Although there are Neoarchean Yudongzi rock groups in South Qinling, the exposed area is too small to reveal the basic characteristics of Neoarchean crustal evolution. However, the Mesoproterozoic strata in this area retain important geological information and are now being further analyzed.

One is the volcanic arc on the continental margin in the early Neoproterozoic.

Part of Wudang Group and Yaolinghe Group in South Qinling belong to early Neoproterozoic volcanic rock series, and their geochemical characteristics show that the volcanic rock series was formed in the tectonic environment of continental margin volcanic arc. Ling et al. (2002) pointed out that Wudang Group belongs to calc-alkaline volcanic rock series, and both basic and acidic volcanic rocks have obvious negative anomalies of Nb, Ta and Ti, showing similar characteristics to island arc volcanic rocks. This set of early Neoproterozoic island arc volcanic rocks can be compared with Mogouxia volcanic rocks in Zhen 'an, Shaanxi Province and Xixiang Group volcanic rocks in the northwest margin of Yangtze Platform in terms of horizon, geochemical characteristics and neodymium isotope characteristics. The zircon U-Pb age of Baimianxia Formation in the lower part of Xixiang Group is (94618) Ma; The upper Sun Jiahe formation is (904 18)Ma. Therefore, Ling et al. suggested that the present South Qinling and Yangtze landmasses once belonged to the same continental plate, and the development of arc volcanic rocks in the early Neoproterozoic indicated that there was subduction of Beiyang crust in this area. The author of this book agrees with the analysis and viewpoints of Ling et al.

Second, the mid-Neoproterozoic continental cracked volcanic rocks.

Another part of the volcanic rocks of Yaolinghe Group in South Qinling is the product of continental cracking in the middle Neoproterozoic, and the geochemical characteristics of volcanic rocks show the characteristics of intraplate volcanism. Among them, the content of lithophile macro-ion elements is obviously higher than that of Wudang Group, and there are no Nb and Ta anomalies, showing the basic characteristics of continental rift volcanic rocks. Xia et al. (1996) mainly expounded the evidence that Yaoling River Group was formed in the background of continental rift from two aspects: first, the natural combination of volcanic rocks showed bimodal characteristics; Secondly, the geochemical characteristics of basic volcanic rocks are similar to those of continental intraplate volcanic rocks, rich in Ba, Th, Rb, La and Ce, and low in Nb and Ta, which are obviously different from island arc and ocean island basalt.

Volcanic strata similar to the formation age and structural background of Yaolinghe Group include Bikou Group and Tiechuanshan Formation in Hanan. The latest isotope chronology results show that the U-Pb isotopic ages of these volcanic rocks are concentrated in the South China period. For example, the SHRIMP U-Pb age of Bikou group basalt is (790 15)Ma and (776 13)Ma (Yan Quan et al., 2003), and the TIMS U-Pb age is (77). TIMS U-Pb ages of Yaoling River Group are (808.65438±0.5.7)Ma (near Shiquan Reservoir in Shaanxi Province) and (744.65438±0.4)Ma (south of Xixia in Henan Province), respectively. These age data are completely consistent with the isotopic age (Wang Jian, 2000) of the lower volcanic rocks of the Nanhua system in the Yangtze Craton, and they are in the same fault tectonic environment.

Three. Intrusive complex in the middle Neoproterozoic

A series of granodiorite bodies have been discovered in the distribution area of Douling Group in the north of Xichuan, Henan Province and its adjacent areas. Although they have different lithologic and geochemical characteristics, their spatial distribution is very close, and their formation age is almost the same, so they should be formed in the same tectonic background. We call these rocks formed in the Nanhua period "Douling intrusive complex", including "Sanpinggou, Gangou, Shangzhangying rock mass" and Tuwushan A-type granite. Their age of formation is between 730 and 71000 ma.

1. Dark granular inclusions in Laojundian granodiorite-quartz diorite

Laojundian rock mass is exposed in Laojundian area, north of Xichuan, Henan Province, and intrudes into Douling rock group. The rock structure is massive and has not been obviously deformed and superimposed. The rock body has a medium-grained structure. A remarkable feature of rock mass is that it contains a large number of dark basic inclusions, which can be round, irregular or banded, and often have a transitional relationship with the mineral composition of host rock. Irregular and banded inclusions often show plastic deformation, indicating that dark basic inclusions and host rocks are in plastic state at the same time, indicating that the formation of rock mass may be the result of mixing bimodal magma or crust-mantle magma.

