East Kunlun suture zone (Kunzhong)

The suture zone in the middle of East Kunlun (hereinafter referred to as "Kunzhong suture zone") starts from the northern shore of Ayakumu Lake in Xinjiang in the west, and reaches the northern Tengger Tage, Kaimuqi Dounan, Dagangou and Wutuo in kajakka in the east longitude, and reaches Qingshuiquan and Jirimai areas in the east. The discontinuity is about 1000km long and 2 ~ 5 km wide, and its two ends are cut by two intraplate strike-slip faults, Wahongshan and the southern margin of Altun Mountain, respectively, and it is generally distributed in NW-NW direction. To the south of the suture zone is the East Kunlun metamorphic terrane, and to the north is the Qaidam block. There are significant differences in landforms between the two sides of the strait, especially aerial (satellite) photos.

First, the characteristics of the suture zone in Kunming

In this round of work, the research group mainly carried out detailed field research and indoor research on Qingshuiquan area on the suture zone in central Kunming. According to the previous research data and our research progress, the characteristics of Kunlun central suture zone are summarized as follows:

First, geophysical data confirm that the suture zone is inclined to the south near the surface, and the deep part is steep to the north, with an inclination angle greater than 80, which is an obvious geophysical interface. The average crust thickness in the north is 40km, and the seismic wave velocity in the upper mantle is 7.8 ~ 7.9 km/s; The average crust thickness in the south is 55km, and the seismic wave velocity in the upper mantle is 7.5 ~ 7.6 km/s; The Bouguer gravity anomaly difference on both sides is 80MnT, and the density difference is 0.07 g/cm3. Generally speaking, gravity is high in the north and low in the south (Huayang, 1986). In the contour map of magnetotelluric sounding, it can be clearly seen that the resistivity step belt that the suture belt passes through south of Golmud is steep to the north, and the electrical horizons on both sides are greatly dislocated.

Secondly, there are significant differences in sedimentary environment, rock assemblage and geological evolution on both sides of the suture zone from Mesoproterozoic to Late Paleozoic. The northern part is a crystalline basement with amphibolite facies (local granulite facies), which is composed of various schists, gneiss, marble and a small amount of quartzite. The protolith is a set of deep-sea sandy argillaceous clastic rocks and intermediate-basic volcanic rocks, which are covered by the unconformity of Neoproterozoic glacial ditch group coastal-shallow sea clastic rocks and carbonate rocks. The early Paleozoic strata in the area are scattered, and only clastic rocks and volcanic rocks from late Ordovician to early Silurian Qimantage Group are seen. It is covered by the unconformity of continental clastic rocks, volcanic rocks and pyroclastic rocks of Chegaisu Group in the middle and late Devonian. The south side belongs to metamorphic basement with low crystallinity (only reaching low greenschist facies), which is similar to the basement of Yangtze block. Therefore, some people refer to the substrates on both sides of the sewing tape as "hard substrate" and "soft substrate" respectively (Jiang Chunfa et al., 1992). Above the soft basement is a set of early Paleozoic deep-water clastic rocks and basic volcanic rocks deposited in the passive continental margin environment, exposed from Cambrian to Silurian. Regionally, it is covered by marine clastic rocks and carbonate rocks of the Lower-Middle Devonian. According to microfossil data, there may be Sinian strata.

Thirdly, there is a granite belt about 800km long from east to west on the north side of the suture zone, and the lithology is mainly gneiss granite and granodiorite; The chemical composition is generally characterized by calcium-alkaline series rich in potassium and poor in sodium; The genesis mostly belongs to the deep remelting product of continental crust which was obviously reformed in the later period (Jiang Chunfa et al., 1992). Therefore, this shows that a large-scale northward subduction occurred along the suture zone, which led to the remelting of the continental crust and the formation of a huge granite belt on the southern edge of the Qaidam block.

