Geochemical characteristics and genetic analysis of the early and middle Cambrian volcanic rocks in Baiyin mining field
白银厂矿田早中寒武世火山岩地球化学及成因分析

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.