Lower crust and mantle structure in Minggang area

Minggang is located at the junction of North Qinling and North China. Because of the complex lithosphere structure of orogenic belt, it has the characteristics of subduction, thrust, superposition and intersection, and it is impossible to restore the deep structure with general methods. It is necessary to study the ownership of the collected samples and the orogenic process.

The crustal structure of the North Qinling Mountains has been studied by predecessors. According to the geophysical data and geological results of the geological profile QB- 1, the understanding that the middle and lower crust of the south Qinling slipped and dived under the crust of the north Qinling was obtained (Zhang Guowei et al.,1988; Cao et al.,1994; Yuan Xuecheng et al., 1996). Geochemical study further confirmed that the source area of the late Paleozoic-early Mesozoic collision granite in the northern part of Shangdan structural belt was not mainly from the basement stratum of North Qinling, but mainly from the middle and lower crust of South Qinling (Zhang Hongfei et al., 1996) on the south side of Shangdan structural belt. From the perspective of material composition, it was confirmed that the lower crust of North Qinling had the composition of the middle and lower crust of South Qinling, which provided direct evidence for the above understanding. However, there is no clear answer whether the subduction is on the whole lithosphere scale or only in the crust, and how far the South Qinling block subducted northward. Through the study of the deep xenoliths in Minggang, we can reconstruct the deep lithospheric structure of the crust and mantle in the North Qinling and further understand the boundary properties of the tectonic units.

6.2.3. 1 Most of the xenoliths in the lower crust in Minggang area represent the composition of the lower crust in South Qinling.

According to the usual methods used by predecessors at home and abroad to study deep-seated xenoliths, the calculated temperature and pressure conditions can be used to restore the columnar section of the upper mantle-lower crust. The obtained peak temperature and pressure of basic granulite metamorphism are 962℃ and 65438 0.45 GPA (48km), and it is speculated that the crust thickness was over 48 km at that time. According to the generally accepted principle, the distribution of the above lithology from the mantle upward should be: spinel lherzolite (mantle part) → basic granulite (lower crust) → acid granulite (upper and lower crust) → amphibole (middle crust), and eclogite and pyroxenite may be located in the lower crust or upper mantle. However, as mentioned above, according to Pb isotope data, granulite xenoliths are not the material components of North Qinling, but belong to South Qinling. Pb isotopes of pyroxenite and eclogite in Figure 6- 12 are also located in the South Qinling range, indicating that most of the lower crust samples obtained in Minggang are part of the South Qinling. It is further confirmed that the lower crust of South Qinling extends to the junction of North Qinling and North China.

Fig. 6- 15 is the reconstructed lower crust section of South Qinling. The basis is as follows: ① The calculation results of temperature and pressure show that the original depth of basic granulite equilibrium is 48km, the current crust thickness is 35km, and the lower crust thickness is 10km. It is estimated that the thickness of the lower crust at that time is16 ~14 km; ② Acidic granulite is related to basic granulite, which may be the differentiation product of the latter and should be located in its upper part; (3) The gabbro is transformed into a basic rock that is underplated after collision orogeny, located below granulite and above mantle; ④ Eclogite is closely related to granulite, which is lenticular and pyroxenite is generally located at the boundary of crust and mantle.

Fig. 6- 15 section of the lower crust of South Qinling.

Lithospheric structure at the northern foot of Qinling Mountains in 6.2.3.2

According to the previous work and our work, the lithospheric model of the northern foot of the Qinling Mountains is put forward (Figure 6- 16). The general feature is that the lithosphere of the North China block is inserted into the Qinling orogenic belt from north to south like a crocodile, and the lower crust and lithospheric mantle of the South Qinling are separated from the upper crust, diving northward under the North China block, and the lower crust and upper crust of the South Qinling and some middle crust (? The subduction pad is placed under the crust of the North Qinling Mountains, forming the boundary lithosphere structure of the orogenic belt, which is separated in the south and wedged in the crocodile shape in the north. The model in Figure 6- 16 can explain the following problems.

Fig. 6- 16 lithospheric model of minggang area at the northern foot of Qinling mountains

NC: North China block, NQ: North Qinling, SQ: South Qinling, M: South Qinling mantle, YZ: Yangtze block, A: Songshugou ophiolite B: Mianlue ophiolite.

