I. Bayankala Stratigraphic Area
(1) Stratigraphic division of Marcand.
It is known that the pre-Sinian metamorphic strata are intermittently exposed at the core of a series of dome structures, such as Qingchuan-Pingwu Jiaoziding, Wenchuan Xuelongbao, Danbagezong and Tolerance, which can be clearly divided into basement and caprock. Its basement consists of Mesoproterozoic Tongmuliang Group metamorphic volcanic rock series and Jinning orogenic-magmatic complex and special lithostratigraphic units formed by metamorphic complex. The unconformity caprock covering different rocks in the basement is the shallow metamorphic gravel sandstone layer of Muzuo Formation in Nanhua system.
1. Tongmuliang Group
The group is a set of low greenschist facies metamorphic marine volcanic rocks. Lower metamorphic intermediate-basic volcanic rocks; Thickness > 1 100 m (bottomless); The upper metamorphic intermediate-acid volcanic rocks are 840 ~ 1600 m thick.
Metamorphic volcanic rocks are mainly composed of marine basic pillow spilite, neutral amphibole, acidic Shi Ying amphibole and spilite-amphibole pyroclastic rocks formed by central volcanic eruption, with a small amount of pyroclastic sedimentary rocks. From bottom to top, the general trend is from basic to neutral to acidic volcanic eruption. In addition, there is an annular crater near Longdong Rock in Qingchuan, which consists of subvolcanic rocks, volcanic lava, volcanic conglomerate-breccia and diorite-quartz diorite-granodiorite intrusions in volcanic rocks. At present, zircon U-Pb ages of diorite and quartz diorite are 865Ma and 829Ma, respectively (Northwest Sichuan Geological Brigade, 1996). Combined with regional comparison, the group is only roughly equivalent to the rock profile of the middle and upper part of Guihuaqiaogou Formation in the middle of Bikou Group in Motianling Division, which was mainly formed in the late Jixian period before 865Ma, or extended to the early Qingbaikou period.
What needs to be explained here is that this book does not adopt the author's view of replacing Tongmuliang Group with Bikou Group and abandoning the latter in Lithostratigraphy of Sichuan Province (1997). The reason is that the formation tectonic environment of volcanic rock assemblage of Bikou Group and Tongmuliang Group is obviously different, and the formation time is also far apart. The volcanic rock series of Guihuaqiaogou Formation in the middle of Bikou Group is mainly formed in the mid-ocean ridge tectonic environment of Jixian period, and it is a bimodal spilite-quartz keratophyre formation composed of sodium-rich alkaline tholeiite series, which is characterized by a general lack of neutral keratophyre volcanic rocks. The volcanic rock series of Tongmuliang Group was mainly formed in the continental volcanic arc tectonic environment of Jixian County in the late Cretaceous. It is mainly composed of intermediate-acid volcanic lava-pyroclastic rocks in potassium-rich subalkaline calc-alkaline rock series, and together with intermediate-acid intrusive rocks in Jinning, it constitutes the "Jiaoziding Magmatic Complex" with basement folds. So the two should not be compared together.
2. Magmatic complex and metamorphic complex
As an objective special lithostratigraphic unit, it is mainly formed by Jinning orogeny and its closely associated plutonic magma intrusion, migmatization and granitization. Taking Gezong granitic magmatic complex and tolerance migmatized gneissic metamorphic complex in Danba area as examples, it is briefly described as follows:
(1) "Each case of granitic magma complex" (AnZGZ)
Exposed in the core of Danbagong dome structure, it is mainly composed of plagiogranite with relatively simple lithology. Three groups of zircon U-Pb surface ages obtained from this rock mass are almost consistent with 864 1 Ma (Hou Liwei et al., 2002), indicating that its diagenetic age belongs to the late Qingbaikou period. In addition, the late basic and acidic shallow diagenetic veins inside and outside the intrusive complex, as well as the existing isotopic dating results, also show that the ancient complex also experienced the superimposed transformation and influence of multiple episodes of magma and metamorphic thermal events in the late Indosinian-early Yanshan and late Yanshan-early Himalayan.
