西藏班公湖-怒江缝合带中段碰撞后A型花岗岩的时代及其对洋盆闭合时间的约束

近年来地质调查在班公湖-怒江缝合带中段发现了一批A型花岗岩,岩体呈面积不大(＜1 km2)的岩株产出,岩性上分为钾长花岗岩和花岗闪长斑岩两种,侵入白垩系地层中.岩石化学上这些A型花岗岩相对富硅,SiO2含量在68.62％～75.36％之间,全碱含量(K2O+Na2O=8.03％ ～ 9.37％)和全铁含量(FeO(T)=0.86％～5.39％)偏高,Al2O3含量(12.76％～ 15.54％)偏低,显示弱过铝质和亚铝质特征.微量元素明显富集大离子亲石元素(LILE)Rb、Th、U、K和Pb,但Ba和Sr相对亏损;同样在高场强元素(HFSE)中,Nb、Ta和Ti亏损明显,Zr和Hf相对富集.这些岩石化学特征与岛弧型花岗岩是明显不同的,显示出A型花岗岩的特征.岩体稀土元素含量总体较高(∑REE=122.37～291.19 μg/g,平均201.31 μg/g),相对富集轻稀土元素(LREE/HREE=4.89～9.58,平均为5.93),Eu负异常明显(δEu=0.14～0.54,平均0.34),球粒陨石标准化分布模式呈向右缓倾的(V)型.三个岩体的锆石U-Pb LA-ICP-MS加权平均年龄分别为(109.6±1.4) Ma (MSWD=3.7)、(112.2±0.9) Ma(MSWD=0.06)和(113.7±0.5) Ma (MSWD=0.21),表明这些A型花岗岩形成时代为早白垩世晚期.从岩石的碰撞后A2型花岗岩特征推测班公湖-怒江中特提斯洋盆的闭合时间至少应该在白垩纪初.

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

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 A1203 （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.