鄂尔多斯盆地南缘上奥陶统赵老峪组岩石磁学研究

Rock magnetism of the Upper Ordovician Zhaolaoyu Formation of the southern Ordos Basin

利用古地磁学手段定量约束华北板块早古生代古地理位置可为原特提斯造山作用及华北板块与冈瓦纳大陆的亲缘性研究提供重要科学依据。但华北板块早古生代以来经历了多期构造运动,其西部鄂尔多斯盆地含丰富油气资源,后期构造运动和油气运移均可能导致其早古生代地层遭受重磁化影响。对鄂尔多斯盆地南缘富平地区上奥陶统赵老峪组灰岩、凝灰岩样品进行了系统岩相学、岩石磁学及逐步退磁研究,包括光学显微镜鉴定实验、扫描电镜(SEM)实验、能谱分析(EDS)实验、饱和等温剩磁(SIRM)实验、三轴等温热退磁实验、磁化率随温度变化(κ-T)实验、磁滞回线实验、一阶反转曲线(FORC)实验、系统热退磁实验以及热-交混合退磁实验,探讨了各类岩石的主要载磁矿物及其是否具备记录原生剩磁信息的潜能。结果表明:灰岩及凝灰岩样品的主要载磁矿物为磁铁矿、磁黄铁矿和(或)胶黄铁矿,部分样品还含少量赤铁矿(含杂质)和针铁矿,矿物颗粒的磁畴类型同时含有SD(单畴)和MD(多畴)两种。结合岩相学及退磁实验结果认为这些主要载磁矿物可能是与后期流体作用(有机质成熟及运移等)有关的次生矿物,其携带的剩磁信号不能直接用于约束板块早古生代古地理位置,或可用于约束鄂尔多斯盆地南缘下古生界油气运移成藏过程。

[Objective] Palaeomagnetic methods can be used to quantitatively constrain the Early Palaeozoic palaeo-position of the North China Block(NCB),which provides a crucial scientific basis for researching the evolution of the Proto-Tethyan Orogeny and the affinity of the NCB for Gondwana.However,the NCB has undergone considerable tectonic activity since the Early Palaeozoic,and the Ordos Basin in the western part of the NCB is rich in hydrocarbon resources.Late tectonic activity and the migration of hydrocarbon resources may have caused remagnetization of Early Palaeozoic strata.[Methods] Thus,in this paper,to determine the main magnetic minerals in various rocks and their ability to record the primary remanent magnetization,we carry out a series of detailed petrography,rock magnetism and stepwise demagnetization experiments on the limestone and tuff samples of the Upper Ordovician Zhaolaoyu Formation in the Fuping area of the southern Ordos Basin,including optical microscope identification,scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),saturated isothermal remanent magnetization(SIRM) acquisition curve,three-axis isothermal thermal demagnetization test,κ-T curve test,hysteresis loop experiment,first-order reversal curve(FORC),and thermal demagnetization and hybrid(TD+AF) demagnetization.[Results] The results indicate that the main magnetic minerals in the limestone and tuff samples are magnetite,pyrrhotite and/or greigite,and some samples also contain a tiny quantity of haematite with impurities and goethite.Mineral particles possess single domains(SDs) as well as multiple domains(MDs) within their magnetic domains.Combining petrographic and demagnetization data,this paper suggests that the main magnetic minerals in the samples are likely secondary minerals associated with later fluid alteration(maturation and migration of organic matter).[Conclusion] The remanent magnetic signals conveyed by these rocks cannot be directly used to constrain the Early Palaeozoic palaeoposition of the NCB but may be used to constrain hydrocarbon migration and accumulation in the Lower Palaeozoic strata of the southern Ordos Basin.