The study of rock geochemistry proves that high and low velocity bodies not only coexist in a certain depth range, but also lead to their interaction under appropriate conditions, that is to say, the interaction between the upwelling asthenosphere or soft fluid and the residual lithosphere is not only possible, but also inevitable. Magmatism and its characteristics are favorable probes to reflect its source area. According to the data of Phanerozoic magma origin in Qinling area in recent 10 years, almost none of them belong to single mantle endmember or crustal origin, but to multi-source origin. The source region has experienced a complex process of layer-circle interaction, which shows that layer-circle interaction plays a key role in the process of magma origin. On the contrary, magmatism and the inversion of xenoliths in it are the most favorable means to reveal the interaction between layers. Based on the data of Qinling orogenic belt, this chapter obtains the characteristics of the source region through geochemical inversion, and further understands the layer-circle interaction of the source region, which is preliminarily divided into the following types:
8.2.2. 1 Interaction between asthenosphere and lithosphere melt/fluid
The interaction between melt/fluid in asthenosphere and lithosphere is very common. These melts did not form an independent magma chamber and ascended into the deep crust, but remained in the mantle. After the action, the metasomatic mantle lithosphere can be melted to form kimberlite, K-Mg lamprophyre and alkaline lamprophyre. Because the alkaline magma with high incompatible elements cannot be explained by the partial melting of normal garnet lherzolite, the reasonable model is to account for the partial melting of mantle. South Africa type ⅰ kimberlite is close to OIB on Sr-Nd isotope diagram, which is considered to be mainly caused by asthenosphere melting. Type ⅱ kimberlite is located on the mantle evolution line in the fourth quadrant, which is considered to be formed by partial melting of the lithosphere after being replaced by carbonate/silicate melt from the asthenosphere. However, in recent years, the study of complex xenoliths in type I kimberlite in South Africa shows that the mantle in this area has obvious heterogeneity and complex evolution history (Zhang, 1998). Multiple mantle metasomatism led to the formation of a series of Ti-bearing water-bearing minerals and opaque minerals, and at the same time, the iron-rich metasomatism edge of olivine and orthopyroxene was formed. The study of oxygen isotope further confirmed that the main reason for the composition imbalance between minerals is metasomatism, and these metasomatic melts/fluids come from the asthenosphere (Zhang, 1998). It can be seen that the source area of type I kimberlite is also the product of the interaction between asthenosphere and lithosphere.
There are two kinds of kimberlites containing diamonds in China, one belongs to type II (Fuxian) and the other is the transition between type I and type II (Mengyin) (Zheng Jianping Lv Fengxiang, 1996). Lithospheric composition plays an important role in the source areas of these two kimberlites, and magma is not a simple asthenosphere source area.
The lamprophyre group in Shaanxi described in Chapter 6, the original author suggested that the magma originated from the abnormal mantle of phlogopite pyroxenite type, which was formed by the melt/fluid metasomatism of deep mantle rich in alkali, titanium, iron and volatile matter (Xu, 1999), and its formation mechanism was similar to that of kimberlite, but the formation depth was less than 65439. It is speculated that both Qinling mantle and North China Craton had the characteristics of lithospheric mantle melt/fluid activity in Paleozoic.
Interaction between Late Mesozoic-Cenozoic asthenosphere and Paleolithosphere in 8.2.2.2
Since the Late Cretaceous, large-scale lithospheric extension and asthenosphere upwelling have occurred in eastern China, resulting in obvious lithospheric thinning. At the same time, with the formation of energy basin and the extensive activities of basalt magma from asthenosphere, there are different explanations for the mechanism of lithospheric thinning, which can be summarized as follows:
(1) The lithospheric thinning in eastern China is closely related to the subsidence of the Pacific coastal zone, including the continental lithospheric roots (Deng et al., 1996).
(2) The lithospheric thinning of orogenic belt is related to the detachment of eclogite and lithosphere in the lower crust (Gaoshan et al.,1999; Li Shuguang et al, 200 1).
