Geochemical Characteristics And Implications For Tectonic Settings Of The Middle Segment And Western Segment Of Bangong Lake-nu River Suture,the Tibet Plateau

Through geological surveys in Tibet, the authors have discovered arc granitebatholiths both in the north side of Shiquan River-Gerze-Dongcuo ophiolite zones andin the south side of Laguocuo ophiolite zones in the western section of BangongLake-Nujiang suture zone. These granite bodies are lithologically dominated bymid-grain granodiorite, and petrochemically rich in large-ion incompatibleelements（LILE, e.g. Rb, Th, U, K, Pb）and depleted in high field strengthelements（HFSE, e.g. Nb, Ta, Ti）, and show essential characteristics of arc magmaticrocks, indicating there were bidirectional subduction in the Bangong Lake MiddleTethys oceanic basin. Zircon U-Pb LAICPMS dating results show thesegranite bodieshave different ages. In the north side of Shiquan River-Gerze-Dongcuo ophiolitezones, the zircon206Pb/238U weighted average ages of Galale and Gerze-northbodies are155.6±1.1Ma（MSWD=1.7）and142.15±0.35Ma（MSWD=2.9）, respectively.And in the south side of Laguocuo ophiolite zones, the age of Zhabuye-north body is134.07±0.77Ma（MSWD=1.8）. This shows that the northward subduction of theBangong Lake Middle Tethyan oceanic basin（west segment） occurred in late Jurassic,whereas the southward subduction occurred in early Cretaceous, with an about8Madelay of the latter.A group of A-type granite has been found in recent years in the middle segment ofthe Bangong Lake-Nu River suture suture. These granite bodies occur as stocks withexposed areas of less than1km2. Lithologically, these A-type granites include twotypes of biotite granite and granodiorite porphyry, which intrudeinto Cretaceoussedimentary rocks. Petrogeochemically the A-type granites have high SiO2contents,varying from68.62%–75.36%, relatively high FeOT（0.86%–5.39%） and lowAl₂O₃（12.76%–15.54%） contents, displaying weak peraluminous and metaluminouscharacteristics. Trace element N-MORB-normalized spider diagram displays apparentenrichments of large ion lithophile elements （LILE） Rb, Th, U, K, Pb with Ba and Srof marked depletions. High field strength elements （HFSE） Nb, Ta and Ti are sharplydepleted with relative enrichments of Zr and Hf. All of these are characteristics ofA-type granite, which are different from arc granite. These granites have high REEcontents （∑REE=122–291μg/g with an average of201μg/g）. Light REE are richrelative to heavy REE （LREE/HREE=4.89–9.58, with an average of5.93） andnegative Eu anomalies are sharp （δEu=0.14–0.54,with a mean value of0.34）, with slightly right-dipping V-type chondrite-normalized rare earth element patterns.ZirconU-Pb LA-ICP-MS dating for the three granite bodies yielded weighted mean ages of（109.6±1.4） Ma（MSWD=3.7）,（112.2±0.9） Ma （MSWD=0.06） and （113.7±0.5） Ma（MSWD=0.21）, respectively, and indicates that they formed in the late stage of theEarly Cretaceous. From their zircon U-Pb ages and post-collisional A2-type signatures,the authors infer that the Bangong Lake-Nu River suture middle Tethys oceanicbasin（middle segment） should be closed at least inthe beginning of the EarlyCretaceous.