Cambrian-Ordovician tectonic background and evolution around Tarim basin

The Neoproterozoic ultraancient land-Rodi Niagu land began to crack in Sinian, and by the time of the Cambrian Tarim block, ocean crust had appeared.

(1) Cambrian tectonic setting and evolution around Tarim

Cambrian is a period of rapid disintegration of the mainland. The split of Siberian ancient land produced the ancient Asian Ocean, and the Salar Mongolian Lake area is the northern branch of the ancient Asian Ocean and has the nature of marginal sea. There are also Sinian-Ordovician ophiolite in Zhaisan-North Junggar area on the south side of Altai, which is the remnant of ancient Asian ocean. The lead isotopic age of the Charleske ophiolite is over 700Ma, and the Hongguleleng ophiolite and amante ophiolite develop eastward, indicating the existence of the ancient Asian Ocean. After the early CAMBRIAN, the Sarayir movement caused intense crustal compression, and late CAMBRIAN molasses, acidic K-type and Na-type post-volcanic rocks, large quartz diorite-granite intrusions and ophiolite nappes appeared on the adjacent paleocrust, indicating that the northern branch of ancient Asia began to close.

The Kazakhstan-Junggar plate includes various landmasses floating on the ancient Asian ocean, such as Yili and Middle Tianshan, including the vast areas of central Kazakhstan-North Tianshan and Balkhash-Junggar. The North Tianshan Ocean developed between the Middle Tianshan Block and the Junggar-Turpan-Hami Block, and was produced in Sinian-Cambrian, with tectonic melange (Chechecheng,1993; Ma Ruishi (1993) and Gangou ophiolite suite are the remnants of oceanic activities in the northern Tianshan Mountains, and their western extension may be connected with the Tangbale ophiolite suite. The Pb-Pb isochron value of light gabbro in Tangbale or sphene plagioclase in plagiogranite is (508±20)Ma, and the U-Pb age value is 520 ~ 480 Ma, which represents the crystallization and formation age of ophiolite (Xiao Xuchang, 199 1). Chen Zhefu and others (1997) believe that the ocean basin began to subduct under the middle Tianshan terrane from the Middle Cambrian until the Middle Ordovician.

Cracking intensified in the southwest margin of Tarim block, and a semi-deep-sea basin appeared in North Kunlun area from Late Sinian to Early Cambrian, and the transformation of Xiangyang basin in rift valley may occur in Middle Cambrian (how, 1994). However, some scholars believe that the North Kunlun Belt may have cracked into oceanic bodies in the early Sinian, which is inferred from the age of the Di Ku North Ophiolite, which is 860.5Ma (Wang Yuzhen, 1982) and 689Ma (Pan Yusheng, 1994) respectively.

(2) The tectonic background and evolution of the Ordovician area around Tarim.

Ordovician is a crucial period for the evolution of ancient Asian oceans. Its southern branch, the South Tianshan Ocean, developed and formed, while the North Kunlun Ocean dived under the Central Kunlun Block, which eventually led to the collision between the Central Kunlun Block and the Tarim Block at the end of Ordovician, forming a complete trench-arc-basin system in the Altun Belt. The change of peripheral tectonic environment led to the strong differentiation and evolution of the basin on the basis of the original tectonic framework.

1. Formation of the South Tianshan Ocean, the southern branch of the ancient Asian Ocean.

There are two ophiolite belts in the southern Tianshan Mountains (Figure 2-2-6). The 40Ar-39Ar age of gabbro in Changawuzi ophiolite in the northern margin of South Tianshan Mountains is (439.4±26.9)Ma (Hao Jie, 1993), which represents the expansion age of South Tianshan Ocean Basin, indicating that South Tianshan Ocean Basin was formed in Ordovician. Ophiolite in the southern belt belongs to Late Silurian-Devonian (Chen Zhefu et al., 1997), which represents the further development of the southern Tianshan ocean basin.

Figure 2-2-6 Schematic diagram of ophiolite distribution in Tianshan Mountain (according to Jia Chengzao et al., 1997)

2. The shrinking and closing of the North Kunlun Ocean.

Ophiolite in West Kunlun Mountain is mainly distributed along the line of Wuytag-Di Ku-Aqi Kekule Lake-Xiangride (Figure 2-2-7), among which Di Ku ophiolite is the most typical.

