摘要
白银厂矿田早 -中寒武世火山岩主要由钙碱性玄武岩和流纹岩及少量的安山岩组成 ,被认为是大陆裂谷环境下的产物。本文的地球化学分析结果显示 :玄武岩具有低Ti,Nb和Ta负异常、富集LREE ,相对亏损HFSE ,Th/Ta比值高 (介于 9～ 15 ) ,La/Nb >1.4 ,低Ni,Th/La为0 .32～ 0 .5 6;低Zr/Nb(<16) ,(87Sr/ 86 Sr) i 为 0 .70 4～ 0 .70 7,εNd(t) =- 6.4 6,δEu≈ 1的地球化学特点 ;流纹岩同样具有低Ti,Nb和Ta负异常 ,Th/Ta为 6～ 12 ,低Ti/Zr比值 (介于 6.1～ 5 3)之间 ;稀土元素配分为MREE负异常 ,δEu为 0 .5～ 1.1,(87Sr/ 86 Sr) i 为 0 .70 2 6～ 0 .70 4 8,εNd(t)为+ 5 .60～ + 6.14。综合两类岩石的地球化学特征 ,可以推测它们是陆缘弧岩浆作用的产物 ,但并非同源岩浆演化的结果。玄武岩源于富集的岩石圈地幔 ,在岩浆上升过程中发生明显的陆壳混染作用 。
The early-middle Cambrian volcanic rocks from Baiyin mining field are composed mainly of calc-alkaline basalts, rhyolites and minor andesites. They were considered to be resulted from magmatism of continental rift. The results of the geochemial characteristics show that, compared with oceanic ridge basalt (MORB), the basalts of study area are characterized by low Ti and Ni ( Ni<25 at Mg'=40), obviously negative anomaly of Nb-Ta, enrichment of LREE, relative depletion of HFSE, high ratios of Th/Ta (9～15), Th/La (0.32～0.56) and La/Nb (>1.4 ), highly variable range of Ce/Zr, La/Zr and Nb/Zr, and with low Zr/Nb ratio (<16). ( 87Sr/ 86Sr) i=0.704～0.707, ε Nd( t )=-6.46 and δEu≈1.0. However, the rhyolitic rocks have ( 87Sr/ 86Sr) i=0.7026～0.7048, ε Nd( t )=+5.60～+6.14, δ Eu=0.5～1.1, Th/Ta=6～12 and Ti/Zr=6.1～53. Their REE distribution patterns have a negative anomaly of MREE, indicating the presence of amphibole as a residual phase in the mafic lower crust (pre-existing island arc basalts). According to the geochemistry of the volcanic rocks, it can be inferred that the basalts and rhyolites were resulted from the magmatism in normal continental margin arc, but with different sources. The basalts stemmed from enriched sub-continental lithospheric mantle and the mantle-derived magma was strongly contaminated by continental crust. However, the rhyolitic rocks were resulted from mixing of mafic lower crust generated by partial melting of depleted mantle wedge and subduction-related mantle-derived underplating magmas at the same tectonic episode.
出处
《高校地质学报》
CAS
CSCD
2003年第1期89-98,共10页
Geological Journal of China Universities
基金
国家自然科学重大基金 ( 10 2 990 40 )
甘肃省自然科学基金资助项目 (ZS0 0 1 A2 5 0 0 6 Z)
关键词
甘肃
构造环境
早-中寒武世
火山岩
地球化学
白银厂矿田
Early-Middle Cambrian
volcanic rocks
geochemistry
continental margin arc
Baiyin Mining Field
