I. Major volcanic zones
According to the geological records of Xinjiang Uygur Autonomous Region (1993) and related research data (Yu Xueyuan et al.,1995; Chen Zhefu et al., 1997) summarized the general situation of major volcanic zones (from north to south) related to the section area as follows:
The early Paleozoic volcanic belt in northern Altai belongs to the early Paleozoic continental margin active belt in northern Altai (including Kelan Devonian-Carboniferous back-arc basin and Kaba-Narem Carboniferous-Permian magmatic arc), with only late Ordovician and middle-late Silurian volcanic rocks. Late Ordovician volcanic rocks are intermittently exposed in the western part of the rock belt along the northern part of Altay Town-Baihaba line, and the thickness in Baihaba area is the largest (up to 927m). The lithology is neutral, intermediate and acidic andesite-dacite-rhyolite combination, with tuff or tuff containing breccia locally. Middle-late SILURIAN volcanic rocks occur in Kulmutu Group on the north side of the western middle section of the rock belt in the form of interlayer, lenticular or irregular rock strains. Lenticular, irregular rock-like or rock-dendritic metamorphic basic rocks are distributed linearly along the Keketuohai-Kuwei-Ayubulak line. These rocks have been metamorphic to amphibolite gneiss or biotite amphibolite gneiss, and their original rocks are mainly basic and neutral basalt-andesite combination, and the cumulative thickness of volcanic rocks is less than 500m (Geology of Xinjiang Uygur Autonomous Region,1993; How, 1994). The volcanic rocks of the Late Ordovician and Middle and Late Silurian are generally low in alkali content, and the basic volcanic rocks of the latter are also very rich in MgO (4.56% ~ 12.73%).
The late Paleozoic volcanic belt in southern Altai belongs to the active belt of the late Paleozoic continental margin in southern Altai, mainly Devonian volcanic rocks, with a small amount of Carboniferous volcanic rocks, mainly distributed in Hongshanzui and Fuyun rock belts in the east. Volcanic rocks are mainly composed of neutral and intermediate acid andesite-dacite-rhyolite. According to the research of Yu Xueyuan et al. (1995), Devonian volcanic rocks in the western part of the southern Altai volcanic belt are distributed in the northwest-southeast direction on both sides of the Habahe River and extend to Kazakhstan in the west, mainly composed of amphibole-amphibole lava and pyroclastic rocks, with a small amount of lepidolite lava and pyroclastic rocks; Middle Devonian volcanic rocks in Ashele area are extremely developed, mainly composed of spilite lava and pyroclastic rocks, and there are also many keratophyre-timely keratophyre lava and pyroclastic rocks in some areas. Generally speaking, Devonian volcanic rocks in the southern Ataixi section are banded, followed by Devonian volcanic rocks and Middle Devonian volcanic rocks from north to south. Devonian is dominated by intermediate-acid volcanic rocks, while Middle Devonian is dominated by basic volcanic rocks, all of which belong to spilite-keratophyre-quartz keratophyre series.
Chen Zhefu et al. (1997) divided the Altai volcanic rocks into five eruption cycles, namely, early Late Ordovician, Devonian, Middle Devonian, Late Devonian and Early Carboniferous. The bottom of the first eruption cycle (Late Ordovician) is dominated by explosive breccia and agglomerate, with a small amount of ignimbrite, the middle is dominated by andesite and dacite, and the upper is dominated by dacite and rhyolite. The second eruption cycle (Devonian) consists of spilite-keratophyre-quartz keratophyre. Their contents are 15% ~ 20%, 30% ~ 35% and 45% ~ 65% respectively, and the contents of silica, sodium oxide and K2O are 47% ~ 53%, 54% ~ 64% and 64% ~ 75% and 4.6% ~ 5.5% respectively. 1% and 0.52 ~ 2.00%; In the third eruption cycle (early Middle Devonian), the rock assemblage of different sections is quite different. Ashele area is dominated by quartz keratophyre, with a small amount of basalt and andesite, most of which belong to tholeiite volcanic rocks. The lower part is dominated by eruptive sedimentary facies (pyroclastic rocks+sedimentary rocks), with less overflow lava. Upward, from explosive facies (agglomerate)+overflow facies+sedimentary facies, lava of overflow facies and clastic rocks of sedimentary facies gradually increase. The clastic rocks in the lower part of Altay Formation near Altay City contain many pillow-shaped spilite, and the contents of SiO _ 2, TiO _ 2, Al _ 2O _ 3 and Na2O are 44% ~ 52%, 1.8% ~ 2.38%, 16% ~ 17.72% and 4.29% ~ 5, respectively. Rb-Sr isochron age (fitting correlation coefficient of 6 samples is 0.989) is 380±27ma, and the initial ratio of 87sr/86sr is 0.7074 1 ~ 0.70906, which indicates that crustal materials are added to the magma, which is close to the initial ratio of Sr of island arc basalt. The fourth eruption cycle (Late Devonian) is only exposed in the area north of Ashele village in