西藏改则热那错地区下—中侏罗统色哇组玄武岩地球化学特征及其构造意义

Geochemical Characteristics and Tectonic Implication of Basalt in Lower-Middle Jurassic Sewa Formation in Renacuo Area of Gaize,Tibet,China

西藏改则热那错地区下—中侏罗统色哇组中含粗面玄武岩,元素含量测试表明,其Si O2含量为45.62%~48.46%,Al2O3含量为14.09%~16.51%;Na2O与K2O含量分别为2.68%~4.12%、1.35%~4.73%,K2O/Na2O=0.34~1.77。岩石富集轻稀土元素、亏损重稀土元素,有弱的铕正异常;总体富集Rb、Ba、U等元素,亏损Sr、Yb、Th、Y等元素。地球化学特征表明其形成于大洋板内洋岛环境,岩浆来源于富集地幔,未受或很少受到地壳物质和陆下岩石圈的混染。玄武岩、玄武质砾石、灰岩构成了洋岛型岩石组合,结合色哇组为海底扇沉积环境的观点,可以推断班公湖—怒江洋盆在早—中侏罗世发育成熟洋壳。

The trachy basalt was discovered in Lower-Middle Jurassic Sewa Formation in Renacuo area of Gaize,Tibet. The contents of Si O2,Al2O3,Na2 O and K2 O are 45. 62%- 48. 46%,14. 09%- 16. 51%,2. 68%-4. 12%,1. 35%- 4. 73%,respectively,and the ratio of K2 O to Na2O（ K2 O / Na2O） is 0. 34- 1. 77. The rocks are rich in LREE,and poor in HREE with a weak positive Eu anomaly,as well as an enrichment of elements such as Rb,Ba,U and a depletion of elements like Sr,Yb,Th,Y as a whole. The geochemical characteristics indicate that they are generated in oceanic island environment within the oceanic slab,and that the magma is derived from enriched mantle without or with a little contamination of crust materials and sub-continental lithosphere. The oceanic island rock combination consists of basalt,basaltic gravel and limestone. Combined with the idea that Sewa Formation is seafloor fan sedimentary environment,it can be inferred that the Bangongcuo-Nujiang ocean was mature oceanic crust in Early-Middle Jurassic.