赣中地区周潭群斜长角闪岩年龄、成因及对华南新元古代构造演化的地质意义  

Ages and petrogenesis of the amphibolites from the Zhoutan Group in central Jiangxi Province: Implication for the Neoproterozoic tectonic evolution of South China

作　　者：程进雄 李武显[1] CHENG Jin-xiong;LI Wu-xian(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学,北京100049

出　　处：《地球化学》2021年第6期537-549,共13页Geochimica

基　　金：国家重点研发计划(2016YFC0600204);国家自然科学基金委(41573026,41373033)。

摘　　要：赣中周潭地区发育的周潭群为一套高角闪岩相变质且强烈变形的火山-沉积岩组合,其形成年龄一直存在争议。本次研究对周潭群中斜长角闪岩和黑云母片岩开展了锆石U-Pb年代学和岩石地球化学研究。斜长角闪岩有两种类型的锆石,第一种变质锆石的定年结果为(417±47) Ma,第二种捕获锆石;Pb/;U表观年龄在873~716 Ma之间。黑云母片岩中全部为碎屑锆石,锆石;Pb/;U表观年龄在861~658 Ma之间。斜长角闪岩具有平坦型的稀土分布模式和正的ε;(t)值(1.72~2.69),其Nb/La(0.73~0.94)、Nb/Y(0.10~0.12)以及Zr/Y(1.61~2.54)比值类似于洋中脊玄武岩(N-MORB);在Nb/Yb与TiO;/Yb、La与La/Nb构造判别图解中,斜长角闪岩全部落在N-MORB区域,表明斜长角闪岩来自亏损地幔。对比扬子东南缘850~760 Ma的N-MORB型基性岩的ε;(t)值、Zr/Y和Sm/Nd比值,斜长角闪岩类似于皖南铺岭组约760 Ma的基性岩,暗示它们形成于相同岩浆岩事件。结合斜长角闪岩和黑云母片岩中小于820 Ma的捕获锆石年龄,表明周潭群的顶界年龄小于760 Ma,因此周潭群不仅包含了类似于四堡群和冷家溪群等相应地层,还应该包含类似于板溪群或丹洲群等相应地层。综合本次报道的周潭群N-MORB型斜长角闪岩和前人报道的850~760 Ma双峰式火山岩、基性-超基性岩墙以及低δ;O锆石,表明华南地块从约900 Ma闭合完毕之后一直处于陆内伸展环境中。The Zhoutan group exposed in the Zhoutan area is a suite of volcanic-sedimentary rocks metamorphosed and deformed to a high-grade amphibolite.However,the formation age of the Zhoutan group is still debated.In this study,we report the SHRIMP U-Pb age of amphibolites and biotite schists of the Zhoutan group and the geochemistry of amphibolites.There are two types of zircons in zircon grains of amphibolite.The first type is the metamorphized zircons whose weighted mean age is (417±47) Ma and the second type is the detrital zircons whose apparent ages range from 716 Ma to 873 Ma.Ages of zircons in biotite schists are from 658 Ma to 861 Ma.Amphibolites are characterized by flat REE patterns and positiveε;(t)=1.72–2.69.In addition,they have Nb/Yb(0.73–0.94),Nb/Y (0.10–0.12),and Zr/Y (1.61–2.54) ratios similar to N-MORB.In the diagrams of Nb/Yb vs.TiO;/Yb and La vs.La/Nb,amphibolites plot within N-MORB field,suggesting that the parental magmas of these amphibolites were derived from a depleted mantle.Comparison ofε;(t) and Zr/Y of Yangtze Block shows that amphibolites and~760 Ma mafic rocks in the South Anhui Province share theε;(t),Zr/Y,and Sm/Nd ratios,an indication that their formation was during the same magmatic episode.Combined with the presence of<820 Ma detrital zircons of amphibolites and biotite schist of the Zhoutan group suggests that the top age of the Zhoutan group may be<~760 Ma.This implies that the Zhoutan group includes not only strata equal to the Sibao and the Lengjiaxi group but also strata equal to the Banxi and the Danzhou group.Synthesizing the geological facts of the Zhoutan group N-MORB-type amphibolites,850–760 Ma bimodal and mafic-ultramafic rocks,and 850–760 Ma zircons of lowδ;O suggests that the Yangtze Block is in an extension setting after~900Ma.

关 键 词：地球化学特征 斜长角闪岩 周潭群年龄 N-MORB 华南 

分 类 号：P597[天文地球—地球化学]