美姑县南红玛瑙矿区玄武岩地球化学特征和岩石成因

Geochemical Characteristics and Petrogenesis of Basalt of Nanhong Agate Orefield in Meigu County

摘要: 凉山州美姑县发育一套与南红玛瑙成矿关系紧密的玄武岩，对该玄武岩元素质量分数和相关性进行分析。研究结果表明：玄武岩ω(SiO₂)为50.33%~52.51%，ω(TiO₂)为2.64%~4.08%，ω(Al₂O₃)为13.10%~13.40%，全碱ω(K₂O+Na₂O)为2.74%~4.25%，ω(MgO)为3.60%~4.94%，Mg指数为32.01~38.17。玄武岩轻稀土元素(LREE)富集、轻重稀土元素分异作用明显，具轻微Eu负异常(δω(Eu)=0.88~0.96)和轻微的Ce正异常(δω(Ce)=1.01~1.05)，富集Rb、Ba和Th等大离子亲石元素，Nb和Ta无明显异常，相对亏损K、P和Sr等元素。矿区玄武岩为碱性玄武岩，形成于板内拉张环境，为大陆裂谷构造背景。岩浆发生了轻度的源区混染，未遭受明显的地壳物质混染；经历了分离结晶作用，且发生部分熔融。玄武岩可能位于地幔热柱边部，是由石榴石二辉橄榄岩地幔部分熔融形成的，且具有由石榴石稳定区向尖晶石稳定区过渡的特征。

Abstract: A set of basalts closely related to the Nanhong Agate mineralization was developed in Meigu county of Liangshan prefecture. Element concentration of the basalts and geochemical correlation analysis were made. According to the results, the ω(SiO₂) of basalts is from 50.33% to 52.51%, ω(TiO₂) is from 2.64% to 4.08%, ω(Al₂O₃) is from 13.10% to 13.40% and ω(K₂O + Na₂O) is from 2.74% to 4.25%, ω(MgO) is from 3.60% to 4.94% and Mg index changes from 32.01 to 38.17. The light rare earth elements (LREE) of basalts are enriched, and the differentiation between LREE and HREE is obvious. Eu of the sample shows slight negative anomaly (δω(Eu) = 0.88~0.96), and Ce shows slight positive anomalies (δω(Ce) = 1.01~1.05). It is enriched in large ion lithophile elements (such as Rb, Ba and Th), shows no obvious anomaly in high field strength elements (such as Nb and Ta), and is relatively depleted in the elements (such as K, P and Sr). The basalts of the orefield are alkaline basalts, formed in an intra-plate environment, which is the tectonic setting of continental rift. The magma has mild contamination of source region, and does not suffer significant continental crust material contamination. The magma underwent the fractional crystallization and partial melting. Basalts may be located in the edge of mantle plume, formed by the partial melting of garnet peridotite mantle, and have the characteristics of transition from garnet stable region to spinel stable region.