Fourthly, ultrabasic rocks, gabbro, diabase and basic volcanic rocks are intermittently exposed along the suture zone, among which Qingshuiquan, Wutuo, Jirimai and Ayakumu are the most widely distributed in Hubei. Predecessors generally defined it as ophiolite (Gao Yanlin et al.,1988; Xie,1998; Wang et al; Zhu et al., 1999). Most of the ophiolite mentioned above are mixed in the Mesoproterozoic and Neoproterozoic metamorphic rock series or lower Paleozoic metamorphic volcanic rocks on the south side of the suture zone in broken structural slices or blocks. The lithologic combination is mainly pyroxenite, olivine, gabbro, diabase, basalt and plagiogranite. The analysis results of major, trace and rare earth elements of various rocks show that peridotite and basalt belong to the same rock combination and have the same origin and provenance. At the same time, it is determined that the peridotite in this area belongs to alpine metamorphic peridotite with insufficient magnesium, alkali and aluminum (Gao Yanlin, 1987).

Fifthly, the isotopic ages of mafic and ultramafic rocks in this suture zone have been determined by various methods. Zheng Jiankang transferred from Qinghai Province is equal to 1.988. The isotopic age of Qingshuiquan metamorphic volcanic rocks is 65,438 297 Ma by Sm-Nd isochron method for the first time. According to the understanding level at that time, the development and evolution model of the Mesoproterozoic Kunlun Ocean was established (Zheng Jiankang, 65,438+0,992). Later, Xie et al. measured the Sm-Nd isochron ages of altered olivine pyroxene in Jirimai area east of Qingshui Spring as 133 1Ma and 1027Ma, which seems to further prove the existence of Mesoproterozoic and Neoproterozoic ancient Kunlun Ocean (Xie, 1998). In the same period, Yang Jingsui, Institute of Geology, Chinese Academy of Geological Sciences, and Bian Bian, Institute of Geology, Chinese Academy of Geological Sciences, respectively measured (518 3) Ma (Yang et al., 1996) and 517.89 Ma (./kloc-). In the last round of research, we also obtained the U-Pb age (522.3 4.1) Ma of zircon from gabbro TIMS method (Lu Songnian et al., 2002), which confirmed the existence of early Paleozoic ophiolite. Therefore, many scholars believe that the tectonic belt should be a suture zone with many times of opening, closing, subduction and collision (Jiang Chunfa et al., 2000; Pan Guitang et al., 2004).

It is worth noting that Pan Yusheng and Zhang Qi from the Institute of Geology, Chinese Academy of Sciences, after investigating ultramafic rocks in Qingshuiquan and Wutuo, etc. Question the ophiolite in the suture zone. The reasons are as follows: ① There are no deep-sea pelagic sediments and ridge basalts associated with ultramafic rocks in this area, but more granulite xenoliths in the lower crust are found in terrigenous clastic rocks; ② The contents of Ti, K and P in Qingshuiquan basalt are particularly high [W (TiO _ 2) = 2.46%, W(K2O)= 2.84%, w (P2O5) =1.07%], and LREE is strongly enriched (where La is1of chondrite. Presumably equivalent to continental tholeiite or alkaline basalt. Therefore, they may be the upper mantle materials emplaced by the diapir under the thinned continental crust, with typical "Yidun-style" rock mass characteristics, and should be formed in the initial continental rift (Pan Yusheng et al.,1996; Zhang Qi et al, 200 1).

Second, the research progress

We have systematically studied the basic granulites in Qingshuiquan ophiolite melange in petrography, geochemistry and chronology, and made many important progress.

(1) Geography, traffic location and regional geological survey

Qingshui Spring is located in Gouli Township, dulan county City, Qinghai Province, 60 kilometers south of Xiangride Town, connected by a simple highway, with an altitude of 3,600 meters (Figure 2-47).