(2) "tolerance migmatized gneissic metamorphic complex" (AnzGC)
It is the most representative metamorphic core complex and constitutes the core of several domed structures in Danba area. The zircon U-Pb age of gneiss is 798±24Ma (Hou Liwei et al., 2002), indicating that the ultra-deep metamorphic thermal event occurred at the end of the late Qingbaikou period, slightly later than the formation period of various magmatic complexes. Then, in Indosinian-early Yanshan period, about 200 ~ 150 Ma ago, it experienced a progressive metamorphic superposition transformation of medium-pressure dynamic heat flow, and another superposition transformation, which occurred in late Yanshan-early Himalayan period, about 124 ~ 63 Ma ago, and was dominated by shallow fine-grained rock-pegmatite dike intrusion.
To sum up, two special tectonostratigraphic events, represented by Gezong magmatic complex and tolerant metamorphic complex, were formed in the continental margin volcanic-plutonic magmatic island arc tectonic environment in Jinning's main orogenic stage, just like the Jiaoziding magmatic complex formed by metamorphic volcanic rocks of Tongmuliang Group and intermediate-acid intrusive rocks mentioned above. Jinning orogeny with important geological significance in western Sichuan Plateau mainly occurred 864 years ago.
3. Muzuozu
It is continuously exposed in Pingwu and Qingchuan, and distributed sporadically in Danbagezong and other places, characterized by metamorphic conglomerate and gravelly metamorphic clastic rocks.
The lithology and lithofacies of this formation vary greatly in the horizontal and vertical directions. The two wings of Muzuo Mupi anticline in Pingwu County are mainly composed of thick massive gravelly metamorphic sandstone and metamorphic tuffaceous sandstone, with a thickness of 694m, and there are few conglomerates in the south wing of the anticline and five conglomerates in the north. The thickness of conglomerate can reach 1 1 16m. The composition of conglomerate gravel is complex, mainly composed of granite, felsic, veined quartz and various greenschists, metamorphic sandstone, dolomite and siliceous rocks. There are etching marks on the gravel surface, which belongs to ice raft marine rockfall deposition. In Huang Yang area of Pingwu County, it is a kind of gravelly tuffaceous phyllite with a thickness of 500 ~1000 meters. From Nijiagou to Heigouli in Qingchuan County, the gravelly metamorphic sandstone is lenticular. The Yangou Formation in Qingchuan County (formerly known as Yangou Formation) contains a set of purple metamorphic tuffaceous glutenite, metamorphic siltstone, black-gray chlorite sericite phyllite with impure limestone lentils or more than a thousand tuffaceous limestone, and the volcanic debris increases rapidly, with a thickness of 0 ~ 1250 m ... To the west, Weimen in Maowen County is a set of conglomerate (formerly known as Weimen conglomerate).
Figure 1- 1 comprehensive stratigraphic zoning map of Songpan Ganzi orogenic belt (modified according to Sichuan lithostratigraphy 1997).
In the area of Qiwoliang-Longdongyan at the junction of Pingwu County and Qingchuan County, the rock group is superimposed on the "Jiaoziding Magmatic Complex" in Jinning Period in a ring unconformity, and the parallel unconformity is located under the Chikoukou Formation of the Lower Sinian or the Crystal Formation of the Upper Sinian. The composition lithology is light gray medium-thick metamorphic tuffaceous sandstone, containing gravels. The timely sandstone contains specularite with a thickness of 20 ~ 270 meters. The gravels are derived from granite and volcanic rocks in the underlying basement, with a diameter of 0.5 ~ 3 cm and sub-angular shape, with a content of 3% ~ 8%, mainly distributed at the bottom and lower part of the group, and the structural attribute is bottom conglomerate.
Table 1- 1 Comprehensive Stratigraphic Division and Stratigraphic System of Songpan-Aba Orogenic Belt and Its Adjacent Areas
sequential
In the Danba area, this formation is exposed intermittently at the periphery of various tolerance domes and unconformity contact with various rock masses (Figure 1-2). It occurs in Jinning basement complex, dolomite marble crystallized in Upper Sinian and medium-pressure metamorphic rock belt with enhanced dynamic heat flow. The protoliths, gravelly tuffaceous composite sandstone and composite sandstone, have evolved into ductile structural rocks such as gravelly mylonite mica schist, gravelly gneiss or felsic schist under the strong structural decomposition and metamorphic differentiation in the later period, and they show structural lenses or even all structures are missing in the mylonite bedding ductile shear structural zone.