Ataixi section. The lower part is amphibole or dacite or Shi Ying amphibole, the middle part is dominated by andesite (or amphibole), and the upper part is dominated by basic basalt with a small amount of neutral basalt, which seems to have reverse sequence characteristics. In the total alkali silicon diagram, it belongs to high aluminum (calcium alkali) series. The eruptive cycles are mostly near-source volcanic rocks, which are composed of multiple eruptive overflows, forming a repetitive rhythm; The fifth eruption cycle (Early Carboniferous) is mainly developed in Noerte superimposed basin, which is a set of acidic volcanic rocks. The main lithology is residual porphyry, rhyolite, dacite, quartz porphyry and felsic, and the upper part is argillaceous siltstone and siliceous slate, with a small amount of limestone and calcareous sandstone. Most volcanic rocks belong to alkaline series.
The Sawuer Late Paleozoic volcanic belt belongs to the Paleozoic continental margin structural belt in the northern margin of Junggar, and is dominated by Devonian and Carboniferous volcanic rocks. It is the region with the longest volcanic activity, the greatest intensity and the widest distribution of volcanic rocks in late Paleozoic in Xinjiang. Volcanic rocks are distributed from the Middle Devonian to the Middle Carboniferous in the Sawuer island arc area in the western part of the northern margin of Junggar, and the Devonian volcanic rocks are basalt-andesite-dacite-rhyolite combination; Carboniferous volcanic rocks are basalt-andesite-dacite combination; Early Permian volcanic rocks are distributed in Jimunai, Ursu and other places. Generally, the lower part is basalt, andesite, intermediate-acid volcanic breccia and tuff, and the upper part is andesite, dacite, rhyolite, volcanic breccia and tuff, forming a basalt-andesite-rhyolite combination. Fold uplift occurred in this area in Permian, and the volcanic rocks are continental volcanic rocks.
Borocco's Nu Paleozoic volcanic belt is distributed in the early Paleozoic Ordovician and Silurian, the late Paleozoic Devonian and Carboniferous, in which the Carboniferous is the most developed. There are few volcanic rocks in the Middle Ordovician, which is a typical basalt-rhyolite (felsic) combination; Middle Devonian volcanic rocks are relatively simple basalts and albite basalts; Carboniferous volcanic rocks are mainly calc-alkaline series (basaltic andesite-andesite-syenite-rhyolite-dacite-rhyolite), accounting for more than 80%, and a small amount of alkaline series (alkaline basalt+basaltic trachyandesite+trachyandesite), accounting for about 15%. Nu volcanic rocks in Borocco belong to a series of potash volcanic rocks, which are the products of different tectonic environments. They belonged to an expanding aulacogen in the Early Paleozoic (Ordovician-Silurian), gathered in aulacogen in Devonian and developed into island arcs from Carboniferous (Chen Zhefu et al., 1997).
Yili basin in Carboniferous-Permian intracontinental rift volcanic belt is the main distribution area of early Carboniferous volcanic rocks in Tianshan area. The lower part of the basin is composed of andesite, dacite, rhyolite and pyroclastic rocks, with occasional basalt, the thickness of which can reach 4543m, and the ratio of pyroclastic rocks to lava is 1: 1 to 1:5, which belongs to marine fissure-central eruption. The distribution of volcanic rocks in the Middle Carboniferous decreased obviously in the Early Carboniferous, and concentrated in the middle of Yili Basin. The main rocks are andesite, dacite, olivine basalt and pyroclastic rocks. The ratio of pyroclastic rock to lava is 1: 1 ~ 5: 1, and its explosive intensity is obviously greater than that of Early Carboniferous, belonging to shallow sea fissure eruption. Early Permian volcanic rocks are still widely distributed in Yili basin. Generally, the lower part is acid lava, dacite mixed with andesite and ignimbrite, and the upper part is basalt, andesite mixed with dacite, quartz porphyry and pyroclastic rock. The thickness of volcanic rocks is 42 1 1m, and the ratio of pyroclastic rocks to lava is 1:2, which belongs to the continental volcanic eruption environment. The distribution of volcanic rocks in the Late Permian decreased obviously, with rhyolite in the lower part, tholeiite in the middle part and andesite clastic rocks in the upper part, which belonged to the eruption in the center of the continent. According to petrochemical discrimination, Carboniferous and Permian volcanic rocks in the rock belt of Yili Basin are mainly calc-alkaline series, and some are tholeiite series and alkaline series. The volcanic rock assemblage is dominated by basalt-andesite-dacite-rhyolite assemblage, and a small amount of basalt-andesite-dacite assemblage (Middle Carboniferous). Carboniferous volcanic rocks have continental rift and island arc tectonic environment, while Permian volcanic rocks belong to continental volcanic eruption in post-orogenic stage.
