扬子陆块北缘汉中地区成冰纪冰期沉积物与锆石U-Pb年龄
- 收稿日期:
2025-01-27
- 网络出版日期:
2025-08-20
摘要: 【目的】华南地区扬子陆块广泛保存了新元古代成冰纪“雪球地球”时期的沉积记录，时间包括Sturtian冰期（720 ~ 660 Ma，相当于南华纪长安冰期或江口冰期）、间冰期（660 ~ 649 Ma，大塘坡间冰期）和Marinoan冰期（约649 ~ 635 Ma，南沱冰期）等。扬子陆块北缘汉中地区钢厂-元坝剖面成冰纪冰期沉积岩曾被认为是南沱组冰碛层，上覆埃迪卡拉系陡山沱组，下伏于莲沱组之上。为理清研究区冰期沉积岩在扬子北缘-秦岭南缘裂谷盆地的发育期次和充填特征。【方法】在剖面实测基础上，对冰期沉积岩底部沉凝灰岩和上部砂岩中的锆石进行了U-Pb定年分析（LA-ICP-MS法），并探讨了冰期持续时间、沉积充填特征及区域对比。【结论】在汉中地区新元古代成冰纪沉积岩中识别出下、上两套冰碛岩，均为陆相冰川，冰碛岩之间和之上发育砂岩和粉砂岩。冰碛岩底部与莲沱组分界处沉凝灰岩锆石最年轻谐和年龄的加权平均值为719 ± 11 Ma （MSWD = 0.75），表明它们可能形成于Sturtian冰期，两套冰碛岩大致对应于南华系长安组和古城组。这套冰期沉积岩不仅可与盆地内镇巴、城口地区同期沉积岩对比，而且跟华南其它地区Sturtian冰期沉积岩的发育规律也有相似性。
The Cryogenian Sediments and Zircon U-Pb Age in Hanzhong Area, North Margin of the Yangtze Block
- Received Date:
2025-01-27
- Available Online:
2025-08-20
Abstract: [Objective] In the Yangtze Block, South China, many places have been found with well preserved sedimentary records of Neoproterozoic “Snowball Earth” (Cryogenian), including the sediments from Sturtian glaciation (ca. 720 ~ 660 Ma, equivalent to Chang'an glaciation and Gucheng glaciation, Nanhuan Period), nonglacial interlude (ca. 660 ~ 649 Ma, Datangpo Age ), and Marinoan glaciation (ca. 649 ~ 635 Ma, Nantuo glaciation). The Neoproterozoic glacial deposits in Gangchang-Yuanba section, Hanzhong area, north margin of the Yangtze Block, previously called “Nantuo tillite”, were formed above the Tonian Liantuo red coarse clastic rocks, and below the Ediacaran Dushanshituo dolomite. In order to clarify the tillite age, how they formed and filled in the small rift basin between the north margin of Yangtze Block and the south margin of Qinling Orogens. [Methods]The field observation and measurement were carried out in the section. Based on it, zircons selected from the bottom tuff layer and upper sandstones were conducted U-Pb dating analysis (LA-ICP-MS).The detailed features of the sedimentary rocks and their regional correlation were also discussed in this paper. [Results and Discussion] Two sets of tillites were first identified within the Neoproterozoic glacial sedimentary rocks in this region, and both were formed from continental glacier. Several layers of sandstones and siltstones were filled between them and above. Zircons from the bottom tuff yield a youngest mean concordant age of 719 ± 11Ma, indicating the beginning of glaciation in this area. Considering with the lithological succession and sedimentary structures, these Cryogenian deposits are thought to be formed in the Sturtian glaciation, corresponding respectively to the Chang'an glaciation and the Gucheng glaciation. The timing of this glaciation event and the sedimentary associations are not only consistent with the contemporaneous sediments in adjacent Zhenba and Chengkou areas, but also show similarities to the Sturtian sediments widely distributed in other parts of South China.
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