江西庐山地区星子群变质时代及变质机制探讨

Study on Age and Mechanism of the Metamorphism of the Xingzi Group in the Lushan Area, Jiangxi Province

江西庐山地区出露的星子群为一套中深变质岩系,前人将其划为古元古代,并认为其在新元古代经历了一期区域变质作用,之后在中生代发生了动力变质-岩浆作用。然而,最近的研究表明星子群地层时代为新元古代,其变质作用时代和机制的研究对解释这一地区甚至华南地区构造演化具有重要意义。本文通过对星子群局部发育的混合岩浅色体中锆石进行SIMS U-Pb定年,得到142.6±1.5 Ma(MSWD=0.49)的变质增生边年龄,应该是星子群峰期变质作用发生的时间,从而否定了前人认为的星子群新元古代的区域变质事件;对星子群石榴二云母片岩中变质矿物白云母进行Ar-Ar定年,得到了136.08±1.78 Ma的坪年龄和等时线年龄,进一步佐证了星子群中生代的变质作用。此外,混合岩各项特征均显示其为流体交代成因,流体可能来源于变质流体和岩浆热流体,结合这一地区存在大量中生代岩脉和岩墙的地质事实,因此推断星子群中深变质作用的发生可能与该地区中生代广泛的岩浆事件有关,是岩浆热动力变质的产物。这一研究成果对江西庐山地区的构造演化格局,特别是对庐山"星子变质核杂岩"的重新认识具有重要意义。

The mesometamorphic strata of the Xingzi Group developed in the Lushan area, whose zircon U-Pb age of 1869i40 Ma is widely accepted, was supposed to belong to Paleoproterozoic, and experienced two stages of metamorphism. The first phase of metamorphism which reached to amphibolites facies was defined as regional metamorphism occurred in the Neoproterozoic and the second phase was dynamic metamorphism occurred in the Mesozoic. However, recent research shows that the Xingzi Group belongs to Neoproterozoic. And this result contradicts the revolution history built by previous researches. So studying the age and mechanism of the metamorphism of the Xingzi Group will have a significant influence in unveiling the revolution history of the Xingzi area and even South China. We acquire the age of 142.6±1.5 Ma（MSWD=0.49）from the metamorphic zircons in the leucosomes of the migmatite from the Xingzi Group by SIMS U-Pb method, and we advocate this age represents the time of the peak metamorphism. For this reason, we argue the regional metamorphism in the Neoproterozoic is not exactly existed. In addition, the age of 136.08±1.78 Ma obtained by Ar-Ar dating of metamorphic muscovite in the schist from the Xingzi Group also supports the conclusion above. Besides, the migmatite shows the characteristic of fluid metasomatism whose fluid might include metamorphic fluid and magmatic fluid. Combining the development of Mesozoic veins in the study area, it can be inferred that the metamorphism of the Xingzi Group might have been induced by the Mesozoic magmatic events in the Lushan area. And the mesometamorphic strata resulted from thermodynamic metamorphism. This conclusion is of great significance on rediscovering the tectonic evolution pattern of the Lushan area, especially on re-examining the Xingzi metamorphic core complex.