Characteristics of basement rock assemblage

This area is one of the main distribution areas of Archean metamorphic rocks in China. Archean metamorphic rocks are mainly composed of gneiss-granulite and granite-greenstone.

1. Zunhua deep metamorphic granite-greenstone area

This area starts from Zunhua Racecourse in the west, extends from Malanyu, Jinchangyu and Badaohe to Qinglong in the east, extending nearly 150km in the east-west direction and 20 ~ 40 km in the north-south direction. There is a set of deep metamorphic rocks in the area, covered by Mesoproterozoic sedimentary rocks in the north and south, and qinglong river granite-greenstone area in the northeast as the east boundary.

This area is mainly composed of metamorphic plutonic intrusive rocks and supracrustal rocks (Figure 1- 1). All kinds of metamorphic plutonic intrusive rocks account for about 40% of the total rocks in the whole region, and metamorphic supracrustal rocks account for about 60%. Due to the emplacement of plutonic rocks and strong deformation and transformation, the distribution of metamorphic crustal rocks is discontinuous. Except for a few areas, most of them are outcrop-scale inclusions, which are unevenly distributed in felsic gneiss, making it difficult to establish a complete stratigraphic system.

Figure 1- 1 geological sketch of early CAMBRIAN in Jidong area (modified according to Geng, 1998)

Zunhua Group (metamorphic supracrustal rocks in this area) is mainly composed of diopside amphibole, amphibole and biotite plagioclase granulite, sometimes mixed with magnetite quartzite. A small amount of garnet plagioclase gneiss (granulite) is also found in the metamorphic plutonic intrusive rocks from Shangching to Yu Jinlong, north of Jinchangyu and Qingshankou. The rock combination of different sections is different. The metamorphic crustal xenoliths from Qingling, Zunhua to Longwan, Qianxi are mainly plagioclase amphibole, with a small amount of amphibole, amphibole, serpentine amphibole and banded magnetite quartzite. The xenoliths in the area from Santunying to Wangchang are mainly amphibolite, followed by garnet amphibole gneiss, banded magnetite quartzite and a small amount of amphibole granulite. The eastern part of Wangchang is dominated by biotite granulite, with a small amount of amphibole and banded magnetite quartzite. Based on this, the sequence of volcanic sedimentary cycles can be roughly established, that is, the Zunhua area in the west is dominated by mafic volcanic rocks-ultramafic rocks; The central part of western Guizhou is dominated by mafic volcanic rocks, including some metamorphic sedimentary rocks; The area east of Wangchang is dominated by metamorphic sedimentary rocks, with a small amount of volcanic lava and volcanic tuff (Wujiashan et al.,1998; How, 199 1). This shows that the protolith of greenstone in this area is mainly mafic volcanic rocks with volcanic sedimentary cycle of pyroclastic rocks. However, due to the emplacement of plutonic rocks, the thickness of volcanic sedimentary cycle is still difficult to determine.

Archean metamorphic plutonic rocks are widely distributed in this area. The rock types of metamorphic plutonic intrusive rocks include tonalite-granodiorite gneiss, metamorphic basic dyke, syenite, diorite, etc. Feldspar granodiorite gneiss is widely distributed and mostly occurs in layers, which is closely related to plagioclase amphibole and plagioclase amphibole gneiss. Among them, plagioclase amphibole, plagioclase amphibole and magnetite quartzite are common inclusions, which are developed in Erbazi, Sibazi and Shibapan. There are also some metamorphic basic rock walls in Wangyeling, Shimen and Wangchang. The rocks have been metamorphic to garnet amphibole and garnet amphibole. They are several meters to ten meters wide and extend for tens to hundreds of meters. The extension direction is mostly NNE or nearly EW, sometimes cutting the surrounding rock gneiss, and sometimes bedding intrusion, which has been metamorphic and transformed, but not strongly folded and deformed. Intrusions such as diorite and quartz diorite are found in xiaoguanzhuang and other places, with weak internal deformation and strong edge deformation. Among them, xenoliths such as plagioclase amphibole, magnetite quartzite and plagioclase gneiss were found, which formed late. Synenite also developed in Qingyang area, with limited distribution and intrusive contact with surrounding rocks.