Figure 1-2 Profile of angular unconformity contact relationship between Muzuo Formation and each rock mass (each river mouth in Danba County, according to1∶ 200,000 Danbatu)
(2) Stratigraphic division of Motianling
Motianling area, located at the junction of Sichuan, Gansu and Shaanxi provinces, is a typical producing area with the widest distribution area, the best extension continuity, the most complete stratigraphic development and the largest number of stratotype sections in the pre-Sinian metamorphic strata in the western Sichuan Plateau. Its representative rock strata are Bikou Group formed in Mesoproterozoic-Qingbaikou period and Muzuo Formation metamorphic strata in Nanhua period.
Bikou Group consists of Mesoproterozoic-Qingbaikou folded basement, including Dashaba Formation, Guihuaqiaogou Formation and Yin Ping Formation from bottom to top. The unconformity of Muzuo Formation above Bikou Group is a part of Nanhua sedimentary cover.
1. Dashaba Formation
Limited to the exposed core of Mupi anticline in Dashaba, Huang Yang-Shuibai Township, northern Pingwu County. The composition lithology is dark gray metamorphic feldspar chronological sandstone, siltstone and silty sericite chronological phyllite, and sericite chronological phyllite rhythmically interbedded, with a small amount of granite and quartzite pebbles occasionally seen, and the exposed thickness is more than 750 ~ 1400 m, with no bottom.
Dashaba Formation is a set of terrigenous clastic turbidite flysch formation, which is divided into deep-sea and semi-deep-sea facies from bottom to top, which is different from the overlying strata because it does not contain pyroclastic materials (rock strata in Sichuan Province). According to the stratigraphic relationship of Guihuaqiaogou Formation (161118ma ~1304196ma), it is inferred that this formation may be mainly formed in the early stage of Jixian County.
2. Guihuaqiaogou Formation
Widely exposed in the core of the middle and eastern section of Mupi anticlinorium and the north and south wings of the western section. Composition lithology is mainly metamorphic basic and acidic volcanic rocks, such as grayish green-dark green albite actinolite schist containing chlorite, albite containing albite, albite containing albite, albite containing albite, albite containing albite, albite containing albite, albite containing albite and biotite. The general thickness is 860 ~2800 m, and the maximum thickness can reach 5300 m.
Guihuaqiaogou Formation is a set of bimodal marine spilite-quartz keratophyre formations on the ocean ridge, which is mainly composed of mantle-derived sodium-rich tholeiite series. In the existing isotopic dating results, the age data that can represent the diagenetic age mainly include: the whole rock Sm-Nd age value161118ma (Zhang et al., 1996). The zircon ages of U-Pb method are 1304 196 Ma and 1367 Ma, and those of whole rock Rb-Sr method are 970 124 Ma (Hu Zhengdong et al., 1988). Accordingly, the diagenetic time was mainly 65438 years ago. Rb-Sr dating of the whole rock obtained from the glaucophane greenschist in this group has two sets of data: one is 977 ~933 Ma, and the other is 837 Ma (Tao Hongxiang et al., 1993), indicating that this group has experienced structural deformation caused by Jinning orogeny and transformation of high-pressure and low-temperature dynamic metamorphism in greenschist facies area. It experienced the intense superimposed transformation and reorganization of late Indosinian structure, metamorphism and magmatic emplacement. SHRIMP zircon U-Pb dating is 840 ~776 Ma (Yan et al., 2003).
3. Horizontal Tone Group (AnZy)
Limited to the north of Pingwu and the north and south wings of the western section of Mupi anticlinorium in the northwest of Qingchuan. The composition lithology is mainly characterized by rhythmic interbedding of gray-green tuffaceous granulite with mica-bearing microcrystalline schist, gray-green tuffaceous chlorite sericite phyllite and silty sericite phyllite, and the thickness ranges from 0-1800 m.
