Wudang Group is the largest and oldest metamorphic volcanic-sedimentary rock series in Wudang area, which constitutes the main body of Wudang Mountain. It is a set of light-colored metamorphic rock series dominated by greenschist facies, which consists of albite schist, biotite schist, felsic granulite, granulite, epidote (or actinolite) chlorite schist, metamorphic sandstone, tuff siltstone and quartzite. , and form several rhythm layers. The protolith of light-colored rocks is mainly acidic volcanic rocks, followed by normal sedimentary clastic rocks; The protolith of dark rocks is mainly basic volcanic rocks. According to the identification, the intermediate-acid rocks are mainly dacite rhyolite, microcrystal and lithic tuff, and the basic rocks are mainly basaltic andesite and basic pyroclastic rocks or tuff.
There has always been controversy about the formation age of Wudang Group protolith. Recent research shows that the group contains volcanic rocks and a small amount of sedimentary rocks formed in different periods. The whole rock Sm-Nd isochron age of the metamorphic volcanic rocks of the North Wudang Group in Shiyan is1927 75 Ma. The Nd mode age (TDM) of rocks is between1913 ~1986 Ma, with an average value of1945 25ma, which is the same as that of rocks. However, the Sm-Nd isochron can't be obtained in the metamorphic volcanic rocks of Wudang Group in Taizipo, Yunxian County and Tucheng, Fangxian County, and the age of Nd mode of rock samples varies greatly, ranging from1175 ~ 2162 Ma, showing11. Through the test of TDM-1/[w (nd)/10-6], it is found that TDM changes with the change of 1/Nd, indicating that volcanic rocks are mixed with ancient crustal materials. The formation age of the volcanic rocks in Taizipo is (1251190) Ma, which was obtained by the Sm-Nd isochron of several samples, and these samples have the least ancient materials in the crust (Zhang et al., 2002). Therefore, Wudang Group should be a set of metamorphic volcanic-sedimentary rock series from Proterozoic to Mesoproterozoic, which is consistent with the geological relationship that there are older Proterozoic crystalline basements (Douling Group and Foping Group) in South Qinling and Wudang Group is covered by Yaolinghe Group.
Yaoling River Group is much more widely distributed than Wudang Group. Yaolinghe Group is mainly composed of marine basic volcanic rocks, accompanied by volcanic-sedimentary rock series of acidic volcanic rocks. Lithology and thickness vary greatly, generally more than 2000 meters thick, occurring in the tectonic environment of rift basin (or rift valley). It is distributed from east to west around Zaoyang, Neixiang and Ankang Niu Shan eruption centers. It is observed that its false conformity and local unconformity cover the Mesoproterozoic (Wudang Group) (Zhang Erpeng et al., 1993), and it is also observed that Yaolinghe Group and Wudang Group are separated by (nappe) ductile shear zone (Ling et al., 2002b) or shear zone. In addition, in South Qinling, Neoproterozoic volcanic-sedimentary rock series dominated by metamorphic basalt is often called Yaolinghe Group, while modern volcanic-sedimentary rock series dominated by metamorphic volcanic rocks is called Yanxi Group. According to the fact that the two groups of volcanic rocks peacefully formed a volcanic dome in Niu Shan, and the intersection of acidic and basic volcanic rocks was interbedded, it can be considered that the two groups of volcanic rocks were the products of continuous volcanic activity in the same period (Xia et al., 1996), indicating that the two groups of volcanic rocks should belong to a bimodal suite. This is consistent with their intracontinental rift tectonic environment. Diabase wall groups have also been found in Wudang area, more precisely, layered rocks intruded into Wudang Group and Yaolinghe Group (Zhang Guowei et al., 200 1), and both Yaolinghe Group and Wudang Group were covered by Sinian unconformity.
The basic volcanic rocks of Yaolinghe Group include spilite, spilite breccia lava and spilite tuff, which are all kinds of greenschists after metamorphism, but basically retain the volcanic characteristics of the original rocks. Acid volcanic rocks are mainly rhyolite, with a small amount of trachyte, tuff and volcanic breccia. After metamorphism, it is felsic schist, granulite and granulite. Normal metamorphic sedimentary rocks are mainly mica schist, quartz schist, carbon silicalite and metamorphic sandstone. In Suixian-Zaoyang area near Dabie, the lower member of Yaolinghe Group is dominated by metamorphic basalt (spilite) with acidic volcanic rocks in the upper part. The middle part is mainly green schist, and the middle and upper part is sodium schist, mica schist and quartz schist. The upper mica schist and greenschist are mixed with marble, and the total thickness exceeds 1600m (Zhang erpeng et al., 1993). Rock metamorphism generally belongs to greenschist facies, and can reach high greenschist facies locally.
