The delamination of lower crust in continental back-arc basin: Evidence from Sr isotope and elemental compositions of plagioclase and clinopyroxene in andesites from Kueishantao, north of Taiwan, China

Abstract

Subduction zones (arcs and back-arcs) are major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting; many models have been suggested by researchers to explain these processes. However, the influence of the overriding lithosphere, especially the lower crust, on the generation of back-arc magmas during the early spreading stage remains largely unknown. Kueishantao (KST), located at the tectonic junction of the southernmost part of the Okinawa Trough (SPOT) and the Ryukyu arc, is an ideal setting to investigate these issues because of the obvious differences between the subducted slab and the overriding continental lithosphere. To better address the contribution of the overriding lithosphere to a nascent back-arc basin, we examined the in situ major and trace element compositions of plagioclase (Pl) and clinopyroxene (Cpx) phenocrysts and the in situ 87Sr/86Sr of Pl phenocrysts in andesites from KST.The Pl and Cpx in KST andesites show obvious evolution (An contents of Pl, 39–91; Mg# of Cpx, 73–92). The valid thermobarometers of Pl and Cpx imply crystallization temperatures and depths of 1059–1117 °C and 1–14 km, respectively, similar to those of the SPOT. The compositions of Cpx suggest that the KST andesitic magma belonged to the subalkaline, tholeiitic and calc-alkaline magma series. The δEu values of Pl are 9.27–25.73, and those of Cpx are 0.50–0.92, indicating that the magma experienced Pl crystallization and separation. The 87Sr/86Sr values of Pl range from 0.705630 to 0.706985 and remain constant with varying An contents (52–85), indicating an enriched magma source not resulting from assimilation of continental crust. Taking into consideration all geochemical, trace elemental, 87Sr/86Sr isotope and geophysical constraints, the principal magma source of KST andesites was probably the lower continental crust, which foundered into the convecting mantle and subsequently melted and interacted with asthenospheric mantle materials. The over-thickened crust, mantle corner flow and addition of subducted fluids in the KST area led to delamination. The structure and geochemistry of Pl and Cpx imply a simple two- or multilayer magma chamber system in the continental crust and a mixing magma process between a relatively silicic magma (H2O-saturated and high fO2) and a primitive mafic magma at shallow depths.