扬子北部三峡地区南沱组冰碛岩的物源特征:锆石年龄和地球化学证据

Source Characteristics of Tillite the Nantuo Formation in Three Gorges,Northern Yangtze Block:Evidences from Zircon Ages and Geochemical Composition

扬子陆块新元古代地层发育完整并广泛分布,三峡地区南沱组冰碛岩是扬子北部南华纪的冰成杂砾岩,不整合地覆盖在新元古代黄陵花岗岩之上.南沱组冰碛岩成分复杂,分选性差,缺乏可对比的标志,其沉积时代和物源组成研究较少且存在争议.对南沱组冰碛岩开展锆石U-Pb定年工作,得到最年轻的锆石年龄为706±7 Ma,印证了南沱组沉积时代应不早于700Ma的说法,并可与Marinoan冰期对应.所获得锆石U-Pb年龄谱峰值主要分为-3.0Ga,2.5-2.3Ga,2.05-1.95Ga和900-700Ma,反映了扬子陆块4次地壳再造事件,其中古元古代和太古代年龄与崆岭杂岩相似,900-800Ma和800-700Ma的锆石年龄与附近黄陵花岗岩和扬子北缘岩体一致.冰碛砂岩的微量元素和同位素组成分异明显,可能是扬子北缘岩体、黄陵花岗岩和崆岭杂岩不同程度混合的结果.南沱组冰碛岩中包裹的花岗质砾石表现出不同的微量元素特征,具有偏低的稀土含量和亏损的重稀土组成,结合其锆石U-Pb年龄和Sr-Nd同位素特征,这些花岗质砾石主体可能来自于附近黄陵花岗岩和崆岭岩基.南沱组冰碛岩的物质组成特征反映了新元古时期,扬子地区强烈的裂谷岩浆作用所导致的北高南低的地势环境.

Neoproterozoic sedimentary sequences from the Yangtze block are completely developed and widely distributed. The Nantuo tillites in the Three Gorge area are glacial diamictites from the northern Yangtze block in the Nanhua period, which unconformably overlie the Neoproterozoic Huangling granites. The compositions of the Nantuo tillites are complicated, and the sedimentary time and components of source are rarely studied and remain in dispute. Zircon U-Pb dating results of the Nantuo tillite in this study yield youngest zircon age of 706±7 Ma. Combining with previous results about zircon U-Pb ages of the tuff from basal Doushantuo Formation and top of Datangpo Formation, the sedimentary ages of the Nantuo Formation are limited to 654-635 Ma, which can be equal to that of Marinoan. The peaks of zircon U-Pb age-spectrum are - 3.0 Ga, 2.5- 2.3 Ga, 2.05-1.95 Ga and 900-700 Ma respectively, which indicate four crustal-reworking events in the Yangtze block. The ages of Paleoproterozoic and Archean are similar to that of the Kongling complex, meanwhile, 900-800 Ma and 800-700 Ma are the same as that of the Huangling granites nearby and the rocks from northern margin of the Yangtze block. The trace elements and isotope compositions of moraine sandstones show obvious differentiation, maybe due to the different degrees of mixture by materials from the northern margin of the Yangtze block, the Huangling granite and the Kongling Complex. Granitic conglom-crates in the Nantuo tillites have different features of trace elements, low REE contents and depleted HREE. Associated with zircon U-Pb age and signature of Sr-Nd isotopes, these granitic conglomerates probably mainly came from the Huangling batholith and the Kongling Complex. The provenance characteristics of the Nantuo tillites reflect intensive rift related magma activities in the Yangtze block during the Neoproterozoic time, which causes the terrain featuring ＂lower in the south and higher in the north＂.