(1) Geological characteristics
Figure 1- 18 Comprehensive profile of Wutaishan-Hengshan granite-greenstone belt.
(In the figure, the generations of folds are numbered according to the deformation period. F2,3 is equivalent to WF 1, F4 is equivalent to HtF 1 or WF2, F5 is equivalent to HTF2, and F 1 is equivalent to WF0, which is not shown in the figure).
1- Archean gneiss complex; 2- Granite gneiss; 3 eyeball gneiss; 4- gneiss granite; 5— Banyukou Formation; 6— Jingangku Formation; 7-Wang Zhuang Group; 8— Wenxi Formation/Baizhiyan Formation; 9— Hong Men Formation; 10-Zhangxianbao Formation; 11-Mohe Formation; 12-Sijizhuang Formation; 13- equal metamorphic zoning boundary; 14- biotite belt; 15- almandine belt; 16- sillimanite-kyanite zone; 17- granite boundary line; 18- ancient fault; 19 stack and its stages; 20— Strain ellipsoid
Although it is clear that granitic rocks in Archean greenstone terrane, especially those composed of tonalite or adamellite (recently called TTG), are dominant and can be classified to some extent (V.R. McGregor, 1932,1951; C.S.Pichamuthu， 1976； D.R.Hunter, 1973), but the nature of rocks is vague, so the terms gneiss complex, plutonic batholith and small plutonic rock mass are often used in field combination.
In Wutaishan area, Wu Tieshan and others divided the middle Precambrian granites in the greenstone belt into five categories: granites without gneiss or gneiss-like structures, gneiss-like composite granites, gneiss-like quartz diorite bodies and unique sodium granite porphyries. Tian Yongqing et al. (199 1) classified granitic rocks into pre-greenstone, syntectonic, post-greenstone (post-structure) and late stage according to the time relationship between the development and structural evolution of rock mass and greenstone. At the same time, according to the characteristics of rock assemblage, it can be divided into complex rock (rock suite, composite rock mass) and rock mass; Then, according to the emplacement mechanism or genesis, the granitic rocks in the greenstone belt are further divided into: granite gneiss complex in the pre-greenstone belt, mainly composed of TTG; Diorite solid diapir complex also has TTG assemblage characteristics; Synclinal hypabyssal granite, mainly porphyritic plagiogranite and adamellite; Syntectonic magmatic diapir, including gneiss adamellite and diorite; Post-greenstone plutonic rock mass, lithology is granite and sodium granite porphyry; Post-greenstone metasomatic remelting granite, including porphyritic biotite granite and potash feldspar granite; Late intrusive bodies refer to Mesozoic plutonic granite and hypabyssal porphyry. However, due to the different understanding of the genesis of granite and its relationship with greenstone, it is not suitable to make a detailed division. According to the isotopic age of granitic rocks (mainly determined by zircon U-Pb method) and their relationship with greenstone, we can divide them into pre-greenstone (pre-structure), syntectonic and post-tectonic types, and then subdivide them according to rock assemblage or lithology, which can be divided into TTG complex (gneiss complex), gneiss granite, quartz diorite, granite and diorite. (fig. 65433) Compared with the brief classification of K.C.Condie( 198 1), the gneiss complex is consistent, but there are few bedrock. Only the Yixingzhai-Lingqiu diorite in Hengshan area can reach 1000km2, which is equivalent to some gneiss complexes (such as Fuping granite and Hengshan granite).
