甘肃北山北部红石山地区泥盆系雀儿山群火山岩地球化学特征及构造环境分析

位于北山北部红石山地区雀儿山群的火山岩主要为一套中酸性火山岩,以安山岩、英安岩和流纹岩为主,属钙碱-碱性系列。地球化学研究表明:火山岩SiO2含量较高(62.29%~75.74%),MgO含量较低(<2.15%),全碱含量较高(Na2O+K2O为6.24%~9.2%),且K2O/Na2O<1。轻稀土元素(LREE)相对于重稀土元素(HREE)呈现较强富集[(La/Yb)N=2.94~6.36],轻稀土内部出现较强分馏[(La/Sm)N=1.94~3.33],并且具有相当明显的Eu负异常(0.50~0.90);在微量元素方面,大离子亲石元素(LILE)相对高场强元素(HFSE)富集,具有明显的Nb、Ta、P、Ti相对亏损和Ba、Th、Zr、Ce、Sm相对富集。上述特征表明该火山岩形成于洋壳俯冲作用下的岛弧环境,可能与哈萨克斯坦板块和塔里木板块之间的古洋壳俯冲消减有直接的成因联系,在俯冲过程中洋壳含水流体引起上覆地幔物质的部分熔融,并在岩浆上升中经历了一定的分离结晶作用和上地壳物质的同化混染。

Geochemical Characteristics and Tectonic Settings of Volcanic Rocks from Queershan Group in Hongshishan Area,Gansu

The volcanic rocks of Queershan Group, located in Hongshishan area, north of Beishan, are mainly composed of intermediate-acid volcanics, andesites, dacite and rhyolite, which belong tocalc-alkaline series. Researches on the geochemistry suggest that, these volcanic rockshave high SiO2 content(62.29%-75.74%, lower MgO content(<2.15%), relatively high alkali content(Na2O+K2O=6.24%-9.2%), and K2O/Na2O<1. These rocksare enrich in LREE relative to HREE((La/Yb)N=2.94-6.36), with a strong fractionation in LREE((La/Sm)N=1.94-3.33), presenting a strong negative anomaly of Eu(0.50-0.90). In the aspect of trace elements, these rocks are enrich in LILE more than HFSE, with relatively depletion of Nb, Ta, P and Ti, as well as the enrichment of Ba, Th, Zr, Ce and Sm. These geochemical characteristics approve that, these volcanic rocks formed in the island arc environment with the effect of oceanic crust subduction, which has a direct genetic relationship with the subduction of ancient oceanic crust between Kazakhstan plate and Tarim plate. During the process of subduction, the partial melting of the overlying mantle material was caused by the hydrous fluids of oceanic crust, and the product created by the rising magma has experienced a certain fractional crystallization and assimilation and contamination of upper crustal material.