According to the available isotopic chronological data of volcanic rocks in Yaolinghe Group, the volcanic rocks in Yaolinghe Group in Hubei Province were mainly formed in 700 ~ 800 Ma (Zhou et al., 1996), in which zircon U-Pb age is 796±77Ma and Sm-Nd isochron age is 806Ma, including Baisangguan, Yuanjiashan, Xijiadian, Yudidian and Chestnut.
. In Niu Shan, Ankang, Shaanxi, the Sm-Nd isochron ages of the metamorphic basalt of Yaolinghe Group and the felsic volcanic rocks of Liaoxi Group are10/6 Ma and 1009Ma, respectively, and the Nd model ages of the two groups of volcanic rocks are similar (TDM is 1225Ma and1respectively. The εNd(t) values of basalt and felsic volcanic rocks are basically the same (+5.98 and +5.97, respectively) (Zhang et al., 2002), indicating that the volcanic rocks of Yaolinghe Group were formed at the turn of Mesoproterozoic, the younger ones in the east belong to Neoproterozoic, and the older ones in the west belong to the late Mesoproterozoic.
Proterozoic crystalline basement in South Qinling is mainly represented by Proterozoic Douling Group (about 2000Ma) and Foping Group (about 2000Ma). Among them, the Douling Group is distributed in Xixia, Xichuan and Neixiang of Henan Province to the east of the South Qinling Mountains, and reaches the junction of Henan and Shaanxi in the west. According to the research of Zhang Shouguang et al. (1996), the composition of Douling complex is very complex, mainly composed of felsic gneiss, amphibolite, diopside metamorphic rock and a small amount of marble, graphite schist and quartzite. Felsic gneiss is the main body of Douling Group, which is formed by metamorphism of supracrustal rocks and plutonic rocks. Petrological and geochemical studies show that biotite plagioclase gneiss and Eryun plagioclase gneiss are accessory gneiss, and their original rocks are argillaceous-sandy sedimentary rocks. The composition of trace and rare earth elements is very similar to the characteristics of many potash granites. The protolith of amphibolite plagioclase gneiss has been proved to belong to volcanic rocks. However, the protolith belongs to deep gray gneiss, which is similar to TTG rock in main element composition. According to the occurrence and petrochemical characteristics, amphibolite can be divided into two types: one is gneiss interlayer, and the composition pattern of rare earth elements is flat. The protolith should be tholeiite or basaltic volcanic tuff, and the magma comes from the source region of depleted mantle. The other is lenticular, the composition pattern of rare earth elements is smooth to the right (L-, M- and HREE decrease continuously), and the negative Eu is weak to not obvious. The original rocks should be basic intrusive rocks, and the magma is strongly contaminated by continental crust materials, which leads to the evolution of rocks into calc-alkaline basalts. Diopside granulite may be formed by metamorphism of Archean tonalite and carbonate Shuang Yuan mixed sedimentary rocks. The early metamorphism of Douling Group rocks belongs to high amphibolite facies of medium-pressure facies system, and the late metamorphism belongs to greenschist facies (Zhang Shouguang et al.,1996; Zhao Ziran et al., 1995).
Chronological study shows that (Zhang et al., 2002), in the Wawuchang Formation in the lower part of Douling Group, 207Pb/206Pb mixed gneiss zircon of 19 12 12 Ma and 193 1 89 Ma were obtained. The Nd model age (TDM) of mixed gneiss is 1890 ~ 2 145 Ma, with an average of 2014 ma; ; The Nd mode age (TDM) of amphibole is between1694 ~ 3911ma, with an average of 2982Ma, and amphibole with mode age of 1694Ma is green. The amphiboles of other samples (mostly Archean with model age) are brown, indicating that the latter rocks are mixed with different amounts of Archean crustal materials. In the Dagou Formation in the upper horizon, the Sm-Nd isochron age of gneiss is/kloc-0 1860±350Ma (due to the narrow distribution range of 147Sm/ 144Nd, the age uncertainty is great), and its Nd mode age (TDM) is between 2096 and 2/kloc. The Nd model age of amphibolite in Dagou formation is 2089～2254Ma, with an average of 265438±097±64Ma. The 207Pb/206Pb age of zircon in gray gneiss is 19 18 2ma, and the average nd model age is 20 14Ma. According to the above chronological data, it is considered that the protolith of the metamorphic complex of Douling Group belongs to the Paleoproterozoic (2000Ma) stratum (Zhang Shouguang et al.,1996; Zhang et al., 2002).
