The Xigaze ophiolite, southern Tibet: a fossil oceanic forearc generated during subduction initiation

Abstract:

The Xigaze ophiolite is located at the central Yarlung Zangbo Suture Zone, southern Tibet representing the remnants of the Neo-Tethyan oceanic lithosphere. It displays well preserved sections from mantle to crustal rocks, together with overlying marine sediments. The Xigaze peridotites have low CaO and Al₂O₃ contents and U-shaped REE patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of LREE-enriched boninitic melts. Two of five massifs from the Xigaze pillow lavas display tholeiitic affinity, while other three massifs show calcalkalic characteristics. The latter lavas show more HFSE-depleted and LILE-enriched compositions than those of the former. The latest diabase dikes possess high SiO₂ and low TiO₂ contents and show flat to LREE enrichment patterns, indicating they are analogue with boninitic rocks. Tholeiitic lavas were formed via decompression melting of a fertile lherzolitic source, whereas calcalkalic lavas and latest stage dikes were generated by melting of residual harzburgite and were influenced by slab-derived fluids. Both mantle and crustal rocks were invaded by plagiogranite dikes. The first plagiogranite group invaded into the crustal rocks lies on a continuation of the chemical trends defined by the mafic rocks and show similar REE patterns with the lavas, indicating they were generated by shallow fractional crystallisation. The second plagiogranite group invaded into the mantle displays different trends on Harker and REE patterns with diabase dike and enclave, but show uniform LREE depletion similar with mafic rocks, suggesting that they might be the product of subducted mafic slabs melting. LA-ICPMS zircon U-Pb and Lu-Hf analyses from the second plagiogranites reveal that they were generated between 125-122 Ma, indistinguishable from those of mafic rocks. The zircons possess positive ε_Hf(t) values ranging from +12.3 to +16.4, suggesting they were derived from depleted source. Taking into account the geological and geochronological characteristics of the Xigaze ophiolite, we propose they were formed in a forearc setting through rapid slab rollback during subduction initiation between 127-122 Ma.