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华南下寒武统一直引起地质学家们的关注。最近,于贵州织金戈仲伍及金沙岩孔2条剖面的下寒武统牛蹄塘组底部发现数层粘土岩,采用X射线衍射分析方法(XRD)、X荧光光谱法(XRF)及电感耦合等离子体质谱法(ICP-MS)对其进行了矿物学和地球化学研究。矿物学分析显示所研究的粘土岩的矿物组合包括:伊利石、伊利石-蒙脱石混层矿物、蒙脱石、高岭石、石英、云母、角闪石、长石、锆石等。主量元素含量分析显示粘土岩w(K2O)介于4.05%～5.75%之间,平均值为4.76%,明显高于典型的以蒙脱石为主要矿物的斑脱岩。粘土岩的微量元素含量特征表现为:相对于地壳页岩,其亲石元素Th、Y、Nb、Zr、Hf、Ga、U等含量较高,亲铁元素Ni、Co含量较低,火山活动产物中的标志元素As含量亦较高。球粒陨石标准化多元素蜘蛛图解显示所有粘土岩样品相对亏损Rb、Nb、Pb、Sr,相对富集Ba、U;球粒陨石标准化稀土元素配分图显示它们具有右倾的轻稀土元素富集、重稀土元素亏损且具负Eu异常的稀土元素配分模式。矿物学及地球化学特征表明它们成因上属于由火山凝灰物质经水解、成岩及蚀变作用形成的钾质斑脱岩,代表了早寒武世时期于贵州地区发生的3期火山活动。通过Nb/Y-Zr/TiO2图解判别,认为研究区钾质斑脱岩的原始岩浆性质为中酸性岩浆;进一步通过(Y+Nb)-Rb构造判别图解投影,获得了钾质斑脱岩的源火山喷发于板内拉张环境的地球动力学信息。由于新发现的钾质斑脱岩与华南下寒武统Ni-Mo-PGE矿床存在密切的时空联系,因此,我们认为钾质斑脱岩的发现为解释由火山作用贡献了成矿物质的矿床成因观点提供了新的研究线索。
The Lower Cambrian in South China has aroused the attention of geologists. Recently, several layers of clay rock were found at the bottom of the Lower Cambrian Niutitang Formation at two sections of the Zhijie Gezhongwu and Jinshayanzones in Guizhou. The XRD, XRF and inductive coupling Plasma-mass spectrometry (ICP-MS) carried out mineralogical and geochemical studies. Mineralogical analysis shows that the mineral assemblages of clay rocks under study include illite, illite-montmorillonite mixed mineral, montmorillonite, kaolinite, quartz, mica, amphibole, feldspar, zircon and the like. Analysis of main elements shows that clay mineral w (K2O) is between 4.05% and 5.75% with an average value of 4.76%, which is obviously higher than the typical bentonite with montmorillonite as the main mineral. The content of trace elements in clay rocks is characterized by higher contents of Th, Y, Nb, Zr, Hf, Ga, U and other elements relative to crustal shale, low contents of Ni and Co, Product logo element As content is also higher. The chondrite normalized multielement spider diagram shows that all claystone samples are relatively depleted in Rb, Nb, Pb, Sr and relatively enriched in Ba, U; the chondrite normalized REE maps show that they have a right-leaning LREE enrichment by weight Rare earth elements with depletion of rare earth elements and Eu anomalies. Their mineralogical and geochemical characteristics indicate that they are genetically belong to the karatitic bentonites formed by the hydrolysis, diagenesis and alteration of volcanic tuffaceous material, representing the three volcanic activities that took place in Guizhou during the Early Cambrian. According to the graph of Nb / Y-Zr / TiO2, it is considered that the original magmatic property of potassic plagioclase rocks in the study area is medium-acid magma. By means of the (Y + Nb) -Rb tectonism, EARTH DYNAMICS OF SOURCE VOLCANO IN EARTH PLACEMENT. Because of the close temporal-spatial relationship between the newly discovered K-Pb rocks and the Lower Cambrian Ni-Mo-PGE deposits in the South China, we believe that the discovery of potassic plagioclase to explain the mineralization contributed by volcanism Genesis point of view provides a new clue.