Figure 1- 19 Schematic diagram of granite distribution in Wutaishan-Hengshan granite-greenstone belt
1- Quaternary coverage area; 2- Wutai greenstone (a) and its subsequent supracrustal rocks (b); 3- Fupingian block; 4- Mesozoic granitic rocks; 5- greenstone post-structural granite (1.6 ~ 2.1ga); 6- greenstone syntectonic diorite (2.3 ga); 7— Syntectonic gneiss granite in greenstone belt (2.3 GA); The 8- greenstone belt is syntectonic TTG rock (2.5 ~ 2.6 ga); 9- greenstone syntectonic diorite-tonalite (2.5 Ga); 10-pre-greenstone (pre-structure) TTG complex (> > 2.5 ga); 11-isotope age sampling point and serial number: (1) 2560ma; (2)2483 ~ 2507ma； (3)252 1Ma； (4)3036 mA; (5)2500 mA; (6)2500 mA; (7)25 10Ma； (8)25 14Ma； (9)2374 mA; (10)2245 ma; ( 1 1)2 130ma； ( 12) 15 17Ma； ( 13)2549ma； ( 14)2607ma； 12— Late failure. Remarks in the picture: h1-Hengshan complex (forest farm TTG); H2—(W) Hengshan complex (Wen Zhuang pluton); H2—(Y) Hengshan complex (Yixingzhai pluton); H2—( 1) Hengshan complex (Lingqiu rock mass); B- Beitai-Zhicungou TTGd- Dazhaikou TTGG—-Guangming Temple rock mass; S- stone Buddha rock mass; L- Lan Zhi mountain rock mass; F- Fuping TTG complex; U-shaped granite at the bottom of the building; Ekou granite; J- Baojiagou granite; Q— Duyu rock mass; West Wang Jiahui rock mass; P— Pingxingguan rock mass; A— Dawaliang rock mass; H- Lianhuashan rock mass
1. gneiss complex
Or granite gneiss complex, is the main body of granite in greenstone belt (accounting for 70% ~ 80%). This kind of rock mass is very large, mostly exceeding 200km2. The area of Fuping granite gneiss complex and Hengshan TTG complex can reach 1000km2. Their relationship with the greenstone belt is very delicate. Most areas are in contact with the bottom of greenstone with ductile shear zone, such as Hengshan complex and Fuping complex. Some of them are located in greenstone, which is also in ductile shear contact relationship, but the interface participates in greenstone folding, such as Beitai-Zhicungou rock mass; Sometimes, it appears in greenstone belts, such as Shi Fo rock mass and rock mass at the bottom of buildings. The intrusive relationship between granitic rocks and greenstone in greenstone is not obvious.
Gneiss complex belongs to TTG type (figure 1-20A, b) and is a compound rock mass, but there is no obvious boundary between rock types, and there is a transitional relationship between potash granite and sodic granite. Gneiss complex has a strong schistosity, sometimes forming a typical banded structure (plate ⅲ-4), with banded width ranging from 0.2 cm to several tens of cm, mainly composed of felsic components and mafic components. There are many basic inclusions (Ⅲ-5 plates) in tonalite-Ordovician granite, which are usually classified as bimodal assemblage. In the weak deformation part, the band is not obvious, only with gneiss texture. Most rocks are subjected to strong deep structural deformation, and besides the flexible folds which are difficult to distinguish stages, they also show strong ductile shear eyeball structure (plate ⅲ-6) and S-C foliation.
Figure 1-20 An-Ab-Or diagram of granitic rocks
(According to O 'Connor 1965)
A- basement complex (Fuping, Yuanshan, Beitai and Baojiagou, etc.). ); B—— Syntectonic gneiss Complex (Lan Zhi Mountain, Stone Buddha, Ekou, etc. ); C- quartz diorite-tonalite base (Lingqiu-Yixingzhai); D— Small intrusions with the same structure (×1000; The dotted line is Wang Jiahui, and the practice is Guangming Temple); E— Post-tectonic small intrusion (× big tile beam; +Lianhua Mountain; Phoenix Mountain); F— Post-structural metasomatic granite (Pingxingguan) (except Fuping complex data quoted from Wang, Yuanshan complex and some data quoted from Wang Renmin, etc.). The rest data are from Shanxi Coordination Group and Shanxi Institute of Geological Sciences).
Gneiss complexes generally have different degrees of migmatization, with siliceous differentiation and metasomatism in the early stage, and veins and matrix are integrated; In the late stage, potassium infiltration and metasomatism dominated, and potassium granite was formed in the parts with strong metasomatism. The appearance of granite pegmatite is consistent with the degree of late migmatization and is the product of mixed metasomatism.
The structural study of gneiss complexes shows that they have more complicated structural deformation history and deeper deformation characteristics than greenstone belts. They have experienced at least three folds, and the superposition of multiple folds has formed a gentle dome structure. Fuping complex and Hengshan complex are the most typical, and Beitai rock mass is also obvious. Gneiss complex and its basic xenoliths not only have strong shear deformation near greenstone, but also have the same metamorphic degree as greenstone, that is, when greenstone is greenschist facies, granite is chlorite granite and contains chlorite schist xenoliths, indicating that gneiss complex has undergone the same low-grade metamorphism as greenstone.
