西藏冈底斯带中段以东得明顶地区晚白垩世花岗岩类锆石U-Pb年代学和地球化学证据

SUBDUCTION-RELATED LATE CRETACEOUS GRANITOIDS IN DEMINGDING,AREA,EAST OF MIDDLE GANGDISE,TIBET:EVIDENCES FROM ZIRCON U-Pb GEOCHRONOLOGY AND GEOCHEMISTRY

得明顶地区花岗岩位于西藏冈底斯火山岩浆弧中段以东,主要岩石类型有:石英闪长岩、石英二长闪长岩、英云闪长岩、黑云花岗闪长岩、斑状黑云二长花岗岩,岩石具钙碱性特征,w(SiO2)在57.19%～71.78%之间,K2O/Na2O=0.39～0.98,相对富钠,A/CNK=0.80～1.02,Al2O3变化于13.64%～18.74%之间,为准铝质岩石,花岗岩体稀土元素总量ΣREE变化于94.69×10-6～227.28×10-6之间;轻稀土元素富集,负Eu异常由不明显到明显,富K,Rb,Ba,Th等大离子亲石元素和亏损Nb,Y,Yb等高场强元素为特征。岩石学和岩石地球化学研究表明,该时期的花岗岩有由中性向酸性演化的规律,为同源岩浆,具俯冲I型花岗岩的特点,形成于板块俯冲下的岛弧环境。冈底斯岩带中东段雪拉岩体花岗闪长岩锆石的SHRIMPU-Pb年龄为70.4Ma±2.2Ma,表明冈底斯岩浆弧带在白垩世从早到晚岩浆均在剧烈地活动,是新特提斯洋向北俯冲作用的产物。

Granitoids occurred in Demingding region,east of middle Gangdise volcanic magma arc in Tibet, was composed of quartz diorite, quartz monzodiorite, leucotonalite, biotite granodiorite and porphyrite biotite adamellite with alkaline calcium-calcium characteristics. SiO2 content of the rocks ranged from 57.19% to 71. 78%, the ratio of K2O/Na2O equals 0.39~0.98,with relatively rich sodium,and A/CNK was equals to 0. 80 ~1.02. The content of Al2O3 of 13.64%-18. 74% indicated that the rocks were of the aluminium type. Granite aggregate FREE for rare earth elements ranges from 95.11 ×10^-6 to 187.21 ×10^-6 with enrichment of light REE. Negative Eu anomaly changed from no obviousness to obviousness. The rocks were rich in K, Rb, Ba, Th and other large-ion stone elements and poor in high field strength elements of Nb,Y,Yb. Study of petrology and geochemistry showed that the granite subjected a magma evolution from acid to neutral and was of characters of I type granite. They formed in the island arc environment during the plate subduction. SHRIMP U-Pb age of the granodiorite in the eastern section of Gangdise was 70.4 Ma±2.2 Ma,implying that the magmatism in Gangdise acted throughout Cretaceous and the magmatic activity was the product of northward subduction of new Tethys.