace element modelling indicates that Group 1 rocks formed from -6 - 13 % partial melts of a spinel-garnet lherzolite with garnet:spinel ratios of 45:55 to 10:90, whereas primitive Group 2 melts were generated through -1 - 3 % partial melting of a garnet-spinel-lherzolite with garnet:spinel ratios of 90:10 to 80:20. We propose a geodynamic model in which a north -dipping Neotethyan slab beneath the western Alborz (Talesh area) continental margin produced arc assemblages of Group 1 rocks and subsequent slab rollback led to the upwelling of an asthenospheric mantle to generate OIB-like magmatism of Group 2 rocks in an extensional intra-arc to back -arc setting.