摘要
对皖南新元古代石耳山花岗岩样品进行了锆石微区U-Pb定年、全岩主微量元素分析和Sr-Nd同位素分析以及矿物氧同位素分析。结果表明,石耳山花岗岩中存在两个时代的岩浆锆石,对应的U-Pb年龄分别为777±9Ma和827±15Ma。总体上石耳山花岗岩的特征为:高SiO2(74.7%~78.5%)、高K2O(3.99%~5.64%)和高K2O/Na2O比值(1.5~3.0),以及很低的基性组分含量(ΣTiO2+FeO+MgO=1.2%~3.0%),显示出高度演化的地壳物质。并具有LREE富集的右倾模式和强烈的Eu负异常(Eu/Eu*=0.2~0.4)。全岩εNd(t)值(-0·53^+0·72),指示其岩浆源区含有显著的亏损地幔组分。样品的锆石δ18O值为2.4‰~7.1‰,显示石耳山花岗岩为低δ18O值岩浆岩;相对较大的δ18O值变化范围,说明其成因与高温超固相热液蚀变作用有关。其余矿物与锆石相比,大多数表现出很大的δ18O值变化范围,表明经历了不同程度的高温热液蚀变。根据这些岩石的同位素年代学和地球化学特征,认为U-Pb年龄为827±15Ma的锆石应为继承来源,而年龄为777±9Ma的锆石可解释为同时代岩浆成因。因此在1000~880Ma中,皖南出现过大规模的幔源岩浆活动,沿着大陆边缘形成初生地壳。约827±15Ma热事件(地幔超柱活动?)使岩石圈地幔及其上覆地壳加热,导致拉伸加厚地壳内部的初生地壳重熔形成岩浆岩。随着裂谷的快速打开,约780Ma幔源岩浆沿裂谷带上涌,启动了中上地壳裂谷带高温热液蚀变,同期喷发了大量的火山岩。裂谷带内的岩浆围岩经受了不同程度的水-岩交换,形成各种低δ18O值的蚀变围岩。在约805℃时,蚀变围岩开始部分熔融并且同期侵位冷却固结,形成花岗岩。较老的继承锆石大多被熔融,大量形成的是约777±9Ma结晶的岩浆锆石。由于处于熔体状态的时间很短,使得岩浆未能均一化,而生成具有不同低δ18O值的花岗岩。石耳山花岗岩初生地壳的形成至少经历了2个阶段部分熔融成岩过程,因而具有高硅、高钾和低基性组分等高度演化的地壳物质特征。
Zircon U-Pb dating, whole-rock elements and Sr-Nd isotopes, and mineral O isotope analyses were carried out for Neoproterozoic granites at Shiershan in South Anhui Province. Zircon U-Pb dating indicates two age phases of magmatic zircons formed at 777 ± 9 Ma and 827 ± 15 Ma, respectively. As a whole the granites are characterized by high SiO2 （74.7 - 78.5% ）, high K2O （3.99 - 5.64% ）, high K2O/Na2O ratios （ 1.5 - 3.0） and very low contents of mafic components ∑TiO2 ＋ Fe2O3^T ＋ MgO （ 1.2 - 3.0% ）, showing the features of highly evolved crust. They have similar REE patterns showing LREE-enrichment and strong negative Eu anomalies （Eu/Eu＊ = 0.2 -0.4）. The nearly neutral whole-rock εNd （t） values of- 0.53 to ＋0.72 indicate a magraatic source with significant proportions of depleted mantle component. The δ^180 values of zircon range from 2.4‰ to 7.1‰, indicating the granites at Shiershan are low δ^18O magmatic rocks; The big variation of δ^18O values reflects their genesis was related to supersolidus hydrothermal alteration. Compared with zircon, the other mineral separates yield considerable variation of δ^18O values, suggesting they suffered different degrees of hydrothermal alteration. On the basis of element and isotope results, we interpret the zircons of 827 ± 15 Ma as of the inherited one, and those of 777 ± 9 Ma as the coeval magmatic genesis. It is inferred that an extensive mantle-derived magmatic activity took place at ca. 1 100 to 880 Ma, and a juvenile crust was formed in the southeastern margin of the Yan- gtze craton. It is assumed that due to anomalous thermal pulse by a mantle superplume event at ca. 827 Ma that heated the overlying lithosphere, the igneous rocks were generated by remelting of the juvenile crust in the thickened orogenic crust. Accompanying rapid opening of the rift, mantle-derived magma moved upward at ea. 780 Ma along the rift zone, resulting in high-T meteorie-hydrothermal alteration within the rift teetonie zone in the level of middle-upper erust, and voluminous volcanic rocks were formed during this period. Due to different degrees of water-rock reaction at high temperature, altered wallrocks of magma chamber within the rifling zone have differently low δ^18O values. At ca. 800℃ the altered wallroeks began to be melted, and almost synchronously emplaced and cooled to form granites. Most old inherited zircons were resorbed, a great deal of magmatic zircons were crystallized at ca. 777 Ma. Because of a very short duration of the melt phase, the granitoid magma could not be uniformized, consequently the granites with differently low δ^18O values were generated. The granites at Shiershan suffered at least two phases of partial melting processes, thus they have the features of highly evolved crust characterized by high SiO2, high K20, and low contents of mafic component.
出处
《高校地质学报》
CAS
CSCD
北大核心
2005年第3期364-382,共19页
Geological Journal of China Universities
基金
中国科学院知识创新工程项目(KZCX3-SW-141)
国家自然科学基金项目(40334036)
西北大学教育部大陆动力学重点实验室联合资助成果
关键词
锆石
氧同位素
新元古代
热液蚀变
阴极发光
zircon
oxygen isotope
Neoproterozoic
hydrothermal alteration
cathodoluminescence