Second, volcanic rock assemblage characteristics
According to the characteristics of volcanic eruption, volcanic rocks mainly include erupting lava and pyroclastic rocks, as well as some hypabyssal-ultrahypabyssal intrusive subvolcanic rocks. Volcanic lava generally represents magma that comes directly from the source area and has less pollution. The volcanic rocks exposed in the geoscience profile studied here are mainly lava-facies volcanic rocks. Through the classification and naming of volcanic lava, we study its source, genesis and evolution, and discuss its structural attributes and plate activity characteristics. Due to the poor crystallinity of volcanic rocks, we have collected more than 800 published analysis data of Paleozoic volcanic rocks in Xinjiang (regional geology of Xinjiang Uygur Autonomous Region,1993; How,1994; Yu Xueyuan et al.,1995; Zhang Xiangbing et al.,1996; Tuguangchi, 1993), combined with the analysis data of volcanic rocks in local geological profile (Table 3- 1), the Paleozoic volcanic rocks in Xinjiang were uniformly classified and named.
Because of the different volcanic age, volcanic rock development and research degree, the distribution of volcanic rock data in various volcanic areas and zones is uneven, but they are all representative. Figures 3-2 to 3-4 show the general trend of chemical composition and rock type distribution of Paleozoic volcanic rocks in Siberia plate area and Kazakhstan-Junggar plate area. It can be seen that the Paleozoic volcanic rocks in different magmatic rock belts in the two plate areas cover all types of basic-neutral-intermediate acid volcanic rocks, but there are differences in each volcanic rock belt. As shown in Figure 3-2, the volcanic rocks in northern Altai are located in basalt and rhyolite areas respectively, with bimodal volcanic rock distribution characteristics. Volcanic rocks in the Erqis-Burgen plate tectonic suture zone are mainly located in basic and intermediate-basic volcanic rocks, but lack of intermediate-acid and acidic volcanic rocks. Devonian-Carboniferous aulacogen volcanic rocks in Dalbout-Kela Miri, Malaysia are mainly concentrated in basic-intermediate-basic volcanic rocks, with a small amount of intermediate-acid volcanic rocks (Figure 3-3). In the past, Paleozoic volcanic rocks in Xinjiang were named basalt, basaltic andesite, andesite, dacite and rhyolite (all belong to normal series), but more than half of Paleozoic volcanic rocks can be found on TAS map as alkaline trachyte, basaltic trachyte, trachyte and alkaline rhyolite.
The increase of total alkali (Na2O+K2O) in volcanic rocks mainly reflects the increase of K2O. On the SiO _ 2-K2O diagram (Figure 3-5 to Figure 3-7), we can see the evolution trend of volcanic rocks from low-potassium tholeiite series to high-potassium calc-alkali series. A few of them have entered Olivain coarse rock series, and most of them belong to calc-alkali series and high-potassium calc-alkali series, the Erqis-Bourgen plate tectonic suture zone and the volcanic rocks of Dalbout. From the FAM diagram (Figure 3-8 to Figure 3- 10), it can be clearly seen that its main series is calc-alkaline volcanic series, but there is no obvious evolutionary relationship, and a considerable number of volcanic rocks belong to tholeiite series. The above shows the compatibility of volcanic rocks with continental margin and island arc environment. We also found picrite (Table 3- 1, XJ970 14) in JiangGuelec village in the south of Altai, with only 4 1.09% SiO2 _ 2 and 24.87% MgO.
