Plateau basaltic volcanism in a syn-collision setting (South Caucasus)

Recent Armenian collision-related volcanism, occurring within the past 5 million years, is characterized by the formation of thick, laterally extensive sequences of Upper Pliocene - Pleistocene mafic lava flows. These flows, primarily consisting of valley-filling ophitic doleritic basalts, are typically attributed to fissure eruptions similar to continental flood basalt volcanism. However, the total volume of plateau basalts in the region is significantly lower than that of typical continental flood basalt provinces. The plateau basalts in the South Caucasus display textures consistent with magma mixing processes. These thick mafic flows have generated several plateaux within the Lesser Caucasus, including the Javakheti Plateau in southern Georgia and northwestern Armenia, and the Lori and Kotayk Plateaux in Armenia. The basalts also extend to the northeastern Kars-Erzurum Plateau in Turkey. Complete cross-sections containing several generations of lavas can be observed in the Dsoraget, Akhuryan, Debed, and Hrasdan river canyons in Armenia, which were sampled during field campaigns in 2011-2012. Most plateau basalts in the region have a narrow silica range (SiO2 = 48-53 wt%), low MgO (<7 wt%), and are primarily mid-K basalts-trachybasalts-basaltic trachyandesites. Most lavas exhibit modest enrichment of LREE over HREE (La/Yb = 8-13). More fractionated dolerites are typical for the Kotayk plateau in central Armenia (La/Yb = 12-28). Increasing SiO2 coupled with increasing LREE/HREE fractionation from the oldest to youngest flows is evident only in the Hrasdan river canyon, Yerevan. Other sections are more homogeneous in composition, suggesting a continuum of magma supply, efficient magma mixing, and limited crustal storage time. Trace element and isotope models suggest that the primary magma formed through approximately 10% spinel-facies melting of a subduction-modified mantle source with a moderately enriched background composition. High Zr/Hf ratios (>50) and low MgO point towards a pyroxenitic source. Isotope signatures also indicate a lithospheric mantle source that inherited its subduction component during recent magmatic events, such as Mesozoic Tethyan arc activity. Compositions of these mafic lavas (87/86Sr = 0.7041-0.7045, 143/144Nd = 0.5128-0.5129) are homogeneous and amongst the most depleted of the Turkish-Armenian-Iranian Plateau. There appears to be little evidence for crustal contamination during magma ascent, even though some lavas have been erupted through ancient basement terranes. The trigger for magmatism, occurring some 25 million years after the initial Arabia-Eurasia collision, may lie in small-scale delamination of lithospheric mantle and heating of the lower lithosphere during asthenospheric upwelling following Tethyan slab break-off.