皖南新元古代井潭组火山岩锆石U-Pb定年和同位素地球化学研究

对皖南江南造山带新元古代井潭组火山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，井潭组火山岩存在二期火山岩，形成时代分别为820±16 Ma和776±10 Ma。所有样品都具有较高的基性组分(ΣTiO2+ Fe2O3 T+MgO) 含量(4.7%~16.36%)，LREE富集，中等Eu负异常，大离子亲石元素K，Rb，Ba，Th，U等富集，Nb，Ta，Sr，Ti等相对亏损。εNd (t )值为-2.79~-1.71，指示岩浆源区含有显著的新生地壳组分。这些岩石具有高A/CNK(1.37~1.61)和高δ18O值(锆石6.52‰~8.98‰)，显示为壳源S型岩浆岩的特点。石英、斜长石和钾长石等矿物氧同位素具有很大的变化范围，表明井潭组火山岩经历了不同程度的亚固相中温热液蚀变。根据锆石U-Pb定年和元素以及同位素分析结果，776±10 Ma井潭组火山岩与石耳山花岗岩为同期岩浆岩，井潭组火山岩直接来源于早新元古代新生地壳重熔，而石耳山花岗岩源岩为约825 Ma岩浆岩，间接来源于早新元古代新生地壳的再造。这些新生地壳是江南弧–陆碰撞造山带的组成部分，在Rodinia超大陆裂解时发生拉张跨塌熔融，既不是地幔柱岩浆活动产物，也不是岛弧岩浆作用的直接产物，而是板块–裂谷岩浆活动产物。

Zircon U-Pb Age and Isotope Geochemistry of Neoproterozoic Jingtan Volcanics in South Anhui

Zircon U-Pb ages, whole-rock elements, Sr-Nd isotopes, and mineral O isotopes were determined for Neoproterozoic Jingtan vocanics in South Anhui. Zircon U-Pb dating indicates: the Jingtan volcanics consist of two episodes of volcanic rocks with the formation ages of 820±16 Ma and 776±10 Ma, respectively. The Jingtan volcanics show high contents of mafic components (ΣTiO2+ Fe2O3T+MgO) of 4.7% to 16.36%, LREE enrichment,moderate negative Eu anomalies, strong enrichment in large ion lithospheric elements (LILE) of K, Rb, Ba, Th and U,but pronounced negative anomalies of Sr and high field elements (HFSE) of Nb, Ta and Ti relative to the neighboring elements. Neutral whole-rock εNd(t ) values of -2.79 to -1.71 imply their derivation from relatively juvenile crust. High A/CNK ratios of 1.37 to 1.61 and high zircon δ18O values of 6.52 to 8.98‰ point to a supracrustal orign characteristic of S-type magmatic rocks. Quartz, plagioclase and K-feldspar show O isotope disequilibria when paired with zircon, suffered different degrees of post-magmatic alteration at subsolidus temperatures. On the basis of the element and isotope geochemistry, 776±10 Ma Jingtan volcanics and Shi’ershan granites are coeval. The Jingtan volcanics were directly derived from the melting of the sediments of juvenile crust, but the main source rocks for theShi’ershan granites are about 825 Ma magmatic rocks, indirectly derived from the resetting of early Neoproterozoic juvenile crust. It is the juvenile crust in the Jiangnan orogen that was repeatedly worked due to extensional collapse of arc-continent collision orogen during the Rodinia breakup, resulting in this episode of volcanic rocks. They are neither the product of mantle superplume activity, nor the direct product of island arc magmatism, but a product of plate-rift magmatism.