Figure 2-2-7 Schematic Diagram of Kunlun Mountain Structural Belt and Ophiolite Distribution (According to Jia Chengzao, 1997, 2004)

On the south side of Wuyi Tage-Di Ku-Aqi Kekule Lake-Xiangride suture zone, there are a large number of early Paleozoic intermediate-acid intrusive rocks and volcanic rocks, including Di Ku North diorite, granodiorite and Kangxi Wabei granite. , intermittent distribution over 600km. These intermediate-acid intrusive rocks were formed in the island arc tectonic environment (Ma Ruishi et al., 1995), and their isotopic ages are concentrated in 449 ~ 494 Ma (Ordovician-Silurian). Its origin is that the North Kunlun Ocean subducted southward along the line of Wuyitage-Di Ku-Xiangride, forming an early Paleozoic island arc magmatic rock belt on the Central Kunlun terrane.

In the Middle Ordovician, this subduction reached its peak, leading to large-scale volcanism and intrusion, so volcanic ash and pyroclastic deposits were widely developed in the Middle Ordovician strata of Tarim Basin. The intrusive horizon of intrusive rocks generally does not penetrate into Silurian system, indicating that its main intrusive age is Late Ordovician (Jia Chengzao et al., 2004), and its intrusive scope is mainly Maingard Depression, with a distribution area of tens of thousands of square kilometers. It is speculated that the North Kunlun Ocean was closed at the end of Ordovician.

3. Formation and extinction of the valley-arc basin system in the northern margin of Altun Mountain.

Archean granulite facies metamorphic rock series is the crystalline basement rock series of Tarnum block, which is exposed to the north of the northern margin fault of Altun Mountain. On the other hand, in the main part of Altun Mountain, the early Paleozoic aulacogen structure can be divided, and the trough may be dominated by Ordovician. The lower part of Ordovician is mainly composed of volcanic lava and pyroclastic rocks, and the upper part is mainly sedimentary rocks, mainly including gray-black slate, siliceous slate (argillaceous rock), gray limestone and other intermediate-basic tuffaceous lava, lenticular magnetite, hematite and fine-grained chronological sandstone. There is a basic-ultrabasic rock belt distributed in the east-west direction in aulacogen, and the Paleozoic strata are in sedimentary contact or structural hybrid relationship with this rock belt, which constitutes the early Paleozoic ophiolite melange belt in the north of Altun Mountain.

There are both intermediate-acid intrusive rocks and acidic intrusive rocks in the early Paleozoic depression trough in the northern margin of Altun Mountain, which appear in the form of bedrock, rock branches and irregular bodies. Chen Xuanhua et al. (200 1) think that they may have formed in the (volcanic) island arc tectonic environment of the active continental margin.

Paleozoic volcanic rocks in this area are associated with ophiolite, which is distributed from west to east in a nearly east-west direction and transited to normal sedimentary rocks in the east. The volcanic rock assemblage is bimodal, mainly basalt, rhyolite, a small amount of trachyte basalt, basaltic trachyandesite and dacite. It belongs to subalkaline volcanic rock series, in which the main type of basalt is island arc tholeiite. Volcanic rocks may have been formed in various oceanic environments, such as island arc, mid-ocean ridge and intraplate. The isotopic age of rhyolite in the west of Lapeiquan is about 480 Ma (Greels, 1999).

According to the tectonic environment of granitoids and volcanic rocks, there may be an "ocean" environment in the northern margin of Altun Mountain in the early Paleozoic, with a complete trench-arc-basin system, which Chen Xuanhua et al. (200 1) called "Al Yang Kim". It is similar to the complete trench-arc-basin system and mature ocean (Feng Yimin, 1997) formed in the early and middle Ordovician in Qilian Mountain, so it may be interlinked.

Xu Zhiqin et al. (2006) linked the North Altun Belt with the North Qilian Belt and the South Altun Belt with the South Qilian Belt, including the corresponding relationship among their volcanic island arc belt, ophiolite belt and (ultrahigh) pressure metamorphic belt. Although there are still different opinions on the study of regional tectonic framework, the study of the tectonic environment of granite and volcanic rocks shows that the northern margin of Altun Mountain may have developed a complete trench-arc-basin system in the early Paleozoic. In recent years, the research on ultrahigh-pressure garnet, lherzolite and granite gneiss containing ultrahigh-pressure garnet in Aerjin Yinglisai area shows that the formation of these ultrahigh-pressure rocks is the product of deep subduction of continental crust (Liu Liang et al., 2005, 2003 and 2002), which further proves the existence of this trench. The back-arc basin is divided into zones along the north and south of Bashikaogong fault: the Suramuning area in the south is dominated by shallow-water facies deposits on the platform and its edge, and the Lapeiquan area in the north is a set of extremely thick trough-basin facies acid-base volcanic rocks and clastic rocks with limestone strata (in time, it is the Middle Ordovician). The back-arc basin died out in the late Ordovician.