According to the research of Liu et al. (1996), this formation is a set of volcaniclastic-terrigenous clastic turbidite strata integrated on Guihuaqiaogou Formation. The microscopic shear strain marks identified by Xu Zhiqin et al. (1992) in this formation in northern Pingwu, such as early bedding recumbent fold, mylonite foliation, early biotite phenocryst domino fabric with shear direction from S→N, and asymmetric spin fabric of timely phenocryst, can echo the high-pressure glaucophane greenschist facies metamorphism developed in Guihuaqiaogou Formation, and are the main orogeny formed in Jinning period in late Qingbaikou period.
4. Muzuozu
The unconformity surface is located on the folded basement composed of metamorphic volcanic-sedimentary rock series of Bikou Group of Mesoproterozoic-Qingbaikou system, and the integration or parallel unconformity surface is located under the metamorphic marine deposits of Lower Qiongkou Formation or Upper Crystal Formation of Sinian system, and it is composed of a set of shallow sea ice raft facies metamorphic glutenite clapboard layers with a thickness of 0 ~ 1250 m ... According to He Shucheng's research (1982). The lower and upper sections of the stratotype section of this formation are mainly grayish green and purplish red metamorphic tuffaceous glacier conglomerate-gravelly tuffaceous composite sandstone, with marine ice raft falling rocks and pyroclastic rocks, and the middle section is mainly grayish black silty slate (or phyllite) with normal marine facies, which seems to be similar to the moraine of Nanhua Gucheng Formation in Xixia area, Hubei Province.
In this group of slate interlayers in Dagu, Qiaozhuang County, Qingchuan County, microfossils were collected, such as Dichondra crassipes, T.SP, Laminaria japonica, Dichondra longifolia, Dichondra glabra, Chlamydomonas sp. and so on. Among them, the representative species have high molecular abundance, such as Phaeocystis, Thallus Laminariae and characteristic molecule Baul-inglla feveolata. It is similar to the fossil assemblage of Trachelospermum-Curvatum and its common genera and species in the Nanhua system of South China, and has certain reference value in chronostratigraphic division.
In addition, the single zircon U-Pb age of tuffaceous cement in the metamorphic calcareous moraine conglomerate at the bottom of Muli Shuiluo area is 748 ~ 777 Ma (lithostratigraphy of Sichuan Province, 1997), which provides an important isotopic age basis for further determining that the formation age of Muzuo Formation began in the early and late South China.
To sum up, Muzuo Formation in Motianling area is a set of polygenic glutenite sedimentary caprock, which was formed in the glacial-interglacial period from the early and late South China. It is composed of shallow sea ice raft rockfall, terrigenous detritus and pyroclastic materials, and has the structural properties of molasses and breccia. Between it and Guihuaqiaogou Formation or Yin Ping Formation in the middle of the underlying Bikou Group, there was an angular unconformity structural interface with important zoning significance formed by Jinning orogeny. The contact relationship that seems to be "pseudo-conformity" or "parallel unconformity" today is actually the result of deep high-medium temperature ductile shear deformation-metamorphism caused by late Indosinian orogeny, which completely replaces the primary foliation and early secondary foliation of pre-existing unconformity structural interface and its upper and lower strata, so this book calls it "hidden unconformity" contact relationship. In view of the stable production horizon and obvious lithologic and lithofacies marks of Muzuo Formation, it is feasible to take the occurrence and bottom boundary of metamorphic glutenite as the boundary between Nanhua System and Qingbaikou System or Jixian System.
(3) Muli District
The pre-Sinian metamorphic strata are limited to the core of Shuiluo dome structure in Muli County, formerly known as Chasi Group (Sichuan Dispatching Team, 1984). 1997 lithostratigraphy of Sichuan province is divided into two formal lithostratigraphic units according to the results of rock assemblage and isotopic dating: the newly established Xiakasha Formation in the lower part and the used Muzuo Formation in the upper part.
