Trace elements and Pb isotope of the mafic rocks from the Xigaze ophiolite of Yarlung Zangbo suture zone, southern Tibet: Constraints on the tectonic setting

A series of ophiolitic massifs are distributed along the Yarlung Zangbo Suture Zone (YZSZ), southern Tibet. However, the formation settings of these ophiolites are still controversial. Trace elements and Pb isotope geochemical features indicate that the magmas that formed the mafic rocks in the Luqu and Dazhuqu massifs of the Xigaze ophiolite in YZSZ were derived from the depleted mantle source. The La/Sm and Sm/Yb ratios of the mafic rocks show that their parental magmas were produced by ~10% partial melting of spinal lherzolite. Combined with the petrographic observations and major elements of the mafic rocks, the authors hold that they were formed from an anhydrous basaltic magma. Furthermore, the normalized patterns of trace elements and REE of the mafic rocks are very similar to those of N-MORB, except for weakly negative Nb-Ta anomalies. These features indicate that the Luqu and Dazhuqu massifs were formed in the MOR environment. Moreover, the Pb isotope of the mafic rocks indicates that their mantle sources have similar geochemical characteristics to Indian MORB mantle domain. N-MORB normalized trace element patterns of the mafic rocks show that weakly negative Nb-Ta anomalies might have resulted from the process that their mantle source metasomatized old subducted materials.