安徽女山麻粒岩包体的地球化学特征:下地壳组成及其构造属性初探

Geochemical characteristics of the Nushan granulite xenoliths, Anhui Province:Implications for composite of lower crust and tectonic affinity

女山麻粒岩包体的Mg#值总体较低(28～51),在化学成分上主要表现为偏中性,没有明显的辉石、石榴子石或Fe-Ti氧化物的堆晶作用,但部分有斜长石的堆晶作用,略具Eu、Sr正异常,部分则有斜长石的分离结晶,略具Eu、Sr负异常。另外,女山麻粒岩包体亏损Rb、Th、U、Nb、Ta,富集Ba、LREE、Zr、Hf等,具较高的Nb/Ta、Zr/Hf比值,麻粒岩相变质作用过程造成了Rb的强烈亏损,但对Nb、Ta、U、Th的影响较弱。女山麻粒岩包体的形成过程中存在岩浆的结晶分异作用,通过分离结晶混染作用模型(AFC)模拟,表明华北克拉通太古宙基性火山岩和中性麻粒岩分别为较可能的初始岩浆和混合源,初始岩浆经过10%～40%的AFC可以形成女山麻粒岩包体的微量元素组成。总体上,女山麻粒岩包体的主元素、微量元素特征均相似于华北克拉通太古宙中性麻粒岩、华北克拉通下地壳平均值,而与扬子克拉通下地壳平均值有明显差别。因此女山地区下地壳可能仍属华北克拉通的一部分,因而支持Li(1994)的碰撞拆离模式。

The Nushan granulite xenoliths are characterized by intermediate composition (Mg#=28～51). There are no evidences for accumulations of pyroxenite, garnet or Fe Ti oxides in these samples. However, weak accumulation of plagioclase is evident in some xenoliths with positive Eu and Sr anomalies. In addition, the Nushan granulite xenoliths are depleted in Rb, Th, U, Nb and Ta and arerich in Ba, LREE, Zr and Hf exhibiting high Nb/Ta and Zr/Hf ratios. The granulite phase metamorphism may result in the strong depletion of Rb, and be partly responsible for the depletions of Nb, Ta, U and Th. Chemical variation of the Nushan granulite xenoliths can be accounted for the fractional crystallization. Trace element modeling shows that the Nushan granulite xenoliths can be simulated by 10%～40%assimilation fractional crystallization (AFC) of different fractionating phases. The Archaean mafic volcanic rocks and the Archaean intermediate granulite of the North China craton are the most possible primary magma and mixing source, respectively. Geochemical characteristics of major and trace elements show that the Nushan granulite xenoliths are very similar to average composition of the lower crust and the Archaean intermediate granulite of North China craton, but distinctively different from those of the Yangtze craton. These considerations lead to the conclusion that the lower crust beneath Nushan is one part of the North China craton, supporting the crustal detached collision model proposed by Li (1994).