河南嵩山地区元古宙五佛山群沉积岩的地球化学特征及其对物源区和构造环境的制约
发布时间：2016-02-29 10:11:29 点击：541
【摘要】：河南嵩山地区位于华北克拉通南缘,保存有较为完整的太古宙-古元古代变质结晶基底和之后的沉积盖层。五佛山群直接角度不整合覆盖于新太古界登封群和古元古界嵩山群之上,为嵩山地区结晶基底之上分布广泛的第一沉积盖层。其主要由(长石)石英砂岩组成,夹少量的粉砂质页岩、泥岩和薄层灰岩。从下而上岩性相变明显,代表两次较大的沉积旋回。地球化学特征上,五佛山群砂岩的SiO2含量(70.18%～95.21%)明显比泥质岩(20.79%～52.27%)高,但Fe2O3、MgO、CaO和K2O含量比泥质岩低,表明泥质岩中含有较多的基性组分和富钾矿物。泥质岩的稀土总量(∑REE=131×10-6～169×10-6)比砂岩(∑REE=7.30×10-6～158×10-6)高,反映泥质岩中较高含量的粘土矿物对稀土元素具有较强的吸附作用。砂岩样品的成分变异指数(ICV=0.89～3.59)较低,说明其成熟度较高,为稳定构造环境下沉积的产物。泥质岩的ICV值为4.04～17.50,SiO2/Al2O3比值(3.90～5.58)和La/V比值较低(0.45～0.65),说明其成熟度较低,形成环境相对比较活动。较低的化学蚀变指数(CIA=7.52～66.1)反映碎屑沉积岩的源区遭受相对较弱的化学风化作用,其中砂岩(CIA=27.6～66.1)的化学风化强度比泥质岩(CIA=7.52～24.9)高。五佛山群碎屑沉积物主要来源于古元古代中酸性组分,有少量基性组分的加入。其下部和中部的砂岩与华北南缘同时代的沉积地层均形成于被动大陆边缘的沉积环境,说明华北克拉通南缘在熊耳群之后进入了一个稳定的沉积阶段。而五佛山群上部泥质岩形成于与大陆岛弧相关的沉积盆地,显示地壳活动性增强,沉积环境上暗示由被动大陆边缘向活动大陆边缘过渡的过程,可能与秦岭造山带形成初期的板块活动有关。
Abstract The Archean-Paleoproterozoic metamorphic crystalline basement and subsequent sedimentary cover well outcrop in the Songshan area along the southern margin of the North China craton (NCC). The Wufoshan Group, covering on the Dengfeng Group and songshan Group with angular unconformity, is the earliest sedimentary stratum extensively distributed in Songshan area. Sandstones have higher SiO2 contents (70.18％~95.12％) than mudstones (20.79％~52.27％),with relatively lower Fe2O3, MgO, CaO and K2O contents, indicating more basic constitutes and potassium-rich minerals in mudstones. All sedimentary samples show that the light rare earth elements (LREE) are enriched relative to the heavy REE with moderately negative Eu anomalies. Mudstones have higher total REE (ΣREE131×10-6~169×10-6) than sandstones (ΣREE7.30×10-6~158×10-6), implying the absorption of REE by clay minerals abundant in fine sediments. The lower index of compositional variability (ICV=0.89~3.59) of sandstones indicate they have higher grade of maturity and formed in a relatively stable tectonic setting, whereas the higher ICV values (4.04~17.50) and lower SiO2/Al2O3 (3.90~5.58) and La/V ratios (0.45~0.65) of mudstones indicate that they have relatively lower grade of maturity and formed in an active sedimentary basin. The chemical index of alteration (CIA=7.52~66.1) of the Wufoshan Group samples display that they have experienced low grade of chemical weathering and alteration .The petrologic and geochemical signatures show that the sedimentary rocks of Wufoshan Group sourced from post-Archean medium-acid components with minor basic rocks. Sandstones in the middle and lower part of the Wufoshan Group formed in a passive continental margin , the same as contemporary sedimentary sequences in the southern margin of the NCC ,implying that the tectonic setting shifted to a stable sedimentary stage after the eruption of Xiong’er Group volcanic rocks .However , mudstones in the upper part formed in a sedimentary basin related to a continental arc. More crustal activities and distinct changes in sedimentary environments from Mesoproterozoic to Neoproterozoic indicated the transition from passive to active continental margin , a possible effect of the early tectonic events of the Qinling Orogenic Belt .
Key words Sedimentary rock ; Geochemistry ; Provenance ; Tectonic setting ; Wufoshan Group ;North China Craton