According to the relationship between gneiss complex and greenstone, structural deformation, development degree of inclusions, rock assemblage and isotopic age, it can be divided into two types: one is TTG complete basement complex (Figure 1-20A), mainly including Fuping complex, Hengshan complex and Baojiagou rock mass (Wutai Group is not integrated on it, Tian Yongqing et al., 65438). The other is the early suite of TTG complex with the same structure, which is mostly in the form of banded or irregular small intrusions (? ) occurs in greenstone strata, such as Shi Fo rock mass, Lan Zhi mountain rock mass and Ekou rock mass. They have TTG evolutionary properties in composition, but lack tonalite, so they are not complete TTG suite (Figure 1-20B). Zircon U-Pb isotopic age of this complex is 2500～2560ma.
2. Rock foundation
Generally speaking, the granitic rock mass with an area greater than or close to 1000km2 is called bedrock. In Wutaishan-Hengshan area, there is only Yixingzhai-Lingqiu rock mass except gneiss complex outside greenstone belt. This is a composite rock mass of granite and diorite-tonalite (Figure 1-20C). Like gneiss complex, there is no clear boundary between different rock types. They have most characteristics of gneiss complex and contain many basic inclusions (lherzolite inclusions discovered in Wen Zhuang). The bedrock is nearly distributed in the east-west direction, located between Hengshan greenstone belt and Wutai greenstone belt, and is in stable contact with the bottom of greenstone belt, with typical diapir positioning characteristics (C.R.Anhaeusser, 1984). The isotopic age of bedrock is 2500Ma, which belongs to the syntectonic period of greenstone belt.
3. Small plutonic rock mass
The same feature of this kind of rock mass is that it occurs in greenstone, and its intrusive relationship with greenstone is obvious (plate ⅲ-7), but its scale is small (< 100km2). Another feature of them is that their composition is single. For example, the Wang Jiahui and Duyu plutons are adamellite (figure 1-20D), the Guangmingsi pluton is dominated by ordinary pyroxene, and the Dawaliang pluton is sodium granite porphyry (figure 1-20E). The composition of Pingxingguan rock mass varies greatly, but it is mainly porphyritic biotite granite (Figure1-20f). The change of composition may be related to the reason of remelting replacement. The field occurrence of rock masses shows that most of them are products of the same structure of greenstone, so they have the same foliation direction as greenstone and the occurrence close to integration with greenstone. However, they are not syngenetic intrusions, and there are two main types: one is a very small-scale integrated rock mass, such as Guangming Temple adamellite and Shuangcaicun porphyritic plagiogranite, which may be shallow intrusions of greenstone at the same time, with isotopic age greater than 2500Ma；; The other kind, with a slightly larger scale (50 ~100km2), such as Wang Jiahui granite and Duyu granite, intruded into the magma diapir and contained greenstone xenoliths with isotopic ages of 2300～2400Ma. These two types of intrusions with the same structure are widely distributed and are important components of greenstone terranes.
Granitic rocks belonging to greenstone post-structure are mainly the products of Lvliang period, and the quantity is limited. Its plutonic intrusions were mainly formed in the late Lvliang movement (1800 ~ 2 100 Ma), which are generally unconformity intrusions, scattered in different parts of the greenstone belt, and some of them can invade the Hutuo Group. This kind of rock mass varies in size, ranging from 3km2 (Dawaliang rock mass) to 1 10km2 (Fenghuangshan rock mass), which is nearly equiaxed in plane and has clear contact boundary with surrounding rock. Generally, rocks do not have gneiss structure or only develop at the edge of rock mass, and it is not obvious. As mentioned above, this kind of small rock mass has a single composition, except for the special Dawaliang rock mass, which is generally rich in potassium, but there are obvious differences in lithology between rocks, such as biotite granite in Lianhua Mountain (Ⅲ-8 plate) and amphibole granite in Fenghuang Mountain (Figure 1-20E). The formation process of metasomatic granite is relatively long, from 2245Ma (Pingxingguan rock mass) in early Lvliang to 1800Ma in late. This kind of granite has different shapes and sizes, represented by Pingxingguan granite, with an exposed area of 80 ~ 90km2 and a clear boundary with surrounding rocks. The lithology of this kind of rock mass is potassium-rich, mainly gneiss biotite granite (figure 1-20F), and the inclusions of various surrounding rocks in the rock mass are mainly supracrustal rocks-magnetite quartzite, amphibole schist and plagioclase gneiss of Pingxing Guanwenxi Formation and Wang Zhuang Formation. Their foliation and granite boundaries are consistent with the direction of regional tectonic lines. Fine-grained granite veins are also interspersed in the rock mass, indicating that it has multi-stage activity.
