四川盆地南北缘志留系的锶和碳、氧同位素演化及其地质意义

Strontium,carbon and oxygen isotopic compositions andtheir geological significance:the Silurianstrata on the northern and southernmargins of the Sichuan Basin

通过对四川盆地北缘广元地区及南缘綦江地区上奥陶统及下志留统地层的８７Ｓｒ／８６Ｓｒ、δ１３Ｃ及δ１８Ｏ的系统测定，取得了沿地层剖面变化的同位素组成规律。研究结果表明，在盆地南缘８７Ｓｒ／８６Ｓｒ值高于地质历史中海水的平均值，其原因是本区为受板块内古陆控制的缓坡环境，大量陆源锶的混入增加了８７Ｓｒ／８６Ｓｒ的值。８７Ｓｒ／８６Ｓｒ值在上奥陶统至下志留统地层交界处、Ｒｈｕｄｄａｎｉａｎ／Ａｅｒｏｎｉａｎ（鲁丹期／艾隆期）交界处具正峰波动，反映了此时海平面的短时下降。但从Ｒｈｕｄｄａｎｉａｎ至Ｔｅｌｙｃｈｉａｎ（特里奇期），８７Ｓｒ／８６Ｓｒ逐渐降低。从Ａｅｒｏｎｉａｎ至Ｓｈｅｉｎｗｏｏｄｉａｎ（舍因伍德期）早期，盆地南北缘的δ１３Ｃ值处于逐渐上升的过程之中，而δ１８Ｏ则相反。同位素的演化特征说明本区当时为海进时期，海平面不断上升，与全球性海平面变化相吻合。由于受区域构造运动的作用，Ｓｈｅｉｎｗｏｏｄｉａｎ之后的全球持续海进对本区无影响。

オhe Sr,C and O isotopic compositions across the stratigraphic section have been obtained in the light of the systematic determinations of 87Sr/86Sr,δ13C and δ18O values for the Upper Ordovician-Lower Silurian strata in the Guangyuan region of the northern margin and Qijiang region of the southern margin of the Sichuan Basin.On the basis of the isotopic data in combination with the sedimentary facies and tectonic data and the published information about the global Silurian regressivetransgressive events,the following conclusions can be drawn:1.The 87Sr/86Sr values are commonly higher than the avearge values of δ87Sr for sea water in the geologic records on the southern margin of the basin.This is because the study area was in a ramp setting governed by intraplate palaeocontinent,and the addition of a large amount of terrigenous strontium led to the increase in 87Sr/86Sr values.2.The 87Sr/86Sr values are positively fluctuated at the Upper Ordovician-Lower Silurian boundary and Rhuddanian-Aeronian boundary,indicating a temporary fall of sea level at that time.The 87Sr/86Sr values decreased gradually in the period from the Rhuddanian to Telychian.3.The δ13C values increased gradually on both the northern and southern margins from the Aeronian to early Sheinwoodian,while the things for δ18O values are contrary.4.The isotopic evolution shows that the Upper Yangtze area was influenced by the transgressive events from the Rhuddanian to early Sheinwoodian,when the sea level was rising in response to the global sealevel changes.However,the persistent worldwide transgressions follwing the Sheinwoodian didn't affect this area due to regional tectonism.