Figure 2-47 Geological Schematic Diagram of Golmud-Qingshuiquan Area

1- Proterozoic; 2- Mesoproterozoic; 3- Neoproterozoic; 4- Lower Paleozoic; 5- Upper Paleozoic to Middle Paleozoic; 6- Neoproterozoic granite; 7- Phanerozoic granite; 8- diorite; 9- gabbro; 10-ultrabasic rock; 11-eclogite; 12-main fault zone of suture zone; 13- ductile shear zone; 14 —— (Geophysical prospecting) Inferring faults

Figure 2-48 Schematic Diagram of Geological Structure in Qingshuiquan Area

1- four yuan; 2- Mesozoic to Upper Paleozoic; 3- Lower Paleozoic; 4-Graphite marble and amphibole mixed granulite; 5- gneiss and migmatite; 6- gabbro; 7- olivine gabbro; 8- olivine; 9- pyroxenite; 10-undivided ophiolite block; 1 1- failure; 12- Dongkunzhong suture zone

Granite is exposed in the central suture zone of East Kunlun. The exposed stratum is a set of metamorphic rock series, mainly composed of migmatized biotite garnet granulite, biotite pyroxene granulite, graphite marble, diopside-containing tremolite marble, diopside marble, biotite amphibole and hypersthene biotite garnet granulite. Granite exists in other metamorphic rocks in lenticular form. The igneous rocks in this area mainly include granodiorite, gabbro, ultrabasic rock, diabase vein, granodiorite porphyry vein and granite vein. Granodiorite is located in the north of metamorphic rock series, with bedrock occurrence and fault contact with metamorphic rock series. Gabbro bodies are exposed in metamorphic rock series and have an intrusive relationship with metamorphic rock series. Ultrabasic rocks are mixed in metamorphic rock series in blocks (slices) of different sizes, and two large blocks can be seen. As for diabase veins and granite veins, they are relatively developed and often intrude along bedding (Figure 2-48). In addition, there are granite gneiss with an age of (828.5 9.1) Ma (Lu Songnian et al., 2002). This area is located in the axis of Qingshuiquan compound anticline (Gao Yanlin et al., 1988) with complex structural morphology and developed fault structures. We think that the metamorphic-igneous rock assemblage is ophiolite-melange.

(2) Petrographic characteristics of granulite

The rocks are mainly composed of plagioclase (30% ~ 35%), diopside (30%), garnet (25% ~ 30%) and hypersthene (5% ~ 10%), with a grain size of 0.2 ~ 0.55 mm and a small amount of titanomagnetite (< 5%). The plagioclase is in the shape of heteromorphic mosaic granules with wave extinction and mechanical twins. Garnet is reddish and granular; Diopside is a light green-brown column, and diopside distributed along cracks has radiation phenomenon; Perilla frutescens is a short column with a reddish green color. Its edges, cracks and cleavage are explained by serpentine, and only a few remain. There are a few cracks in the rock, diopside distributed along the cracks is radioactive, and plagioclase is sericitized. The main minerals are closely intertwined and evenly distributed, and the contact angle of the three minerals is nearly 65438+020. Rock has columnar granular metamorphic structure and massive structure. The rock is garnet lherzolite granulite (Figure 2-49).

In this paper, the chemical composition of main mineral components of granulite is analyzed by electron probe, and it is estimated that the temperature and pressure conditions for the formation of granulite in Qingshuiquan are T=800℃ and p = (9.3 ~11) ×108 pa (Holland & Powell, 6508 Pa). This is basically consistent with the temperature and pressure conditions estimated by Chen Nengsong et al. (1999).

(3) Geochemical characteristics of granulite

In the continental dynamics laboratory of Northwest University, we analyzed the major and trace elements of three granulite samples by X-ray fluorescence spectroscopy and plasma mass spectrometry (Table 2-8). Judging from the content of major elements, Qingshuiquan granulite and basaltic igneous rock have similar chemical composition. The standardized REE model of chondrite is flat, there is basically no difference between light and heavy rare earths, and there is no Eu anomaly. The REE content of chondrite is 10 times that of chondrite. According to the original mantle analysis and N-MORB standardized incompatible element distribution map, Qingshuiquan granulite has similar trace element geochemical characteristics to submarine plateau basalt (Figure 2-50).

