粤中地区基底变质岩地球化学特征、锆石U-Pb年代学及其地质意义

Geochemical Characteristics,Zircon U-Pb Geochronology and Geological Significance of Basement Metamorphic Rocks in Central Guangdong

粤中地区经历了多期次的区域变质作用改造,形成一套从绿片岩相到角闪岩相的局部有序总体无序的地质体,其中以浅变沉积岩类为代表的云开岩群构成了该地质体的主体。本文对粤中地区基底变质岩进行了岩石地球化学、锆石U-Pb同位素分析。分析结果表明该变质基底具有比上地壳平均成分更高的Al2O3、TiO2以及更低的SiO2、Fe2O3、Na2O、CaO,岩石成熟度中等。Zr,Hf,Th,U等高场强元素轻度富集,说明沉积物的源区以长英质组分为主或主要由再循环物质组成。综合本文和前人的锆石U-Pb定年数据,认为区内云开岩群的形成时代为新元古代晚期。在锆石年龄频率直方图上,3个样品均出现~1000Ma的峰值,年龄峰值相对较单一而且年龄谐和性较好,表明原岩主要为格林威尔(Grenville)期碎屑物质组成,综合华夏地块其他地区以往的研究资料,本文认为华夏地块南缘可能存在或极其靠近一个Grenvillian期的造山带,其时代可以与全球性的Grenville造山运动对比。

Central Guangdong has undergone several stages of regional metamorphism transformation, forming a set of locally ordered and totally disordered geological bodies from greenschistfacies to amphibolite facies, in which the Yunkai Group represented by epigenetic sedimentary rocks constitute the main body of the geological body. Rock geochemistry and zircon U-Pb isotope analysis of basement metamorphic rocks in central Guangdong have been carried out in this paper. The results show that the metamorphic basement has higher Al2O3, TiO2 and lower SiO2, Fe2O3, Na2O and CaO than the average composition of the upper crust, and the rock maturity is medium. High field strength elements Zr, Hf, Th and U are slightly enriched, indicating that the source area of sediments is dominated by felsic components or mainly composed of recycled materials. Based on the zircon U-Pb dating data of this paper and predecessors, it is considered that the formation age of the Yunkai Group in this area is late Neoproterozoic. On the zircon age-frequency histogram, the peak values of ~1000 Ma appear in all three samples, which are relatively single and age-harmonic, indicating that the original rocks are mainly composed of clastic materials of Grenville period. Based on previous studies in other areas of the Cathaysian massif, this paper suggests that there may be once existing or very close to a Grenvillianorogenic belt in the southern margin of the Cathaysian massif. Its age can be compared with the global Grenville orogeny.