Relicts of the Early Cretaceous seamounts in the central-western Yarlung Zangbo Suture Zone, southern Tibet

Despite more than 30 years' studies in the Yarlung Zangbo Suture Zone (YZSZ), southern Tibet is still a region of discovery and the geological evolution of the Neo-Tethys here remains controversial. In this paper we present the new field observation, petrography, zircon U-Pb dating and geochemistry of the Zhongba mafic rocks in the western segment of the YZSZ. Zircon U-Pb analyses from a diabase using SIMS (Secondary Ion Mass Spectrometry) yielded a concordant age of 125.7 ± 0.9 Ma. All the Zhongba mafic rocks exhibit LREE enrichment (LaN/YbN = 4.7-15.8) without Eu anomalies (δEu = 0.97-1.01). Their Primitive-Mantle-normalized trace element patterns are similar to those of the average OIB and Hawaii alkaline basalts, as well as other OIB-type rocks already reported in the central-western YZSZ. These observations lead us to suggest the existence of a series of seamounts within the Neo-Tethys during the Early Cretaceous. These OIB-type rocks could be originated from (1) hotspot mantle; (2) near-ridge seamounts by melting-induced mixing of two distinct mantle sources; (3) late-stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination. Combining literature data from the entire YZSZ, which include studies of the MORB- and IAB-type rocks and mantle peridotites, we prefer the third explanation that the OIB-type rocks represent a late-stage magmatic activity above an intra-oceanic supra-subduction zone within the Neo-Tethys.