In addition, the unconformity above the crystalline basement of Douling Group in Xichuan-Neixiang area of western Henan Province is Maotang Group composed of metamorphic volcanic rocks. Yaoyingzhai Formation in the lower part of the group is metamorphic acid volcanic rock, and Matoushan Formation in the upper part is metamorphic basic volcanic rock. The original rocks of metamorphic volcanic rocks of both rock groups were formed in 1794Ma. It is based on the existence of zircon with the age of 1794 1 1ma in the metamorphic volcanic rocks of Yaoyingzhai Formation, and the Sm-Nd reference isochron age of the metamorphic volcanic rocks of Matoushan Formation is about 1780Ma. According to the nd model age (TDM) (2079 ~ 2117ma) and Nd isotopic characteristics of acid volcanic rocks in Yaoyingzhai Formation, it is speculated that acid volcanic magma may be the product of remelting the ancient continental crust rocks in Douling Group. The calculated values of metamorphic basic volcanic rocks in Matoushan Formation according to the age of 1780Ma (5.79 ~ 6.69, with an average of 6.08) are basically consistent with the values of the source region of MORB depleted mantle (6. 1) in the same period, indicating that its magma should be a partial melting product of depleted mantle. Although the Maotang Group is similar to the Yaolinghe Group and Yunxi Group in rock characteristics, their age of formation cannot be compared with the Sm-Nd isotopic composition (Zhang et al., 2002). As for the relationship between Maotang Group and Wudang Group, it remains to be verified.
When discussing the crustal structure-rock composition of Qinling Mountains, predecessors pointed out that the lower crust of South Qinling Mountains is the same as Yangtze landmass, the middle crust is composed of Archean continental core or basement, and the unconformity between it and Sinian-Phanerozoic caprock is composed of the upper crust. It should be composed of ancient-Mesoproterozoic rocks such as Maotang Group and Wudang Group, and the Yaolinghe Group, whose main body is Neoproterozoic, should be composed of the upper or top of the middle crust.
3.2. 1.2 Geochemical characteristics of Wudang Group, Yaolinghe Group and crystalline basement rocks.
As a comparative reference with Dabie, the geochemical characteristics of volcanic rock series of Wudang Group and Yaolinghe Group are emphatically introduced below.
(1) According to the TAS diagram of constant element (Cox et al., 1979) and the diagram of high-field element Nb/Y-Zr/TiO2 (Winchester&Floyd, 1977), it is found that the volcanic rocks of Yaolinghe Group have obvious bimodal volcanic suite (Figure 3-6). This feature is more obvious in the relatively stable high field intensity element diagram (Figure 3-6b). Moreover, a considerable proportion of samples belong to alkaline rocks. The overall characteristics of gabbro-diabase of dike group (which should be rock mat, the same below) are similar to those of basic volcanic rocks of Yaolinghe Group. The volcanic rocks in Wudang Group are mostly acidic (SiO _ 2 > 70%) and basic (SiO _ 2 < 53%), with a certain proportion of neutral andesite, indicating that the magma is in a continuous evolution sequence, and the volcanic rock series generally shows the characteristics of calcium and alkalinity (Figure 3-6 a) (Ling et al., 2002b).
Fig. 3-6 TAS(a) and Nb/Y-Zr/TiO _ 2 (b) diagrams of volcanic rocks of Wudang Group and Yaolinghe Group in Wudang area.
(2) According to the chemical composition of the original mantle of Sun He mcdonough (1989), the normalized elemental composition maps of the original mantle of Wudang Group and Yaolinghe Group are compiled (Figure 3-7). The results show that the acidic and basic volcanic rocks of Wudang Group (see Figure 3-7a and Figure 3-7b respectively) have obvious negative anomalies of Nb, Ta and Ti, indicating that their magmatic processes (Figure 3-7b) are different. However, the basic volcanic rocks and acidic volcanic rocks of Yaolinghe Group (see Figure 3-7c and Figure 3-7d respectively) generally have no obvious high-field elements (Nb, Ta, etc.). ), only a few basic volcanic rocks with low HREE show weak negative Nb, Ta, Zr and other anomalies (Ling et al., 2002b). It is speculated that the latter situation may be related to the pollution of crustal materials during magma intrusion and crystallization. The elemental composition characteristics of diabase in dike group are very similar to those of basic volcanic rocks in Yaolinghe Group. In addition, under the condition of similar SiO2 _ 2 content, the content level of lithophile macro-ionic elements in Yaolinghe volcanic rocks is obviously higher than that in Wudang volcanic rocks, regardless of basic rocks or acidic rocks.
