大容山-十万大山花岗岩带中A型紫苏花岗岩、麻粒岩包体的发现及意义

The Discovery and Significance of A-type Charnokite in Southeast Guangxi Province, China

桂东南大容山-十万大山花岗岩带南部发现的紫苏花岗岩体及其中的大量基性麻粒岩、片麻岩包体的研究表明,麻粒岩包体普遍发育等温减压后成合晶变质结构,其主要变质反应为:Ga+Sill→Spl+Q;Ga+Q→Opx+Crd和Gt+Sill+Q→Crd,显示麻粒岩经历了快速抬升揭顶作用。基性麻粒岩中锆石主要由内核和外壳组成,内核以变质结晶特征为主,外壳为岩浆结晶特征,内核变质结晶年龄集中分布于(950-1100)Ma,外壳岩浆结晶年龄为248 Ma,而紫苏花岗岩的Sm-Nd矿物-岩石等时线年龄为202 Ma,并具有典型A型花岗岩岩石化学特征,标志着印支期造山作用的结束。据此,提出桂东南印支期A型紫苏花岗岩的源岩为中元古末四堡期造山事件(格林威尔)形成的麻粒岩相变质岩,印支期碰撞造山后的快速抬升伸展揭顶部分熔融作用形成了A型紫苏花岗岩,这为华南中元古末四堡期(格林威尔)江山-绍兴造山带的南延和晚古生代特提斯造山带的东延,以及印支期造山作用的结束提供了重要证据。

The charnokite body and a lot of granulite, gneiss xenoliths were found in southern of Da-rongshan-Shiwandashan granite belt in Southeast Guangxi. The study on charnokite and granulites show that the isotermal-depression symplectic textures are generally developed in the granulites xenoliths, and main metamorphic reactions are Ga + Sill→Spl + Q, Ga+Q→Opx+Crd and Ga+Sill + Q→Crd. This midi-cates that the granulite is experienced the fast uplift and extensional exhumation once more. The research on genetic texture for zircons in the granulite and SHRIMP dating show a great deal of zircons are consist of the core and rim. The core is generally characteristic of metamorphic crystallization, and the rim is characteristic of magmatic crystallization. The ages of the core are concentrated abount ( 950-1100) Ma, and the magmatic crystallization age of the rim is at 248 Ma. the charnokite is characteristic of A-type granite which marks the end of Indosinian orogeny. According to this, It is consided that the protoliths of Indosin-ian A-type charnokite are the granulite facies metamorphic rocks formed in Sibao (Grenville) orogenic e-vent in the later Mesoproterzoic. A-type charnokite is formed in the fast lift and exhumation partial melting in post-collision orogenic environment during Indosinian. This provides the important evidence for the south extending of the Sibao(Grenvilla) Jiangshan-Saoxin orogenic belt in the late Mesoproterzoic and the east extending of the Tethys orogenic belt in late Paleozoic in Southchina, and indicates the end of Indosinian orogeny.