30 Myr of Cenozoic magmatism along the Tethyan margin during Arabia–Eurasia accretionary orogenesis (Meghri–Ordubad pluton, southernmost Lesser Caucasus)

Three magmatic series of substantially different ages and compositions were successively emplaced to form the composite Meghri–Ordubab pluton (MOP) in the southernmost Lesser Caucasus. The protracted incremental assembly during 30Myr, from Middle Eocene to Early Miocene, renders this location particularly suitable to characterize the petrogenetic evolution of Cenozoic magmatism during the final stage of the Neotethyan subduction. Based on whole-rock geochemistry, two main transitions tightly constrained in time are recognized. The first transition from Middle Eocene medium-K calc-alkaline to Late Eocene–Middle Oligocene shoshonitic magmatism corresponds to a marked increase in LREE and MREE and more juvenile 87Sr/86Sr and 143Nd/144Nd ratios. The second transition to Late Oligocene–Early Miocene high-K calc-alkaline “adakite-like” magmatism is coeval with a marked increase in Mg#, and Ni and Cr contents together with a depletion in MREE and HREE. Although the three differentiation series are derived from lower to mid-crustal hydrous magma fractionation, temporal variations of the magmatic source conditions are required to explain the contrasting chemistry of the parental magmas over time. Medium-K calc-alkaline parental magmas were generated by high degree of partial melting (~15%) of a garnet lherzolite. The shoshonitic and adakitic magmatic series represent magmas produced by low degree (1–5%) of partial melting of a garnet lherzolite, but a higher amount of residual garnet is required to reproduce the "adakite-like" signature. The timing of the two main geochemical transitions in the MOP is correlated with a progressive evolution from a compressional to an extensional stress regime linked to (1) the final stage of the Neotethyan subduction followed by the Arabia–Eurasia continental collision during the Eocene–Oligocene, and (2) the transition toward post-collisional magmatism, combined with a switch toward transcurrent tectonics during the Late Oligocene–Early Miocene.