扬子西缘江浪穹窿核部里伍岩群的形成时代及构造背景

Formation Age and Tectonic Setting of the Liwu Group at the Core of Jianglang Dome in the Western Margin of Yangtze Block

江浪穹窿位于扬子陆块西缘,核部地层里伍岩群为一套变质的火山-沉积岩系。为精确厘定里伍岩群的形成时代并探讨其成岩构造背景,本文进行了LA-ICP-MS锆石U-Pb年代学以及岩石地球化学研究。定年结果表明变沉积岩中碎屑锆石年龄介于2521~262 Ma,其中最年轻一组岩浆成因锆石的^(206)Pb/^(238)U加权平均年龄为552.8±5.7 Ma(MSWD=2.1,n=6)。变沉积岩SiO_2含量变化很大(53.48%~96.08%),显示明显的Eu负异常(Eu/Eu*=0.50~0.67),相对富集大离子亲石元素(Rb、Ba和U等)、亏损高场强元素(Nb、Ta、Zr、Hf和Ti等),其稀土、微量配分模式与大陆上地壳完全一致。变基性火山岩具有很低的SiO_2含量(39.92%~48.27%),其稀土、微量配分模式平坦,类似于E-MORB。结合变基性火山岩的锆石U-Pb定年结果(542±9.0 Ma),本文指出里伍岩群应当形成于新元古代晚期(553~542 Ma),而并非前人所认为的古元古代或中元古代。变沉积岩的物源区主要为扬子西缘新元古代地质体,与变基性火山岩均形成于Gondwana超大陆聚合之后的板片俯冲环境。此外,碎屑锆石的年龄谱反映江浪穹窿可能存在太古宙—古元古代变质基底,并且具有Rodinia超大陆的会聚-裂解事件以及加里东运动的地质年龄记录。

The Jianglang dome is located in the western margin of the Yangtze block,with metavolcanic and metasedimentary rocks of the Liwu Group occurring in its core.In order to accurately constrain the forming age and tectonic setting of these strata,we performed LA-ICP-MS zircon U-Pb geochronological and geochemical studies in this paper.The zircon U-Pb dating results show that the detrital zircons in the metasedimentary rocks have ages ranging from 2521 Ma to 262 Ma,with a weighted mean -（206）Pb/-（238）U age of 552.8±5.7Ma（MSWD =2.1,n =6）for the youngest magmatic zircons.The metasedimentary rocks have variable SiO2contents（53.48%-96.08%）with obvious negative Eu anomalies（Eu/Eu＊= 0.50-0.67）,enrichment in large ion lithophile elements（e.g.,Rb,Ba and U）and depletion in high field strength elements（e.g.,Nb,Ta,Zr,Hf and Ti）;these rocks exhibit similar REE and trace element patterns to the continental upper crust.The metavolcanic rocks have low SiO2contents（39.92% -48.27%）with flat REE and trace element patterns,similar to those of E-MORB.Combining with zircon U-Pb dating results of the metavolcanic rocks（542±9.0 Ma）,we propose that the Liwu Group was formed in the Late Neoproterozoic（553-542Ma）,rather than the Mesoproterozoic or Paleoproterozoic as previously documented.The source of metasedimentary rocks was mainly from the Neoproterozoic geological bodies of the western Yangtze block.The metavolcanic and metasedimentary rocks were both formed in a subduction-related tectonic setting after the convergence of Gondwana supercontinent.In addition,the age spectrum of detrital zircons reveals that the Archean to Palaeoproterozoic metamorphosed basements may exist in the Jianglang dome,where also have geological age records of the Rodinia supercontinent convergence-disintegration processes and the Caledonian movement.