(2) Rock characteristics
According to the comprehensive characteristics, although the granitoid rocks can be divided into several types, the lithology of these rocks is mainly tonalite, monzogranite, granodiorite, quartz diorite, monzogranite, Shi Ying monzogranite, potash granite and sodium granite porphyry. Their petrological and mineralogical characteristics are different, which is a sign of their genesis and evolution.
(1) tonalite-adamellite tonalite and adamellite are rock types with the same genesis and similar composition, which are composed of plagioclase (64% ~ 75% and 35% ~ 48%), Yanshi (12% ~ 3 1% and 21. Ordovician granite often contains a small amount of potash feldspar (5% ~ 10%). Generally, it has a medium-coarse scale metamorphic structure, and in some cases it has a giant porphyritic structure, and the porphyritic crystals are composed of potash feldspar. The schistosity is well developed, but it is not obvious in the middle of individual rock masses, so there are often gneiss, eyeball and banded structures. The plagioclase is sodium, plagioclase (an = 5 ~ 20) to intermediate feldspar (an = 32 ~ 35), and from tonalite to augite, the Ab/An ratio gradually increases. Plagioclase is mostly medium-grained, semi-self-shaped, with various twins, and generally has stress phenomena, such as fine grain at the edge and dislocation of twins. It is shaped like a medium grain, sometimes slightly light blue, and the particles are slender. Its long axis is parallel to the C axis, and there are different degrees of wave extinction-band extinction, some time-dependent edges are refined, and even cleavage occurs. Variant Shi Ying is a polygonal granular aggregate with a polygonal mosaic structure and no wave elimination. The content of potash feldspar is low, which belongs to the transition type from orthoclase to microcline. Among them, there are different degrees of lattice twins, metasomatic texture with suture, vermiculite, metasomatic plagioclase and other minerals, containing more residues and often albite bands, and the order degree is 0.75 ~ 1. Biotite is polychromatic, light yellow, light brown (Np), dark brown and dark brown (Ng). The accessory minerals are magnetite, zircon, apatite, sphene and various metal sulfides, and sometimes there are more garnet and barite.
(2) Granodiorite Granodiorite is mainly composed of potash feldspar (15%), plagioclase (40% ~ 50%), quartz (15% ~ 20%), biotite and amphibole (15% ~ 20%). Potassium feldspar is mostly plagioclase and plagioclase stripe feldspar, which has lattice twins and often contains plagioclase. Plagioclase is semi-self-shaped, about An35, with various twins developed, with twin fracture, dislocation and other stress and deformation structures. It should always appear in the form of streaks, and its long axis is consistent with gneiss, which is an aggregate of unequal grain ages. Biotite and amphibole are mostly arranged in giant spots, which constitute the foliation of rocks. The accessory minerals are mainly magnetite, zircon, apatite, sphene, rutile and sulfides of various metals, such as pyrite and galena.
(3) Quartz diorite is mainly composed of plagioclase (50% ~ 60%), syenite (15% ~ 20%), biotite and hornblende (10%). Sheet and block structure. Scaly and medium-coarse grained granulite structure, redundant semi-self-shaped plate structure. Plagioclase is intermediate feldspar, An≈30. The crushing structure is obvious, including fragmentation, bimorph bending, dislocation, wave extinction and so on. There are two kinds of plagioclase: primary plagioclase and metamorphic plagioclase. Primary plagioclase is authigenic, while metamorphic plagioclase is heteromorphic and irregular, which is mostly a collection of fine particles and distributed in parallel sheets. Most of the time is metamorphic Shi Ying, which is abnormal, some of them are flat and have obvious wave absorption, and some of them are banded and lenticular time aggregates, and the wave absorption of biotite kink is obvious. The accessory minerals mainly include magnetite, zircon, apatite, sphene, barite, tourmaline, rutile, metal sulfide minerals and limonite. Zircon contains biotite inclusions.
