桂东南马山杂岩体地球化学与锆石U-Pb同位素年代学特征及其对构造背景的指示意义  

Geochemical and Zircon U-Pb Geochronological Characteristics of the Mashan Complex from Southeastern Guangxi Province and Their Significance to the Tectonic Setting

作　　者：段瑞春 凌文黎[2] 李青 邱啸飞 杨红梅 刘重芃 卢山松 

机构地区：[1]武汉地质调查中心同位素地球化学研究室,武汉430205 [2]中国地质大学地球科学学院,武汉430074 [3]广西地质矿产勘查开发局,南宁530023

出　　处：《地质学报》2013年第9期1221-1232,共12页Acta Geologica Sinica

基　　金：地质调查项目"中南地区重大地质事件同位素年代学研究"(编号1212011121102);国家自然科学基金面上项目"扬子克拉通神农架-黄陵地区古元古代晚期岩浆事件与地壳增生的地球化学研究"(编号40673025)联合资助的成果

摘　　要：对桂东南马山杂岩体中的橄榄岩进行了锆石原位LA-ICP-MS U-Pb分析,获得了157.8±2.4Ma的^(206)Pb/^(238)U加权平均年龄,表明该岩体形成于燕山早期的晚侏罗世。两件样品具有极低的SiO_2含量,指示其为堆晶成因,无Nb、Ta亏损表明它们形成时没有受到明显地壳物质混染,因此其+4.46和+4.24的ε_(Nd)(158Ma)值反映了岩浆来自中度亏损的地幔源区,是对幔源物质亏损程度的最大估计;中基性岩具有类似于OIB的微量元素特征,不同程度的弱Nb、Ta亏损以及Sr、Nd同位素的变化,指示了幔源岩浆受到了不同程度的地壳物质的混染;相对于华南大多数同期或中生代花岗岩,马山杂岩体中花岗岩具有较小的ε_(Nd)值和年轻的t_(DM2)年龄,指示岩体形成时有幔源物质的加入。因此,桂东南地区在燕山早期为拉张环境,马山杂岩体为板内拉张环境下中等亏损程度的地幔源区和下地壳物质部分熔融作用导致的岩浆作用产物。The I.A ICP MS U-Pb zircon dating for gabbro of the Mashan Complex from Southeastern Guangxi yields a weighted mean 206Pb/Z3sU age of 157.8±2.4Ma, the early Yanshanian of late Jurassic age. The extremely low contents of SiO2 for two samples are indicative of accumulate origin. Along with the lack of depletion in Nb and Ta, it is suggested that the role of crustal contamination during their crystallization is insignificant. Thus their eNd（158Ma） values of ＋4.46 and ＋4. 24 are likely to indicate a mantle source with moderate depletion, a maximum estimation for the depletion degree of the regional mantle source. The （）IB-like elemental patterns, different degrees of depletion in Nb and Ta as well as the variation of Sr Nd isotopes of mode-basic samples indicate the existence of crustal contamination. Relative to the typical contemporaneous granites, the granitic samples of the Mashan Complex have smaller end （t） values and younger trove ages, which are indicative of involvement of mantle materials. Therefore, it is an extensional setting of Southeastern Guangxi in the early Yanshanian. The Mashan Complex formed within an extensional setting through a magmatism derived by partial mehing of moderately depleted mantle and regional lower crust.

关 键 词：桂东南 马山杂岩体 U-Pb同位素年代学 地球化学 

分 类 号：P597.3[天文地球—地球化学]