华南东部陆缘侏罗纪岩浆作用特征及其构造背景——来自浙东南毛弄组火山岩的新证据

华南东部内陆地区在侏罗纪处于陆内伸展背景已逐渐成为学术界共识,但对于该时期东部陆缘的构造环境仍存在争议。本文以浙东南毛弄组火山岩为对象,开展了岩石学、SHRIMP锆石U-Pb年代学、锆石微量元素、全岩元素地球化学和Sr-Nd同位素等研究,为认识华南东部陆缘构造背景提供制约。浙江侏罗纪火山岩以松阳毛弄剖面较具代表性,总体上是一套以英安质晶屑凝灰岩和流纹质玻屑凝灰岩为主的火山岩组合。本次研究在毛弄组下段获得的流纹质玻屑凝灰岩的SHRIMP锆石U-Pb年龄为153±2Ma。综合前人研究表明,毛弄组火山岩主要形成于180～148Ma。毛弄组火山岩属于镁质钙碱性系列,且表现出明显的轻稀土元素富集特征,其中早侏罗世毛弄英安质凝灰岩Eu负异常不明显,而晚侏罗世毛弄流纹质凝灰岩具有显著的Eu负异常;两组凝灰岩均富集K、Rb和Th等大离子亲石元素,亏损Nb、Ta、Zr和Hf等高场强元素。毛弄英安质火山岩的源区残留相组合可能为"斜长石+石榴石+角闪石",而毛弄流纹质凝灰岩则可能为"斜长石+角闪石",这一差异可能代表它们具有不同的源区深度。毛弄组火山岩的Sr-Nd-Hf同位素特征与华夏陆块古元古代基底岩体相似,其可能主要起源于基底地壳物质的部分熔融,并有少量地幔新生物质的加入。结合构造背景分析认为,毛弄组火山岩等陆缘岩浆岩形成环境为相对低温的"安第斯型"俯冲环境。通过不同时代区域岩体锆石氧逸度计算发现,早侏罗世东南海域岩浆岩的氧逸度比大陆陆缘更高,且大陆陆缘岩浆岩的氧逸度从晚三叠世至晚侏罗世不断升高,可能标志着陆缘地区俯冲流体的影响有着从东南海域向大陆陆缘扩散的趋势。综合分析认为,在侏罗纪华南东部陆缘可能发育一个与古太平洋俯冲有关的"安第斯型"陆缘岩浆弧,其时代可能最早可以追溯到早侏罗世早期,并持续扩大到中-晚侏罗世。这一时期浙闽沿海地区总体处于弧后挤压主导的构造环境,与同期内陆地区构造伸展背景形成鲜明对比。

The Jurassic magmatism and its tectonic setting in the eastern margin of South China: Evidence from the volcanic rocks of the Maonong Formation in the SE Zhejiang

Intraplate extension has been considered as a consensus to explain the setting of the interior of eastern South China during Jurassic, however, there are still arguments on the tectonic setting in the eastern margin. Therefore, analyses on the volcanic rocks of the Maonong Formation from the southeastern Zhejiang (including petrology, SHRIMP zircon U-Pb dating, zircon trace elements, bulk elements geochemistry and Sr-Nd isotope analyses) are employed to constrain the tectonic setting of the eastern margin of South China. The Maonong profile located in Songyang is a representative of the Jurassic volcanic rocks in Zhejiang Province. The volcanic rocks in this profile are mainly composed of dacitic crystal tuff and rhyolitic vitric tuff. The SHRIMP U-Pb dating shows that the age of rhyolitic vitric tuff from Maonong is at 153±2Ma. Coupled with previous researches, the Maonong Formation was probably formed at 180~148Ma. Volcanic rocks from the Maonong Formation are predominantly magnesian and calc-alkaline series and show prominent pattern of relative enrichment in LREE. Early Jurassic dacitic tuff has inconspicuous Eu negative anomalies whereas Late Jurassic rhyolitic tuff has prominent one. Tuffs in both periods are enriched in LILE (K, Rb and Th) and depleted in HFSE (Nb, Ta, Zr and Hf). Early Jurassic dacitic tuff was generated at conditions with a residual assemblage of plagioclase + hornblende + garnet, but Late Jurassic rhyolitic tuff was generated at conditions with residual assemblage of plagioclase + hornblende, presumably indicating their distinct source depths. The Sr-Nd-Hf isotope compositions of the Maonong volcanic rocks are akin to the Paleoproterozoic basement of the Cathaysia Block. Therefore, they presumably originated from the partial melting of the Paleoproterozoic basement with minor involvement of juvenile material. According to the analyses of the tectonic setting, they were formed in Andean-type subduction environment with relatively low temperature. By calculations of zircon oxygen fugacity of rocks at different ages and regions, it was found that the oxygen fugacity of the Early Jurassic magmatic rocks from sea areas of SE China was higher than that from continental margin, and oxygen fugacity of magmatic rocks from continental margin was rising from Late Triassic to Late Jurassic. The tempo-spatial variation of oxygen fugacity may suggest the impact of subducted fluids with a tendency to expand from sea areas of SE China to the continental margin. The comprehensive analyses suggest that an "Andean-type" margin arc related to the paleo-Pacific subduction was probably developed in the eastern margin of South China, which could be dated back to the early Early Jurassic and expanded in Middle-Late Jurassic. The coastal region was predominantly under a retro-arc compressive setting, in contrast to the synchronous extensional setting in the hinterland.