Three. Rare earth elements and trace elements
The instrumental neutron activation analysis results of rare earth elements and trace elements in representative volcanic rocks in 24 profile areas are shown in Table 3-2, and their distribution patterns are shown in Figure 3-1~ Figure 3- 13. According to the contents and distribution patterns of rare earth elements and trace elements, it can be basically divided into three types: basic basalt, neutral andesite, intermediate-acid dacite and rhyolite. The content of rare earth elements in basalt is generally less than 50μ g/g (22 ~ 45μ g/g), which is 20 times that of chondrite 10 ~ 20 times. Among them, two kinds of basalts (one is oceanic basalt) in the Carboniferous-Permian magmatic arc zone of Kalba-Narem have the lowest REE content, which is only about 10 times that of chondrites, and they have oceanic basalts. Generally, basalts show slightly positive or no Eu anomalies, which may be related to a small amount of plagioclase crystal differentiation, except that oceanic basalts show obvious negative Eu anomalies. The rare earth content of individual basalts can be as high as 10 1 μ g/g, and the rare earth content of andesite is generally less than 100μ g/g (73 ~ 99μ g/g), which is 40 ~ 60 times that of chondrite, showing a moderately separated distribution pattern of rare earth elements, showing EU and EU to varying degrees. The content of rare earth elements in individual andesite (YX97057) reaches145μ g/g, and the content of rare earth elements in dacite and rhyolite is the highest, with a great change of (74 ~ 177) μ g/g, which is generally 50 ~1g of chondrite. The distribution patterns of rare earth elements and trace elements have obvious negative anomalies of EU and Ba. The REE content (765,438+0 μ g/g) and distribution pattern of spilite (YX97002) are similar to those of intermediate-basic basalt-andesite, and there is no negative Eu anomaly. The rare earth content (129μg/g) of a quartz keratophyre (YX97005) is equivalent to 100 times that of chondrite, and its rare earth distribution pattern is similar to that of acid rhyolite, showing obvious negative Eu anomaly. The characteristics of rare earth elements and trace elements of basalt show that it is mainly produced by partial melting of the upper mantle. However, andesite-dacite-rhyolite all experienced different degrees of plagioclase crystallization separation.
Table 3- 1 Analysis Results of Major Elements of Volcanic Rocks in Geological Profile (wB/%)
Methods: X-ray fluorescence spectrum; Analyst: Institute of Geology, Chinese Academy of Sciences; XJ is the analysis sample of this topic, and YX is the analysis sample of Yang Zhun 'en of this topic, the same below.
Figure 3-2 TAS Classification Map of Paleozoic Volcanic Rocks in Siberia Plate
Figure 3-3 TAS Classification Map of Volcanic Rocks in Kazakhstan-Junggar Plate Area (I)
Figure 3-4 TAS Classification Map of Volcanic Rocks in Kazakhstan-Junggar Plate Area (II)
Figure 3-5 Volcanic rocks in the Siberian plate region
Figure 3-6 SiO _ 2-K2O Diagram of Volcanic Rocks in Kazakhstan-Junggar Plate Area (I)
Figure 3-7 SiO _ 2-K2O Diagram of Volcanic Rocks in Kazakhstan-Junggar Plate Area (I)
Figure 3-8 AFM diagram of Paleozoic volcanic rocks in Siberia plate area
Figure 3-9 AFM Diagram of Paleozoic Volcanic Rocks in Kazakhstan-Junggar Plate Area (I)
Fig. 3- 10 AFM Diagram of Paleozoic Volcanic Rocks in Kazakhstan-Junggar Plate Area (II)
Figure 3- Distribution Pattern of Rare Earth Elements in Paleozoic Volcanic Rocks in Siberia Plate Region +0 1
Table 3-2 Trace Element Analysis Results of Volcanic Rocks in Geo-section (wB/ 10-6)
Analysis method: instrumental neutron activation; Analyst: Institute of High Energy Physics, Chinese Academy of Sciences.
sequential
Analysis method: instrumental neutron activation; Analyst: Institute of High Energy Physics, Chinese Academy of Sciences.
Figure 3- 12 Distribution Model of Rare Earth Elements in Paleozoic Volcanic Rocks in Kazakhstan-Junggar Plate Area
Fourthly, the tectonic environment of volcanic rocks.
