The formation of the Jia-Meng massif and its geotectonic significance

The proposal of the Variscan geosynclinal fold belt or collision orogenic belt is mainly based on the fact that this area is composed of an arc-shaped geosynclinal fold belt on the southern edge of the Siberian platform or the Central Asian orogenic belt between Siberia and the North China plate. partial understanding. More direct evidence comes from the understanding of the formation age of large areas of granite in the Northeast and the metamorphism of Paleozoic strata. In the past, based on the regional tectonic background and some K-Ar and R b-Sr isotope chronological data, the formation age of these granites was determined to be the late Paleozoic, and it was believed that the widely distributed Paleozoic suffered from Variscan greenschist phase metamorphism. function (Dong Shenbao et al., 1986).

(1) Phanerozoic granite in Northeast China

1. Chronological framework of granite

In order to reveal the chronological framework of granite in Northeast China, the project The group conducted systematic high-precision zircon dating on granite in key areas, and also made statistics on zircon dating data published in recent years, and based on this, established a chronological framework for Phanerozoic granite in Northeast China ( Figure 1-2), and compiled the spatiotemporal distribution map of granite (Figure 1-3). The results show that granite distributed over a large area in Northeast China has two obvious characteristics. First, the main body of granite was formed in the Mesozoic Era, not the Late Paleozoic as traditionally believed; second, the spatiotemporal evolution of granite has obvious stages and divisions. According to the peak age of granite and the regional tectonic evolution background, the Phanerozoic granite in Northeast China can be clearly divided into five tectonic-magmatic events. The peak age of magma corresponds to the time when important regional tectonic events occurred.

Figure 1-2 Zircon U-Pb age statistics of Phanerozoic granite in Northeast China

(1) Early Paleozoic tectonic-magmatic event

Peak period The age is 500 Ma, the age range is 530-450 Ma, and the corresponding geological age is Middle Cambrian-Middle Ordovician. This event has been recorded in the Jiamusi Massif, Songnen Massif and Ergun-Xing'an Massif. This tectonic-magmatic event is basically consistent in time with the occurrence of what is known as the Xingkai Movement in the region, or the Salail Movement and the Pan-African Movement in the larger region.

(2) Late Paleozoic tectonic-magmatic events

The peak age is 330 Ma, and the age range is 340-310 Ma, corresponding to the geological age from the late Early Carboniferous to the early Late Carboniferous, mainly It is developed in the Ergun-Xing'an massif and spreads northeast along the Greater Xing'an Mountains. This event is basically consistent with the time of the merger of the Songnen block and the Ergun-Xing'an block.

(3) Late Paleozoic-Early Mesozoic tectonic-magmatic events

The peak age is 260 Ma, the age range is 270-240 Ma, and the corresponding geological age is the Late Permian-Early Third Age World after generation. This event is basically consistent with the collision and merger time of the North China Plate and the Northeast Plate.

(4) Early Mesozoic tectonic-magmatic event

The peak age is 190 Ma, the age range is 210-160 Ma, and the corresponding geological age is the Late Triassic-Middle Jurassic. Mainly distributed in the Xiaoxinganling-Zhangguangcai Ridge and Dongning-Yanbian areas. In the past, this set of granites was classified as the Hercynian granite belt. This event is basically consistent with the time when the ancient Asian Ocean tectonic domain transformed into the peri-Pacific tectonic domain.

Figure 1-3 Spatiotemporal distribution map of Phanerozoic granite in Northeast China

(5) Late Mesozoic tectonic-magmatic event

Peak age 115 Ma, age range 140~110Ma, corresponding to the geological age of the Early Cretaceous. Granite is mainly distributed in the Greater Khingan Mountains area, spreading in a northeast direction. This event coincides with the important basin-forming period of the Mesozoic in Northeast Asia and the formation time of the Cretaceous large volcanic province in Northeast Asia.

2. Early Paleozoic granite tectonic environment

Early Paleozoic granite is distributed in the Jiamusi Massif, Songnen Massif and Ergun-Xing'an Massif, but the types are different.

