Rapid cooling history of a Neotethyan ophiolite: Evidence for contemporaneous subduction initiation and metamorphic sole formation

摘要

The Beywhir-Hoyran Nappes, including Mesozoic carbonate platform rocks, deepsea sediments, and ophiolite-related units, crop out extensively on the western limb of the Isparta Angle in the Central Taurides, Turkey. The ophiolite-related rocks are represented by variably serpentinized harzburgitic mantle tectonites, tectonically underlain by a subophiolitic metamorphic sole and melange. The harzburgitic mantle tectonites and metamorphic sole are intruded by undeformed isolated dikes. Protoliths of the metamorphic sole are similar to within-plate alkali basalts and associated sediments. The isolated dikes were geochemically derived mainly from tholeiitic magma and, to a lesser extent, from alkaline magma. Five isolated dike samples yielded U-Pb ages ranging from 90.8 +/- 1.6 Ma to 87.6 +/- 2.1 Ma (zircon) and from 102.3 +/- 7.4 Ma to 87.5 +/- 7.9 Ma (titanite). Seven amphibolite samples yielded U-Pb age ranges of 91.1 +/- 2.1-88.85 +/- 1.0 Ma (zircon) and 94.0 +/- 4.8-90.0 +/- 9.4 Ma (titanite) and a Ar-40-Ar-3(9) age range of 93.7 +/- 0.3-91.4 +/- 0.4 Ma (hornblende). U-Pb and Ar-40-Ar-39 ages of mineral phases with different closure temperatures (similar to 900-500 degrees C) from the isolated dikes and metamorphic sole rocks are almost identical and overlapping within 1 sigma, suggesting that both the magmatic growth of oceanic crust and formation of metamor- phic sole were contemporaneous and cooled very rapidly. Hence, all the data should be interpreted as the crystallization ages of the ophiolite and metamorphic sole pair. Genesis of suprasubduction zone-type oceanic crust, genesis and exhumation of the metamorphic sole, and postmetamorphic dike emplacement within the Inner Tauride Ocean can be best explained by subduction initiation and rollback processes during the Late Cretaceous based on petrological and geochronological data obtained from the ophiolitic rocks of the Beywhir-Hoyran Nappes.