(3) Large-scale lithospheric thinning and stretching in eastern China are closely related to asthenosphere upwelling. Different people elaborated and emphasized different aspects. Xu Yigang and others emphasized that the upwelling asthenosphere material not only has thermal and mechanical erosion on the paleolithosphere, but also has great significance in chemical erosion. There is no obvious contradiction between the new mantle material proposed by Zheng Jianping and the "mushroom cloud" model proposed by Lv Fengxiang.
(4) This book holds that the second view is supported by a large number of research results on rock geochemistry of orogenic belts and has been widely recognized. In the continental craton, however, due to the low density, the extremely thick paleolithospheric roots are difficult to sink due to gravity (Lv Fengxiang et al., 2000). In this way, due to the small-scale extension of the crust from JBOY3 -K 1, the large-scale extension of the lithosphere after K2 and the influence of global mantle thermal events, the upwelling of the thermosphere and the accompanying settlement of paleolithospheric debris are balanced, complementary and passive. Part of the remaining paleolithosphere is still preserved in the asthenosphere mantle, which is the L/A interaction zone proposed in this chapter, and it also causes the pattern of high-speed and low-speed interbedding or vertical juxtaposition in eastern China. The replacement and mixing of the newly upwelling asthenosphere and the old lithosphere is an integral part of L/A interaction. Three years ago, we only provided evidence of the main elements of L/A interaction. Recently, trace elements and Re-Os isotopes have been used to trace the residual paleolithosphere and newborn mantle, and similar conclusions have been reached (Zheng et al., 2001; Gaoshan et al., 2003; Wu et al, 2003).
The interaction between upwelling asthenosphere mantle and paleolithosphere (L/A interaction) that we emphasize is very obvious in this period. This is not only reflected in the characteristics of mantle samples, but also in the composition of mantle-derived magma. A typical example in eastern China is the genesis of potash volcanic rocks in Wudalianchi. 1992, Zhou Xinhua proposed that the high-potassium volcanic rocks in the northern margin of Northeast China have an EMI-type LOMU endmember, and EMI is obviously related to the ancient land mass. Zhang et al. (1998) proposed through detailed rock geochemical work that the source area of potash volcanic rocks in northeast China is mainly Archean-Proterozoic garnet peridotite containing phlogopite, and the lithospheric mantle formed mantle endmembers with EMI characteristics through ancient metasomatism, and then the upwelling asthenosphere mantle containing Dupal anomaly caused thermal erosion to the thick lithosphere in the late Mesozoic, and melted to form magma. This viewpoint explains the genesis of potash volcanic rocks distributed in many areas of eastern China, and may also have certain reference significance for the deep function of basalt source areas with EMI components.
The predecessors have done a lot of work on Cenozoic basalts in Nvshan area (Chen Daogong,1992; Chung, 1999) provides more comprehensive isotope data, and also reflects that the source region contains EMI components. We choose representative Sr-Nd isotope data for interpretation. According to the range occupied by the throwing point of Nvshan basalt in Figure 8-5, the necessary components of the mantle source area of magma are DMM (depleted oceanic ridge mantle) and EMI (enriched mantle I). Most people think that the electromagnetic interference of Cenozoic basalts in eastern China is provided by the continental lithospheric mantle. These basalts distributed in the Sino-Korean Craton (Archean-Proterozoic basement) generally show that the source region contains EMI isotope characteristics, with a lower 206Pb/204Pb(Chung, 1999), which is different from that of South China. The figure also shows that the basalt in Nvshan area keeps pace with the times. From the end of Paleogene (36.2Ma) to the end of Neogene (5.9Ma), the EMI component gradually decreased, while the Dmm component gradually increased, that is, the component representing the paleolithosphere decreased and the component representing the asthenosphere increased. The source area of basalts ejected during the 5.9Ma period is close to the typical asthenosphere, indicating that the interaction between asthenosphere and paleolithosphere does exist. The Pb isotope map published by Chung( 1999) also shows that the source area of Cenozoic basalts, including the Tancheng-Lujiang fault zone, is composed of DMM and EMI in different proportions, so it can be considered that Pb isotope data also support the above conclusions.
