The western boundary between the Yangtze and Cathaysia blocks, new constraints from the Pingbian Group sediments, southwest South China Block
The western boundary between the Yangtze and Cathaysia blocks, new constraints from the Pingbian Group sediments, southwest South China Block
复制标题
扬子与华夏地块西界，屏边群沉积新制约，华南地块西南部
DOI：
10.1016/j.precamres.2019.105350
复制
发表时间：
2019
影响因子：
3.8
通讯作者：
Wang Yadi
中科院分区：
文献类型：
--
作者：
Zhu Guanglei;Yu Jin-Hai;Zhou Xueyao;Wang Xiaolei;Wang Yadi
云南东南部的屏边群是华南地块西南角唯一出露的前寒武纪地层单位。这一群的性质对揭示该区基底成分、进一步认识华南板块的前寒武纪演化和扬子-华夏地块边界具有重要意义。锆石U-Pb年龄表明，屏边群沉积岩的形成时间晚于772-710 Ma。屏边群附近的“寒武系”郎木桥组沉积岩具有与屏边群相似的最大沉积年龄和碎屑锆石年龄谱，可能属于屏边群。屏边群沉积岩成分变化较大，SiO2/Al 2 O3比值较低（3.7 ~ 9.7），成熟度较低。稀土配分模式以轻稀土富集为特征，（La/Yb）n为5.3 ~ 10.3。这些沉积岩的地球化学和岩石学特征与扬子地块南部新元古代沉积岩相似。屏边群沉积岩的碎屑锆石年龄谱为单峰型，峰值在844-808 Ma之间，并有少量的古老碎屑。其年龄谱与扬子地块南部和西部新元古代沉积岩相似，但与华夏地块和印支地块不同，表明屏边群与扬子地块有密切关系。碎屑锆石U-Pb年龄和Hf同位素综合表明，扬子地块西南部发生了五次主要岩浆事件。古元古代早期、古元古代晚期和中元古代晚期岩浆活动主要是对古地壳的改造，而1.7- 1.5Ga和0.9- 0.7Ga岩浆活动则伴随着大量幔源物质的输入。新元古代幔源岩浆活动的连续性表明其形成的构造背景与俯冲作用有关。沉积岩地球化学特征和碎屑锆石年龄分布表明，屏边群沉积于弧后前陆盆地，与扬子地块西部新元古代沉积岩相似，应归属于扬子地块。因此，扬子陆块与华夏陆块的边界应位于研究区以东。
The Pingbian Group of southeast Yunnan Province is the only exposed Precambrian unit in the southwestern corner of the South China Block (SCB). The nature of this group is crucial to reveal the basement component in the area, and to further understand Precambrian evolution of the SCB and to constrain the boundary between the Yangtze and Cathaysia blocks. Zircon U-Pb ages indicate that the Pingbian Group sedimentary rocks were deposited later than 772–710 Ma. “Cambrian” Langmuqiao formation sedimentary rocks near the Pingbian Group have the maximum deposition age and age spectra of detrital zircons similar to the Pingbian Group, suggesting that they probably belong to the Pingbian Group. The Pingbian Group sedimentary rocks have variable compositions with low SiO2/Al2O3(3.7 ∼ 9.7), implying relatively low maturity. Their REE patterns are characterized by enrichment in LREE with (La/Yb)n of 5.3–10.3. Geochemical and petrologic characteristics of these sedimentary rocks are similar to those Neoproterozoic sedimentary rocks in the southern Yangtze Block. The Pingbian Group sedimentary rocks show unimodal age spectra of detrital zircons with a peak at 844–808 Ma and minor old detritus. The age spectrum resembles those of Neoproterozoic sedimentary rocks in the southern and western Yangtze Block, but different from those in the Cathaysia and Indochina blocks, suggesting the Pingbian Group’s affinity with the Yangtze Block.The integration of U-Pb ages and Hf-isotopes of detrital zircons indicates that there were five main magmatic events occurring in the southwestern Yangtze Block. Early Paleoproterozoic, late Paleoproterozoic and late Mesoproterozoic magmatism mainly involved the reworking of ancient crusts, whereas 1.7–1.5 Ga and 0.9–0.7 Ga magmatic activities were accompanied by the input of significant mantle-derived materials. Continuous Neoproterozoic mantle-derived magmatism suggests a subduction-related tectonic setting. Geochemistry of sedimentary rocks and age distribution of detrital zircons suggest that the Pingbian Group was deposited in the back-arc foreland basin.All observations suggest that the Pingbian Group is similar to those Neoproterozoic sedimentary rocks in the western Yangtze Block, and consequently the Pingbian area should belong to the Yangtze Block. Thus, the boundary between the Yangtze and Cathaysia blocks must be located at east of the study area.