Zunhua deep metamorphic greenstone belt has experienced a complicated deformation history. It is preliminarily determined that there are three stages of fold deformation. In the first stage, the deformation scale is small, forming bedding recumbent fold and rootless hook fold. The second stage of deformation formed a large-scale continuous syncline inversion composite syncline and anticline structure; The third stage of deformation produces an open oblique fold, which is superimposed on the first two stages. It should be pointed out that Zunhua deep metamorphic greenstone belt has experienced ductile deformation belt of deep structure and ductile-brittle deformation of middle-shallow structure. The ductile deformation of deep structure is characterized by the elongation of basic volcanic inclusions in felsic gneiss, which eventually forms strips, and the retrogression of amphibolite facies is characterized by the deep ductile deformation process; The ductile-brittle deformation zone in the middle-shallow part is a structural zone in the process of structural uplift after the emplacement of late granitic rocks, and the metamorphic facies is amphibolite facies-greenschist facies. The ductile-brittle deformation event is an important gold mineralization period in this area.

The degree of regional metamorphism in this area is roughly as follows (Wujiashan et al., 1998): the area from Erbazi to Weijinhe in the west is dominated by high amphibolite facies, and banded granulite appears locally; From Weijinhe to Santunying in the middle, it is mainly granulite facies, with a small amount of amphibolite facies metamorphic rock belt; Xiabancheng area and Qinglong-Wangchang area in the east belong to high amphibolite facies.

At present, the isotopic ages of Zunhua rock group are mostly between 2800 and 2700 Ma. The Sm-Nd isochron age of five garnet diopside samples obtained by Guo Jinghui (1993) in Toudaogou section of Qinglong Wangchang is (2786.5±39.7)Ma, and the end point (T) is 3.4 0.8, ε nd. According to the data of Zhang Guicun and others, the Sm-Nd isochron age of amphibole in adamellite in Jianli area is (2756.2177) Ma. These isotopic ages indicate that amphibole in this area was mainly formed in 2800～2700ma.

The second is qinglong river, a shallow metamorphic granite-greenstone area.

This area is located in the eastern part of Yanshan Mountains and Shuangshanzi, Qinglong County, eastern Hebei Province, with an exposed area of about 2700km2. Among them, granite accounts for about 75% of the total exposed area, and greenstone belt is distributed in the northeast direction, about 40km long and nearly 10km wide. The north and south ends are unconformity covered by Mesoproterozoic Great Wall sedimentary rocks, and the east and west sides are surrounded by faults (Figure 12).

The layered rock series in the greenstone belt consists of metamorphic strata of Shuangshanzi Group and Zhuzhangzi Group, passing through Shuangshanzi to Huangzhangzi in the north and Miaoling in the south. Shuangshanzi Group is a metamorphic volcanic-sedimentary formation with a total thickness of about 2000 meters, which is divided into Ciyushan Formation and Luzhangzi Formation from bottom to top. The main rock types of Ciyushan Formation are biotite amphibole granulite mixed with a small amount of amphibole, biotite granulite, magnetite quartzite and chlorite schist, with the top contacting with Luzhangzi Formation as a whole and the thickness being greater than 1260m ... The rock types of Luzhangzi Formation include metamorphic pillow amphibole, metamorphic porphyritic amphibole, flaky biotite amphibole, metamorphic porphyritic plagioclase granulite and metamorphic porphyritic sericite. The structure of residual sedimentary debris is still preserved in the granulite of Ciyushan Formation. Above Shuang Shan subgroup is Zhang Zhu subgroup, which is unconformity contact. Zhangzhuya Formation can be divided into Zhangjiagou Formation and Boluotai Formation. The Zhangjiagou Formation is dominated by metamorphic conglomerate, with thin layers of plagioclase granulite, mica schist and garnet-bearing mica schist. The rock strata extend stably, with a general thickness of 50 ~170 m. The gravel composition in the conglomerate is complex, mainly composed of granite, granodiorite and granite fine-grained, as well as siliceous rocks, biotite granulite and magnetite quartzite, with good roundness and ellipsoid shape. Gravel content is 20% ~ 70%. Cements are biotite plagioclase granulite and intermediate-basic tuff and marl composite sandstone. The thickness of metamorphic gravel layer varies greatly, ranging from several meters to more than 400 meters. In Zhangjiagou area, the maximum thickness is more than 400 meters, and it tends to become thinner when it extends north to south. The lithology of Baluotai Formation is biotite granulite mixed with garnet biotite granulite, amphibolite, amphibolite schist, chlorite schist, granulite, magnetite quartzite, etc. , intercalated with complex metamorphic conglomerate. The thickness of this formation is more than 2500m, which is in integral contact with the Zhangjiagou Formation and in angular unconformity contact with the overlying Changzhougou Formation of Mesoproterozoic Great Wall System. The rock types of greenstone belt include amphibole, flaky biotite amphibole, porphyritic biotite granulite, porphyritic sericite schist, carbonaceous sericite, garnet biotite schist, biotite granulite and magnetite quartzite. The protoliths of these rocks are basalt, andesite, dacite, rhyolite, argillaceous rock, miscellaneous sandstone and chemical sedimentary rock.

