南祁连党河南山地区鸡叫沟复式岩体岩石地球化学特征及构造环境

Lithogeochemistry of Jijiaogou intrusive complex in the Danghenanshan area, South Qilian Mountain and its tectonic implications

通过对党河南山北坡东段鸡叫沟岩体的岩体地质特征和岩石地球化学特征研究,发现该岩体为一加里东期中酸性复式岩体,岩浆活动有4期,依次为岩枝(脉)状产出的辉石闪长岩、辉石正长闪长岩,岩株状产出的(角闪)石英二长岩,岩脉状产出的黑云二长花岗岩和脉状产出的细粒二长花岗岩.第1,2期(石英二长岩锆石U-Pb年龄为455.3±5.6 Ma)构成该复式岩体的主体部分.总体上,4期岩体均富含碱质(碱度率平均为2.72%)、高钾(平均值为4.24%)、贫钛(均小于1.3%),属钾玄岩系列.岩石富集大离子亲石元素和轻稀土元素,具负Eu异常.进一步分析发现,从第1期到第4期,岩石铝质、钾质质量分数渐次升高,稀土总量降低.同时发现前2期、后2期岩体岩石地球化学特征分别大体相似,而且后2期岩体比前2期岩体贫稀土和不相容元素(第4期比第3期尤甚).岩浆演化程度从第1期到第4期逐渐增加,尤其后2期岩浆演化程度很高(DI平均值达95.9).综合判断显示前2期岩体岩浆源岩为基性岩熔融,形成环境为岛弧环境;后2期岩浆原岩为变质沉积岩,形成于造山环境.

The geologic features and geochemical characteristics of the Jijiaogou intrusive complex in the east of the north slope of the Danghenanshan were summed up and it was found that it is a caledonian neutral-acidic plutonic complex with the four phase of magmatic activities. The first phase is the apophysis-shaped pyroxene diorite and pyroxene syenite diorite, the second is the stock-shaped quartz monzonite, the third is the apophysis-shaped biotite monzogranite and the fourth is the vein-shaped fine-grained monzogranite. The first and second phases (quartz monzonite with the zircon U-Pb age of 455.3±5.6 Ma) constitute the main part of the complex. In general, all rocks are rich in alkali (with an averaged alkalinity rate of 2.72%), high potassium (the averaged value is 4.24%) and poor in titanium (less than 1.3%), and subjected to the shoshonitic series. The rocks are also rich in the large ion lithophile elements and light rare earth elements, with negative Eu anomalies. Furthermore, the content of the aluminum and potassium are gradually increasing but the rare earth elements decreasing in the rocks from the first phase to the fourth, and the rock geochemical characteristics are similar between the first two and the last two phases. In detail, the rocks of the last two phases are less light rare earth elements and incompatible elements (the fourth is less than the third). The degree of magma evolution increases gradually, especially the last two magmatic evolution degree is very high (with the averaged DI of 95.9). The first two phase rocks originated from the partial melting of mafic rocks in the island arc settings, and the last two phase rocks originated from the metamorphosed sediments in the orogenic belts.