Tectonic and magmatic evolution of the southernmost Mariana convergent margin: Constraints from geochronology and geochemistry of the Challenger Deep sediments

The southernmost Mariana convergent margin (SMCM) is a distinct segment of the Izu-Bonin-Mariana (IBM) subduction system with unique east-west orientation, however, its nature and tectonic history remains enigmatic. Here, for the first time, we present the detailed geochemical data of detrital minerals and UPb ages of accessory minerals from two sediment samples recovered at the deepest part of Southern Marianna Trench including the Challenger Deep. Detrital magmatic zircon UPb ages can be divided into three groups. Group 1 zircons show a weighted mean age of 50.3 ± 1.2 Ma, contemporary to the published boninites from northern IBM. The ε_Hf(t) of these zircons (11.1–16.4) are also similar to the boninites (12.6–16.8) and volcanic arc rocks (4.2–14.2), but lower than forearc basalts (17.8–22.1), suggesting that these zircons are from boninitic or arc magmatism. The apatite UPb dating obtained a similar age of 55.0 ± 5.5 Ma. Therefore, if they are all from boninites, then subduction initiation magmatism ended at SMCM was likely coeval with northern IBM. Group 2 and Group 3 zircons have ε_Hf(t) values similar to Group 1 zircons, but recorded younger ²⁰⁶Pb/²³⁸U ages of 16.8 ± 1.8 Ma and 11.0 ± 0.2 Ma, respectively, which are most likely related to the arc volcanism at West Mariana Ridge (WMR). Additionally, a rutile age of 19.0 ± 0.6 Ma is identified, which is also likely related to WMR. The chemical composition and calculated P-T conditions of clinopyroxenes indicate that they are mainly from the Mariana forearc basalts and arc volcanic rocks. In situ Sr isotopic compositions of plagioclases also show strong affinity to the Mariana forearc basalts and arc volcanic rocks. Therefore, our study indicates that sediments at the deep trench can record the long-term tectonic and magmatic evolution of the convergent margin.