西藏日土白垩纪酸性侵入岩年代学及地球化学特征——对班公湖-怒江中特提斯洋盆闭合时限的制约

Geochronology and geochemistry of Cretaceous acid intrusive rocks in the northeast Ritu,Tibet:Constraints on the closure time of Middle Tethys oceanic basin in Bangong Lake-Nujiang

以西藏日土县北东地区的小型酸性侵入体为研究对象,结合岩石地球化学分析及锆石年龄测试,探讨岩石的成因类型及构造环境,确定班公湖-怒江中特提斯洋盆闭合的时限。研究区酸性岩体的岩性为黑云二长花岗岩,其主要组分的质量分数分别是:SiO_2为63.89%~77.49%,全碱(K_2O+Na_2O)为8.06%~8.95%,全铁(FeOT)为0.48%~4.37%,Al_2O_3为12.43%~16.17%;大离子亲石元素中Rb、Th、U、K明显富集,但Ba、Sr明显亏损;高场强元素中Nb、Ta、P、Ti明显亏损,而Zr、Hf相对富集;轻稀土元素相对富集、重稀土元素相对亏损,负Eu异常明显(δEu=0.09~0.67,平均为0.47),球粒原始标准化分布模式为向右缓倾的V型,属于A型花岗岩。其LA-ICP-MS锆石206Pb/238 U年龄加权平均值为(117.8±1.2)Ma,属于中生代早白垩世中期。从岩石碰撞后A1型花岗岩特征推测,班公湖-怒江中特提斯洋盆在日土地区的闭合时间应在早白垩世中期。

The petrogenesis and structural environment of small intrusive acid rock masses in the northeast Ritu County of Tibet are studied so as to investigate the closure time of the Bangong Lake- Nujiang Middle Tethys oceanic basin. Lithological and geochemical studies show that the rock type of the acid rocks in this area is biotite monzonitic granite. These rocks consist mainly of SiO2, （K2O＋Na2O）,（ FeOT）andAl2O3withthemassfractionsof 63.89%-77.4r 9%,8 . 0 6 %-8 . 9 5 %,0.4：8 %-4. 37%, 12. 43%-16. 17%. Large ion lithophile elements （LILE） Rb, Th, U, K obviously enriched but Ba,Sr obvious depleted; high field strength elements （HF SE） Nb,Ta,P ,Ti obvious depleted and Zr,Hf relative enriched; LREE relatively enriched, HREE relative loss; high degree of fractionation and negative Eu anomalies （δEu = 0- 09 -0- 67, an average of 0. 47） , with primitive chondrite normalized distribution pattern of being gently right sloping V-type. Above all, the acid rock belongs to Ai-type granite. LA-ICP-MS dating shows that the zircon 206 Pb/23SU weighted mean age of biotite monzonitic granite is （117. 8±1. 2） Ma, the early Cretaceous period in Mesozoic. From the zircon U-Pb ages and post-collisional A-type features, the Bangong Lake-Nujiang Middle Tethys oceanic basin in Ritu County area is considered to be closed in the middle phase of early Cretaceous.