滇西昌宁—孟连地区依柳组、平掌组地层初议

<正> 滇西昌宁—孟连出露的火山岩系,1/100万普洱幅(1965)因其伏于中石炭统之下而称之下石炭统,1/20万凤庆幅(1981)、孟连幅(1982)分别创立平掌组、依柳组用以表示北部和南部的早石炭世火山岩,后被广泛采用并认为时代与地层对比问题已经解决(云南省地质矿产局,1990)。近年来有不少专事火山岩研究的工作者,如李涤辉(1989)、杨开辉(1989),也认为是一套地层并视为下石炭统。

A preliminary discussion on the stratigraphy of the Yiliu and Pingzhang Formations in Changning-Menglian Area, western Yunnan

The Changning-Menglian area is situated in the southern segment of the Sanjiang Indo-sinian Orogen, where the well-developed thrusting and nappe-structuring constitutes a complicated pattern. The so-called Yiliu (Menglian region) and Pingzhang(Changning region) Formations might actually be a series of overlapped slabs.The sandwich of limestone and tuff (previously, the upper member of the formations) of Early Carboniferous age, might be overthrusted from the east and make up, together with the Middle-Upper Carboniferous rocks, the nappe outliers in the area. The similar Lower Carboniferous in Simao is called the Manbing Formation. The mafic lavas (previously, the lower member of the formations), according to the recently collected fossils in the sedimentary interlayers and others, could be distinguished, at least, into four components: (1) The Lower Carboniferous mid-ocean ridge basalt,is distributed in the valley of the Nanlei River south of Menglian; (2) The Lower Permian oceanic island tholeiite (Hawaiian-type) is to the east of the Nanlei River. The basalt in Guodazhai might be correlative with it; (3) The basalts in the ophiolite suite is distributed in Tongchangjie and Mengyong. Its age might be Late Permian to Early Triassic;(4) The Laochang volcanic rocks, named by the author, is an alkaline assemblage of olivine basalt, basalt and trachyte andesite. Its tectonic setting might be Late Permian to Early Triassic oceanic islands (Azores-type), with the relevants being rocks in Laba, Sipaishan and Tuanjie. In addition, the Yingpan volcanic rocks, with which the Wenquan volcanic rocks might be correlative, and the Yutangzhai volcanic rocks, are still uncertain on their ages and tectonic settings, and remain to be clarified by further work.It seems that the stratigraphic works in the erogenic area should combine paleontologic data on structure geology, geochemistry, petrotectonics, isotopic dating and sedimentology, and the Changning-Menglian area might present an example to the "orogenic belt stratigraphy".