Fig. 1-2 regional geological map of granite-greenstone belt in qinglong river (according to qi Honglie et al., 1999).

1- Quaternary alluvium; 2- Louzishan unit; 3- Populus superunit; 4- Jurassic volcanic rocks; 5- Great Wall conglomerate, sandstone and dolomite: 6- Niuxinshan gneiss (metamorphic fine-grained granite); 7- Liugezhuang gneiss (metamorphic granodiorite); 8- Baihesong gneiss (metamorphic granodiorite); 9- Anziling gneiss (metamorphic diorite); L0- metamorphic granite porphyry; 1 1 —— Wuyun granulite with metamorphic conglomerate lens in Baluotai Formation; 12- metamorphic conglomerate of Zhangjiagou Formation; 13- Wuyun plagioclase granulite with plagioclase amphibole in Luzhangzi Formation; 14- biotite plagioclase granulite with mica schist in Ciyushan Formation; 15- plagioclase amphibolite gneiss of Luanxian Formation; The Great Wall System is higher than the L6-dolomite of the Zhuang Formation; 17- actually measured geological boundary; 18- measured unconformity interface; 19- Normal fault; 20- reverse fault; 2 1- ductile shear deformation zone; 22— Reversing syncline structure; 23- Inverted anticline structure; 24— Formation occurrence; 25- Reverse occurrence

Granitic rocks are the main components of granite-greenstone area in qinglong river, collectively known as Anziling gneiss suite. It is a granite complex with micro-gneiss structure, distributed in the east of granite-greenstone area, with a small amount exposed in the west. According to the results of1:50,000 regional geological survey, Anziling complex is composed of four metamorphic plutonic units: Anziling gneiss, Baihesong gneiss, Liugezhuang gneiss and Niuxinshan gneiss (Figure 1-2), which are in intrusive contact with greenstone.

There has been great controversy about the formation age of greenstone and granite in this area. Generally speaking, scholars of Shuangshanzi Group mostly divide the era into Neo-Archean, while Zhuzhangzi Group has two opinions: Proterozoic and Neo-Archean. In recent years, the geological team of Qinhuangdao, Hebei Province (1995) collected three groups of zircon U-Pb age samples from Liugezhuang gneiss in Dazhangzi area, which are (25 15 38) Ma, (2525 16)Ma and (2532 54)Ma respectively. Yang Chunliang et al. (2000) reported that the U-Pb age of a single zircon in metamorphic volcanic rocks of Shuangshanzi Group is (264±L 33)Ma, and that of metamorphic acidic volcanic rocks is (2546±40)Ma. Geng et al. (1999) determined the Sm-Nd isotopic ages of pillow amphibole (containing a small amount of amphibole plagioclase, biotite amphibole and sericite albite schist) 10 samples, and obtained the isochron age of the whole rock as (2793±69)Ma, with IND = 0.50966. U-Pb isotopic dating of a single zircon from the granite porphyry of Shuangshanzi Group was carried out, and the age value of (2492±8)Ma was obtained. From the above data, it can be concluded that from 2.8 billion to 2.9 billion years, the mantle source area was depleted, basic magma differentiated, greenstones of Shuangshanzi Group and Zhuzhangzi Group were formed from 2.6 billion to 2.7 billion years, and granite magma invaded from 2.52 billion to 2.5 billion years, with sodium TTG intrusive rocks in the early stage, potassium intrusive rocks in the late stage and granite porphyry in the late stage. Accompanied by tectonic magmatic events, greenstone

Three. Gneiss-granulite area in eastern Hebei Province

This is a widely distributed Archean rock type in eastern Hebei, usually called high-grade metamorphic area, and it is the main component of the ancient continental core of this block.

