Abstract
Subduction of oceanic lithosphere is the primary driver for plate tectonics on earth, but a paucity of geological records hinders our understanding of how subduction is initiated. Two general mechanisms have been proposed-induced and spontaneous nucleation of subduction zones. For both mechanisms, subduction inception begins with initial lower plate burial, but whether upper plate extension and magmatic activity coincide with, or are later than, lower plate underthrusting is the key to identifying spontaneous or induced subduction initiation. Here we present geological, geochemical and geochronological characteristics of mafic dikes and amphibolites from the Bursa ophiolite in NW Turkey, to identify the mechanism of the subduction initiation of the Neo-Tethys Ocean. The mafic dikes are depleted in light rare earth elements (LREE) and slightly enriched in large-ion lithophile elements (LILE), which together with high ε_(Nd)(t) values and low ~(207)Pb/~(204)Pb ratios, indicate a MORB-like mantle source similar to the earliest forearc basalts (FAB) of the Izu-Bonin-Mariana subduction initiation system. Zircon U-Pb geochronology shows a crystallization age of 89.8 ± 0.9 Ma. In contrast, the amphibolites are enriched in LREE and LILE, and have low ε_(Nd)(t) values and high ~(207)Pb/~(204)Pb ratios, suggesting an E-MORB-like geochemical affinity. Zircon U-Pb ages of the amphibolites reveal that the metamorphism occurred at 96.4 ±1.9 Ma, representing the initial burial of the lower plate. Thus, subduction predated upper plate extension and formation of the Bursa oceanic crust by 9.4-3.8 Myr, arguing for induced subduction of Neo-Tethyan lithosphere that formed the Bursa ophiolite.
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