The characteristics of constant or trace elements of igneous rocks in Daxing 'anling are displayed, and the formation background of rocks is discussed in combination with them, and the magma source is speculated. The Sr-Nd isotopic system of igneous rocks can more accurately reflect the material source and source characteristics of magma. In order to know more about the strontium and neodymium isotopic compositions of granites and volcanic rocks in Daxing 'anling, this book collects relevant information as much as possible. Although some data do not match, from the initial values of Sr and nd shown in Table 5- 13, they are very close, do not change with time, and are concentrated in a small interval, and the ISr value is 0.704 1 ~ 0.7087. εNd(t) is mostly positive, concentrated between 1 ~ 4, and a few are -3, and εSr is mostly concentrated between 0 ~ 20 (Table 5- 13). As can be seen from Figure 5-27, the low value of 87Sr/86Sr aNd the positive value of εNd(t) make these points concentrated near the primitive mantle, which reflects that the materials from the primitive mantle participated in the formation of granite. From the early Mesozoic to the late Mesozoic, such concentrated Sr and Nd isotope distribution means the long-term uniformity of the mantle source area. The data in Table 5- 13 are mainly taken from Daxing 'anling, north of Xilamulun River, with an area of about 30000km2, which is the Paleozoic continental crust accretion area in the northern margin of North China Craton. The mantle source here is new and has not been obviously polluted by the continental crust, which is different from the lithospheric mantle enriched in the North China Craton in Mesozoic.

Table 5- 13 Sm-Nd, Rb-Sr isotopic data of granitic volcanic rocks in Daxinganling.

Note: *ISr=0.707 1 quoted from Sharla Cheung (1986) Rare Earth Elements and Oxygen Isotope Geochemistry of Inner Mongolia 80 1 Alkaline Granite (master's thesis); The age of biotite syenite granite in Maanzi, linxi county, according to Shenyang Institute of Geology; Haobugao Wulanba is a potash feldspar granite; Plagiogranite is the flower of Butun in Xing 'an League.

From the point of view of igneous rocks and elemental composition, the tectonic environment and material source of magma formation are discussed above, and a general understanding is obtained: igneous rocks are formed in intraplate, non-orogenic and extensional environment, and in a long-term relatively stable environment, there is a continuous supply of magma, which has enough time to complete the separation and crystallization differentiation of magma in deep magma chamber. During the formation of crust-mantle mixed magma, there are not only remelting of continental crust materials, but also unmixed primitive mantle materials in the north area and enriched lithospheric mantle materials in the Karaqin area in the south area. The early Mesozoic showed more remelting of continental crust materials, forming a set of muscovite granite and diorite remelted from basic rocks in the lower crust (see Chapter 4 for details). In the late Mesozoic, especially in the early Cretaceous, the alkalinity of intrusive rocks and volcanic rocks with deep source and shallow intrusion increased, and granites were mostly non-orogenic A-type granites, volcanic rocks showed bimodal characteristics, and basalts erupted in the later period. Generally speaking, the extension gradually strengthened and the magma source area deepened.

Figure 5-27 Strontium-Neodymium Isotope Comparison Diagram

Deficient mantle; PM- primitive mantle; Himu-high μ mantle; Emi-I enriched mantle; The mantle rich in emii-ⅱ; 1- granite; 2- volcanic rocks