(Fujian Geological Survey and Research Institute, Fuzhou 3500 1 1)

In the past, the Xiadu Formation of Douling Group in western Fujian was compared with the Nanyuan Formation in the eastern part of Late Jurassic, and it was considered that the lithology of the two formations was different, with the same age and horizon. Therefore, it is considered that the lithology of Mesozoic volcanic rocks in eastern Fujian is different from that in western Fujian. Through investigation and comprehensive analysis and comparison in recent years, it is confirmed that Xiadu Formation should be above Nanyuan Formation, and there are two sets of volcanic rocks in western Fujian and eastern Fujian, one set of light gray-gray volcanic rocks is Nanyuan Formation in late Jurassic, and the other set of purple sedimentary-volcanic rocks is Shimaoshan Group in early Cretaceous. The lithology, rock assemblage and sequence of the eastern and western volcanic rocks are basically similar and can be compared. It should be the product of volcanic activity in different periods (stages) under the same geological structure background.

Keywords: Xiadu Formation; Nanyuan Formation; Early Cretaceous; Shimaoshan group

1 preface

The Mesozoic volcanic activity in Fujian province was strong, forming a set of eruptive sedimentary volcanic rocks with great thickness. How to reasonably divide this volcanic stratum has long been controversial. Based on the regional adjustment results of1:250,000 Longyan city sheet, this paper focuses on the division and correlation of late Jurassic to Cretaceous volcanic strata in eastern Fujian and western Fujian, and discusses the age and horizon attribution of Xiadu Formation.

Step 2 ask questions

For a long time, people thought: "The lithology of Mesozoic volcanic rocks in eastern Fujian is different from that in western Fujian. Comparing the Xiadu Formation in the western Douling Group and the Nanyuan Formation in the eastern part of the Late Jurassic, it is considered that the lithology is different and the age and horizon are the same. " Is that so? Lack of sufficient basis, the focus of the problem is the horizon and age attribution of Xiadu Formation. With this geological problem, on the basis of analyzing and studying the previous data, the author lists it as a key geological problem with the help of the favorable conditions of1:250,000 Longyan city scale adjustment that spans two major structural units in eastern Fujian and western Fujian (southwestern Fujian depression and eastern Fujian volcanic rift zone).

3 Investigation situation

Through the field investigation, profile measurement and comprehensive analysis and comparison of Longyan City Sheet of 1: 250,000 in recent years, the stratigraphic sequence, age and their relationship between the eastern and western volcanic rocks have been basically found out, and the stratigraphic division has gradually become consistent (table1Jurassic-Cretaceous stratigraphic division evolution table of Fujian Province). Volcanic rocks of Nanyuan Formation in the east extend westward along Yong 'an-Jinjiang fault zone to Chengkou and other places. The lithology is a set of light gray and gray rhyolitic microcrystalline tuff, rhyolite, etc. The obtained Rb-Sr isochron age is139.1.1.4ma. It shows that the volcanic rocks of Nanyuan Formation in the west are consistent with those in the east and have the same age. Like the east, it is a set of purple-gray and purple-red sedimentary volcanic rocks (Huangkeng Formation and Zhaixia Formation). For example, the Lianyuan Volcanic Basin (Figure 1) in the west of China is considered by predecessors to be a set of volcanic rocks of Xiadu Formation. At present, it is basically found that its lower part is gray-dark gray rhyolitic volcanic rock of Nanyuan Formation in Late Jurassic, and the Rb-Sr isochron age value is141.9 2.9 Ma, which is covered by purple tuffaceous conglomerate, glutenite, gray fine sandstone and siltstone in the lower part of Huangkeng Formation and purple gray, purple pyroclastic rock and siltstone in the upper part of Huangkeng Formation.

Figure 1 Schematic diagram of stratigraphic section of late Jurassic-early Cretaceous volcanic basin in Lianyuan area.

Table 1 Jurassic-Cretaceous Stratigraphic Division and Evolution Table of Fujian Province

* According to Longyan1:250,000 (2004) map, Nanyuan Formation in this paper is divided into acidic volcanic rock segment J3n 1, intermediate acidic volcanic rock segment J3n2 and acidic volcanic rock segment J3n3.

The lower part of the sequence of Xiadu Basin in Yong 'an (Figure 2) is a set of purplish red tuffaceous conglomerate, glutenite, light gray fine sandstone and siltstone, the so-called disc stratum, on which are purplish gray and purplish red pyroclastic rocks and rhyolite, the so-called Xiadu Formation. The Rb-Sr isochron age is110.6 0.69 Ma ... Coincidentally, the lithology of the stratotype section of Yong 'an Jingshanpan Formation in Sanming in the adjacent area is also a set of purplish red tuffaceous conglomerate, glutenite, gray fine sandstone, siltstone and mudstone. The upper part is also the original purple-gray and purple-red pyroclastic rocks and rhyolite of Xiadu Formation, and the obtained K-Ar isochron age value is 12 1Ma. It is not difficult to see from the above that the stratigraphic sequence, lithology and rock assemblage characteristics of volcanic rocks in Xiadu and Jingshan areas are similar to those in Liancheng Laiyuan volcanic basin.

Schematic diagram of Cretaceous volcanic eruption basin profile in Xiadu area, Yong 'an.

