Geochemical characteristics and tectonic implications of Hongjishan ophiolite in central and western part of Qiangtang, northern Tibetan Plateau

Taking metamorphic basalt as the main research object, the authors conducted detailed studies of petrology and geochemistry of the Hongjishan ophiolite. The geochemical characteristics indicate that the content of major elements is quite similar to that of typical N-MORB, the Hongjishan ophiolite exhibits low ∑REE and light rare element depletion, and δEu is normal (δEu=1.05~1.16). These characteristics are quite similar to those of the typical N-MORB primitive mantle-normalized trace element patterns and also quite similar to features of typical N-MORB. These data indicate that the Hongjishan ophiolite originated from a depleted asthenosphere mantle with distinctive geochemical features of N-MORB. However, compared with the typical mature mid-ocean ridge basalt, this basalt exhibits the enrichment of some lithophile elements (Ra, Ba, U）and the depletion of Nb, Ta (especially Nb) on the curves of N-MORB normalized trace element distribution pattern (Nb=1.67×10-6~2.26×10-6, mean value of Nb is 1.97×10-6), quite different from features of both the typical mature mid-ocean ridge basalt and the typical island-arc basalt. These unique geochemical characteristics indicate the influence of subduction material and suggests that the Hongjishan ophiolite was formed in a marginal basin tectonic setting resulting from the back-arc mantle convection and back-arc oceanic crust formation which constituted a spreading center near the subduction zone. The results achieved by the authors provide important basis for the study of tectonic evolution of the middle-western part of the Lungmu Co-Shuanghu-Lancang River suture zone.