拉萨地块中北部尼雄地区早白垩世火山岩的成因及构造意义
The Petrogenesis and Tectonic Significance of Early Cretaceous Volcanic Rocks in Nixiong Area from the Central and Northern Lhasa Terrane

以往的研究多侧重于拉萨地体中南部，对拉萨地块中北部地区的火山岩浆活动的分布特点、火山岩成因及构造意义关注相对较少，且对该地区中生代火山岩的成因机制存在不同认识.尼雄地区广泛发育的白垩纪火山岩保存了大量青藏高原新生代之前的地质演化信息.岩石学和锆石U-Pb定年研究表明，火山岩类型主要为玄武安山岩、粗面安山岩和流纹岩，其SiO2含量为55.76%～77.78%，铝饱和指数(A/CNK)为0.89～3.04，属高钾钙碱性-碱钙性、偏铝质-过铝质岩石；其富集Th、U，亏损Nb、Ta等高场强元素，显示出A型花岗质岩石特征；此外，流纹岩具有较高的SiO2含量和极低的MgO、TiO2、P2O5含量及δ Eu值，相对亏损Ba、Nb、Ta、Sr和Eu等元素，与高分异的A型流纹岩特征一致.从1个玄武安山岩、1个粗面安山岩和2个流纹岩样品中获得的岩浆锆石U-Pb年龄分别为117 Ma、127 Ma和126～127 Ma，代表了尼雄地区早白垩世火山岩的形成年龄，否定了前人把尼雄地区火山岩全归属为始新世林子宗群年波组或渐新世日贡拉组的认识.综合研究表明，玄武安山岩、粗面安山岩和流纹岩可能为壳幔熔体混合的结果，并伴随着一定的分离结晶作用.它们可能同时受到班公湖-怒江洋壳向南、雅鲁藏布江新特提斯洋壳向北双向俯冲的影响.
Many studies have been focused on the central and southern Lhasa terrane, but it remains controversial as to the genetic mechanism of the Mesozoic volcanic rocks in the central and northern Lhasa terrane due to less attention paid to the distribution characteristics of volcanic magmatism, the origin of volcanic rocks, and the tectonic significance of the volcanic rocks.Early Cretaceous volcanic rocks are widely exposed in Nixiong area, which record abundant pre-Cenozoic evolutionary geohistory of the Tibetan Plateau. Petrological and zircon U-Pb dating analyses show that the volcanic rocks are mainly composed of basalitic andesite, trachyandesite and rhyolites. They have variable SiO2 contents ranging from 55.76% to 77.78%, and alumina saturation index (A/CNK) of 0.89-3.04, indicative of highK calc-alkaline to alkalinecalc and metaluminous to peraluminous. They are characterized by the enrichment of Th and U, and the depletion of HFSEs (such as Nb and Ta), typical of A-type granitoids. In addition, rhyolites show distinct high SiO2, but low MgO, TiO2, P2O5 and δEu, and display fiercely negative Ba, Nb, Ta, Sr and Eu anomalies, suggesting that they are highly fractionated A-type rocks. LA-ICP-MS U-Pb dating of magmatic zircons from one basalitic andesite, one trachyandesite and two rhyolites samples indicate that they were formed at 117 Ma, 127 Ma and 126-127 Ma, respectively. It is proved that results of previous studies are wrong in that the volcanic rocks in Nixiong area are all Eocene Nianbo Formation of Lingzizhong group or Oligocene Rigongla Formation. In addition, it is found that the basalitic andesite, trachyandesite and rhyolites are likely derived from partial melting of a crust-mantle mixed source, and have experienced significant fractional crystallization. We speculate that the studied rocks have been affected by double subduction of southward subduction of Bangong Co-Nujiang Tethys oceanic crust，and northward subduction of Yalung-Zangbo oceanic crust