Quartz diorite has medium-grained granite structure and massive structure, and its main mineral components are plagioclase (50% ~ 55%), amphibole (40%) and quartz (> 5%). Plagioclase is authigenic granular, with a particle size of 3 ~ 5 mm Cleavage and flaky twins can be seen in some crystals, and there is strong kaolin on the surface. Hornblende is self-shaped, with particle size of 2 ~ 3 mm, light green-green-dark green multicolor, rhombic cleavage developed, cleavage angle of 56, and twin crystals can be seen. It is of medium-fine particle size, with particle size of 1 ~ 3 mm, colorless, clean surface and wavy extinction. Granodiorite is another important rock type of Laojundian pluton, which has medium-fine grained granite structure and massive structure. Its main mineral components are plagioclase (40% ~ 45%), amphibole (25% ~ 30%) and amphibole (20% ~ 25%), and a small amount of magnetite. It is worth noting that the rock mass contains accumulated amphibole xenoliths in a porphyritic structure, and the rocks are green-brown. The amphibole is accumulated and embedded with plagioclase in amphibole cracks. The amphibole particle size is 0.5 ~ 10 mm, and it is self-shaped, cleavage is developed, and the cleavage is filled with timely aggregates.

The SiO2 _ 2 content of granodiorite (DQ07) and quartz diorite (QL93TW0 1) in Laojundian rock mass is 63.63% and 68. 18% respectively. The Al2O3 content is 65438 06.08% and 65438 05.04% respectively. The content of Na2O is slightly higher than that of K2O (3.065438 0% and 2.72%, 3.87% and 2.49% respectively). On ACF map, they all fall in S-type granite area. The distribution characteristics of rare earth elements and incompatible elements in granodiorite are similar to those of volcanic arc granite or collisional granite in the northern magmatic belt, so it is impossible to distinguish them. However, according to isotopic dating data, Laojundian pluton, Yaolinghe Group and typical intraplate Tuwushan granite pluton were formed at the same time, which should be the product of unified cracking mechanism. The geochemical characteristics of trace elements and rare earth elements may inherit the characteristics of source rocks, that is, Laojundian rock mass is the product of remelting of high basic rocks originally formed in volcanic arc or collision environment under the later tensile mechanism.

The zircon U-Pb SHRIMP age of this sample is (71710) ma (the weighted average of the surface ages of 206Pb/238U at10).

2. Tuwushan A type granite

Tuwushan A-type granite is located in the southeast of the outcrop distribution area of Douling Group. Its surface is nearly elliptical, with an area of about1.5km2. There are a few amphibole gneiss and marble surrounding rocks of Douling Group in the north, and the rest are Quaternary. The main body of the rock mass is albite aegirine granite with a small amount of potash feldspar granite. Rock has graphic structure, porphyritic structure and massive structure. The main minerals are potash feldspar (50% ~ 60%), quartz (30%), a small amount of plagioclase, sodium amphibole and aegirine (5%), and the auxiliary minerals are magnetite, zircon and apatite. According to the data of Lu Xinxiang et al. (1999), the SiO2 content of Tuwushan A-type granite is 74.4 1%, w(Na2O+K2O)=9. 19%, σ=2.68, w (Al2O3)/w (Cao The contents of F and Cl in rocks are very high, F is 90 1× 10-6, Cl is 105× 10-6, [w (y)/w (nb)] n = 2./kloc-0. The content and distribution characteristics of rare earth elements are typical A-type granites. Trace element geochemistry shows the characteristics of typical intraplate granite.

Lu Xinxiang et al. once measured the zircon U-Pb age of this rock mass by TIMS method as (725±39)Ma (Lu Xinxiang et al., 1999). We used the high-resolution and high-sensitivity ion probe mass spectrometer for verification, and the statistical average of surface age and weight of 206Pb/238U was (7 1 1 65439).

Four. Hannan basic-ultrabasic intrusive complex

Wangjiangshan basic rock mass in Hannan area, northwest margin of Yangtze Craton, is an important geological record of continental disintegration in Nanhua period. The geotectonic position of Hannan area belongs to the north protruding part of the northwest margin of Yangtze Craton, also known as "Bashan Arc" Micang Mountain System. Its northern edge is adjacent to Mianlue structural belt of Qinling orogenic belt, and the east and west sides are Longmenshan and Dabashan nappe structural belts respectively. Wangjiangshan basic rock group is composed of several rock masses (with a total area of about 400km2), of which the exposed area of the main rock mass is about 120km2, which is the largest South China basic intrusive rock mass in Qinling orogenic belt and its two continental margins. The main lithology of Wangjiangshan basic rock body is gabbro, olivine gabbro and gabbro-diabase gabbro. The main rock-forming minerals are augite (a small amount of diopside and hypersthene) and intermediate-basic feldspar (an = 40 ~ 80, with an average of about 65), and a small amount of olivine, amphibole, biotite and opaque minerals. The most prominent geochemical feature is that most samples lack obvious negative anomalies of Nb and Ta. Wangjiangshan rock mass generally shows the geochemical characteristics of intraplate tectonic environment. Based on the analysis of the scale (> > 400km2) of the basic rock group, its genesis should be similar to continental overflow basalt, which indicates the beginning of large-scale intracontinental rift magmatism in the northern margin of the Yangtze Craton in South China. Ling et al. (200 1) once measured the Sm-Nd isochron age as (785±88)Ma, and inferred that the emplacement age of Wangjiangshan rock mass was in early South China.

