鄂尔多斯地块基底研究新的思考与认识

New thinking and understanding for the researches on the basement of Ordos Block

鄂尔多斯地块基底岩芯岩石组合、花岗片麻岩的锆石U-Pb年龄、Hf同位素和全岩Nd同位素组成以及航磁异常和地壳速度结构特征表明,该地块中部沿大同-华池基底断裂带存在一航磁异常梯度带,沿该带南缘发育一条2.2~2.0Ga古元古代弧岩浆活动带,并经历了1.96~1.85Ga的变质作用改造,与东缘中部带古元古代中晚期构造演化过程一样,可能代表该地块内部古元古代中晚期形成的一条构造活动带。该活动带以北为东西向展布的正航磁异常带,基底由混合岩化的变泥质和杂砂质片麻岩、片岩及大理岩和石英岩等变质表壳岩和一些花岗片麻岩类构成,记录了新太古代末~2.50Ga和古元古代中期2.2~2.0Ga两期岩浆活动及古元古代末期1.95~1.85Ga变质作用等地质事件,为一主要由新太古代陆壳物质构成的陆块。活动带南部为北东向不规则正、负相间航磁异常带,基底除少量副变质岩外,多为花岗片麻岩类,记录了古元古代中期2.2~2.0Ga岩浆热事件以及1.95~1.85Ga的变质作用改造,除新太古代物质外,还存在古、中太古代陆壳物质,是由一定规模中太古代和部分古太古代陆壳物质以及新太古代新生陆壳添加构成的古老地块。因此,鄂尔多斯地块中部沿大同-华池断裂带成为分割该地块一条重要分界带,其北部地块与其北缘孔兹岩带构成同一构造带,可能代表新太古代陆壳基底之上于古元古代晚期发育的一条大陆边缘活动带。南部则为古、中太古代陆壳与新太古新生陆壳增生而成的古老地块,代表鄂尔多斯地块古老基底陆壳的主体。

Based on the comprehensive petrographic observations for the basement samples from deep drilling cores and their zircon U-Pb geochronological data,Hf isotopes and whole rock Nd isotopes of the granitic gneisses as well as aeromagnetic anomaly and velocity distribution of upper crust,a 2.2~2.0Ga arc-magmatism belt is recognized along the NE trending Datong-Huachi fault which goes with the southern fringe of a gradient belt of aeromagnetic anomaly in the basement of central Ordos Block.This arc-magmatism belt had been experienced 1.95~1.85Ga metamorphism similar to those of central part(Trans-North China Orogen)of North China Craton,leading to a two-fold subdivision of the basement of Ordos Block into the northern and southern parts,rspectively.Of which,the northern part displays an EW trending distribution zone of aeromagnetic anomaly,and contains a basement composed by supracrustal rocks including metapelite and metagraywackes gneisses,schists,marbles and quartzites associated with strong migmatization as well as granitic gneisses.It is a terrane with a Neoarchean basement that underwent multiple phases of 2.5Ga and 2.2~2.0Ga magmatic and 1.95~1.85Ga metamorphic events.The southern part is characterized by a NE trending distribution zone of aeromagnetic anomaly and a basement with the reports of strong modification of the 2.2~2.0Ga granitic magmatism and a 1.95~1.85Ga metamorphism,suggesting an old block with a basement composed of Paleo-to Meso-Archean and Neoarchean juvenal crustal materials.Consequently,we proposed that the Datong-Huachi fault is most likely a main boundary separated the basement of the Ordos Block.The northern part likely constructed an activity belt of continental margin formed on the Neoarchean basement together with Konderite belt to the north during Late Paleoproterozoic.While,the southern part is a block with a basement composed of some Paleo-to Mesoarchean and Neoarchean crust,most likely representing a main part of Ordos Block basement.