Ages of post-collisional A-type granite and constraints on the closure of the oceanic basin in the middle segment of the Bangonghu-Nujiang suture, the Tibetan plateau

Abstract

A group of A-type granite has been found in recent years in the middle segment of the Bangonghu-Nujiang suture.These granite bodies occur as stocks with exposed areas of less than 1 km2.Lithologically,these A-type granites include two types of biotite granite and granodiorite porphyry,which intrude into Cretaceous sedimentary rocks.Petrogeochemically the A-type granites have high SiO2 contents,varying from 68.62% – 75.36%,relatively high FeOT(0.86% – 5.39%) and low Al2O3(12.76% – 15.54%) contents,displaying weak peraluminous and metaluminous characteristics.Trace element N-MORB-normalized spider diagram displays apparent enrichments of large ion lithophile elements(LILE) Rb,Th,U,K,Pb with Ba and Sr of marked depletions.High field strength elements(HFSE) Nb,Ta and Ti are sharply depleted with relative enrichments of Zr and Hf.All of these are characteristics of A-type granite,which are different from arc granite.These granites have high REE contents(∑REE=122 – 291 μg/g with an average of 201 μg/g).Light REE are rich relative to heavy REE(LREE/HREE = 4.89 – 9.58,with an average of 5.93) and negative Eu anomalies are sharp(δEu=0.14 – 0.54,with a mean value of 0.34),with slightly right-dipping V-type chondrite-normalized rare earth element patterns.Zircon U-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 the Early Cretaceous.From their zircon U-Pb ages and post-collisional A2-type signatures,the authors infer that the Bangonghu-Nujiang middle Tethys ocean should be closed at least in the beginning of the Early Cretaceous.