The understanding of volcanic rocks of the Lower Carboniferous Liu Yi Formation (southern segment) and the Pingzhang Formation (northern segment) in Changning-Menglian belt is consistent. However, there are different opinions on its tectonic background.

According to the different geographical distribution and lithologic combination, the volcanic rocks in this area are divided into three areas: Yingpan in Fengqing-Tongchang Street in Yunxian County-Yongde Banka District, Mengsheng in Cangyuan-Xiaoheijiang-Lancang Laochang and Manxin in Menglian.

1. Yingpan-Tongchang Street-Banka District

It is a medium-basic volcanic rock. The results of petrochemical analysis show that the volcanic rocks in Tongchang Street belong to calc-alkaline area, the volcanic rocks in Wumulong Highway belong to calc-alkaline and weakly alkaline area, and the volcanic rocks in Banka Yutangzhai and Yingpan belong to alkaline-alkalescent area. In a word, Tongchang Street (low medium titanium) and Wumulong Highway (low titanium) belong to calc-alkaline series and alkaline series, while Yingpan (high titanium) and Yutangzhai (medium high titanium) belong to alkaline series. Trace elements show that Tongchangjie basalt has the genetic characteristics of oceanic ridges, while Yingpan and Yutangzhai are typical intraplate alkaline basalts. Rare earth elements indicate the alkalinity of basalt in Yingpan and Yutangzhai, while there are two kinds of basalt in Tongchang Street: oceanic ridge and alkaline basalt.

2. Meng Province-Xiaoheijiang-Old Factory

This area is mainly basalt and andesite, which are high titanium or high potassium alkaline basalt and belong to intraplate basalt. Trace elements have the characteristics of ocean island (Xiaoheijiang) and intraplate alkaline basalt (Laochang). The distribution pattern curve of rare earth elements is a typical alkaline basalt with high left and low right.

3. Menglianman New District

Manxin basalt can be divided into two sets: one is almost a ridge, which is atypical compared with Tongchang Street; The other set is intraplate alkaline basalt. Trace elements reflect that the basalt near the ocean ridge is more similar to the similar rocks in Xiaohe River than the typical rocks in Tongchang Street. Rare earth elements show that there are intraplate basalt, ocean island basalt and mid-ocean ridge basalt in this area.

(2) basic rocks and ultrabasic rocks

Ultrabasic rocks and basic rocks in this area are mainly distributed in the area of Tongchang Street in the northern section, and also in the middle and southern sections. According to the mineral and geochemical characteristics, Tongchangjie ultrabasic rock is a mafic ultrabasic rock with poor aluminum and alkali, and the geotectonic background reflected by rare earth and trace elements in gabbro is similar to basalt.

The age of ultrabasic rocks in the middle section (Xiaoheijiang) is unknown. Volcanic rocks in this area are typical alkaline basalts, and the characteristics of rare earth elements in ultrabasic rocks are similar to those in Tongchang Street, but the ratio of magnesium to iron is 4. 16 ~ 4.34, which belongs to iron ultrabasic rocks with poor aluminum and alkali.

The ultrabasic rocks in the southern section are iron-rich, iron-rich, alkali-weak, alkali-poor and aluminum-poor rocks, which are different from those in the northern section (Tongchang Street). The basic rocks are different from those in Tongchang Street.

The basic rocks and ultrabasic rocks in Changning-Menglian area reflect different tectonic settings and do not have the characteristics of the whole plate combination zone.

(3) On foreign islands.

Liu Benpei and others believe that Changning-Menglian is the product of a multi-island ocean subduction collision. Multi-island oceans include haishan island, oceanic islands, oceanic island arcs, deep-sea basins and mid-ocean ridges related to oceanic crust.

Specifically, most of the volcanic rocks of Liu Yi Formation and Pingzhang Formation distributed in Changning-Menglian area (except the oceanic crust section mentioned by Liu Benpei and others) must be classified as ocean islands and island arcs due to the platform carbonate deposits of the Middle Carboniferous and the high energy band deposits at the top of Liu Yi Formation and Pingzhang Formation. Literally, it looks like Hawaiian volcanic rocks in sea island. The Hawaiian volcanic rocks are oceanic tholeiite in the lower part and alkaline basalt in the upper part, but Changning-Menglian volcanic rocks do not have this feature. Volcanic rocks of Liu Yi Formation and Pingzhang Formation are carbonate rocks, which are continuously deposited 105 ~ 180 m, mostly the products of high-energy environment. In the long geological history from Middle Carboniferous to Late Permian, these "ocean islands" should always maintain a shallow and high-energy environment, which requires volcanic islands to sink continuously to meet this condition. In addition, the sources of plant debris and argillaceous materials commonly found in Liu Yi Formation and Pingzhang Formation are even more puzzling.