In terms of Nd model age of crustal rocks (sedimentary rocks, metasedimentary rocks and granitoids) in the North Qinling Mountains, in addition to the concentrated distribution area of ??samples in the range of 2.2 to 1.8 Ga, another concentrated distribution area of ??samples is 1.4 to 1.4 Ga. 0.8Ga, indicating that the period from the late Mesoproterozoic to the early Neoproterozoic was the second period of intense crustal accretion in the North Qinling Mountains. So far, no rocks with Nd model ages less than 0.8 Ga have been found in the North Qinling Mountains, and even 1.3 to 0.9 Ga is the concentrated distribution area of ??Nd model ages for metamafic volcanic rocks and intrusive rocks in this area (Zhang Hongfei et al., 1995; Zhang Benren et al., 1996). This fact shows that the actual accretion of crust by mantle-derived material in the North Qinling basically stopped about 0.8 Ga ago, and the period from 1.3 to 0.8 Ga was also a period when mantle-derived magma was strongly active to form the crust. There are a large number of mantle-derived volcanic rock series in this area, such as the metatholeiitic basalt of the Kuanping Group (1015-1142Ma), the metamafic rocks intruding at the bottom of the Qinling Group (822-978Ma), the metabasalt of the Danfeng Group (984±67Ma), Erlangping Group metabasic volcanic rocks (708±63~822±80Ma) (Zhang Zongqing et al., 1994, 1996), and even MORB-type metatholeiitic basalt (1000~1252Ma, Li Shuguang, 1997) in the Songshugou ophiolite sheet, etc. The Sm-Nd isochron ages are all concentrated in this period, which is good evidence. Although there are also mafic rocks formed after 0.8 Ga in this area, such as the Junmiao Suzhou-Gabbro formed before 402 Ma, they have Nd model ages of 0.99 to 1.18 Ga (Li Shuguang et al., 1993). This shows that the magma that formed these rocks was not directly derived from the mantle at that time, but should be the product of remelting of material in the crust.
As for the mode of crustal accretion during this period, research shows that there are obvious records of lateral crustal accretion at the continental margin in plate convergence zones, and geochemical clues of vertical accretion of mantle-derived magma underplating.
9.1.3.1 Geological-geochemical record of lateral accretion
Most researchers generally believe that the North Qinling has a development history of the active continental margin of the North China Plate in the Early Paleozoic, but in Shangdan However, the suture zone produces ophiolites formed and emplaced at different stages in the early and late Neoproterozoic - Songshugou ophiolite (983Ma, Li Shuguang et al., 1991b) and Shangzhou-Xiaguan ophiolite (832Ma, Jiang Changyi et al., 1997). At the same time, on the north side of the Shangdan Suture Zone and on the basement of the Paleoproterozoic Qinling Group in the North Qinling Mountains, the following rock assemblages or suites formed in different periods and with different geochemical characteristics are also spliced ??and superimposed.
(1) The early Neoproterozoic Danfeng Group volcanic rock system (mainly metabasalt) is the main component of the northern edge of the Shangdan fault zone. Its metabasalt has a whole-rock Sm of 984±36Ma -Nd isochron age (Zhang Zongqing et al., 1996) is similar to the emplacement age of the Songshugou ophiolite. Geochemical studies have consistently identified it as arc volcanic rock (Sun Yong et al., 1988; Zhang Benren et al., 1994; Xue Feng and Zhang Guowei, 1993; Zhang Chengli et al., 1994; Zhou Dingwu et al., 1995b; Zhang Qi et al., 1995).
(2) Water-rich mafic rock intrusions with island arc geochemical characteristics in the late Neoproterozoic (761Ma and 561Ma, Yan Zhengfu et al., 1989; Zhang Zejun 1991), and granites formed later Among them, the slightly older Dehe pluton (794±30Ma) belongs to S-type granite and shows the geochemical characteristics of syn-collision type granite (You Zhendong et al., 1991b; Zhang Benren et al., 1994), while the later-formed Cai The Ao and Huangbaiyu granitoids, with ages of 659±50Ma and 670±40Ma respectively, belong to I-type granite and have the geochemical characteristics of island arc granite (Zhang Hongfei et al., 1993). These rock masses all intruded into the Qinling Group.
(3) The Qinwangshan-Zhumiao gabbro-diabase pluton exposed on the north side of the main Shangdan suture zone in the Early Paleozoic has obtained whole-rock Sm-Nd of 402.6±17.4Ma. The isochron age (Li Shuguang et al., 1989) is proved to be formed in an island arc or active continent environment based on geochemical characteristics (Li Shuguang et al., 1993; Zhou Dingwu et al. 1995b; Han Song et al. 1993).
