A preliminary framework for magmatism in modern continental back-arc basins and its application to the Triassic-Jurassic tectonic evolution of the Caucasus

In contrast to intraoceanic back-arc basins, extension within a continental back-arc basin initiates within continental rather than oceanic lithosphere. The geochemical characteristics of magmatic rocks within continental back-arc basins are poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five modern continental back-arc basins – the Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatolia – to establish a geochemical framework for continental back-arc magmatism.  This analysis shows that continental back-arcs yield geochemical signatures more similar to arc magmatism than intraoceanic back-arcs. We apply this framework to published data for Triassic-Jurassic magmatic rocks from the Caucasus arc system, which includes a relict continental back-arc basin, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains unexplained. 40Ar/39Ar and U-Pb ages indicate Permian-Triassic arc magmatism from ~260-220 Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (~220-210 Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc front and opening of the Caucasus Basin. This activity was followed by Jurassic magmatism in the Caucasus Basin from ~180-160 Ma and in the Lesser Caucasus arc from ~180-140 Ma. Trace element and Sr-Nd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental back-arcs and that the mantle source to the Lesser Caucasus arc became more enriched at ~160 Ma, likely from the cessation of back-arc spreading.