朝鲜平南盆地古元古界-下古生界沉积岩碎屑锆石年龄谱对比及意义

中朝古陆(华北古陆)平南盆地面积~25000km~2,位于朝鲜半岛中部,发育从中元古界到下古生界地层,但经历了低级变质作用(绿片岩相及以下)。变质基底岩石中有一套角闪岩相-麻粒岩相的变质的古元古界地层。本文根据盆地不同时代沉积岩碎屑锆石/变质锆石U-Pb LA-ICP MS年龄数据讨论沉积源区的变化,并对区域演化进行制约。甑山群/杂岩为盆地基底岩系,变质砂岩样品中碎屑锆石出现ca.2500~2100Ma的年龄峰值。另外,36.5亿年的碎屑锆石是朝鲜迄今发现的最古老碎屑锆石;夕线榴片麻岩样品记录了~1850Ma(1859±9Ma)的变质年龄;推测甑山群沉积于ca.2100~1900Ma,变质于1850 Ma。黄海群局限分布于朝鲜半岛中部,碎屑锆石年龄谱显示~1850 Ma的峰值,可见~1250 Ma的年龄,推测对应物源为古元古代基底岩浆岩和变质岩系;结合其上覆直岘群的沉积时代,推测地层沉积于ca.1250~1000Ma。直岘群是平南盆地分布最广的地层之一,底部长峰组样品显示明显的~1850Ma的峰值,而其上第二个和第三个组则显示明显的ca.1400~1600Ma和ca.1000~1200 Ma年龄峰值,~1850 Ma年龄很少;推测直岘群开始沉积时,物源主体是盆地基底岩系,但之后出现大量中元古代物质;推测其沉积时代为ca.1000~900Ma。黄州群有~1850Ma和~2500Ma的峰值,另外,还有较少的ca.1000~1200Ma及1400~1600 Ma年龄,表明沉积物源主体仍是基底岩系,可能有中新元古代沉积岩(黄州群-直岘群)的再沉积。这些沉积岩碎屑锆石年龄峰值与辽东和山东半岛沉积地层相似,并且中新元古代地层中均有大量1000~1200Ma及1400~1600Ma的物质,推测可能来自华北古陆之外,如圣弗朗西斯科克拉通。

Comparison on ages of detrital zircons from the Paleoproterozoic to Lower Paleozoic sedimentary rocks in the Pyongnam Basin, Korea.

The Pyongnam Basin of the Sino-Korean paleocontinent (the North China paleocontinent) is about~25000km² and lies in Middle Korean Peninsula, with the low-grade metamorphosed (up to greenschist facies) Mesoproterozoic to Lower Paleozoic strata. There are also the Paleoproterozoic high-grade (amphibolite to granulite facies) strata in the basement. This paper reports LA-ICP MS U-Pb ages of detrital/metamorphic zircons to constrain the provenances of different strata, with implications for the regional evolution. The Jungsan Group/Complex belongs to the basement of the basin. The meta-sandstone shows distinct ca.2500~2100Ma ages, and the~3650Ma age was the oldest age known so far in Korean Peninsula. The sillimanite-garnet-bearing gneiss shows a prominent~1850Ma age peak (1859±9Ma), which likely represents the age of metamorphism. It is thus suggested that the Jungsan Group was formed at 2100~1900Ma and metamorphosed at~1850Ma. The Hwanghae Group distributes only in the center of Korean Peninsula, detrital zircons show significant~1850Ma age peak, with minor~1250Ma ages, and their provenance could be the Late Paleoproterozoic igneous and metamorphic rocks. Considering the deposition age of the Jikhyon Group, the Hwanghae Group was possibly deposited at 1250~1000Ma. The Jikhyon Group is widely distributed in the basin. Ages of detrital zircons from the first formation, the Jangbong Fm., show a distinct~1850Ma peak, but there are only ca.1000~1200Ma and ca.1400~1600Ma peaks with few ages >1800Ma in the samples from the second and third groups, this indicates that the initial provenance of this group was the Paleoproterozoic basement, but then changed to the Mesoproterozoic rocks. It is further concluded that the deposition age of the Jikhyon Group is ca.1000~900Ma. The Hwangju Group has two age peaks, i.e., the~1850Ma and the~2500Ma, with minor ca.1000~1200Ma and 1400~1600Ma ages, this indicate that the provenance is the Late Archean-Paleoproterozoic basement with some re-deposited materials possibly from the Mesoproterozoic sediments (e.g., Jikhyon Group and/or Hwanghae Group). These age spectrums of the Pyongnam Basin are quite similar to those coeval sediments in the Liaodong and Shandong peninsulas, and the Mesoproterozoic-Neoproterozoic strata show distinguish ca.1000~1200Ma and ca.1400~1600Ma provenances. These rocks might be from other block(s) such as the São Francisco craton other than the North China continent itself.