摘要
通过龙首山超基性岩带5个代表性岩体岩石化学、稀土元素地球化学的研究,取得如下认识:1.岩带内各岩体的化学成分以贫Si0_2、MgO和富TiO_2为特征,属于铁质超基性岩;2.各岩体稀土元素的配分模式有富集型、平坦型、亏损型3种。具前两种模式的岩石是地幔部分熔融形成的熔体中由分离结晶作用形成的,而后者则具有地幔残余的某些特征;3.稀土元素定量模式计算表明,金川、塔马子沟岩体的原始熔浆起源于尖晶石二辉橄榄岩15％、1％的部分熔融,而青井子等岩体则起源于石榴石二辉橄榄岩≥40％的部分熔融;4.分离结晶作用过程的定量计算表明,金川岩体中的橄榄辉石岩和斜长二辉橄榄岩分别由77％和25％的晶体分离形成。最后对各岩体的含矿性做了讨论。
The ultramafic belt of Longshoushan occurs in the southwestern parts of Alxa uplift. In this paper, the chemical composition and REE geoche- misty of five typical bodys from ultramafic belt in Longshoushan have been studied and the resttlts reveal as follow: 1. chemical compositions of each body in ultramafic belt are marked by poor-SiO_2 and MgO and rich- TiO_2, and the rocks belong to iron-ultramafie body; 2. REE patterns of ultramafic rocks in this belt display enriched-type, plate-type and depleted-type. The rocks with enriched and plate-type patterns are formed by fractional crystallization from the fused mass generated by partial mel- ting of mantle and the rocks with depleted-type patterns are in some chara- cters of mantle residue; 3. Calculation of the partial melting model shows that the parental magma of Jinchauan and Tamazigou bodies with enriched- type patterns have been originated in 15%and 1%partial melting of spine- lherzolite, and Qinjingzi with plate-type pattern in 40%partal melting of garnet-lherzolite; 4. Computation of fractional crystallzation shows that olivine-bearing pyroxenite and plagioclase-bearing lherzolite were formed by 77% and 25% fractional crystallzation respectively.
