湖盆中心地带湖底扇砂体物源追踪及地质意义

湖盆中心地带湖底扇砂体物源方位具多向性，发育位置和层位具不确定性，规模和分布特点不一，确定物源方位成为掌握砂体发育规模、展布的关键。以泌阳湖盆中心地带程店湖底扇砂体为例，利用碎屑锆石LA-ICP-MS U-Pb定年技术，探讨其物源方位。分析结果显示: 程店湖底扇区C1、B365井与侯庄三角洲区B155、B296井砂岩样碎屑锆石测点年龄数据分布具相似特征，碎屑锆石表面年龄主要集中分布在早白垩－中侏罗世峰期,年龄为120～140 Ma，显示单峰特征。早白垩－中侏罗世两区锆石均显示两组类型：第一组锆石占主体部分，呈自形晶，并具典型的振荡生长环带，Th/U值介于0.21～2.94，暗示岩浆成因；第二组类型锆石相对较少，显示变质成因。同时在210～238 Ma、416～451 Ma、663～813 Ma、1 702～2 442 Ma 几个主要年龄区间亦具一定数量的碎屑锆石分布。结合区域地质背景，得出如下初步结论：程店湖底扇砂体与北部侯庄三角洲砂体为同物源沉积组合，显示南北向展布，源岩主体形成时代为早白垩－中侏罗世，对应燕山构造阶段；主体物源岩系为伏牛山燕山期花岗岩，含该时期热变质作用形成的变质岩系，同时前晋宁期、晋宁期、加里东期及印支期热事件形成的变质岩系或岩浆岩亦可能具不同程度的贡献。

Source Tracing and Its Geological Significance of Sub-Lacustrine Sandbody in the Center of Lake Basin

The source area of sub-lacustrine sandbody in the center of basin is generally multidirectionality,with uncertain position and horizon,and different scale and distribution.Determining the direction of provenance quickly and accurately is the foundation and the key in mastering sandbody scale and distribution.The authors took the Chengdian sub-lacustrine sandbody in Biyang Lake basin for example,studied the direction of provenance and sandstone distribution based on clastic zircon dating analysis of sandstone samples from well C1 and B365 in Chengdian sub-lacustrine and samples from well B155 and B296 in the Houzhuang delta.The result shows:zircons microareaapparent age of Chengdian sub-lacustrine and Houzhuang delta mainly similarly distributes between Early Cretaceous and Middle Jurassic with single age peak of 120-140 Ma.Zircons during this period of two areas can be divided into two types.The first type occupies the main part.Zircons of the first type are euhedral in shape and show the typical oscillatory zoning,which indicates their magmatic origin.Their Th/U ratios ranging from 0.21 to 2.94 implies that they derived from crystallization of magma.The second type is metamorphic with relatively small number.Also,there are some zircons distributing in the age populations of 210-238 Ma,416-451 Ma,663-813 Ma and 1 702-2 442 Ma.In combination with regional geological settings,the authors come to the following preliminary conclusions: the sub-lacustrine sandbody derived from Houzhuang delta and the sub-lacustrine sandbody distributes from north to south.The main formation period of source rock is Early Cretaceous to Middle Jurassic,corresponding to Yanshan tectonic stage.Moreover,the source area is mainly Yanshanian granite in Funiushan region,including Yanshanian metamorphic rocks and the metamorphic rocks or magmatic rocks of Prejinningian period,Jinningian period,Caledonian period and Indosinian period may offer contributions at different degree.