Five, Hannan acid intrusion

Hannan intrusive complex is exposed in Xixiang-Hanzhong area of Shaanxi Province on the northern margin of Yangtze Craton, with a distribution area of about 2 100km2. The intrusive complex is dominated by different types of granitic rocks, mainly plagiogranite, followed by quartz diorite and gabbro. The Wangjiangshan basic rock mass is one of the constituent units of Hannan complex. Plagiogranite is mainly composed of plagioclase (50% ~ 60%), syenite (20% ~ 25%), biotite and amphibole (5% ~ 10%). The average percentage content of major elements in five samples (Yan Zhen et al., 1985) is shown in Table 4-4.

Table 4-4 Average percentage content of major elements in samples (wB/%)

(According to Yan Zhen, 1985)

In this study, tonalite samples (sample number QL65TW0 1) were collected from Sanxiguan, Xixiang, Shaanxi. The tonalite here is of medium-grained structure and massive structure, with undeveloped gneiss, and the main minerals are inlaid with plagioclase. Besides Jishi (35%) and plagioclase (50%), there are amphibole (8%), biotite (5%) and muscovite (2%).

Predecessors have actively explored the age and origin of the Hannan intrusive complex. Ru et al. (1985) reported an Rb-Sr isochron age data, and Zhang et al. recalculated it with the isochron age of (806 140) Ma. He also obtained the 40Ar-39Ar age (796±20)Ma of plagiogranite intermediate biotite and the Sm-Nd isochron age (837±26)Ma of intermediate-acid intrusive rocks. According to the above age data, it is inferred that the Hannan complex was formed in 837-800 Ma (Zhang et al., 2000).

The U-Pb age of zircon in tonalite (sample number: QL65TW0 1) was determined by TIMS method, and the average weight of surface age of 206Pb/238U was (723.3±2.7)Ma. This age value indicates that at least some intrusions in Hannan intrusive complex were formed in the Nanhua period, which was the product of the initial cracking stage of Rodinia supercontinent.

See Table 4-5 for isotopic dating results of magmatic belt in southern Qinling orogenic belt.

Table 4-5 Isotopic Dating Results of Magmatic Belt in the South of Qinling Orogenic Belt

There are various basic and acidic intrusions and volcanic rocks in the rift zone, which indicates the formation of crust thinning environment. The bimodal volcanic rocks, gabbro and A-type granite in these geological bodies clearly show that they were formed in intraplate extensional environment, and the formation age is concentrated between 8 10 ~ 700 Ma. However, some granites that are geographically adjacent to the above geological bodies and have similar formation ages show geochemical characteristics of island arc magmatic rocks. We don't think that the magmatic rocks in the same area and the same era are the products of two tectonic settings, but should reflect the dynamic mechanism of the same extensional environment, and the difference in geochemical characteristics is mainly restricted by the original rocks.

The rift zone extends from Shangdan structural belt in the north to Hannan area of Yangtze Craton in the south, and connects with Nanhua rift-type volcanic rocks (Tang Jiafu et al., 2003) and widely developed Nanhua plutonic granitic intrusive rocks in Tongbai-Dabie Mountain and Sulu orogenic belt to the east. The southern part of the Yangtze Craton may be connected with the Kangdian Rift in the Nanhua period (that is, the "Kangdian-Longmenshan-Micangshan Volcanic Rift Belt" called by Liu Hongyun 199 1). Although the west extension is covered by the sedimentary strata from Late Paleozoic to Mesozoic in the West Qinling, the possibility of the existence of the trigeminal rift centered on Hannan cannot be ruled out. The east and south branches of this trigeminal rift extend more than 1000km, and Hannan, located in the center of the trigeminal rift, may be a "hot spot", so there are large-scale basic rock intrusions such as Wangjiangshan. If this inference holds, the Trigeminal Rift may be the most intact and largest Neoproterozoic Rift System in China. Because the disintegration of Rodinia supercontinent is related to the disintegration of Rodinia supercontinent, it will provide important clues for studying the dynamic mechanism of Laurenci, Australia and Tarim-Yangtze craton in Rodinia supercontinent.