Effect of melt–rock interaction on geochemistry in the Kudi ophiolite (western Kunlun Mountains, northwestern China): implication for ophiolite origin

Abstract

Major, trace and rare earth elements (REE), and platinum group elements (PGE) geochemistry have been studied for the Kudi ophiolite in the northwestern part of the western Kunlun Mountains, northwestern China, in order to determine the relationship between the mantle peridotites and mafic lavas and the origin of the ophiolite. The ophiolite consists of harzburgites, dunites, cumulate dunites, pyroxenites and gabbros, diabase dykes, and pillow and massive lavas. The mantle peridotites are rich in Mg and Ni and poor in Al and Ca. The pyroxene is low in Al and Ti, and the spinel is high in Cr# (Cr#>0.6), similar to those from ophiolites in supra-subduction zone environments. The cumulate rocks are mainly pyroxenites and gabbros and are Nb- and Ti-depleted. The Kudi mafic lavas with marked Nb and Ti depletion can be divided into two groups as shown by their trace, rare earth element and isotopic characteristics: group 1 basalts with more pronounced arc signature, and diabase dykes and group 2 basalts, respectively. The mantle peridotites, cumulates, diabase dykes and group 2 basalts probably constitute an upper mantle and crustal section of a back-arc basin in a SSZ environment. The enrichment of incompatible trace elements (such as Rb, Ba, Th), light REE and Pd in the mantle peridotites indicates that an interaction between the mantle peridotites and the parental magma of group 1 basalts has probably occurred. This interaction obviously affected the trace and rare earth elements, and PGE geochemical characteristics of the mantle peridotites. The parental magma contained a distinctive Pd-rich melt and may be spatially related to arc magmatism.