The subduction zone is located at the closed edge of the plate, which is the transition area from the ocean to the mainland, as well as the folding orogenic area and magmatic activity area. They are characterized by calc-alkaline volcanic rocks. The subduction zone includes two environments: island arc and continental margin (active continental margin). The typical volcanic rock assemblage characteristics, composition and associated intrusive rocks of island arc and active continental margin are different (Jakes et al.,1972; Con-die， 1976； Pearce, 1984), island arc volcanic rocks: low-potassium (K2O average 1.60%) andesite, SiO 250% ~ 66%, K2O/Na2O；; 0.6(K2O generally varies between 0.6- 1. 1), and the associated intrusive rocks are mainly composed of quartz monzonite and granite. Island arc basalt (mainly tholeiite) and continental margin basalt (mainly calc-alkaline rocks) are produced during plate collision and subduction. With the increase of crustal maturity, the chemical composition of magma changes from tholeiite to calc-alkaline rocks. Paleozoic volcanic rocks in Xinjiang are a set of island arc-continental margin basaltic-andesite magma and its evolution products, so they often show transitional characteristics, and their petrological and geochemical discrimination also shows dual and transitional characteristics. Their main series is calcareous alkaline volcanic rocks with continental margin characteristics, followed by tholeiite and andesite with island arc characteristics.
Figure 3- 13 Distribution Model of Rare Earth Elements in Paleozoic Volcanic Rocks in Kazakhstan-Junggar Plate Area
Chen Zhefu et al. (1997) t index ((Al2O3-Na2O)/TiO2) based on Gautier (1968) >: at 10, it is an orogenic volcanic assemblage, and the Gautier index [T] of Altai volcanic rocks is greater than/kloc. It is considered that Paleozoic volcanic rocks in Altai originated from continental margin orogenic belt.
How (1990) divides the volcanic rocks in Altai into tholeiite series (dacite-rhyolite combination), high alumina (calc-alkali) series (andesite-dacite-rhyolite combination) and alkaline series (basalt (spilite)-andesite (amphibole)-dacite (Shi Ying amphibole). Han et al. (199 1) studied the Devonian volcanic belt in the southern margin of Altai Mountain, and compared it with the volcanic rocks in the Andean continental margin arc and the Japanese island arc on both sides of the modern Pacific Ocean, pointing out that the Devonian volcanic rocks in the southern margin of Altai Mountain have the nature of continental margin volcanic belt. High-alkali tholeiite series volcanic rocks are mainly developed in the volcanic arc area of the Andes continental margin, and there are few alkaline tholeiite series and low-alkali tholeiite series volcanic rocks. Most volcanic rocks in Japan island arc belong to high alkali tholeiite series and low alkali tholeiite series, and there are few volcanic rocks in alkaline basalt series. Devonian continental volcanic belt in the southern margin of Altai Mountain is mainly characterized by the development of alkaline basalt series and high-alkali tholeiite series volcanic rocks, especially with high alkali content, which is different from Andean continental arc volcanic rocks and Japanese island arc volcanic rocks.
On the map 1gι- 1gσ (Figure 3- 14 to Figure 3- 16), almost all volcanic rocks are concentrated in the volcanic rock area of subduction zone (orogenic belt), except the Sawuer island arc and Chemise Tower-Kurankaz in the Kazakhstan-Junggar plate area. The basaltic volcanic rocks in the profile area we analyzed are all located in the D area where basalts and differentiated rocks converge at the plate edge (Figure 3- 17), which is related to the selective enrichment of Th in the mantle above the subduction zone, which makes this component move to the Th angle and enter the D area. The most prominent geochemical characteristics of island-arc volcanic rocks are that Th > is higher than that of North MORB, and the Th > of the whole rock is100%. Ta, Nb and Ta are deficient relative to Th and La, which is attributed to the addition of subduction zone materials, mainly pyrophyllite and calc-alkaline basalt.
Fig. 3- 14 Rietmann-Gottignies diagram of Paleozoic volcanic rocks in Siberia plate area
Fig. 3- 15 Rietmann-Gottignies diagram of Paleozoic volcanic rocks in Kazakhstan-Junggar plate area (I)
Fig. 3- 16 Rietmann-Gottignies diagram of Paleozoic volcanic rocks in Kazakhstan-Junggar plate area (2)
Fig. 3-basaltic volcanic rocks Hf-Th-Ta (wood, 1980) in section 3- 17.
Rare earth elements have obvious negative anomalies of Nb and Ta, showing the characteristics of volcanic rocks in island arc or active continental margin, and the Hf-Th-Ta diagram reflects the environment related to the convergent plate margin.