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

Major, trace and rare earth elements (REE), and platinum group elements (PG E) geochemistry have been studied for the Kudi ophiolite in the northwester n part of the western Kunlun Mountains, northwestern China, in order to det ermine the relationship between the mantle peridotites and mafic lavas and the origin of the ophiolite. The ophiolite consists of harzburgites, dunite s, 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# (C r#>0.6), similar to those from ophiolites in supra-subduction zone environm ents. The cumulate rocks are mainly pyroxenites and gabbros and are Nb- and Ti-depleted. The Kudi mafic lavas with marked No and Ti depletion can be d ivided into two groups as shown by their trace, rare earth element and isot opic characteristics: group I basalts with more pronounced are signature, a nd 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 en richment of incompatible trace elements (such as Rb, Ba, Th), light REE and Pd in the mantle peridotites indicates that an interaction between the man tle peridotites and the parental magma of group I basalts has probably occu rred. This interaction obviously affected the trace and rare earth elements , and PGE geochemical characteristics of the mantle peridotites. The parent al magma contained a distinctive Pd-rich melt and may be spatially related to arc magmatism. (C) 2001 Elsevier Science B.V. All rights reserved.