白银厂矿田早中寒武世火山岩地球化学及成因分析

白银厂矿田早中寒武世火山岩主要为玄武岩－碱性玄武岩－安山岩－玄武安山岩－玄武粗安岩－英安流纹岩－流纹岩。其中基性和中基性岩类为碱性系列和亚碱性系列，碱性玄武岩主要为钾质玄武岩系列。酸性岩类为钙碱性系列。岩石全碱含量平均为５．７６％，Ｋ２Ｏ／，Ｋ２Ｏ／Ｎａ２〈１，Ｆｅ２Ｏ／ＦｅＯ平殚为０．７４；ＴｉＯ２绝大部分样品含量小于１％（在酸性火山岩中含量较低：０．２％～０．８３％，平均为０．５３％；基性和中

Geochemical characteristics and genetic analysis of the early and middle Cambrian volcanic rocks in Baiyin mining field

The early and middle Cambrian volcanic rocks from Baiyin mining field consist mainly of basalts, alkalibasalts, andesites, basaltic andesites, basaltic trachy-andesites, dacites, rhyolites and dacite-liparites in which the basic and medium basic rocks belong to alkali and subalkali series with TiO2 of 0.53% to 1.01%(wt%), but the acidic rocks to calcalkali and low k calcalkali series with low TiO2 of 0.2% to 0.83%(average value 0.59%). Besides, the volcanic rocks are characterized by total alkalinity of 5.76% and Fe2O3/FeO ratio of 0.74 on the average, and K2O/Na2O ratio below 1.0. Compared with NMORB, the basic and medium basic volcanic rocks are more enriched in Ba, Rb, Th, U and obviously negative anomaly of Ti, and the acidic rocks are highly enriched in Ba, Rb, Th and U, but slightly depleted in Ta and Sr, and obviously depleted in Ti. Relative to the upper crust, the acidic volcanic rocks are slightly enriched in HREE, but variously depleted in other imcompatible elements. Relative to the lower crust, they are more enriched in Rb, Th, U, K and slightly enriched in Zr, Hf and HREE, but differently depleted in Ba, Sr, Nb, Ta, Ti and LREE. The REE pattern for the volcanic rocks shows the enrichment of LREE and negative anomaly of Eu. They have 87Sr/86Sr and 143Nd/144Nd initial ratios of 0.707049and 0.511992, respectively, fSm/Nd=-0.35-0.48 and Nd=-0.48. The above evidences suggest that the volcanic rocks should be products of partial melting of the mantle associated with plate subduction, formed in the progressive mature volcanic arc.