西藏过铝花岗岩的岩石化学特征及成因探讨

Petrochemistry Characteristic and Petrogenesis of Peraluminous Granite in the Tibet

西藏班公湖-改则-安多-怒江带-线以南分布有61个主要过铝花岗岩体，其岩石类型主要有电气石花岗岩、白云母花岗岩和二云母花岗岩，它们的主量元素组成为SiO2=65．70％～79．52％、Kz0＋NazO=2．20％～12．51％、K。O／Na2O=0．49～1．04及A／CNK=1．04～1．38。随着SiO2的增加，岩石中A12O3逐渐下降，而其他的氧化物较离散。岩石系列主要为含钾较高的钙碱性系列，具典型的（强）过铝花岗岩特征。铝饱和指数图解、QAP图解投点大多数落在CCG区，表明其为大陆碰撞构造环境。Ab—Or—Q—H2O相图表明过铝花岗岩形成的压力为0．05～0．2GPa，推断温度低于700C。根据岩石化学分析，可以得出这样的推论，西藏过铝花岗岩类发生在两个大陆地壳板块的碰撞阶段，源岩来源于上地壳的重熔．成因为S型花岗岩。其中南冈底斯带和拉轨岗日带具有许多相似的性质，这些相似的性质可能暗示二者有相同的岩浆源区或（和）相同的构造环境。

There are 61 major peraluminous granitic bodies in Tibet （TPGs） to the south of the Bangong Co-Gerze- Amdo-Nu Jiang suture, and the include tourmaline granite, muscovite granite, and two-mica granite. The TPGs have SiO2= 65.7%-79.52%, K20 ＋ Na20 = 2.20%-12.51%, K20/Na20 = 0.49-1.04 and A/ CNK= 1.04- 1.38. Al2O3 gradually decrease and the other oxide disperse along with the increase of SiO2. The rock series is mainly calc-alk series with high Potassium. It has typical characteristic of strong peraluminous granite. From the aluminum saturation index and QAP plots, the peraluminous granites plot mostly within the Continental Collision Granite （CCG） field, indicating that the peralumious granites in Tibet formed in a continental collisional setting. Ab-Or-Q-H2O phase diagram indicate the pressure of 0.5-2 k bar in TPGs. It can be deduced the form temperature is under 700℃. The TPGs is mainly occurred the collision stage between two continental crust plates, original magma is rootted in the remelting from upper crust. It is the S-type granite in petrogenesis. There are similar character between the Southern Gandise belt and the Lhagoi Kangri belt. These similar character suggest it has the same magma source and the same tectonic setting in the two belts.