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    桂东北地区鹰扬关群的构造属性:来自下龙组条带状磁铁石英岩的年代学约束

    Tectonic affinity of the Yingyangguan Group in northeastern Guangxi: geochronological constraints from banded iron formation (BIF) of the Xialong Formation

    • 摘要: 桂东北地区作为重建华南大陆早期构造演化的关键区域,该地区鹰扬关群的时代归属及构造属性,长期以来学术界存在诸多分歧。为了厘清鹰扬关群的时代归属与构造属性,文章对鹰扬关群下龙组中连续分布的条带状磁铁石英岩展开了锆石U-Pb年代学与Hf同位素分析。3件磁铁石英岩样品(编号分别为:YY21063、YY21074和YY2302)的碎屑锆石U-Pb测年结果呈现出相似的年龄分布特征,大部分年龄集中分布于0.72~0.90 Ga,少部分年龄分布于0.90~1.30 Ga、1.62~1.99 Ga、2.12~2.36 Ga和2.43~2.64等年龄区间。2件磁铁石英岩样品(编号分别为:YY21063和YY2302)的碎屑锆石Hf同位素组成特征相似,其εHf(t)值分别为-21.92~11.68和-12.80~10.73,二阶段Hf模式年龄(TDM2)分别为0.92~3.85 Ga和1.09~3.31 Ga,表明下龙组碎屑物质主要源自中—古元古代及太古代地壳物质的再循环。3件样品中最年轻碎屑锆石的年龄数据高度吻合,共同限定了下龙组的沉积上限约为0.72 Ga,进而确定鹰扬关群的地质时代归属为新元古代。此外,下龙组条带状磁铁石英岩中碎屑锆石U-Pb年龄的主体分布于0.72~0.90 Ga,进一步暗示了其物源主体可能为扬子东南缘新元古代岩浆活动的产物。下龙组中连续分布且可追溯的条带状磁铁石英岩,为鹰扬关群的整体有序沉积提供了有力证据,表明其构造属性可能并非传统观点所认为的混杂成因,而是形成于伸展裂谷构造背景。

       

      Abstract: The Yingyangguan group in northeastern Guangxi, serving as a crucial area for reconstructing the early tectonic evolution of the South China Block, has long been subject to academic debate regarding its chronological classification and tectonic affinity. To address this, we conducted detrital zircon U-Pb dating and Hf isotope analysis on banded iron formation (BIFs) from the Xialong Formation of the Yingyangguan Group, which exhibit continuous and traceable stratigraphic sequences. Three BIFs samples (YY21063, YY21074, and YY2302) yielded similar detrital zircon U-Pb age spectra, with dominant age peaks at 0.72~0.90 Ga, and subordinate populations at 0.90~1.30 Ga, 1.62~1.99 Ga, 2.12~2.36 Ga, and 2.43~2.64 Ga. Two samples (YY21063 and YY2302) displayed comparable zircon Hf isotopic compositions, with εHf(t) values ranging from -21.92 to 11.68, and -12.80 to 10.73, respectively. Their two-stage Hf model ages (TDM2) varied between 0.92~3.85 Ga and 1.09~3.31 Ga, indicating that the detrital materials were predominantly derived from the reworking of Mes-Paleoproterozoic and Archean crustal sources. The youngest detrital zircon ages from all three samples consistently constrained the maximum depositional age of the Xialong Formation at ~720 Ma, thereby confirming the Neoproterozoic age of the Yingyangguan Group. The dominant 0.72~0.90 Ga zircon age population suggests that the primary provenance was likely associated with Neoproterozoic magmatic events along the southeastern margin of the Yangtze block. The continuous and traceable banded iron formations in the Xialong Formation indicate that the Yingyangguan Group represents a coherent sedimentary sequence rather than a tectonic mélange. Combined with the provenance signatures, we propose that the Yingyangguan Group formed in a rift-related tectonic setting, further elucidating the Neoproterozoic tectonic evolution of the South China block.

       

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