1. Xiakasha Formation (Qbx)
Unconformity located under the metamorphic conglomerate layer of the Jimuzuo Formation in Nanhua is a set of metamorphic volcanic sedimentary rocks in greenschist facies. Its composition lithology: the lower part is gray-gray green thick metamorphic felsic siltstone, silty sericite phyllite, carbonaceous phyllite, metamorphic tuff and other unequal thickness interlayers; The middle part is interbedded with nano-felsic granulite, nano-felsic schist and mica-quartz schist with unequal thickness; The upper part is gray-green green curtain rising schist, chlorite nano-schist and nano-long sub-schist, containing plagioclase granulite lens, and the thickness is >1000 m; 52 1 m, not bottomed out. In the chlorite sericite quartz schist about 100 m from the top boundary of the group, the U-Pb ages of two single zircon are 855±8ma and1083 2 Ma, respectively, indicating that their formation age is roughly limited to the late Jixian (exposed middle and lower strata) to the early Qingbaikou (upper strata), which is roughly similar to Tieling. In addition, the study on the recovery and formation environment of metamorphic rocks also shows that the group is a volcanic arc subalkaline-calc-alkaline intermediate-acid interbedded volcanic rock-sandy argillaceous clastic rock formed in the environment of active continental margin basin, which constitutes the southwest extension of the post-Jinning Longmenshan-Muli continental margin magmatic island arc orogenic belt. Later, under the strong transformation of late Indosinian deep ductile shear deformation and metamorphism, the primary bedding and unconformity structural interface were completely replaced by late structural foliation and disappeared, and evolved into the illusion of "false integration" with the overlying strata, which is called "hidden unconformity" contact relationship in this book.
2. Muzuozu
The concealed unconformity surface covers the upper part of Jixian Formation and the lower part of Kasha Formation, and the parallel unconformity surface is located under the Upper Sinian Crystal Formation. It consists of gray-gray green massive metamorphic moraine conglomerate, gravel green mud silk Junna schist, gravel green mud green episodic nanoschist, etc. , and contains a small number of suspected microfossils, such as Cysticercus. And Leiopsophosphaera sp. with a thickness of 65438.
These microfossils are very similar to those found in the Muzuo Formation of Motianling section where the stratotype section is located, and belong to the common microfossil genera and species represented by coccidia community in South China. In addition, the zircon U-Pb ages of quartz diorite gravel and cement (matrix) in the gray-green massive metamorphic tuffaceous conglomerate at the bottom of the group are 808 ~886 Ma and 748 ~777 Ma, respectively (Sichuan lithostratigraphy, 199 1). It can be seen that the formation time of this formation belongs to the Nanhua period, and there is an angular unconformity structural discontinuity interface with the underlying Xiakasha formation, which was formed by Jinning orogeny. Under the action of deep ductile shear deformation-metamorphism and structural foliation replacement in the late Indosinian period, it was transformed into a hidden unconformity contact relationship that was difficult to identify macroscopically.
Second, South Qinling-Dabie Mountain Stratigraphic Area
Represented by the so-called greenschist facies metamorphic rock series of Baiyigou Group, it is confined to the core of Baiyigou anticlinorium at the border of Sichuan and Gansu. The group is a cross-system lithostratigraphic unit, formed from Nanhua to Early Sinian, and changed from continental deposits to marine deposits.
1. Saiyikuo Formation (Nhs)
This formation is located under Xianglongchang Formation, which has obvious rhythmic structure and can be roughly divided into upper member and lower member according to lithologic combination characteristics.
Interlayer between lower metamorphic calcareous glutenite and tuffaceous slate: it is unequal-thickness interbedded between gray-gray-green thick-massive metamorphic (gravelly) feldspar complex and medium-thick layered metamorphic tuffaceous siltstone, with lenticular metamorphic limestone conglomerate, and a small amount of tuffaceous slate interbedded with tuff. The exposed thickness is >: 500 m, with no bottom.
Upper metamorphic tuffaceous sandstone mixed with tuffaceous slate: the middle and lower part is gray-gray-green thick-massive metamorphic feldspar complex sandstone mixed with metamorphic tuffaceous siltstone, and the upper part is gray-green-purple thin-medium thick layered metamorphic tuffaceous siltstone mixed with metamorphic tuffaceous thin-layer, with a thickness of 568m. ..
