晚中新世红河海底扇碎屑锆石SHRIMP U-Pb年龄：物源区制约及红河袭夺事件探讨

SHRIMP U-Pb ages of detrital zircons: Discussions on provenance control and the Red River Capture Event

关键词:  红河海底扇  碎屑锆石  SHRIMP U-Pb年龄  物源区  红河袭夺

Key words:  Red River submarine-fan  detrital zircon  SHRIMP U-Pb age  provenance  Red River Capture

摘要:

为确定晚中新世红河海底扇物源区以及“红河袭夺”事件，对钻遇红河海底扇的C2井中的53颗锆石颗粒进行了SHRIMP U-Pb年龄的测定。结果表明，该扇体碎屑锆石存在6个年龄段：230~300Ma，400~500Ma，700~1000Ma，1700~1900Ma，2400~2500Ma和~2700Ma。研究结果表明，红河海底扇碎屑锆石年龄频谱特征与红河最为相似，230~300Ma的锆石主要来自呵叻高原-昆嵩地块；400~500Ma、1700~1900Ma的锆石主要来自松潘甘孜地块；700~1000Ma的锆石来自扬子克拉通；2400~2500Ma和~2700Ma的锆石主要来自松潘甘孜地块古老的扬子克拉通的基底；年龄为400~500Ma、1700~1900Ma以及大于2.4Ga的锆石说明了古红河源头曾经延伸至松潘-甘孜地区，红河袭夺现象确实存在。红河物源的存在，为在琼东南盆地深水区中央峡谷水道实施油气勘探提供了重要的理论基础。

Abstract:

A huge Red River submarine fan has been discovered recently in Upper Miocene along the northwestern margin of the South China Sea. To further explore the provenance of the submarine fan and the controversial issue of the Red River capture, SHRIMP U-Pb ages of detrital zircons have been measured using the samples recovered from the well S which intersected the fan. The result shows:1. Six main age intervals have been recognized in the zircon grains of the Red River submarine fan, which are 230-300Ma, 400-500Ma, 700-1000Ma, 1700-1900Ma, 2400-2500Ma and ~2700Ma, respectively. The age distribution spectrum of zircons displays a good similarity with the Red River drainage area. It thus can be inferred that the zircons between 230-300Ma are mainly from the Khorat Plateau-Kon Tum block. Zircons of 400-500Ma and 1700-1900Ma are linked with the Songpan-Ganze block, and the 700-1000Ma zircons are considered to be from the Yangtze Craton. Zircons with the age of 2400-2500Ma and ~2700Ma are related with the ancient basement of Yangtze Craton within Songpan-Ganze block. 2. The zircon grains of the Red River submarine fan, including the ages of 400-500Ma, 1700-1900Ma and older than 2.4 Ga, are proved to exist in the Songpan-Ganze block. It indicates that the Red River had once extended into the Songpan-Ganze block, holding a far more lager drainage area than today. The general decrease of the drainage area today implies that the Red River be once captured. 3. The samples of zircon grains are mainly from the late-Miocene Red River fan, suggesting the Red River had not been captured by the Late Miocene. Once took place, this capture would be inevitable to cause a dramatic decrease in the drainage area and the according loss of the sediment supply, both of which would be recorded in the basin fill. Subsequently, the timing of the Red River capture can be identified through the careful investigation on the evolution history of the Red River depositional systems, as well as the calculations of parameters such as the sediment flux or total volume.