(4) The Early Paleozoic S-type drift pool and Anjiping granite (485-452 Ma) have the geochemical characteristics of syn-collision granite (You Zhendong et al., 1991b; Zhang Benren et al., 1994, 2002), As well as granites with basic characteristics of I-type granite, such as large intrusions such as Huichizi, Da'ao and Taibai, and small rock bodies such as Shimen, Xuzhuang and Zaoyuan in the Danfeng area. The age of the rocks ranges from 444 to 380 Ma. (Shang Ruijun et al., 1988; Zhang Benren et al., 1994). Lithogeochemical studies have consistently proven that these granitoids have volcanic arc-type characteristics. Most of them invade the Qinling Group, and a few are found in the Erlangping Group (Zhang Benren et al., 1994; Zhang Guowei et al., 2001).
(5) The late Neoproterozoic to early Paleozoic Erlangping Group volcanic-sedimentary rock system is distributed in the north of the Qinling Group, that is, the back-arc area.
A whole-rock Sm-Nd isochron age of 708±63-822±80 Ma has been obtained for the lower part of the group (Zhang Zongqing et al., 1994). Early and Conodont and radiolarian microfossils from the Middle Ordovician (Sun Yong et al., 1996). Based on the fact that metabasic volcanic rocks are composed of basalts with island arc-type chemical characteristics and rocks with MORB-type chemical characteristics, and metaclastic rocks have geochemical characteristics of back-arc deposition, it is unanimously believed that this volcanic-sedimentary rock suite is Products in back-arc basins (Zhang Benren et al., 1994; Xia Linqi et al., 1991, Qiu Jiaxiang and Zhang Zhufu, 1994, 1996; Sun Yong et al., 1996).
What is particularly significant is that in recent years, research has made progress in the following two aspects: First, it has been proved with more evidence that the Danfeng Group volcanic rock system is composed of intraoceanic island arcs. In terms of geochemical characteristics, This can be compared with the basalts of intraoceanic island arcs such as Aleutian and New Britain, that is, the basalt mantle source area is basically free of terrestrial sediment pollution (Figure 9-2 and Figure 9-3) (Zhang Qi et al., 1995; Zhang Benren et al., 2002 ), while the Zhanmiao Suchang-gabbro shows the geochemical characteristics of Andean-type continental arc basalt rocks, with obvious addition of terrestrial sediments in the mantle source area of ??the rocks (Figure 9-4) (Li Shuguang, 1993, 1997), indicating that the two production conditions are different, and they cannot be the product of magma of the same era and same source. Secondly, through large-scale detailed mapping, the conclusive contact relationship between the Early Paleozoic Suzhou-Gabbro intrusion into the volcanic rocks of the Danfeng Group was discovered (Zhou Dingwu et al. 1995b; Han Song et al. 1993), which also ruled out the previously thought Danfeng Group. The relationship between basalt and Junmiao Su Chang-gabbro belongs to the magmatic products of the same period.
Figure 9-2 Th/Yb-Ta/Yb diagram of Danfeng Group metabasalt
(According to Pearce, 1983, quoted from Zhang Qi et al., 1995)
< p>DM: depleted mantle; MORB: mid-ocean ridge basalt; OIB: ocean island basalt; TH: tholeiitic basalt; CAB: calc-alkaline basalt; SHO: potash basalt○ is thirty miles Pave basalt; ● is Guojiagou basalt; /p>
(According to Li Shuguang, 1994)
Basic data are quoted from Zhang Qi et al., 1995
The above-mentioned Neoproterozoic and early Paleozoic rock assemblages and suites and their The geochemical characteristics not only strongly prove that the North Qinling Mountains had a history of active continental margin development during this period, but also imply that subduction of oceanic crust may have occurred more than once from the Neoproterozoic to the Early Paleozoic, which should be a very important The long process of ocean-continent interaction, including the formation of intra-oceanic island arcs and continental island arcs, possible two arc-continent collisions (according to two phases of small amounts of syn-collision granite), and the formation of continental margin arcs and back-arc basins Formation and development, and reflects the lateral accretion of the North Qinling microcontinent (Zhang Benren et al., 2002). Of course, the details of these structures await further study and testing.
9.1.3.2 Vertical accretion - information on underplating
Underplating, also known as underplating, is a method of continental crust accretion. Since the underplated rock layer is located at the bottom of the earth's crust and is less likely to be lifted to the surface by tectonic forces, it is not only difficult to directly observe and study it, but even the Nd model age of sedimentary rocks is rarely likely to reflect this underplated material. Generally, it can only be revealed through the geochemical traces of the source areas of basic volcanic rocks and granulite inclusions and crust-derived magmatic rocks (such as granitoids). The identification mark of underplating rock formations is mafic rocks located at the bottom of the earth's crust that are younger than the overlying rock formations. In the following, based on previous research (Zhang Benren et al., 2002), we will briefly discuss the basis for the crustal accretion through underplating of mantle-derived magma during the transition between the Mesozoic and Neoproterozoic in the North Qinling Mountains.