Fig. 8-5 Strontium and Neodymium Isotope Map of Cenozoic Basalt in Nvshan, Anhui Province
The figures in the picture represent the formation age of basalt at this time.
8.2.2.3 crust-mantle interaction
The interaction between crust and mantle is wide, but the action related to basic-intermediate basic magma is mainly the participation of lower crust components. The mantle part includes both asthenosphere mantle and lithosphere mantle, so it may be the interaction of lower crust, lithosphere mantle and asthenosphere mantle, or the action of the former two. Because the lithosphere also includes the crust, it still belongs to the category of L/A interaction. Typical examples are:
Table 8-2 Representative Strontium and Neodymium Isotopes of Basalt in Nvshan Area
(1) Kuang, 2000, doctoral thesis, China Geo University (Wuhan).
It is considered that the source areas of mafic-ultramafic rocks after collision in Dabie orogenic belt, such as Tongjiachong, Zhujiabao, Qingshan and Qingshuihe, are the interaction of DMM, EMI and EMI, and the enrichment of mantle is related to the addition and recycling of subduction plates. Huang Fang, Li Shuguang and others (2002) clearly pointed out that the Mesozoic Zhujiabao mafic-ultramafic rock source area after Dabie Mountain collision is characterized by the mixing of three end-member components: asthenosphere mantle with loss, lithosphere mantle with ancient enrichment and lower crust material, and the intrusion time of the rock mass is about 130Ma. The dynamic process proposed by the original author is that the thickened lithosphere of the orogenic belt is detached after the collision, which leads to the upwelling of the asthenosphere with losses, and both of them occur.
(2) Another possibility is partial melting of asthenosphere. Because the lithosphere is in a transitional state of compression or weak extension, the melt does not reach the surface directly when it rises, but partially stays in the transition zone between the crust and the mantle, and at the same time heats the lower crust to promote its melting. The source of this magma is characterized by the mixing of continental crust and asthenosphere mantle. The above two situations are sometimes difficult to distinguish and need comprehensive judgment.
(3) In the Qinling orogenic belt, the Yangtze Craton dived under the North China Craton in the early Mesozoic, resulting in land-land collision and ultrahigh pressure metamorphism. As mentioned in chapter 6, the boundary between the two blocks is complex, with the characteristics of crossing, overlapping and detachment. Due to the addition of EMII in the middle and upper crust, the composition of the basic magma source area may be more complicated. The Late Cretaceous basalts such as Xinzhou and Huangpi shown in Figure 6- 10 contain EMII components.
According to this book, according to the current data, the Qinling orogenic belt began to be dominated by the crust-mantle interaction at the depth of 140 ~ 152Ma, and the most important sign was the magmatic activity in this period, especially the trace elements of basic-intermediate volcanic rocks showed obvious negative anomalies in the spider web map, which was different from the vast areas in eastern China. From the Sr-Nd isotope diagram (Figure 8-6), it can be seen that the typical points of 127Ma Zhujiabao gabbro in this period are far away from the mantle materials and magma-inducing points dominated by asthenosphere-lithosphere interaction erupted in Paleozoic Gao Lan, Shaanxi, Dahongshan, Huangpi and Yangxin 8 1.3Ma, and the Cenozoic Nvshan, indicating Mesozoic control. Fig. 8-7 shows the ε(Sr-Nd) isotopic positions of basic and intermediate basic rocks from 127Ma to 26Ma in Qinling area. Gabbro-pyroxenite such as Zhujiabao and Qingshuihe are widely distributed, and the first area is gabbro with high magmatic differentiation. Gabbro-pyroxenite such as Zhujiabao has an abnormal value (ε (Nd) is the highest and ε(Nd) is the lowest). It is mainly distributed in gray areas, which is consistent with the distribution areas of other basic and intermediate-basic volcanic rocks from late Jurassic to early Cretaceous. It seems that although the AFC effect experienced by plutonic rocks during emplacement is more complicated and the source inversion effect of volcanic rocks is better than that of intrusive rocks, better results can be obtained if the main components of intrusive rocks are reasonably determined. The figure shows that the composition of the source region is between DMM and the lower crust of North China, suggesting that magmatism is controlled by crust-mantle interaction, and the genetic model is described in this section. However, whatever the possibility, that is, the mantle directly participates in melting or the mantle plays a role in providing heat energy, the key is that the composition of the crust is the first, otherwise there will be no loss of high-field elements and low nd isotope values.