(4) Granite-adamellite-potash granite This is a kind of granite rich in potassium. They are all composed of potash feldspar (20% ~ 40%, sometimes as high as 70%), quartz (25% ~ 35%), plagioclase (20% ~ 40%, sometimes lower than 10%), biotite and amphibole (5% ~ 15%). With different grain sizes, it is generally a medium-grained unequal-grained granite structure and a medium-coarse grained porphyritic structure. Granite generally does not develop foliation, and sometimes it only has inconspicuous gneiss structure. The rocks are biotite granite and amphibole granite. Feldspar granite and potash feldspar granite are mostly gneiss granite, with medium-grained granulose gneiss structure, broken block structure and eyeball structure. The metasomatism between minerals is generally developed, such as stripes, strips, chessboards, peristalsis and clear edge structure. Potassium feldspar is dominated by microcline, and most of them form self-form plate-like porphyry, with albite stripes and many inclusions in the crystal. Plagioclase is albite (an = 5 ~ 20), semi-authigenic, metasomatic texture, and obviously sericitized. Orthoclase is tabular and highly kaolinized. It is irregular and granular, evenly distributed, and sometimes there are obvious wave-absorbing and crushing phenomena. The biotite is yellow-green brown, the polychromatic is not obvious, sometimes it is completely chloritization, and the feldspar has a high degree of order, δ≥0.95. In addition to magnetite, there are many accessory minerals of granite, such as sphene, apatite and zircon, and a small amount of epidote, ilmenite, monazite, pyrite and fluorite. Relatively speaking, apatite and sphene are not as important as adamellite and mixed granite, and sometimes they contain garnet.
(5) Sodium granite porphyry is only found in Dawaliang rock mass. The porphyritic crystals have banded microcline (10 ~ 20 mm)15% ~ 20%; Plagioclase (2 ~ 5 mm)15%; The time is (2 ~ 3 mm) 1% ~ 2%, and the matrix includes (0. 1 ~ 0.5 mm): sodium orthoclase15%; 25% ~ 30% albite; The response time is 20%; 5% ~ 7% of biotite; Muscovite 1%. Rock foliation is not developed, and it is dominated by massive structure. Streak feldspar is a kind of thick plate-like automorphic crystal with lattice twins and albite stripes with metasomatic origin inside, which generally cover plagioclase, orthoclase, quartz and biotite. Albite is authigenic, with slight sericitization, curtain fossilization and twin development. It is shaped in time, and the edge is melted, which explains orthoclase and constitutes a false graphic structure. Sodium orthoclase is in other plates, and there are albite stripes produced by solid melting and decomposition in the crystals. The biotite has completely chloritization. The secondary minerals are limonite, zircon, apatite and anatase, and the secondary minerals are rutile and metal sulfide minerals.
(3) Geochemical characteristics
1. Constant component
Except for the late intrusive rocks such as Dawaliang and Fenghuangshan, most petrochemicals (Al2O3, Fe2O3, FeO, MgO, Cao, TiO2, P205, etc. ) has a negative linear relationship with SiO2 (figure 1-2 1, 1). The relationship between K2O and Na2O and SiO _ 2 is not obvious, and the content of Na2O is stable and does not change with the increase of SiO _ 2. K2O in basement complex tends to increase with the increase of SiO2 _ 2, but it has an opposite relationship with Pingxingguan rock mass (Figure 1-2 1), so granitoids can be clearly divided into two types: potassium-rich type and potassium-poor type. The former mainly includes granite, adamellite and granodiorite, and K2O/Na2O is greater than1. The latter are mainly quartz diorite, tonalite, augite and sodium granite porphyry, and the ratio of K2O/Na2O is less than 1.