In addition, the stratigraphic sequence, lithology and rock assemblage characteristics are basically similar to those of Dayoushan, Shanghang, Longan, Taining and Shaxian. They were originally classified into the Xiadu Formation in the Late Jurassic and are now Shimaoshan Group, indicating that Xiadu Formation was originally called a set of purplish red and purplish gray volcanic rocks equivalent to Shimaoshan Group, and its next set of gray-dark gray volcanic rocks is equivalent to Nanyuan Group.

4 division and comparison

Based on the analysis of the current survey data, it is confirmed that there are two sets of volcanic rocks in the west and east of Fujian Province, one set of light gray-gray volcanic rocks belongs to the late Jurassic, and the other set of purplish red-purple sedimentary-volcanic rocks belongs to the Shimaoshan Group in the early Cretaceous. The upper and lower volcanic rocks in the west are basically similar to the two volcanic rocks in the east. Therefore, the original division of the Panpan Formation and Xiadu Formation is not equal to the Nanyuan Formation in the eastern part of the late Jurassic, but above the Nanyuan Formation, equivalent to the Shimaoshan Group (Huangkeng Formation and Zhaixia Formation), and the age is early Cretaceous. Volcanic rocks related to the ancient stele in Shanghang, Dayoushan in Shaxian and Longan in Taining were originally divided into Late Jurassic, equivalent to Xiadu Formation. With the deepening of geological work, they are all confirmed to belong to the early Cretaceous Shimaoshan Group strata. The above data show that the stratigraphic sequence, rock assemblage characteristics and formation age of Mesozoic volcanic rocks in the east and west are basically consistent and can be compared (Figure 3). The eastern and western parts of Fujian Province can be divided into a set of gray-dark gray sedimentary volcanic rocks in the lower part and a set of purplish red-purple sedimentary-volcanic rocks in the upper part. They are the products of volcanic activities in different periods (stages) under the same geological structure background. The climate and environment in the east and west are similar to volcanic products. Not surprisingly, due to objective historical reasons, the predecessors compared the volcanic rocks in the west with those in the early Cretaceous Nanyuan Formation in the east, and thought that the volcanic rocks in the east and west were different in lithology. Different conclusions will inevitably be drawn, which will naturally cause difficulties compared with surrounding areas and provinces. Therefore, the determination and age attribution of Xiadu Formation will be of certain significance to the division and correlation of Mesozoic volcanic strata and geological prospecting in Fujian Province and even neighboring provinces. Xiadu area in Yong 'an: Xiadu Formation is covered with a set of gray-dark gray-light purple-purplish red sedimentary rocks, and the bottom is purplish red tuffaceous glutenite, which is unconformity with volcanic rocks of Xiadu Formation. The ancestors called it Bantou Formation, which contains a large number of animal and plant fossils. Previous people believed that the age was late Jurassic. In recent years, many well-known experts and scholars, as well as this investigation, think that the Cretaceous has formed the knowledge of * * * through the study of animal and plant fossils in this horizon. For Mesozoic volcanic strata, we should first straighten out the stratigraphic sequence, find out the spatial distribution characteristics of stratigraphic contact relationship, and combine geological structural events and isotopic geological age data. On this basis, we should select the genera and species with short duration and wide distribution among the biological fossils preserved in different horizons, and combine the age significance of this biota to make it in harmony with geological facts, and the isotopic age data are consistent with the paleontological age. In this way, the determined geological age may be more objective and closer to reality.

Fig. 3 Lithologic columnar correlation map of Mesozoic volcanic rocks in Fujian Province

5 conclusion

(1) Volcanic rocks in eastern Fujian and western Fujian are the products of volcanic activity under the same tectonic background, and the volcanic strata and lithologic combinations formed are basically similar. From late Jurassic to Cretaceous, it can be divided into upper and lower rock series. The lower volcanic rock series, represented by Nanyuan Formation, is characterized by the development of a set of gray-dark gray sedimentary volcanic rock series, which is the product of reducing climate environment and belongs to Late Jurassic-Early Cretaceous. The upper volcanic rock series, represented by Shimaoshan Group and marked by the development of a set of purple sedimentary volcanic rock series, is the product of oxidizing climate and belongs to Cretaceous.

(2) The volcanic rocks in the east and west of Fujian Province can be compared, and the formation of volcanic rock series has the same stratum, lithology and age, which can be compared not only in the province but also with neighboring provinces.

(3) Volcanic rocks in the east and west of Fujian Province are generally distributed in northeast belts, but the difference is that they are widely distributed in the east, while they are produced in the form of basins in the west.

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Stratigraphic division and correlation of volcanic deposits in eastern Fujian and western Fujian and determination of horizon and age of Xiadu Formation

Li Jintang, Feng Zongzhi, Zhang Zhenguo

(Fujian Geological Survey Institute, Fuzhou 3500 1 1)

Abstract: According to previous studies, Xiadu Formation of Douling Group in western Fujian can be compared with Nanyuan Formation of Late Jurassic. They are of the same age, but their petrography is different, which leads to the conclusion that there are differences in the petrography of Mesozoic volcanic rocks in western Fujian and eastern Fujian. Recent studies show that Xiadu Formation should be in a higher horizon than Nanyuan Formation. There are two sets of volcanic rocks in eastern Fujian and western Fujian. One set is Nanyuan Formation in Late Jurassic, which is composed of light gray to gray volcanic rocks. The other is the Early Cretaceous Shimaoshan Group, which consists of purple to purplish red volcanic rocks. Therefore, the two sets of volcanic rocks in western Fujian and eastern Fujian are similar and formed in the same tectonic environment.

Keywords: Xiadu Formation; Nanyuan Formation; Early Cretaceous; Shimaoshan group