Jiao Shan Great Wall Route

The main observation contents of Jiao Shan Great Wall route are as follows: ① Volcanic lava and pyroclastic rocks of Yixian Formation of Lower Cretaceous; ② Suizhong granite and its main geomorphological features.

1. Volcanic lava and pyroclastic rocks of Lower Cretaceous Yixian Formation

(A) the purpose and requirements of observation

To observe the characteristics of volcanic rocks of Lower Cretaceous Yixian Formation (K 1y) in the Great Wall of Jiao Shan.

(II) Main observation contents

1) Through a brief analysis of geological maps, we can understand the structural background and occurrence state of volcanic rocks in Jiao Shan.

2) Main rock types of Cretaceous volcanic rocks in Jiao Shan and various dikes developed in volcanic rocks.

(3) observation tips

Jiao Shan, located about 4 kilometers north of Shanhaiguan, is the first peak in the east of the Great Wall of Wan Li. The Houshihushan pluton developed in northern Jiao Shan is Mesozoic alkaline granite. Volcanic rocks of Yixian Formation of Lower Cretaceous are distributed annularly around Houshihushan rock mass. Archean gneiss granite (Suizhong granite) is widely distributed in the low-level coastal zone from Suizhong to Qinhuangdao, forming the Shanhaiguan ancient land. The volcanic eruption of Mesozoic and Early Cretaceous formed the ring-shaped volcanic vent of Houshihushan, and then alkaline granite magma invaded along the caldera, forming Houshihushan rock mass. The volcanic rocks of Yixian Formation of Lower Cretaceous are hard and have strong weathering resistance, and often form steep peaks. The ancient Great Wall was built on such a dangerous peak. Please pay attention to reading related maps. The Great Wall of Jiao Shan extends northward in a semicircle along the volcanic rocks of Yixian Formation around Houshihushan rock mass. The route starts at the foot of Jiao Shan and ends at the end of the cableway near the top of the mountain.

Volcanic rocks of the Lower Cretaceous Yixian Formation (Ky 1) are exposed along the Great Wall of Jiao Shan. Rock types mainly include: volcanic agglomerate, andesite breccia lava and andesite porphyrite. These volcanic magma penetrated the crystalline basement of the gneiss of the pre-Great Wall system, and brought the lumps and breccias of the surrounding rocks to the surface, thus forming the volcanic rocks at the foot of the ancient Great Wall, which directly covered the gneiss (Figure 2- 13- 1, Figure 2- 13-2).

Figure 2- 13- 1 Geological Profile of Jiao Shan Great Wall in Shanhaiguan (modified according to1:200,000 Shanhaiguan Sheet (K-50-36) and Qinhuangdao Sheet (J-60-6))

Volcanic agglomerates are distributed in the lower part of volcanic rocks. The agglomerates are mainly composed of gneiss, most of which are about 30cm in diameter, and the largest is over 1m, mostly in the form of breccia. So many large gneiss surrounding rocks are densely distributed, indicating that this place is close to the crater, which is probably the volcanic outlet 65.438+0.3 billion years ago.

1. andesite breccia lava

Fresh surface is grayish brown, weathered surface is gray, breccia structure, lava cementation and massive structure. Most of the breccias are 3~5cm in size and have complex compositions, including gneiss, granite, quartz, feldspar and other mineral crystals. This layer is thick and is the main body of volcanic rocks at the foot of the ancient Great Wall in Jiao Shan.

Figure 2- 13-2 Geological Map of Jiao Shan Great Wall in Shanhaiguan (revised according to1:200,000 Shanhaiguan Sheet (K-50-36) and Qinhuangdao Sheet (J-60-6))

2. andesite porphyrite

The fresh surface is brown, the weathered surface is light gray, with porphyritic structure and fluid structure, and the feldspar porphyritic crystals are arranged directionally.

Judging from the above volcanic characteristics, it has a complete volcanic eruption cycle. In the early stage, it was a strong volcanic eruption stage, forming volcanic aggregates and volcanic breccia, and in the later stage, it was a calm overflow stage, forming andesite lava.

In addition, granite porphyry veins with a width of about 3m extending from east to west are also seen in volcanic rocks.

Homework and thinking

1. Why was the ancient Great Wall of Jiao Shan built on Mesozoic volcanic rocks?

2. What is the approximate age of Jiao Shan volcano eruption?

3. How many volcanic facies can be seen in Jiao Shan volcanic rocks? What are the representative rocks?

Two. Granite in Suizhong and Its Main Geomorphological Features

(A) the purpose and requirements of observation

Understand the spatial distribution of granite in Suizhong and its relationship with topography.

(2) observation tips

As mentioned in the teaching guide of Lee Tae Coastal Route, there are a lot of granitic rocks in the southwest coastal area of Xingcheng, which was once called Suizhong Mixed Granite (ArSγ). According to the current research viewpoint, the so-called Suizhong granite may contain many rock types, mainly including granite, biotite adamellite, granodiorite, alkali-feldspar granite, biotite granite, porphyry granite, gneiss granite, gneiss adamellite and gneiss tonalite. Suizhong granite (ArSγ) is a plutonic magmatic intrusion, which contains the older granite on the earth (Archaean formed 2.5 billion years ago).

Most of the granite in Suizhong forms low hills with strong weathering, mostly granite weathered sand, and the thickest weathering layer can reach several meters. The aerial photographs are mostly dendritic water systems with gray tone, uniform tone and medium density, and the cross section of gullies is mostly wide and gentle "U" shape.

Cleavage and gneiss in Suizhong granite strike nearly east-west, which constitute the ancient crystalline basement of the original Great Wall system in this area.

On both sides of expressway, which is more than 0/00km from Xingcheng to Shanhaiguan/Kloc-00, there are low hills and wide gullies on the landform, and the underground is the distribution area of Suizhong granite. The dark red weathered sand and red soil seen along the way are the products of granite weathering in Suizhong. The dark red cliff encountered along the way is also the outcrop of granite in Suizhong. Most of the large apple orchards and peanut fields we saw on the way were planted on the weathered granite layer in Suizhong, which may be related to the high potassium content in granite.

Homework and thinking

What are the concepts and structural features of Shanhaiguan ancient land?