准噶尔盆地南缘芦草沟组碳质细粒沉积岩地球化学特征及地质背景

Geochemistry and its geological background of the carbonaceous fine-grained sedimentary rocks in Lucaogou formation,southern Junggar basin,NW China

采用地球化学分析方法,研究准噶尔盆地南缘博格达北缘西段上二叠统芦草沟组碳质细粒沉积岩的地球化学特征、源岩风化程度、古气候和氧化还原环境等。结果表明:各主量元素质量分数变化范围大,SiO_2、Al_2O_3、TiO2质量分数均值由下段往上段升高,Fe_2O_3~T、MgO、Na_2O质量分数均值由下段往上段降低。SiO_2与Al_2O_3、K_2O质量分数呈较好的正相关关系,与CaO、MgO质量分数呈较好的负相关关系;Al_2O_3与K_2O质量分数呈较好的正相关关系,与CaO、MgO质量分数呈较好的负相关关系;CaO与MgO质量分数呈较好的正相关关系,与K_2O质量分数呈较好的负相关关系。微量元素Ga、Nb、Ta、Rb、Zn与Al_2O_3质量分数呈较好的正相关关系。稀土元素总量(REE)差异大,轻、重稀土元素分异较明显,但分馏弱于澳大利亚后太古代页岩(PAAS)。自下而上,芦草沟组源岩遭受的化学风化作用越来越强,下段源岩弱风化,中段源岩逐渐变为低风化,上段源岩过渡为中等风化。沉积早期至晚期,芦草沟组气候由略为干热转变为温湿,各段碳质细粒沉积岩沉积于水体分层不强的缺氧还原环境。

Through the geochemical analysis,we studied the geochemical characteristics,source weathering,provenance,paleoclimate and the redox environmental setting of the carbonaceous fine-rained sediment in the western part of the Bogda mountain in the southern Junggar basin.The results show that:there is a wide range about the content of every major element and the average content of SiO_2,Al_2O_3,TiOT2 gradually increase from lower to upper member,but the content of Fe_2O_3,MgO,Na_2O gradually decrease from lower to upper member;Al_2O_3,K_2O display positive correlations with SiO_2;CaO,MgO display negative correlations with SiO_2 and Al_2O_3;K_2O displays positive correlations with Al_2O_3;CaO displays positive correlation with MgO and negative correlation with K_2O.Some trace elements,such as Ga,Nb,Ta,Rb and Zn display strong positive correlations with Al_2O_3.The variation of total rare earth element and the differentiation between LREE and HREE are obvious,but the fractionation between LREE and HREE is weaker than PAAS.From the lower to middle Lucaogou formation,the chemical weathering in the provenance increased from weak to moderate,and it became to be even stronger till the upper member.From early to late of Lucaogou formation,the paleoclimate changed slightly from dry to a humid,and each member's fine-grained carbonaceous sediments are deposited within reducing environment in unstratified water.