Arthur (1979) divided tonalite-adamellite * * into high Al2O3 type and low Al2O3 type with Al2O3 content 14.5% ~ 15.0% as the boundary, and gave it certain tectonic environmental significance. Besides metasomatic origin, the potassium-rich granitic rocks in Wutaishan area are generally low in Al2O3, so the potassium-rich granitic rocks in Wutaishan greenstone belt before the same tectonic period can be divided into high Al2O3 type and low Al2O3 type, which fully reflects the differences between different types of rocks (Figure 1-22). As far as rock types are concerned, tonalite, adamellite, granodiorite and adamellite can be divided into two types: high Al2O3 type and low Al2O3 type, while quartz diorite is mostly high Al2O3 type. If we consider the types of rock mass, basement complex and solid diapir complex, syngenetic hypabyssal granite and regenerated metasomatic granite, a rock mass usually has high Al2O3 and low Al2O3, except Ekou granite, which are almost all low Al2O3 rocks. Magmatic diapir granite is generally of low Al2O3 type. The contents of MgO, CaO, TiO2 _ 2 and P2O5 in most granites with high Al2O3 content are relatively high, and vice versa, so they are all negatively correlated with SiO2 _ 2 (Figure 1-2 1, 1-22).
Figure 1-2 1 Diagrams of granite rocks SiO _ 2-TiO _ 2, p2o _ 5, CaO, K2O, Na2O and MgO.
BD- Dazhaikou rock mass; Bb— Beitai rock mass; S— Early rock formations (Lan Zhi Mountain, Guangming Temple, Stone Buddha and Ekou); D— Dawaliang rock mass; P— Pingxingguan rock mass; West Wang Jiahui rock mass; F— Fenghuangshan rock mass
2. Trace components
Except for rare earth elements, there is little research on the trace components of granitoids in Wutai Mountain greenstone belt. Previous studies (Wu Tieshan et al., 1984) only used spectral semi-quantitative analysis data for comparison, lacking a certain amount of high-precision analysis data. We only sampled and analyzed individual rock masses and rock types. Compared with the typical Archean granitoid rocks in the world (K.C.Condie, 198 1), it has roughly similar characteristics: high Al2O3 TTG rocks have low SiO2 _ 2, Rb, th, Ba/Sr, HREE and HREE, and have strong separation characteristics and low (Yb/GD) N. The loss degree of HREE and Eu positive anomalies seems to increase with the increase of SiO2 _ 2 (which is fully reflected in four kinds of rocks in basement complex). The contents of Sr, Sc, Cr, Ni and Co in rocks with low Al2O3 content are relatively low, which has the characteristics of weakly differentiated HREE partition, and the (Yb/Gd)N is relatively high, generally ranging from 0.72 to 0.99. However, by further comparison, it is found that the early Precambrian TTG granite in Wutai Mountain contains more transition elements than the Archean TTG granite in other parts of the world (high aluminum type or low aluminum type), and Cr, Ni, Co and large ion pro-MagmaElemental K, Rb and Sr are more similar to the post-Archean granite.
Figure 1-22 Diagram of Granite SiO _ 2-Al _ 2O _ 3
(Except quartz diorite is low alumina type, other rock types and most rock masses can be divided into two types: high Al2O3 and low Al2O3).
1- basement complex; 2-Early rock cover; 3- Wang Jiahui rock mass; 4- Pingxingguan rock mass; 5- Dawaliang rock mass; 6— Fenghuangshan Rock Mass
The difference between different rock types is mainly between TTG complex and adamellite, and adamellite has high pro-MagmaElemental abundance and low transition element abundance. The ratios of Rb/Sr and Ba/Sr are high, and there are obvious negative Eu anomalies, but there is no obvious difference among the rocks in TTG complex-tonalite, augite and granodiorite, which are roughly the same as the typical rocks in the world.
