METABENTONITES FROM EARLY CAMBRIAN MEISHUCUNIAN STAGE IN EASTERN YUNNAN, CHINA AND THEIR IMPLICATIONS

滇東早寒武世梅樹村期變斑脫岩及其地質意義

張俊明(JUNMING ZHANG) ； 虞子冶(ZIYE YU) ； 朱茂炎(MAOYAN ZHU) ； 周傳明(CHUANMING ZHOU) ； 李國祥(GUOXIAN LI)

《Collection and Research》 10期 (1997/12) Pp. 161-173

摘要

The characteristics o f lithology, mineral and geochemistry demonstrate that over ten layers of light color clay beds within phosphatic sequence of the Early Cambrian Meishucunian Stage in eastern Yunnan are volcanogenic metabentonites. These metabentonites representing volcanic eruption events are important isochronous markers for stratigraphic correlation of the area. Metabentonites retain high concentrations of relatively immobile trace elements, particularly Hf, Nb, Y., Th and U in comparison with non-volcanogenic mudstone, and contain elss concentrations of Fe, Ni, Co and Cr that those of the mudstone of the Yu'anshan Formation, TiO_2/AI_2O_3, Ti/Th and Zr/Hf ratios of all metabentonites suggest characteristic of acidic magmas. Both chondrite-normalized REE distribution patterns and middle to large Eu negative anomalies of the metabentonites are similar to those of granites. The temporal and spatial distributions of the metabentonites indicate that volcanic activities occurred frequently during the Early Cambrian Meishucunian Stage. These volcanic eruption events could be related to the separation of an old continent and extension in a intra-plate setting.

關鍵字

metabentonites ； volcanic eruption event ； Meishucunian Stage ； eastern Yunnan

並列摘要

滇東地區早寒武世梅樹村期地層中出現的十多層淺色粘土岩層為酸性火山灰蝕變形成的變斑脫岩，這些變斑脫岩的穩定微量元素Hf、Nb、Y、Th、Zr、含量高於非火山成因的粘土岩，Fe、Mg、Ni、Co、Cr、Cu含量低於節竹寺組泥岩，TiO_2/Al_2O_3、Zr/Hf、Ti/Th比值屬於酸性火山岩範圍，球粒隕石標準化稀土元素配分模式和具中至大的Eu負異常也類似於花崗岩稀土元素配分模式。變斑脫岩在地層序列上和區域上分布顯示了早寒武世梅樹村期火山活動頻繁，它們是火山噴發事件和高分辨事件地層對比的重要標誌。

並列關鍵字

變斑脫岩 ； 地球化學 ； 梅樹村期 ； 雲南東部