诸广山南部白云岩体岩石地球化学特征及成因

Geochemical characteristics and genesis of Baiyun pluton in southern Zhuguangshan mountain

白云岩体位于诸广山南部岩体中东部,是一个重要的产铀岩体,其岩性为粗粒斑状黑云母花岗岩。在主量元素方面,该岩体的岩石富硅(SiO2平均为73.47%)、富铝(A/CNK值平均为1.09)和高的K2O/Na2O值(平均为1.99);在微量元素方面,富集大离子元素Rb、Th,而Ba、Sr、Ti、Nb、Ta、P亏损明显,具有高的Rb/Sr(平均为6.22)和Rb/Nb值(平均为29.07),富含铀(平均为9.04×10-6),可为岩体内铀矿床的形成提供铀源;轻稀土元素富集和轻、重稀土元素分馏相对明显,稀土元素配分曲线呈右倾型,Eu亏损明显;在同位素方面,εNd(t)值低(平均为-10.6),(87Sr/86Sr)i值高(平均为0.71688),Nd模式年龄古老(1865～1874 Ma)。这些特征表明,白云岩体属于典型的壳源型花岗岩,是在华南地块和印支地块碰撞结束后不久形成的伸展构造环境中,位于地壳中-下部的古-中元古代地壳组分在伸展、减薄作用产生的减压、导水和地幔上涌等因素的综合影响下,由泥质岩源区发生部分熔融而形成。

Located in the east-central of the southern Zhuguangshan complex pluton,Baiyun pluton is an important uranium producing pluton.In lithology,it is coarse grain black mica granite.In major elements,the pluton is with rich silicon（SiO2 average for 73.47%）,rich aluminum（A/CNK value average 1.09） and high ratio of K2O/Na2O（average for 1.99）.It is rich in large-ion trace elements Rb,Th,but poor in Ba,Sr,Ti,Nb,Ta,P.Rock samples have high Rb/Sr（average for 6.22） and Rb/ Nb ratio（average for 1.447）,and rich uranium（average for 9.04×10-6） which can act as the source for the formation of uranium deposit.In REE chemistry,the pluton is rich in LREE and relatively high fractionation between LREE and HREE,and bear the right-wing-type distribution patten with obviously Eu deplation.In Isotopic chemistry,the pluton has low εNd（t） value（average for-10.6）,high（87Sr/86Sr）i value（average for 0.71688）,and old Nd mode age（1865～1874 Ma）.These characteristics consistently show that Baiyun pluton belongs to typical crustal source granite type and formed in an extensional tectonic environment shortly after the collision between South China and Indo-China plate from partial melting of argillaceous rocks in the middle-lower crust due to the comprehensive action of the decompression,water-prodution and mantle upwelling induced by crust stretching and thinning.