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 consi st mainly of basalts, alkalibasalts, andesites, basaltic andesites, basalti c trachy - andesites, dacites, rhyolites and dacite - liparites in which th e basic and medium-basic rocks belong to alkali and subalkali series with T iO2 of 0.53% to 1.01% (wt %), but the acidic rocks to calc-alkali and low-k calc-alkali series with low TiO2 of 0.2% to 0.83% (average value 0.59%). B esides, the volcanic rocks are characterized by total alkalinity of 5.76% a nd Fe2O3/FeO ratio of 0.74 on the average, and K2O/Na2O ratio below 1.0. Co mpared with N-MORB, the basic and medium-basic volcanic rocks are more enri ched in Bar 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 acid ic volcanic rocks are slightly enriched in HREE, but variously depleted in other imcompatible elements. Relative to the lower crust, they are more enr iched in Rb, Th, U, K and slightly enriched in Zr, Hf and HREE, but differe ntly depleted in Ba, Sr, Nb, Ta, Ti and LREE. The REE pattern for the volca nic rocks shows the enrichment of LREE and negative anomaly of Eu. They hav e Sr-87/Sr-86 and Nd-143/Nd-144 initial ratios of 0.707049 and 0.5119921 re spectively, f(Sm/Nd) = - 0.35 similar to -0.48 and Sigma(Nd)(t) = -0.48. Th e above evidences suggest that the volcanic rocks should be products of par tial melting of the mantle associated with plate subduction, formed in the progressive mature volcanic arc.