Figure 9-4 Illustration of εNd-Nb/Th, εNd-La/Nb and εNd-Ba/Nb of Junmiao Su Chang-gabbro
(Quoted from Li Shuguang, 1997 )
As mentioned above, the oldest continental crust basement in the North Qinling Mountains (Qinling Group) had granitic magma activity in two eras, the late Neoproterozoic and the early Paleozoic. Among them, the scale and intensity of magmatic activity in the Early Paleozoic Era are much greater than that of the Neoproterozoic, and each period of magmatic activity is characterized by the first formation of S-type granite with syn-collision type chemical characteristics, and then the formation of rocks with island arc type chemical characteristics. I-type granite. All these granitic rocks (feldspar) are characterized by significant enrichment of radiogenic lead, and their Pb isotope ratios are close to those of various basement rocks in the North Qinling Mountains, indicating that the source areas of their magmas are limited to the north. Within the range of Qinling crustal basement rocks. Geochemical studies have proven that both phases of collision-type granite should be the crystallization product of magma formed by anatexis of the Qinling Group rock formation, because they both inherited the unique Pb, Nd, and Sr isotope system of the Qinling Group rock formation (overall) (Zhang Benren et al., 2002 ). However, it is impossible for the Neoproterozoic (670-659 Ma) and early Paleozoic (440-382 Ma) island arc (I) type granitoids to have the Qinling Group as the main source rock.
The main basis is: ① The Nd model age of Neoproterozoic and Early Paleozoic granitoids is between 0.93 and 1.3Ga, with an average of 1.183Ga, which is significantly smaller than the paragneiss of the Qinling Group and the plagioclase angle it contains. Nd model age of amphibolite (1987Ma) (2.02~1.77Ga); ② Calculated εNd values ??of Qinling Group paragneiss at 670Ma and 400Ma (roughly equivalent to the formation ages of two periods of granitoids) (-8.19 and -10.80 respectively ) are significantly lower than the initial εNd values ??of the Neoproterozoic and Early Paleozoic granitoids (+1.12~+2.97 and +2.53~-4.45, respectively), while the calculated 87Sr/86Sr values ??(0.7199 and 0.7366, respectively) are significantly higher than the two The initial 87Sr/86Sr ratios of the granitoids in the early Qinling period (0.7041~0.7064 and 0.7055~0.7085 respectively); ③The main body of the Qinling Group is a metasedimentary rock series, and it is impossible to form granitoids with I-type characteristics. The Nd and Sr isotope system and lithology of the Qinling Group rock series do not match the required conditions of rocks from the granite source area, ruling out the possibility of it being the main source rock of this type of granite. So what is the main source rock of these granites?
There have been many discussions on the source areas and genetic types of the island-arc granitoids of these two periods. Among them, the early Paleozoic island-arc granitoids, especially the Huichizi pluton, have been discussed in more detail. For example, Xie Hongjie et al. (1993) used Pb, Nd, and Sr isotope tracing and combined with the structural background of the North Qinling active continental margin, proposed that the magma of the Huichizi granite body may have differentiated from the mantle 1.0 to 1.2 Ga ago. The outgoing islands and/or oceanic crust basalts were formed by high-level partial melting; Zhang Hongfei et al. (1994) believed that the Neoproterozoic Cai'ao and Huangbaiyu intrusions and the Early Paleozoic Huichizi and Zaoyuan intrusions are island-arc granitoids. The magma should mainly come from the mantle and belong to the mantle-crust synfusion type; and Chen Yuelong et al. (1995) based on the composition points of this type of early Paleozoic island arc granite in the εNd(t)-εSr(t) diagram (t=400Ma) It basically falls between the metamafic rocks (amphibolite, 978Ma) produced at the bottom of the Qinling Group and the gneiss composition region of the Qinling Group. It is proposed that these granitic rocks are metabasaltic rocks of the Qinling Group. and the products of partial melting of gneiss to varying degrees and in different ways to form melt mixtures, etc.