The granites of this age in the Jiamusi Massif can be divided into two types. One type is garnet granite that emplaced at the same time as granulite phase metamorphism (Wilde et al., 2001). The age is 502~507Ma; the other type is porphyry and giant porphyry granite that were emplaced slightly earlier than the peak of granulite phase metamorphism, with an age of 515~534Ma. This type of granite was once regarded as Neoproterozoic mixed granite. Representative rock bodies include Dapandao and Baoqing Sanchahe.

The Early Paleozoic granite in the Songnen massif developed in the Yichun-Zhangguangcai Ridge area. "Regional Geology of Heilongjiang Province" (1993) reported that the R b-Sr isochron age of the Jiling, Xiaoxilin and Chaoshantun granite bodies is 440-370 Ma, and based on this, the Caledonian movement of Heilongjiang Province was established and proposed. The Jiamusi block and the Songnen block were collaged together by the Caledonian movement. High-precision zircon U-Pb dating results of these granites show that their formation time is 508 to 460 Ma. Early Paleozoic granite was also found on the main peak of Zhangguangcai Ridge, with an age of 497-445 Ma. Among them, inherited zircons with ages of 912 Ma and 1700 Ma were also found, indicating that there may be a Precambrian basement deep in the area.

The Early Paleozoic granites in the Ergun massif are mainly monzogranite and monzodiorite. Representative rock bodies include the Tahe rock mass, the Luoguhe rock mass in the west of Mohe, and the Gulian Rock mass, etc., zircon U-Pb age is 504~517Ma. There are also light-colored subalkaline granites with a U-Pb age of 472-467 Ma in the Ergun Massif in Russia.

The above evidence shows that Early Paleozoic granite is prevalent in various basement blocks. Although the types of granite formed are different due to different compositions of basement blocks, it reflects the existence of Early Paleozoic orogeny in the area. It should be yes. The amphibolite-granulite phase metamorphism and granite magma intrusion that occurred with the Early Paleozoic orogeny indicate that the basement blocks in this area experienced an important crustal consolidation event in the Early Caledonian. From the Late Ordovician to the Early Carboniferous (450-340 Ma), the Northeastern region was in a relatively quiet period of granitic magmatic activity, indicating that the basement blocks in the Northeast entered a relatively quiet period after experiencing the early Caledonian crustal consolidation event. Stable tectonic environment, accepting stable shallow marine sedimentation.

3. Late Paleozoic granite tectonic environment

Zircon U-Pb age test results show that the traditional Variscan granite belt in the Greater Khingan Range should be disintegrated into two parts, one part is Mesozoic granite , part of which is late Paleozoic granite. Based on the comprehensive analysis of new chronological data and petrographic data of representative rock bodies, late Paleozoic granites are divided into two categories: one is the calc-alkaline diorite-granodiorite-monzonitic granite combination, which belongs to I type granite. It is mainly distributed in Xing'an, Taguan, Baoshan, Langfeng and Tarqi in the central region of the Greater Hinggan Mountains, and was formed between 340 and 320 Ma. Geochemical characteristics show that they were formed in the subduction environment before plate collision and have the typical characteristics of active continental margin calc-alkaline magma arcs. Late Paleozoic gabbro appears locally, such as the Tahe pile crystal gabbro (340-333 Ma) with high-calcium plagioclase (An=90), which also shows the structural properties of active continental margin gabbro. The second type is high-potassium calc-alkaline monzonite-K-feldspar granite. The rock assemblage spatially overlaps the first type of rock assemblage, and the zircon U-Pb age is 310-299 Ma. Geochemical characteristics show that the composition is Type I, which is of post-collision origin. They are widely distributed but appear sporadicly. These two types of granites are distributed along the northeastern direction of the Greater Khingan Range. They evolve continuously in time, overlap in space, and gradually decrease in scale. In particular, the continuous evolution characteristics of the genetic types from subduction to post-collision fully illustrate that along the There is an important collision and amalgamation zone on the eastern edge of the Greater Khingan Range.