The rock assemblage in this area is dominated by metamorphic plutonic intrusive rocks, with a small amount of metamorphic supracrustal rocks (figure 1- 1), in which metamorphic intrusive rocks account for more than 80% of the exposed area of rocks, mainly including the Santunying-Taipingzhai magmatic complex area and the Qian 'an gneiss dome area in the southwest, including the Middle Archean Cao Zhuang rock series.

According to the data (199 1), metamorphic plutonic intrusive rocks are mainly intermediate-acid intrusive complexes, which can be divided into periclase-feldspar granite gneiss, periclase granite, quartz diorite-adamellite and biotite granite. There are four types of basic rock walls in Taipingzhai area, namely amphibole garnet granite. The Taipingzhai area was first invaded by perilla plagioclase granite gneiss, then by perilla granite and quartz diorite-adamellite gneiss, and finally by biotite granite, most of which were transformed by the first metamorphism, and then developed basic rock walls. Geng (1998) proposed that the Archean metamorphic granite in Qian 'an area can be divided into at least six stages: ① Archean Huangbaiyu gray gneiss; ② Middle Archean granite; ③ Neoarchean Yangshan granite; ④ Caozhuang Neoarchean red granite; ⑤ Neoarchean perilla granite and ⑤ Neoarchean granodiorite and monzonite.

Metamorphic supracrustal rocks are mainly distributed in metamorphic plutonic rocks, including Archean Cao Zhuang Formation and Middle Archean Qianxi Formation. Caozhuangyan Formation is distributed in Jishan, Huangbaiyu, Cao Zhuang and other places in Qian 'an City, with discontinuous and incomplete sequence. The lower part is biotite plagioclase gneiss containing garnet quartzite and cordierite quartzite, the middle part is biotite plagioclase gneiss containing amphibole, and the upper part is biotite schist, plagioclase amphibole and banded iron formation (BIF). On the northern slope of Huangbaiyu Village, the lower part is mainly amphibole, the middle part is composed of chromite quartzite, biotite plagioclase gneiss and garnet gabbro mixed with impure marble, and the upper part is composed of biotite plagioclase gneiss, biotite granulite and thin banded iron. The U-Pb isotopic age of detrital zircon in chromite quartzite is 3.6 billion-3.8 billion years (Liu Dunyi, 1994).

The Middle Archean Qianxi rock group is mainly distributed in Shuichang-Songting area on the western edge of Qian 'an gneiss dome, extending in the north-south direction, with a length of about 50km and a width of about 10km. Qianxi rock group is mainly composed of dark granulite, biotite plagioclase gneiss containing pyroxene, light-colored granulite, banded pyroxene magnetite quartzite and other interlayers, and sillimanite garnet plagioclase gneiss and cordierite garnet plagioclase gneiss appear in the upper part of iron ore layer. Due to the metamorphism and transformation of granulite facies, the primary structure is difficult to preserve. Its primary rock assemblage is mainly basic-intermediate acid volcanic rocks and hard sandstone, with siliceous rocks, clay rocks and a small amount of carbonate rocks locally developed. Combined with the characteristics of rare earth elements, its formation environment is similar to that of modern continental margin island arc.

This area is one of the oldest Archean high-grade metamorphic areas in China, which has experienced multi-stage folding deformation, especially high-strain plastic deformation, accompanied by strong magmatism. There are many structural forms in the area, such as oval structure, dome structure, arc fold, large ductile shear zone and nappe structure, which reflect the structural style in the deep crust. Regional structures generally constitute arc fold belts, but the specific fold styles in different parts are different, ranging from open asymmetric folds to syncline inversion, and even large recumbent folds appear. The ductile shear zone is mostly developed in the late Archean, mostly in the northeast direction, and the phenomenon of dynamic retrogression and metamorphism generally occurs in the shear zone. In addition to large ductile shear zones, there are many mylonite zones, all of which show the characteristics of deep horizontal shear deformation in late Archean.