Changdu-Simao block

The early Paleozoic development history of Changdu-Simao block in the north is different from that of Simao block in the south, but the evolution of the two blocks in the late Paleozoic and beyond is basically similar. This paper focuses on the Changdu block. The formation and evolution of Changdu block can be roughly divided into the following five stages.

1. Formation stage of Precambrian crystalline basement

It is equivalent to the gneiss and granulite of Ningduo Group in Paleoproterozoic, only exposed in the north of Changdu block in summer. The U-Pb age of Duoduo in summer is 2200ma (1:200,000 radome area survey report), and the U-Pb age of Dawumang and Xiaomumang is 1870 ~ 65438+. In Little Su Mang, the U-Pb age of a set of shallow metamorphic rocks overlying Caoqu Group is over 875Ma, and there is a set of conglomerate at the bottom. The Xiongsong Group (Hao Taiping, Tan Zhengyi, 199 1) distributed in Jiangda area is also a set of gneiss and schist, and its original rocks are a set of clastic rocks, carbonate rocks and basic volcanic rocks. The Sm-Nd isochron age of garnet biotite plagioclase gneiss in Xiongsong area is (1594 240) Ma, and the Rb-Sr isochron ages of gneiss and schist are (65 1 152) Ma, (670 24) Ma and (670 24) Ma, respectively. The appearance of conglomerate at the bottom of Caoqu Group and the shallowness of metamorphic degree seem to indicate that there is Jinning movement similar to Yangtze block, and its crystalline basement may have been formed in Jinning movement.

2. Early Paleozoic fold basement formation stage

The early Paleozoic strata of Changdu block are mainly exposed in Boge, Qingnidong, Haitong and Yanjing Duojiban of Jiangda. The Borg Group determined during the Seventh Five-Year Plan period and its age may include Precambrian-Early Paleozoic, which is a set of island arc volcanic-sedimentary rock series and intruded into the island arc tonalite (Rb-Sr age is about 462Ma). The Lower Middle Ordovician in Qingnidong and Haitong areas is a set of continental slope turbidite fan-lower shelf facies clastic deposits and upper open platform facies carbonate deposits. Deep in the north and shallow in the south, clastic turbidites and trough structures developed in Ma Bao sequence A and E and A, B and E are mainly distributed in the lower part of Lower Ordovician in the north of Qingnidong. Overall obvious retrogradation sedimentary sequence. According to the trough model, the paleocurrent direction is 2 10 ~ 220, which is not covered by Devonian. Silurian is a set of clastic rocks and carbonate rocks (No.3 Brigade of Yunnan Bureau of Geology and Mineral Resources, 1990), which are mainly distributed in Duojiban area of southern Yanjing. Its sedimentary characteristics show the evolution process from bottom to top from coastal sandbars to confined bays to lagoons and then to open platforms, forming a progradational sedimentary sequence. The above situation shows that the plate subduction in the middle Ordovician Jiangdagobo area led to the development and orogeny of the island arc belt in Qingnidong area. There is an original Tethys ocean in the northeast of Changdu block, and the Silurian progradation in the south of Yanjing is not suitable for the orogeny and uplift of Changdu block after the early Ordovician, and its deposition may be more closely related to the original Tethys Lancangjiang ocean in the west of Changdu block.

3. Late Paleozoic stable block to volcanic arc and back-arc basin.

After Caledonian movement, Changdu block began to enter the stage of stable block development. Devonian system evolved from early river and coastal clastic deposits to middle and late Devonian open platform facies carbonate deposits, and layered insect reefs developed in the south, forming a complete progradation sedimentary sequence. In Qingmudong, Devonian is not integrated on the strongly folded Lower Ordovician. According to the oblique bedding of lower Devonian sandstone, the direction of water flow is from east to west, reflecting that there is ancient land in the east and transgression in the west. This progradational sedimentary sequence seems to be related to the Silurian progradational sedimentary sequence in Duojiban area of Yanjing, and also to the transgression of Lancang River on the west side of PaleoTethys. From Middle Devonian to Late Devonian, basic to intermediate-acid volcanic activities occurred in Jiangda area on the east side, including altered basic basalt, basic intermediate-acid volcanic breccia and tuff. The petrographic and geochemical characteristics of these volcanic rocks and their tectonic setting are still unclear. But there are two possibilities: ① it is located in the ancient island arc belt formed by the early Paleozoic primitive Tethys Ocean, which may be a set of lagging arc volcanic rocks; ② Since the middle and late Devonian, deep-water turbidites, radiolarian cherts and thin argillaceous limestone with conodonts have appeared in Jinsha River belt. Therefore, this volcanic activity is probably the product of the block edge extension mechanism during the formation of Jinsha River Ocean from rift to ocean basin. What is right and wrong needs further research.