According to the standardized REE distribution pattern of chondrites in a large number of granite rocks in this area, according to the loss (Eu/Eu *) of Eu, it can be basically divided into 3 ~ 4 types, namely, Eu obvious loss type, Eu slight loss type, EU normal type and EU enriched or slightly enriched type (Figure 1-23). The distribution patterns of these rare earth elements and their corresponding characteristics of other rare earth elements, such as the total amount of rare earth (∑REE), the separation degree of light and heavy rare earth (LREE/HREE) and the separation degree of heavy rare earth from light rare earth, are also different, which can be summarized as table 1-6. In the table 1-6, (La/Sm)N represents the separation characteristics of light rare earth, while (Yb/Gd)N represents the separation characteristics of heavy rare earth. According to the existing research results, the original rock and its formation process can be directly inferred without quantitative simulation (Niu Laizheng,1975; Hansen,1978; Kalle et al.,1984; Martin et al., 1983, 1987).
In figure 1-23, most rock types have drawn the characteristic range of REE, that is, the envelope range determined by the high and low values of ∑REE. It can be seen from the figure that the basement gneiss complex, early TTG suite and syntectonic hypabyssal granite are quite different, but the separation degree of light and heavy rare earths is the same, and the curves are parallel to each other; The distribution patterns of rare earth elements in the only two plutonic granites are very similar. Therefore, the distribution pattern of rare earth elements in granite not only reflects the differences between rock types, but also marks the origin and evolution of rocks.
Figure 1-23 Standard Rare Earth Distribution Map of Granite-chondrite in Wutaishan Greenstone Belt
B- basement complex; S- early rock cover; I- hypabyssal intrusive granite (I 1- Guangming Temple rock mass; I2— Shuangcaicun rock mass); Syntectonic potash granite; P metasomatic granite; I3 and I4—— Post-tectonic intrusive granitoids (Dawaliang rock mass and Lianhuashan rock mass respectively).
Table 1-6 Table of Rare Earth Element Classification Types of Granite in Wutaishan Greenstone Belt
For a granite complex, different rock types have the sequence of formation, showing the evolution characteristics of lithology and composition. Basement gneiss complex and early suite, the rock types appearing from morning till night are granodiorite-tonalite, quartz diorite-adamellite-adamellite, showing the trend of evolution along the arrow in figure 1-20, and metasomatic granite also has a similar evolution from sodium to potassium. The evolution of chemical composition in the complex is mainly reflected in Q-Ab-Or diagram, K2O-Na2O-CaO diagram and FeO-(Na2O+K2O)-MgO diagram (Tian Yongqing, 199 1). Gneiss complex, basement complex and syntectonic TTG suite all show the trend of adamellite series first, and the potassium replacement in the later period is adamellite series first.
Therefore, according to the current discussion on the origin of sodium granite magma (G.N.Hanson,1972; F. Barker,1979; J.G.Arth， 1979； B.M.Jahn， 198 1， 1984； H. Martin et al., 1976,1983; Condie et al., 1976, 1983), think that TTG magma may come from basaltic materials. Tian Yongqing et al. (199 1) calculated the Sr and K abundances of TTG with high Al2O3 in Wutaishan greenstone belt by Xiao's simple partial melting equation (1970), and the source rock only needs about 60% of the minimum value of separation crystallization or 40% of the maximum value of partial melting. Potassium-rich young granite may be the result of partial melting of immature sandstone, siliceous granulite and ancient calcareous gneiss granite. Based on the distribution pattern of rare earth elements of various granites in Wutai Mountain greenstone belt, according to the existing research results of granite magma genesis, the genetic series is represented by figure 1-24, and the diagram of rare earth elements of various rock types in the figure is a simplification of figure 1-23. Its source rocks include eclogite and garnet amphibole in the lower crust (metamorphic from TH2), siliceous granulite and granulite facies adamellite; Metamorphic sedimentary rocks in the upper crust and early sodic granite. The way of rock formation is mainly crystallization of granitic magma produced by partial melting of source rock, but it is not excluded that crystallization differentiation in this process will generate similar granites with different distribution types of rare earth elements in the same rock mass (such as the relationship between I 1a and I 1b). Although the origin and evolution of this series are qualitative, in fact, within the temperature and pressure range outlined above, granite rocks in various places have basically partially melted to varying degrees (less than 50%).
Figure 1-24 Rare Earth Element Model of Precambrian Granite in Wutaishan Greenstone Belt
(See Figure 1-23 for details)
B- basement complex; S- syntectonic TTG complex; M- syntectonic adamellite; P- regenerated metasomatic granite; T— Post-structural plutonic granite