The discovery of the Early Cretaceous Zenong Group volcanic rocks and geological significance in Jiwa area in south of the Central Lhasa subterrane
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摘要: 西藏尼玛县吉瓦地区措勤-多瓦后陆拗陷带内分布的火山岩LA-ICP-MS锆石U-Pb年龄为120.3~126.5Ma左右,重新厘定为早白垩世则弄群,否定了前人归属为上新世乌郁群(N2wy)及始新世帕那组(E2p)的认识。岩石以酸性火山岩为主,中基性火山岩为辅,酸性岩类主要为火山碎屑岩类和熔岩类,典型岩石类型为流纹质熔结凝灰岩和流纹岩等,中基性岩主要为玄武安山岩,安山玄武岩等,杏仁构造普遍发育。研究区大量的流纹质熔结凝灰岩的出现反映了吉瓦地区的火山岩主要为陆相火山喷发形成。地球化学特征显示轻稀土富集,负Eu异常明显,富集K、Rb、Th、U等大离子亲石元素,相对亏损Nb、Ta、P、Ti等高场强元素。酸性火山岩具有A型花岗质岩浆岩特征,基性岩具有板内玄武岩亲缘性,这一特征可能与班公湖-怒江洋壳岩石圈南向俯冲过程中发生的板片断离有关。中部拉萨地体南侧早白垩世火山岩的发现,使班公湖-怒江洋壳南向俯冲在晚侏罗世-早白垩世的岩浆活动在原来的基础上向南延伸70~80km,火山岩地层时代的重新归位对研究冈底斯带早白垩世地球动力学背景及建立地质年代学格架提供了新的约束资料,具有重要的科学意义。吉瓦地区早白垩世则弄群火山岩可能受到了班公湖-怒江特提斯洋壳向南、雅鲁藏布江洋壳向北的双向俯冲制约。Abstract: The volcanic rocks LA-ICP-MS zircon U-Pb ages of Coqen-Dowa backland depression in Jiwa area, Nima County, Tibet, were formed around 120.3~126.5Ma. We denied the understandings of predecessors that volcanic rocks were Pliocene Wuyu Group (N2wy) and Eocene Pana Formation (E2p) and redefined volcanic rocks as Early Cretaceous Zenong Group (K1zn). Rocks which have the characteristics of acid volcanic rocks are major, and intermediate-basic volcanic rocks are supplement. Acid rock mostly was pyroclastic rocks and lava typical rock type was rhyolite ignimbrite and rhyolite. Basic rock mostly was basaltic andesite and andesite-basalt amygdaloidal structure development. Plenty of welded tuff reflecting the geological environment mainly was the continental volcanic eruptions. These rocks are rich of LREE, with an apparent negative anomaly of Eu, and LILE like K, Rb, Th, and U are rich, and HFSE like Nb, Ta, P, and Ti are depleted. Acid volcanic rocks show geochemical features similar to A-type silicic rocks basic volcanic rocks exhibit geochemical affinity of within-plate basalts, which can be attributed to the slab break-off at that time. The discovery of Early Cretaceous volcanic rocks in south of the Central Lhasa subterrane pushes the position of magmatism of Late Jurassic-Early Cretaceous southward subduction of Bangong-Nujiang 70km or 80km to the south. It gives new constraint information to the geodynamic background of Gangdese belt in Early Cretaceous from the perspective of volcanic rock. It seems that Early Cretaceous volcanic rocks in Jiwa area that happened after double subduction of Bangong-Nujiang Tethys oceanic crust southward, and Yalung Zangbo oceanic crust northward.
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