2. Xianglongka Formation (Nh-Zx)
According to the lithologic combination characteristics, it can be divided into lower metamorphic glutenite section and upper metamorphic siltstone-phyllite interbedded section.
Lower metamorphic glutenite section: green-gray thick layer, mixed with massive metamorphic limestone conglomerate and (gravelly) tuffaceous debris composite sandstone, mixed with a small amount of metamorphic limestone siltstone and silty slate, with a thickness of 547m ... The conglomerate is lenticular in appearance and its strike extension is extremely unstable. Gravel content is generally 40% ~ 70%, and the proportion of granite, granite porphyry and quartzite in gravel composition is obviously increased, with good roundness; The content of volcanic rocks dominated by dacite is relatively small, mostly sub-angular-sub-circular. The matrix is filled with coarse sand and cemented with sandy argillaceous (sericite) matrix. Metamorphic sandstone has a low maturity in composition and structure, which is mainly supported by miscellaneous foundation and cemented by basement.
Upper metamorphic siltstone-phyllite interbedded section: it is gray-gray green metamorphic sericite-bearing timely sandstone, silty sericite-bearing timely phyllite and sericite-bearing timely phyllite interbedded with unequal thickness, with a thickness of 480-570 meters. Compared with the underlying stratum-the lower section of Saiyikuo Formation and Xianglongchang Formation, this rock section is obviously different, mainly in the following aspects: the debris particles are obviously thinner.
Mao (1990) and (1992; 2003) shows that:
1) The Saiyikuo Formation and its upper integrated metamorphic glutenite section of Xianglongchang Formation were constructed by fluvial alluvial-diluvial metamorphic pyroclastic-terrigenous coarse clastic rocks formed in the intracontinental rift tectonic environment between "Ruoergai micro-ancient land" and "Taohe micro-ancient land". According to its occurrence horizon, lithologic association, sedimentary facies and geochemical characteristics of pyroclastic rocks, it can be compared with terrestrial acidic volcanic rocks mixed with pyroclastic silt-sandstone strata and tuffaceous glutenite strata of the sixth group in Xiaoxiangling-Luojishan area, southwest Sichuan. However, the shallow ice raft facies metamorphic complex glutenite clapboard group of Muzuo Formation in Motianling sub-district belongs to different correlation systems in the same period.
2) The lithologic association and sedimentary facies characteristics of the metamorphic siltstone-phyllite interbedded rock section in the upper part of Xianglongka Formation are obviously different from the metamorphic glutenite section+Saiyikuo Formation in the lower part of Xianglongka Formation, but similar to the sericite-bearing phyllite in the adjacent area, with metamorphic siltstone-fine sandstone layer, which can be compared with the Doushantuo Formation of the Lower Sinian in Yangtze area.
3) Zircon U-Pb age of rhyolite gravel and Rb-Sr isochron age of metamorphic tuff are 734±63Ma and 737.6±22.4ma (gross, 1990), which can represent their diagenetic ages respectively. In view of this, combined with the results of regional stratigraphic correlation analysis, the lower limit of the formation age of Baiyigou Group can be determined as the middle and late stage of early South China, slightly later than 800Ma and slightly earlier than 740Ma, which is roughly similar to the formation time of metamorphic gray conglomerate at the bottom of Muzuo Formation in the adjacent Mulishuiluo area (before 777 ~ 748 Ma). In the profile of metamorphic siltstone-phyllite interbedded rocks in the upper part of Xianglongka Formation, the Rb-Sr isochron age of tuffaceous slate is 640±40Ma (Mao et al., 1990), and the K-Ar age of tuffaceous slate is 619 46ma (Northwest Sichuan Geological Brigade, 65438+).
To sum up, Baiyigou Group is a cross-system lithostratigraphic unit, which was formed in South China-Early Sinian, and transformed from alluvial and diluvial facies of inland rift rivers to normal marine sediments, which can be compared with Muzuo Formation and Centipede Formation in the adjacent Motianling area.