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A Preliminary Framework for Magmatism in Modern Continental BackāArc Basins and Its Application to the TriassicāJurassic Tectonic Evolution of the Caucasus
Abstract Extension within a continental backāarc basin initiates within continental rather than oceanic lithosphere, and the geochemical characteristics of magmatic rocks within continental backāarcs are poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five exemplar modern continental backāarc basinsāthe Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatoliaāto establish a geochemical framework for continental backāarc magmatism. This analysis shows that continental backāarcs yield geochemical signatures more similar to arc magmatism than intraoceanic backāarcs do. We apply this framework to published data for TriassicāJurassic magmatic rocks from the Caucasus arc system, which includes a relict continental backāarc, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains debated. Our analysis of 40Ar/39Ar and UāPb ages indicates PermianāTriassic arc magmatism from ā¼260 to 220Ā Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (ā¼220ā210Ā Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc and opening of the Caucasus Basin. This activity was followed by Jurassic arc magmatism in the Lesser Caucasus from ā¼180 to 140Ā Ma and backāarc spreading in the Caucasus Basin from ā¼180 to 160Ā Ma. Trace element and SrāNd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental backāarcs and that the source to the Lesser Caucasus arc became more enriched at ā¼160Ā Ma, likely from the cessation of backāarc spreading