Fig. 8-6 Sr-Nd isotope map of Phanerozoic representative basic igneous rocks in Qinling orogenic belt.
1 —— the average value of Cenozoic basalt and spinel peridotite xenoliths in Nvshan, Anhui Province (upper left); 2— Average value of Paleozoic K-Mg lamprophyre and garnet peridotite xenoliths in Dahongshan, Hubei Province (lower left); 3— Average value of Paleozoic K-Mg lamprophyre and pyroxenite xenoliths in Gao Lan, Shaanxi (I); Huangpi and Yangxin 4-8 1.3Ma basalts; 5— Represents Zhujiabao 127Ma gabbro-pyroxenite.
Fig. 8-7 Sr-Nd isotope diagram of basic igneous rocks in Qinling orogenic belt after collision.
1-Gabbro-pyroxenite in Zhujiabao and other places, 2-Huangpi and Xinzhou basalts, 3-Ruyang and Yichuan basalts, 4-North Huaiyang Late Mesozoic basalts and 5-North Huaiyang Late Mesozoic coarse andesite. NCLC is the lower crust of North China, and DMM is the source of the mantle of the depleted oceanic ridge. See I and the text in the gray area.
In the Late Cretaceous (8 1Ma) in 8.2.2.4, the interlayer interaction between Huangpi and Xinzhou basalts was complex, involving many components.
(1) is composed of asthenosphere, which indicates that the main elements of magma are similar to Cenozoic alkaline basalt in eastern China. In Figure 7- 10, an end-member component with DMM is shown, which is the main body of the source region.
(2) There are EMII components in the mantle of Qinling Mountains (Figure 7- 10), but the source of EMII components is not very clear at present.
(3) There are crustal components, but the number is small or limited to a few areas. Figure 7-8 shows that some K2 basalts on the southern slope of Dabie also have negative anomalies of high field strength elements.
(4) Figure 9-9 in Chapter 9 of this book shows that the late Cretaceous basalt may be a mixture of eclogite and depleted mantle.
It can be seen that the deep action in Qinling area in this period has transitional characteristics, which is different from the deep action dominated by asthenosphere-lithosphere interaction in Cenozoic.
The asthenosphere-lithosphere transition from Mesozoic to early Cenozoic in 8.2.2.5.
At the beginning of this chapter, it has been explained that "the lithosphere mantle is the residual part of the original mantle or the weakly depleted mantle after partial melting", and now a part of the mantle lithosphere should be transformed from the asthenosphere upwelling after the extraction of basalt from Late Cretaceous to Early Tertiary to the new lithosphere at the stage of magma stopping from Oligocene to Early Miocene. The evidence is that among mantle xenoliths contained in Neogene (Late Miocene)-Quaternary alkaline basalts, there are samples with weak main elements, weak trace elements and isotope losses, and the equilibrium temperatures of some samples are low, which proves that the above speculation is in line with reality. In other words, today's lithosphere has both Cenozoic lithosphere caused by the transformation of upwelling asthenosphere; And the remnants of the ancient lithosphere. According to the actual data in eastern China, the asthenosphere/lithosphere transformation does not need a long time, but it can be completed in millions of years.