Island arc tholeiite to boninitic melt evolution of the Cretaceous Kizildag (Turkey) ophiolite: Model for multi-stage early arc-forearc magmatism in Tethyan subduction factories

摘要

The Kizildag ophiolite in Turkey is a remnant of the late Cretaceous suprasubduction zone (SSZ) oceanic lithosphere that was developed in Southern Tethys, and is part of a peri-Arabian ophiolite belt including the Troodos (Cyprus), Baer-Bassit (Syria) and Semail (Oman) ophiolites in the eastern Mediterranean region. The internal structure, petrology and geochemistry of the Kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite (IAT) to boninitic compositions. Similar structural architecture and geochemical trends in the coeval Troodos and Baer-Bassit ophiolites suggest that this multi-stage SSZ magmatism was common in most Tethyan ophiolites. The sheeted dikes and the majority of extrusive rocks in Kizildag are composed of basalt, basaltic andesite and andesite with relatively low La/Sm_N (0.5-0.7), low MgO (7-9 wt%) and high TiO2 (0.7-1.0 wt%) and exhibit high Dy/Yb_N ratios. These IAT suites also include rare, younger dacitic dikes showing strong enrichment in the LREE. The late-stage basaltic dikes and lavas (sakalavites) have high La/Sm_N (0.8-1.0), high MgO (9-11 wt.%) and low-TiO2 (0.3-0.4 wt.%) contents and display strong depletion in both LREE and HREE. This suite of rocks represents the boninitic component of the Kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment. The progressive evolution of Kizildag magmas from IAT to boninitic compositions in time and space was a result of variable mixing, aggregation, and differentiation of geochemically different melt batches, which formed in different levels (polybaric) within the melting column. Rapid slab rollback-induced corner flow, mantle return flow, and asthenospheric diapirism (providing more fertile melt) directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle, producing refractory liquids of the boninitic stage. These geochemical patterns of the Kizildag ophiolite reflect a multistage evolution of SSZ magmatism in Tethyan subduction factories that is analogous to the earlier arc volcanism in the Eocene-Oligocene Izu-Bonin-Mariana system.