苏北盆地高邮凹陷始新统戴南组一段稀土元素特征及其物源指示意义

REE Characteristics of the 1st Member of Eocene Dainan Formation in Gaoyou Depression of North Jiangsu Basin, and Its Significance in Provenance Instruction

稀土元素(REE)的物源对比表明,苏北盆地高邮凹陷戴南组一段沉积岩的物源以其西侧张八岭隆起的元古宙浅变质岩基底为主,并与大别和苏鲁造山带的浅变质岩基底存在亲缘关系,推断研究区的母岩类型为高钾I型花岗片麻岩。而张八岭隆起的新元古代细碧—石英角斑岩和绿片岩、中生代火成岩、大别山南部大范围的榴辉岩以及宁镇山脉的中生代中酸性侵入岩对研究区的物源影响很小。在高邮凹陷内部的物源方向分析中,轻重稀土元素分馏值[(La/Yb)N]与矿物成熟度指数(MMI)形成很好的吻合,(La/Yb)N值结合稀土元素总量(∑REE)及铕异常值(δEu)可对研究区内物源方向的精确判别具有指示意义。戴南组一段沉积时期,高邮凹陷的物源主要来自4个方向:西北部柘垛低凸起方向、东部吴堡低凸起方向、南部通扬隆起方向和西南部菱塘桥低凸起方向。高邮凹陷戴南组一段的沉积相类型与REE特征的相关性分析显示,重稀土元素分馏值[(Gd/Yb)N]与沉积相间存在较为规律的变化,表现在近岸水下扇相、扇三角洲相和三角洲相随其碎屑岩的成熟度升高,(Gd/Yb)N平均值呈逐渐升高的趋势,该趋势同样表现在相同沉积相的不同沉积亚相之间,说明(Gd/Yb)N平均值可较好地反映沉积相特征。

REE compositions in the Gaoyou Depression, North Jiangsu Basin, eastern China, are compared to the REE fingerprints of their possible sources. The REE patterns show that source rocks were mainly derived from the Proterozoie granite--gneiss from a low-grade metamorphic basement of the Sulu--Dabie orogen belt and the Zhangbaling Uplift to the west. It is inferred that the source rocks in studied area are I-type high potassium granitic gneiss. Whereas, the Neoproterozoic spilite--quartz-keratophyre and the Mesozoic igneous rocks widely distributed in the Zhangbaling Uplift, the eclogite distributed in the southern Dabie Mountains and the Mesozoic intermediate-- acid intrusive rocks widely outcropped in Ning--Zhen （Nanjing--Zhenjiang） Mountains have negligible effect on provenance of the Gayou Depression. Source direction analysis discovered that the ratios of light REE （LREE） to heavy REE （HREE） [ （La/Yb）N ] are coincident well with the mineralogical maturity index （MMI）. The （La/ Yb） N values can correspond well with the sum of REEs （ ∑ REE）, and the Eu anormaly values （δEu） can be used in exact judgement of source direction. In period of E2d1 , source rocks in the Gaoyon Depression mainly come from the next four orientations： the northwestern Zheduo Salient, the eastern Wubao Salient, the southern Tongyang Uplift and the southwestern Lingtangqiao Salient. Correlative analysis between sedimentary subfacies and REE parameters reveals that, fractional degree of REE[ （Gd/Yb） N ] changed regularly with sedimentary facies. The average （Gd/Yb）N values increase with the maturity indexes of detrital rocks decreasing from delta to fan delta and to nearshore subaqueous fan. This trend is also revealed in different sedimentary subfacies of the same sedimentary facies. Hence, REE parameters especially the （Gd/Yb）N values can well respond to the types of sedimentary subfacies.