华南印支期弱过铝质和强过铝质花岗岩中黑云母的矿物化学及其岩石成因制约

Mineral chemistry of biotites from the Indosinian weakly peraluminous and strongly peraluminous granites in South China and their constraints on petrogenesis

华南印支期岩浆活动强烈,并形成了大量的花岗质岩石,按矿物组合可将其分为含典型过铝质矿物白云母、电气石、黄玉的强过铝质花岗岩(SPG)和含角闪石、黑云母等镁铁质矿物,而缺乏典型过铝质矿物的弱过铝质花岗岩(WPG)两大类。黑云母是花岗岩类岩石中最常见的镁铁质矿物之一,其成分特征可以有效示踪花岗岩的成岩物理-化学条件和源区性质。以赣南印支期龙源坝岩体和湘中白马山岩体为例,对华南印支期花岗岩中两类过铝质岩石的黑云母矿物化学成分进行研究,结果显示WPG中黑云母富镁、贫铝、贫铁,属镁质黑云母,而SPG中黑云母贫镁、富铝、富铁,属铁质黑云母。黑云母平衡结晶温度和氧逸度估算结果显示,WPG比SPG具有更高的岩浆温度和氧逸度。WPG属于壳幔岩浆混合产物,而SPG则来自于接近纯地壳沉积物质部分熔融产物。花岗岩中黑云母的化学成分可以有效判断源区性质,无法单独判断构造背景。

Indosinian magmatic activities are widely developed in South China and a great amount of granitoids were produced. The granitic rocks can be divided into two types by mineral assemblages. One category is strongly peraluminous granites （SPG） containing typical peraluminous minerals such as muscovite, tourmaline and topaz. The other category is weakly peraluminous granites （WPG） bearing hornblende and other mafic minerals such as biotite, and lacking typical peraluminous minerals. Biotite is an important ferromagnesian mineral in the granitoids, and its composition can he used as a powerful tool in evaluating the physical-chemical conditions and identifying the magmatic source natures. In this paper, the biotites from the two types o[ granitic rocks from the Indosinian Longyuanba complex in southern Jiangxi Province and Baimashan complex in central Hunan Province were chosen to study. The results suggest that the biotites in the WPG, which are rich in Mg, and poor in A1 and Fe, belong to Mg-biotites, whereas the biotites in SPG, which are poor in Mg, and rich in AI and Fe, belong to Fe-biotites. The temperature and fugacity of oxygen were also calculated using compositions of biotites, and the results show that the WPG have higher magma temperature and oxygen fugacity than the SPG. On the basis of the composition of biotites, the WPG are the result of the mixing of crust and mantle-derived magmas, while the SPG are likely to he derived from the partial melts of nearly pure sedimentary materials in crust. Therefore, the compositions of biotite in granitic rocks can be employed in tracing the nature of the magma source, but can not be alone used to discriminate tectonic environment.