GEOCHEMISTRY OF LATE CENOZOIC COLLISION VOLCANISM IN THE LESSER CAUCASUS, LACHIN – KALBAJAR AREA OF THE REPUBLIC OF AZERBAIJAN

The Lesser Caucasus in Azerbaijan is a segment of the collisional Alpine – Himalayan orogenic system. It records a long history of subduction related to closing of the ancient Tethys and more recent intraplate magmatism associated with continental collision of the Eurasian and Arabian plates. Here, we examine >40 Miocene-Pliocene lavas from the Lesser Caucasus in Azerbaijan to investigate temporal changes in the geochemistry of lavas associated with collisional tectonics in the Alpine-Himalayan orogenic system.

Our geochemical sampling covers >40 km² and documents the geochemistry of an Upper Miocene rhyolite-dacite suite and an Upper Pliocene trachybasalt-trachyandesite suite. Upper Miocene lavas have SiO₂ values ranging from 58–70 wt.% and are characterized by dominantly peraluminous, ferroan, and alkali-calcic to alkali compositions. They display a broad range in Sr concentrations (100-900 ppm), but typically have low Sr/Y values (<50). In contrast, the younger Pliocene suite has lower SiO₂ values (45–60 wt.%), and are distinct from Miocene lavas, being metaluminous, magnesian and also alkali-calcic to alkali. They are also distinct in having higher Sr concentrations (~1000-2000 ppm) and Sr/Y values (typically >50). Relative to NMORB, both suites display large ion lithophile enrichment, negative Nb and positive Pb anomalies. This apparent transition in geochemistry suggests an important change in source and or depth of partial melting during progressive collision. One possibility is that Miocene lavas were dominantly derived from shallow-level partial melting of crustal sources; whereas, younger Pliocene lavas reflect deep-crustal partial melting of a plagioclase-poor, garnet-rich source region. Lower SiO₂ values in the Pliocene lavas may indicate an influx of mantle-derived magmas or partial melting of more mafic source rocks.