Geochemical and geochronological study of early Paleozoic volcanic rocks from the Lajishan accretionary complex, NW China: petrogenesis and tectonic implications

Abstract

The Lajishan accretionary complex cropped out in the Central Qilian belt in Northwest China, consists of ophiolite, volcanics, chert, carbonate and clastic rocks. Here we report new geochemical, geochronological and Sr-Nd isotopic data of the andesite, subalkaline and alkaline basalt blocks, with a view to evaluate the subduction – accretion process of the Lajishan accretionary complex. Zircon grains from the andesites yield crystallization ages of 476 ± 6 Ma, 468 ± 2 Ma and 460 ± 5 Ma. The andesites display high concentrations of SiO₂ and MgO, and low TiO₂ and P₂O₅ contents, and are enriched in large ion lithophile elements and light rare earth elements, with obvious Nb, Ta and Ti negative anomalies. The data indicate that the arc volcanics were probably derived from a metasomatized depleted mantle in a normal subduction environment. The subalkaline basalts have moderate SiO₂, Al₂O₃ and high MgO contents, and slight enrichment in LREEs, Rb, Ba, Th and U, and weak depletion in Nb and Ta. They also show consistently positive εNd(t) values of +6.09 ~ + 6.97, and are similar to E-MORB. The alkaline basalts show lower contents of SiO₂, higher (K₂O + Na₂O) and TiO₂ contents, and are enriched in LREEs and depletion in HREEs, in the absence of Nb, Ta and Ti anomalies, suggesting an ocean island basalt affinity, with a restricted range of positive εNd(t) values of +5.63 and +5.84. These basalts might correspond to accreted seamount fragments, and may represent mantle transition zone plume- related magmatism within an intra-oceanic setting. Our new results, combined with published data, track the history of formation of oceanic basin to subduction and accretion during early Paleozoic.