(2) Upper Paleozoic sedimentation and its structural-thermal evolution characteristics

1. Late Paleozoic structural-sedimentary environment

Late Paleozoic structural-sedimentary environment It is an important scientific issue that restricts the understanding of geotectonic attributes and basin evolution in Northeast China. Research shows that after the early Caledonian metamorphism and magmatism, there are mainly two sets of Upper Paleozoic with regional distribution characteristics in Northeast China. One is the Devonian-Lower Carboniferous of marine sedimentation, which is mainly developed in the Greater Hinggan Mountains. , distributed along the Heihe-Arong Banner-Wunur-Yirbi-Dongwu Banner line (represented by the Niluhe Formation and Hongshuiquan Formation). There are also a small number of exposures on the eastern edge of the Jiamusi block, distributed along the Baoqing-Mishan line. (Represented by the Heitai Formation), but the biological fossil assemblage characteristics of the two are obviously different ("Regional Geology of Heilongjiang Province", 1993); the other set is the Middle Permian, which is dominated by marine sediments, mainly distributed in the Northeast In the central and southern parts of the region, exposures are widespread along the Songliao Basin and the Erlian Basin.

Well Du 101 in Duerbot County, northwest of the Songliao Basin, found fossils of Spiriferella in the gray-black marl at a depth of 1662-1946m. Its characteristics are similar to those of the Middle Permian Spiriferella exposed around the Songliao Basin. The fossils of the Si group are the same. In the Xiaoxing'anling area in the northern part of the Songliao Basin, fossils of Middle Permian clams were also found in contact metamorphic rocks originally thought to be Precambrian. These evidences fully demonstrate that there is no doubt that the Middle Permian of marine sediments exists under the Songliao Basin, and that the range of marine sediments extends northward to the Lesser Khingan Mountains (Figure 1-4). Combining regional geological data from the three provinces (regions) of Heilongjiang, Jilin and Inner Mongolia, it can be seen that from the late Early Carboniferous to the beginning of the Late Carboniferous, except for the development of marine sediments in the southern region, the entire north was in an uplift and denudation environment; from the Late Carboniferous to the Middle Second During the interstitial epoch, the whole region was in an extensional tectonic background. From the Late Carboniferous to the Early Permian, a basin dominated by volcanic-normal clastic sedimentation was formed. The northern part of the basin was still a continental environment, and the southern part was a marine environment. The continental sedimentation range moved towards To the west, it covers the northeast-trending Heihe-Zhalaite Fault Zone (Figure 1-5); in the Middle Permian, the marine sedimentary range expanded to the north, forming a marine sedimentary basin that was narrow in the north and open to the south. , contemporaneous continental sediments developed in the northwest and east of the basin (Figure 1-6). The above characteristics prove that the sedimentary paleogeographic pattern of the Hokuriku and the South China Sea in the Northeast region had already appeared in the Late Carboniferous, and the marine sedimentation range was the largest in the Middle Permian.

Figure 1-4 Distribution map of the Upper Paleozoic in Northeast China

Figure 1-5 Simple map of Late Carboniferous lithofacies paleogeography in Northeast China and adjacent areas

Rocks The facies paleogeographic characteristics show that after the formation of the Early Carboniferous (359-318 Ma) magmatic arc in the Greater Khingan Range, the late Paleozoic sedimentary environment in the Northeast region underwent major changes, with the main body uplifted into a continent, and the south connected with the ancient Asian Ocean. In the north, a continental volcanic-clastic sedimentary basin developed during the Late Carboniferous; during the Early to Middle Permian, the basin further subsided and received marine sedimentation. The southern part was always in a marine sedimentary environment from the Late Carboniferous to the Middle Permian, spreading east-west. This structural-sedimentary pattern shows that a huge late Paleozoic sedimentary basin developed in the Northeastern region during the Late Carboniferous to Middle Permian. The southern part of the basin was always connected to the ocean (Paleo-Asian Ocean) and continued to receive marine sedimentation. This implies that there was a continent on the north side of the ancient Asian Ocean. According to the analysis of the relationship between basement tectonic units and basins, this continent was composed of the Ergun-Xing'an block, the Songnen block and the Jiamusi block.

Figure 1-6 A simplified diagram of the early to middle Permian lithofacies paleogeography in Northeast China and adjacent areas

2. Thermal evolution of the Upper Paleozoic

In the wild Investigations and indoor studies have shown that, except for the partial impact of contact metamorphism or dynamic metamorphism, the Upper Paleozoic in Northeast China is mainly in the diagenetic to advanced diagenetic stages and has not been affected by regional metamorphism.