We consider that the late Neoproterozoic and early Paleozoic arc-type granitoids have the same Nd model age of 0.9-1.2 Ga, reflecting that both should be related to the mid-Neoproterozoic in the North Qinling Mountains. It is related to the crustal accretion material that was handed over. Therefore, if it is believed that the island arc granitoids of the Early Paleozoic were derived from the subducted oceanic crust at that time, we will encounter a source area of ??a small number of late Neoproterozoic island arc granitoids that separated from the mantle 1.0 to 1.2 Ga ago. How could the oceanic crust (which indicates that subduction had occurred at that time) be preserved until the early Paleozoic and then undergo partial melting? At the same time, in the εNd(t)-εSr(t) diagram (t=400Ma), the calculated values ??of the Nd and Sr isotopic compositions of the late Neoproterozoic island-arc granite when it evolved to 400Ma are the same as those of the same type of rocks in the Early Paleozoic. Distributed between the metamafic rocks (amphibolite, 978 Ma) produced at the bottom of the Qinling Group and the gneiss of the Qinling Group (mainly garnet biotite), and mostly close to each other. Metamafic rock endmembers (Figure 9-5) indicate that granite magma is mainly formed by partial melting of metamafic rocks, but the magma of different rock bodies is contaminated to varying degrees by the Qinling Group rock layers when it intrudes upward, resulting in isotopic Certain changes in composition. This also shows that the source rocks of the two periods of island arc granitoids are the same. In addition, although the basic volcanic rock series of the Kuanping Group and Danfeng Group, which are related to the crustal accretion event at the turn of the Mesoproterozoic, are in the Pb, Nd, and Sr isotope systems
Figure 9-5 North Qinling Neolithic Illustration of εNd(t)-εSr(t) of ancient and early Paleozoic island arc (I) type granitoids and related basement rocks (t=400Ma)
Rock mass name code: ZY—Zaoyuan; HBY- CO—Huangbaiyu-Cai'ao; HBC—Huangbaicha; SM—Shimen; HCZ—Huichizi; XZ—Xuzhuang. QL1—biotite plagioclase gneiss of the Qinling Group; QL2—garnet biot plagioclase gneiss of the Qinling Group; QL3—metamafic rock (amphibolite) with an age of 978 Ma formed at the bottom of the Qinling Group. Small ■, ● and ▲ represent the composition points of single samples of QL1, QL2 and QL3 respectively; large ■, ● and ▲ represent the average composition points of QL1, QL2 and QL3 respectively, which are also close to the two island arc granitoids. However, their metamorphism only reaches the low amphibolite phase, which excludes the possibility of partial melting. Based on the comprehensive analysis of the above data, it can be considered that the metamafic rocks produced at the bottom of the Qinling Group should be the most reasonable source rock for the two-stage island arc granitoids.
The Qinling Group is the oldest continental crust basement rock layer in the North Qinling Mountains, and no older continental crust rocks have been found beneath it so far. In view of the fact that the rocks of the Qinling Group have metamorphosed up to high amphibolite phase and even granulite phase in some places, the TPt trajectory study of the metamorphic process proves that the metamorphic complex of this group should be formed at a depth of about 26km underground (You Zhendong et al., 1991a; Liu Liang et al. , 1996), therefore the Qinling Group should have been a component of the lower crust.
Therefore, the main body of this mafic rock produced at the bottom of the Qinling Group should be located in the lower part of the lower crust. Furthermore, the formation age of this metamafic rock at the bottom of the Qinling Group (978 Ma) is much younger than that of the overlying Qinling Group (about 2000 Ma). Therefore, this rock is likely to be the top component of the rock formation formed by underplating mafic magma, and this underplating is likely to be closely related to the intense crustal accretion event that occurred in the North Qinling Mountains from the late Mesoproterozoic to the early Neoproterozoic. connect.
In recent years, Li Wuping (2001), through detailed studies of elemental and isotope geochemistry, proposed that the granitoids of the Early Paleozoic Huichizi pluton in the North Qinling Mountains have similar characteristics to adakitic rocks and should be composed of the deep lower crust. Adakitic rocks formed by partial melting of Neoproterozoic basaltic rocks. This view coincides with our above-mentioned understanding that this type of granite originated from underplated mafic rocks at the turn of the Mesozoic and Neoproterozoic.
As for the North Qinling block, it was later strongly uplifted through collisional orogeny, and was placed under the upper crust of the North Qinling due to the deep detachment and subduction of the middle and lower crust of the northern margin of the Yangtze Plate (South Qinling) to the north. The transformation process that resulted in the Qinling Group and the rock layers above forming the upper crust of the present-day North Qinling Mountains and the loss of the original underplating rock layers will be discussed later. I would like to point out in passing that the North Qinling Mountains have been proved to have the properties of a microcontinent of oceanic island origin. Accordingly, it is speculated that its extension should be limited, and this study shows that it has extended eastward to the Dabie area, so it is unclear whether it can still extend. It is worth noting to go to the Su-Lu area in the east and the West Qinling Mountains in the west.