滇西南昌宁—孟连构造带火山岩地层学研究

昌宁—孟连构造带火山岩非常发育,是重要的含矿岩系。在构造带南段,火山岩统称为依柳组,北段统称为平掌组下段,以往时代均归属早石炭世。本文工作表明,该区火山岩可分为两套:一套为早石炭世火山岩,分布于该构造带西部,以拉斑玄武岩系列为主,代表大洋中脊、洋岛残留物,仍称为依柳组;另一套为晚二叠世火山岩,分布于构造带中、东部,以碱性玄武岩系列为主,可能属亚速尔型洋岛火山岩范畴,而非大洋蛇绿岩套,新命名为老厂组。

STRATIGRAPHIC INVESTIGATION OF THE VOLCANIC ROCKS IN CHANGNING-MENGLIAN TECTONIC BELT,SOUTHWESTERN YUNNAN, CHINA

The Changning-Menglian belt, located between Baoshan massif and Lincang massif in southwestern Yunnan, represents the main branch of Palaeo-Tethys polyisland ocean in Hercynian-Indosinian stage. The volcanic rocks well-developed in the belt, are important lithostrata containing mineral deposit and main part of Changning-Menglian ophiolite belt. According to the results of stratigraphy, paleontology and petrology in recent years, the volcanic strata is subdivided into Yiliou Formation of Early Carboniferous and Laochang Formation of Late Permian. Yiliou Formation is only distributed in western part of the belt, and includes the vol- canic strata in Manxin, Yiliou, Tongchangjie and so on. It is subdivided into lower member which is composed of basalts and less volcaniclastic rocks, radiolaria-bearing siliceous rocks, and upper member which consists of volcaniclastic rocks and less basalt, sandstone, shale and limestone. The formation is overIaid in conformity by limestone of Middle Car- boniferous. The volcanic rocks belong to tholeiitic basalt series on the basis of petrochemi- cal study and they should be formed in mid-oceanic ridge and oceanic island within ocean ba sin. Laochang Formation is distributed in middle and eastern parts of the belt, and com- posed of basalt, andesites, tuffaceous rocks and less radiolarian cherts, siltstone. The for- mation is overlaid by Carboniferous limestone in fault, and may be conformity with Zhongzhai Formation. Petrochemica1 studies show that they belong to alkaline basalt se- ries. The genetic background forming the rocks may be determined in the Azores-type o- ceanic is land.