In Carboniferous, especially in the early period, the passive marginal zone in the eastern part of Changdu block was still in a slope environment, and a set of clastic turbidite of channel sand body, clastic rock and floodplain sand body can be seen in Deqin area. From the west to Qingnidong, it is still an ancient land. A set of fluvial facies-coastal swamp-tidal flat facies-limited platform sponge reef-limited platform facies clastic rocks and carbonate rocks are deposited in Qingnidong area, and the river still flows from east to west. Late Carboniferous carbonate rocks in open platform facies. Coal-bearing clastic rocks appear in the lower Carboniferous in the west of Changdu-Kaixinling area, and the upper Carboniferous is mainly carbonate rocks mixed with volcanic rocks. It shows that the Changdu block uplifted in the early Carboniferous, and ancient lands appeared on the east and west edges, and both of them were transformed into open carbonate platforms in the late Carboniferous. Combining Jinshajiang-Ailaoshan Ocean in the east and Lancangjiang Ocean in the west, it seems that the uplift of Changdu block in the early Carboniferous was a dome with lithosphere extending on both sides, and the appearance of open platform facies in the late Carboniferous was caused by the expansion of marine basin transgression on both sides.

In Permian, due to the subduction of the oceanic crust on the east and west sides of the ocean basin to the Changdu-Simao landmass, the volcanic arc belts on both sides developed, and the early open platform facies carbonate deposits evolved into clastic rocks, coal-bearing clastic rocks and transitional carbonate rocks between land and sea, accompanied by basic-intermediate-basic-intermediate-acid island arc volcanic activities. The middle part of the block is transformed into a back-arc basin, and the Tuoba area is mainly a set of coal-bearing clastic rocks without volcanic rocks. The Changdu block is generally in the state of archipelagic uplift.

4. Foreland basin development stage

In the early Triassic, except for clastic rocks and volcanic rocks deposited in local depressions, such as the pre-arc zone of Jiangda volcanic arc and the east-west and NW-trending depression basins, most areas and the south of Simao block lacked deposits. In the Middle Triassic, a set of clastic rocks and intermediate-acid volcanic rocks were deposited in the piedmont depression on both sides, and the southern Guanguang area of Zhuka-Weixi-Manghuai-Jinghong in the west and the south-central area of Weixi-Ailaoshan in the east were relatively developed or well preserved, and the middle part of the block was missing. Strong orogeny began in the early Late Triassic, which led to a large number of molasses deposits in front of the mountain, and the early red beds were redeposited into conglomerates (as seen in the Yiwanshui Formation), reflecting the rapid uplift of the mountains and their continuous advancement to the basin.

Large-scale transgression occurred in the late Triassic Expo Lira period, and a set of platform carbonate rocks developed. The deposition of feldspathic sandstone and coal-bearing clastic rocks in Adoula and Duogaila periods in the late Late Triassic shows that there is rapid accumulation of mountain uplift and denudation in both basins, but the slope of the mountain is not high, so coarse clastic molasses deposition cannot be formed. Since then, except for the second transgression in the Middle Jurassic, the late Jurassic-Cretaceous is a continental red bed, and the Cretaceous is a gypsum layer. Changdu-Simao Mesozoic basin is in the stage of continuous contraction since JBOY3-Kelongbang.

5. Development stage of strike-slip pull-apart basin

The geological history progressed to Tertiary, when the Yarlung Zangbo ocean basin was closed and the Indian plate collided with the Eurasian plate. When the northeast corner of Indian landmass formed in Sanjiang area collided obliquely with the southwest corner of Yangtze landmass, the collision of the two landmasses caused the central part of Sanjiang area to shrink sharply, forming a bee waist, and the Changdu and Simao landmasses were pushed away from the north and south ends. Due to the compression of the block, a series of strike-slip pull-apart basins were formed under the control of X-type strike-slip faults. Tertiary red clastic rocks, with gypsum salt, peat or lignite deposits in some areas and periods, are mostly accumulated in strike-slip pull-apart basins except for some residual lake basins after the late Cretaceous.