(1) shows the results of previous studies on the crustal structure of the Qinling Mountains: the middle crust of the South Qinling Mountains was placed under the crust of the North Qinling Mountains, and the North Qinling Mountains rose to the North China Block in Mesozoic.

(2) The lower crustal xenoliths carried by volcanic rocks in Minggang area are mainly the lower crustal materials of South Qinling, which are composed of granulite, eclogite and pyroxenite. At the end of Paleozoic (273 ~ 205 Ma), South Qinling experienced basic magma underplating, and the representative sample was gabbro, and the magma crystallized under high pressure cooling.

(3) Under the southern margin of North China block, the lower crust of South Qinling subducted under the wedge mantle of North China. According to the above xenolith data, there are acid granulites besides mafic granulites. Due to the influence of deep warming, the silicon-rich melt/fluid is released, which occupies the wedge-shaped mantle of North China block, forming lepidolite instead of serpentine peridotite commonly found in mantle, indicating that the lepidolite belongs to the wedge-shaped mantle sample of North China block embedded in Qinling orogenic belt. This not only explains the genesis of peridotite, but also explains why the host rocks with the lower crust as the main source area carry a considerable amount of mantle xenoliths.

(4) Berlin et al. (1996) proposed that the source of Paleocene tholeiite (original sample Dy5) in Hefei basin was the product of partial melting of mantle wedge formed by subduction of Yangtze block to North China block in early Mesozoic. This area is located in the west of Hefei Basin, adjacent to it. Although the main elements of volcanic rocks are different from the former, the characteristics of trace elements and rare earth elements are similar. We believe that the xenolith-bearing basaltic andesite in this area comes from the boundary zone between the lower crust and mantle of South Qinling, which subducted under the North China block. Qualitative speculation shows that the composition of the lower crust in the magma source area accounts for an important proportion, forming volcanic rocks similar to the continental margin, and the proposed model also explains the genesis of ore-hosting rocks well.

(5) According to the main geological profile data in eastern China and a large number of sampling analysis, it is estimated that the overall composition of the lower crust in eastern China is 70% neutral granulite, 65,438+05% metamorphic argillaceous rock and 65,438+05% mafic granulite, and mafic granulite is developed in most parts of the lower crust, with the wave velocity ranging from 6.98 to 7.265,438+0m/.

(6) The influence of collision orogeny of Qinling Mountains and northward subduction of South Qinling Mountains is a concern of geological circles. As mentioned above, the source area of Paleocene tholeiite in Hefei Basin is the product of partial melting of the "mantle wedge" formed by the subduction of Yangtze plate to North China plate in the early Mesozoic, so the influence of subduction extends to Hefei Basin, about 70km away from the northern boundary of Qinling Mountains. According to the latest data (Xu et al., 2002), there is Jurassic magmatism in the southwestern Shandong-northern Jiangsu area, and the emplacement age is 188 ~ 19 1 Ma. The lithology is amphibole-diorite-diorite-monzodiorite-monzoporphyry, in which eclogite xenoliths are found, and the rock mass group is about 400km away from the northern boundary of Qinling Mountains. If these eclogite xenoliths belong to the material composition of South Qinling subduction to the north rather than Sulu subduction to the west, then the deep impact of Qinling collision orogeny is much greater than originally estimated, and this conclusion needs more data to confirm.

6.2.3.3 abstract

The volcanic breccia containing deep xenoliths in Minggang area (1) was formed in178 5438+0 3.77 Ma, which has the characteristics of continental subalkaline volcanic rocks and is the product of melting the lower part of the lower South Qinling lithosphere in the North China block.

(2) The mafic granulites in the deep xenoliths are high-pressure granulites, and the characteristics of trace elements and lead isotopes indicate that they belong to the South Qinling block. Other trace elements of xenoliths in the lower crust also have the characteristics of South Qinling massif.

(3) Slippery peridotite is a wedge-shaped mantle of North China block, which has been replaced by acidic melt/fluid released by the underlying South Qinling subduction plate.

(4) The deep lithosphere structure in Minggang area is the detachment and subduction of the South Qinling to the north in the Late Paleozoic, and its upper part is placed under the North Qinling; The lower part dived under the North China block, and the North Qinling thrust northward in the early Mesozoic, and the North China block wedged southward into the Qinling orogenic belt like a crocodile.

(5) According to the distribution of similar rocks, the space-time scale and influence range of the northward subduction of the South Qinling lithosphere in the study area may be relatively large.