Figure 2-49 Electron backscattering image of granulite slices in Qingshuiquan area

Cpx—- orthopyroxene; Grt—- garnet; ILM—- ilmenite; PL- plagioclase; OPX-Tetragonal Pyrite

Table 2-8 Analysis and Test Results of Macroelements, Rare Earth Elements and Trace Elements in Qingshuiquan Granite

sequential

Note: The samples were tested in the Continental Dynamics Laboratory of Northwest University; The unit of major elements is mass percentage, and the unit of rare earth elements and trace elements is 10-6.

Figure 2-50 Rare Earth Elements and Trace Elements Atlas of Granite in Qingshuiquan Area

(4) U-Pb isotopic age of zircon

We collected zircon U-Pb isotopic dating samples from granulite, and the selected zircon is colorless, transparent, round and granular. Cathodoluminescence photography shows mottled zoning characteristics and is a typical granulite facies metamorphic zircon (Figure 2-5 1). In-situ U-Pb isotopic data of zircon microdomain of 14 were measured by SHRIMP in Beijing Ion Probe Center. The consistent U-Pb surface age was obtained within the error range, and all data points overlapped on the harmonic line. The weighted average surface age of 206Pb/238U at all 14 points is (506.18.9) ma, indicating that granulite facies metamorphism occurred in the Middle Cambrian (Figure 2-52, Table 2-9). It is worth pointing out that the w(Th)/w(U) ratio of most zircons is relatively high (0.36 ~ 0.66), which reflects the characteristics of the w(Th)/w(U) ratio of zircons inherited from primary igneous rocks. Only two test data points show that the w(Th)/w(U) ratio of metamorphic zircon is 0.03 and 0. 1 1 respectively. According to the analysis of CL image and w(Th)/w(U) ratio characteristics, it is considered that the zircon in Qingshuiquan granulite should be metamorphic recrystallization zircon formed in the process of granulite facies metamorphism, thus inheriting the w(Th)/w(U) ratio characteristics of original igneous zircon to some extent.

Fig. 2-5 1 zircon CL image of granite in Qingshuiquan area

Fig. 2-52 SHRIMP U-Pb age harmonic diagram of zircon from granulite in Qingshuiquan area.

Table 2-9 SHRIMP test results of zircon from Qingshuiquan granulite (04QD02-0 1)

Note: the error is1σ; Pbc and Pb* represent ordinary lead and radioactive lead respectively; All isotope ratios have been corrected for the measured 204Pb.

Combined with the age data of 5 18 ~ 522 Ma (Yang et al.,1996; Lu Songnian et al., 2002), we think that Qingshuiquan high-grade metamorphic rocks and basic-ultrabasic schists are ophiolite melange formed in the early-middle Cambrian, which marks a very important plate tectonic boundary. Granite is the product of uplift and thermal relaxation in the post-collision orogeny.

Three. abstract

The central suture zone of East Kunlun is an important structural boundary of geophysical and geological evolution characteristics on both sides. High-grade metamorphic rocks such as migmatized biotite garnet granulite, biotite pyroxene granulite, graphite marble, tremolite marble containing tremolite, diopside marble, biotite amphibole and hypersthene biotite garnet granulite, and igneous complexes such as ultrabasic rocks, gabbro, diabase and basic volcanic rocks are developed in the belt. The high-grade metamorphic rocks are isomorphic with igneous complexes to form ophiolite-* *. Although some predecessors thought that Qingshuiquan ophiolitic melange belt was a suture zone with many times of opening, closing, subduction and collision, the reliability of previous age data deserves serious consideration and appraisal, because some older Sm-Nd and Rb-Sr isochrones are unreliable, and we think that only collision orogeny in early Early Paleozoic can have reliable age basis.