Heterogeneous oceanic arc volcanic rocks in the South Qilian Accretionary Belt (Qilian Orogen, NW China)

Abstract

Primitive arc magmas in oceanic island arcs are probes of sub-arc magmatic processes and are crucial for understanding oceanic subduction. We report data for an Early Paleozoic oceanic arc volcanic complex in the Lajishan-Yongjing terrane, South Qilian Accretionary Belt (SQAB), Qilian Orogen, including zircon U-Pb dating and Hf-O isotopes, mineral and whole-rock geochemistry, and Sr-Nd isotope compositions. New zircon ages of ∼455-440 Ma constrain the timing of arc volcanism and the subduction of the Qilian Ocean. Based on petrography and bulk-rock composition, five lithological types have been identified, including: (1) ankaramite; (2) high-Mg basaltic andesite; (3) high-Al andesite; (4) boninite; (5) sanukite. The volcanic sequence thus is one of the few island arcs where three types of near-primitive arc rocks including boninite, ankaramite and sanukite have been simultaneously produced. All these rocks have variably enriched Sr-Nd isotopic compositions, positive to slight negative zircon εHf(t) values and elevated zircon δ18O values. Boninites, ankaramites and sanukites are interpreted as contemporary, near-primitive, melts generated from different sources and conditions within an island arc setting. Boninites are characterized by low Ti, REE concentrations and high Cr# chrome spinel, and are interpreted as melts of refractory, Cpx-poor, spinel lherzolite or harzburgite at > 25% partial melting. Anomalous zircon δ18O values of 6.57‰-7.61‰ and Sr-Nd mixing calculations suggest less than 2% incorporation of subducted oceanic sediments into the mantle source of the magmas. The ankaramites are characterized by low SiO2, high MgO (Mg#), Cr, Ni and La/Yb ratios, and have similar isotopic ratios to tectonically adjacent OIB lavas. The ankaramite lavas are likely to have derived from mantle sources similar to those of OIB, i.e., pyroxenite-bearing garnet peridotite enriched in incompatible elements. High-Mg basaltic andesites and high-Al andesites may be derived from parental ankaramite magmas. Sr-Nd-Hf isotopic mixing modeling constrain the amount of silicic melt to ∼1-4% for ankaramite magma. Sanukites are of andesitic-dacitic composition with high Mg#, Cr and Ni, and enriched LILE and high La/Yb ratios. They are interpreted as having been generated by reaction of mantle peridotite with a silicic melt, itself derived from subducted sediments. Enriched Sr-Nd-Hf isotopic compositions constrain the amount of silicic melt to ∼10-15% for sanukite. Large compositional variations among the volcanic rocks from the same arc reflect heterogeneous mantle sources and variable degrees of mantle metasomatism by sediment-derived hydrous fluids or silicic melts, accompanied by secondary AFC processes during magma ascent to the surface.