(3) According to Ta-Hf-Th(Wood et al., 1979) and Y-Ti-Zr (Pearce & Cann, 1973), the composition points of basic volcanic rocks in Yaolinghe Group are all located in intraplate tholeiite basalt area; The basic volcanic rocks of Wudang Group are located in island arc volcanic rocks. According to Y-Nb(Pearce et al., 1984) diagram, the acid volcanic rocks of Yaolinghe Group belong to intraplate granite area, and the acid volcanic rocks of Wudang Group belong to island arc granite area. In the Ga/Al(× 100)Zr diagram (Whalen et al., 1987), the acid volcanic rocks of Yaolinghe Group are mainly distributed in alkaline (A-type) granite areas, while the acid volcanic rocks of Wudang Group are mainly distributed in I-type or S-type granite areas. In a word, the volcanic rocks of Wudang Group have similar geochemical characteristics to those of continental island arc volcanic rocks, while the rocks of Yaolinghe Group and gabbro dyke group show the same geochemical characteristics as those of continental rift magma (Ling et al., 2002b).
Figure 3-7 Standardized Element Composition Model of Primitive Mantle of Mesoproterozoic Volcanic Rock and Basic Rock Wall in Wudang Area
α -acid volcanic rocks of Wudang Group; B- Wudang group basic volcanic rocks; C- acid volcanic rocks of Yaolinghe Group; D basic volcanic rocks of Yaolinghe Group; E- basic dyke group rocks
3.2. 1.3 Composition characteristics of trace elements in metamorphic complex of Douling Group and rocks of Maotang Group
The composition characteristics of rare earth elements in the main rocks of Douling Group are introduced. According to the existing literature, in felsic gneiss, only accessory gneiss has trace element analysis data (4 samples, Zhao Ziran et al., 1995). The cobweb diagram of trace elements compiled by the standardization of primitive mantle reveals that the rocks have losses of Nb, Ta, P, Ti and Y in different degrees relative to the neighboring large ion pro-MagmaElemental (sketch). The standardized element composition model of plagioclase amphibolite ridge basalt in Douling Group shows (Zhao Ziran et al., 1995) that most samples of plagioclase amphibolite (tholeiite) produced in layers have no obvious loss of high-field elements such as Nb, Ta, Zr, Hf and Ti, but some samples have obvious negative anomalies of Ta and Hf, and some samples have weak negative anomalies of Nb and Zr. However, because the original rock belongs to amphibole (amphibole is green) which invades the basic rock, most samples show negative anomalies of Nb, Ta, Zr, Hf and Ti in different degrees. The protolith of Wawuchang Formation belongs to plagioclase amphibolite of tholeiite, and its protomantle standardization element cobweb diagram (Zhang et al., 2002; Zhao Ziran et al., 1995) also showed two composition modes; Nb, Zr and Hf have obvious losses, accompanied by strong losses of u and Th, while high-field elements have no obvious losses, but they are accompanied by losses of u and th. The cobweb diagram of normalized elements in the primitive mantle of plagioclase amphibolite originated from the intrusive rocks of Dagou Formation (Zhang et al., 2002, data quoted from Zhao Ziran et al., 1995) shows that all rocks have losses of Nb, Zr, Hf and Ti in different degrees.
Zhang et al. (2002) The spider web diagram of primitive mantle standardization elements of metamorphic volcanic rocks in Yaoyingzhai Formation of Maotang Group shows that the rocks obviously lack Nb, Ta and Ti, but not Zr and Hf. The same spider web diagram of metamorphic volcanic rocks in Matoushan Formation shows the loss of Nb, Sr, Ti and Y. ..
In a word, the geochemical characteristics of Proterozoic rocks in South Qinling show that most rocks show high-field element combinations with different degrees of loss, even some rocks produced in rift tectonic environment are no exception. Therefore, this may be an inherent feature of the lithosphere in this area, and magmatism is polluted by continental crustal materials in many cases. Therefore, it is not certain that rocks were formed in the island arc environment only according to the signs that rocks have high field strength elements. Of course, this is a problem that needs further study in the future.