【摘要】 笔者等在内蒙古白云鄂博铁铌稀土矿床东南约25km处的达尔罕茂明安联合旗黑脑包腮林忽洞群下部发现一个厚30余米、呈北西向延伸的滑塌岩块。滑塌岩块的岩性主要为薄层黄灰色微晶灰岩与青灰色泥质微晶灰岩互层,单层厚多为1～2cm。风化面上可见青灰色泥质灰岩层抗风化能力较强,呈突出状。青灰色薄层泥灰岩在宏观上即见大量层间破碎,几乎层层都被小的同生断裂错开,有的薄层被分割成叠瓦状的碎块,有的被邻层的泥质贯入而分割成一个个竹节状块体(即布丁构造)。同生小断裂和布丁构造为同沉积期的地震造成,因此,发育同沉积布丁构造的薄层微晶灰岩可以看作是典型的震积岩。整个岩块发生了强烈的褶皱变形,且变形强度自下而上逐渐变弱。岩层在下部强烈褶皱,向上规模和强度都逐渐变小,直到变为未变形的平直岩层,具有明显的不协调性。这种褶皱是岩块受地震影响而整体滑塌过程中的变形记录。震积岩和滑塌褶皱都揭示了盆地边界的拉张背景,同时指示盆地自南东向北西水体变深的地貌格局。这一发现表明,腮林忽洞群沉积的早期就已有地震事件发生,与已知的腮四组震积岩和腮五组微晶丘一起,表明腮林忽洞群沉积时期,本地先后至少有两个阶段处于不稳定的地震活跃阶段。腮林忽洞群可能相当于白云鄂博群的下部,后者中产著名的白云鄂博铁铌稀土矿床,成矿的同位素年龄值有2.1～0.4Ga多种不同的结果,但多位微体古生物学者均认为是震旦纪至奥陶纪,且在腮林忽洞群中还发现了三叶虫碎片,故作者等在本文取信腮林忽洞群为寒武纪—奥陶纪。在腮林忽洞群顶部产有厚近百米的微晶丘,与白云鄂博矿区的赋矿白云岩相当,可能均是热水沉积形成。所以,本文对于认识白云鄂博地区腮林忽洞群沉积时(寒武纪—奥陶纪?)的大地构造性质及白云鄂博矿床的成因具有重要意义。更多还原

【Abstract】 A 30 m-thick block of earthquake slump-fold block was discovered in lower part of the Sailinhudong Group (Cambrian—Ordovician?), Heinaobao, Darhan Muminggan Qi, 25 km to the southeast of famous Bayan Obo Fe—Nb—REE deposit, Inner Mongolia, northern China. The slump block is different from the underlying and overlying strata in appearance and compositions, so it should have been an exotic block derived from another shallower sedimentary facies when deposited. The slump-fold block is composed of two kinds of very thin layers, pale-gray micrite (microcrystalline limestone) of 1~2 cm in thickness interbedded with gray muddy micrite layers with the same thickness. Almost every thin muddy micrite layer was cut into imbricate blocks or boudins by abundant tiny contemporaneous faults, while the interbedded micrite remains in the original continuity. Boudins form as a response to layer-parallel extension and/or layer-perpendicular flattening of stiff layers enveloped on top and at the bottom by mechanically soft layers. In this case, the imbricate blocks cut by the tiny contemporaneous faults are the result of horizontal extension of the crust in the SE-NW direction accompanied with earthquakes. Thus, the rock block is, in fact, a kind of seismites. The strata in the lower to middle part of the slump block are strongly folded while the strata in the upper part remain straight. In other words, the block shows gradual upward transition from strongly folded strata, through mildly folded strata, to the undisturbed strata. This means that the folds are not the result of later tectonic deformation but only slump-folds induced by a large earthquake. The orientation of the fold axes are in accordance with the direction of imbricate tiny blocks as well as that of the foreset beds of the sandstone in the bottom part of the Sailinhudong Group, showing that the basin bottom was higher in the southeast side and deeper in the northwest side. The seismites and the slump-fold indicate that there was at least a strong earthquake belt in the southeast side during the early stage of the Sailinhudong Group. It is considered that the Sailinhudong Group is equivalent to the lower part of the Bayan Obo Group, which is the host rock of the Bayan Obo Deposit. Even through there many isotopic ore\|forming ages from 2.1~0.4 Ga, all conclusions about the time of Bayan Obo Group or Sailinhudong Group by micropalaeontologists are from Sinian to Ordovician. And at the top of the Sailinhudong Group there are a near 100m\|thick mudmound, siminar to the ore\|hosted dolostone in Bayan Obo orefield, may be sedimented from hotwater at sea bottom. So, this discovery may be a good constraint on the tectonic evolution of the Bayan Obo area, Inner Mongolia, northern China for the Early Paleozoic and the genesis of the Bayan Obo deposit.更多还原