(1) Stratigraphic and biological fossil evidence

The late Paleozoic marine strata in Northeast China are mainly Devonian-Lower Carboniferous and Middle Permian, and the lithology is mainly clastic. Mainly rock and limestone, among which dark mudstone and bioclastic limestone are developed. The strata are stable in appearance, well stratified, rich in fossils, well preserved, and have not been transformed by metamorphism. There are many types of fossils in the Devonian-Early Carboniferous strata (Figure 1-7). The representative fossil features are that the trilobite fossils are huge, with head, thorax and tail not separated (Figure 1-7A); brachiopods are highly differentiated, with more than 35 genera (Figure 1-7B); corals and bryozoans are single The body is well preserved (Figure 1-7C, D); biological reefs are developed; and the bioclastic structure of bioclastic limestone is well preserved (Figure 1-8). The Middle Permian is characterized by the Zess brachiopod fauna, with well-preserved fossils (Figure 1-9). Well Du 101 in the northwest of the Songliao Basin found well-preserved small stone swallow fossils from the Zhesi Formation in the Middle Permian, indicating that even the Upper Paleozoic beneath the basin with a burial depth of nearly 2000 m was not affected by metamorphism.

Figure 1-7 Main fossil characteristics of Devonian to Lower Carboniferous in Northeast China

Figure 1-8 Microstructure characteristics of Upper Paleozoic bioclastic limestone in Northeast China

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Figure 1-9 Representative fossil characteristics of the Zhes brachiopod group in the Middle Permian

(2) Evidence of rock thermal evolution indicators

In order to understand the Upper Paleozoic diagenesis To study the relationship between the degree of metamorphism and the formation of metamorphism, 47 mudstone samples were collected from the late Paleozoic strata and analyzed for illite crystallinity. The results showed that the illite crystallinity distribution range is 0.28 to 0.89.

According to the currently commonly used classification standards of greater than 0.42 as high-grade diagenesis and less than 0.42 as low-grade near-metamorphism (Table 1-1), 66% of the samples tested have illite crystallinity greater than 0.42, indicating that most mudstones are in advanced diagenesis. stage. R of Paleozoic rocks in 19 existing wells in Songliao Basin. In the data, 11 wells have Ro values ??less than 2.5% (Figure 1-10). According to the diagenesis and metamorphism classification scheme of Frey (1987) (less than 2.5% is the diagenesis stage, and greater than 2.5% is the metamorphism stage), the main body of the Upper Paleozoic in the basin has not reached the metamorphism stage.

Table 1-1 Zoning of rock diagenesis-very low-grade metamorphism

Figure 1-10 Characteristics of vitrinite reflectance (Ro) of Upper Paleozoic rocks in Songliao Basin< /p>

The above-mentioned macroscopic and microscopic evidence shows that neither the outcrop area nor the Upper Paleozoic in the basin suffered from regional metamorphism in the Northeast. In the past, regional metamorphic rocks that were regarded as Paleozoic were mostly contact metamorphic rocks or dynamic metamorphic rocks related to Mesozoic rock mass intrusion or fracture activities. Contact metamorphic rocks are characterized by the occurrence of characteristic metamorphic minerals such as andalusite and cordierite and hornbeamization ( Figure 1-11, Figure 1-12). As we move away from the rock mass, the thermal contact metamorphism gradually disappears and transitions to normal sedimentary strata. The width of the contact metamorphic zone is usually 300 to 700m. Dynamically metamorphic rocks are mainly developed in and near large fault zones, with varying degrees of ductile-brittle deformation.

Figure 1-11 Microstructure of the andalusite hornfels in Qianxinglong, Moridawa Banner, Inner Mongolia

Figure 1-12 Actual measurement of the contact metamorphic zone of the Upper Carboniferous Bembatu Formation in the Greater Hinggan Mountains Section

It can be determined from this that the late Paleozoic structural attribute of the Northeastern region was not an active fold orogenic belt, but a relatively stable tectonic unit.