Paleoproterozoic Oleshan magmatic active zone

The striking feature of Paleoproterozoic in western Shandong is the development of a large number of collision granites, which are GMS assemblages and strongly superimposed with Taishan granite-greenstone terrane to form a huge Paleoproterozoic magmatic active zone.

Calcareous alkaline granitoids

It consists of Oleshan granite and Hongmen diorite. The isotopic age of the former is mostly 2560～2424ma, which is between NeoArchean and Paleoproterozoic. The isotopic age of the latter is 2 1 17Ma.

3.5. 1. 1 Oleshan granite

It is composed of adamellite with medium-coarse grain-fine grain and porphyritic structure, which is widely distributed, accounting for about 53% of the area of early Precambrian intrusive rocks in western Shandong. Songshan rock mass, the late intrusive body of Oleishan granite, has the widest distribution and the largest exposed area, which reveals the strong magmatism in the late stage. From the early intrusive body to the late intrusive body, it has the characteristics of migration from northeast (Lushan-Yishan area) to southwest (Mashan-Sihaishan area) in geographical distribution. The rock changes from medium-grained structure to fine-grained structure, and the particle size changes from coarse to fine. The degree of rock deformation is from strong to weak, from strip structure to block structure; The mineral composition shows that plagioclase decreases gradually, while chronocline and microcline increase slightly, and dark minerals decrease gradually.

The measured mineral composition is located near the continental collision granite area in the QAP map (Figure 3. 15a). Generally speaking, the chemical composition of rocks shows that they are rich in Si, K and poor in Fe, Mg and Ca from morning till night, belonging to the calc-alkaline evolution series. In the K2O-Na2O-CaO diagram, the obvious evolution trend of calc-alkaline granite is shown (Figure 3. 15B). On the K2O- silica diagram (Figure 3. 16a), most of the samples are located in the high-potassium calc-alkaline rock series area. In the ACF diagram (Figure 3. 16b), the projection point is near the boundary between S-type and I-type granite, and most samples are located in S-type granite area. In the R 1-R2 diagram (Figure 3. 17) for judging the tectonic environment of granitoids, the main samples are located in the granite area and its vicinity in the same collision period.

Fig. 3. 15 paleoproterozoic granite QAP map and K2O-Na2O-CaO map (a).

(Basemaps are based on Magnard & Picoli, 1989 and Bakr & Arthur, 1976 respectively).

Fig. 3. 15 (a)QAP diagram and (b)K2O-Na2O-CaO diagram of Paleoproterozoic granite.

(After Magnard & Picoli, 1989 and Buck & Art, 1976 respectively).

1-Oleshan granite; 2- Sihaishan granite; 3- Motianling granite; 4- Double-topped granite; 5- Hongmen diorite; See Figure 3. 10 and Figure 3. 1 1 for descriptions of symbols in the figure.

Fig. 3. K2O- silica diagram and ACF diagram (a) and (b) of16 Proterozoic granite.

(The base map is according to Rickwood, 1989 and Nakata Festival, 1979 respectively).

Fig. 3. Diagram of (a)K2O-Na2O-CaO and (b)ACF of16 Proterozoic granite body.

(After Rickwood 1989 and Miyajiro 1979 respectively)

1-Oleshan granite; 2- Double-topped granite; 3— Hongmen diorite

Fig. 3. 17 Paleoproterozoic granite map R 1-R2

(According to the basemap of Batchelor & Bowdden, 1985)

Figure 3.17 r1vs. R2 discriminant diagram of Proterozoic granite

(After Batchelor & Bowden, 1985)

1-Oleshan granite; 2- Sihaishan granite; 3- Motianling granite; 4— Double-top gneiss casing; 5— Hongmen diorite

The rock assemblage, lithological characteristics and geochemical composition of the Oleishan granite are obviously different from those of the Neoarchean TTG granite, which is characterized by the development of a large number of biotite adamellites and represents the formation of mature continental crust. The distribution pattern of rare earth elements is different from the Neoarchean rare earth pattern with obvious enrichment of light rare earth and obvious negative anomaly of uranium, and it has the genetic characteristics of continental crust [10]. Generally speaking, the Oleishan granite is a kind of syncollision continental crust remelting granite, which is a continental collision granite formed by partial melting of continental crust during arc-continental collision and basin closure. The appearance of a large number of collision granites indicates that Shandong continental crust has entered a typical plate tectonic system.

3.5. 1.2 Hongmen diorite

Hongmen diorite is mainly composed of diorite, quartz diorite and granodiorite, with a small amount of gabbro and granodiorite accounting for more than 70% of the distribution area. It is mainly distributed in the northwest of Paleoproterozoic Oleshan magmatic active zone, and its exposed area accounts for about 65438 0.3% of the area of early Precambrian intrusive rocks in western Shandong. From the early intrusive body to the late intrusive body, the magmatic activity gradually became stronger; The rock composition evolved from basic to acidic, the dark minerals amphibole and biotite gradually decreased, the light minerals chronological and potash feldspar gradually increased, while plagioclase decreased.

The chemical compositions of the main elements in the rocks are classified into calc-alkaline granite evolution trend area, calc-alkaline rock series-high-potassium calc-alkaline rock series area, I-type granite area and granite area near post-collision uplift-late orogenic area (Figure 3. 15b, Figure 3. 16, Figure 3. 17). The chemical composition of gabbro has the characteristics of calc-alkaline basalt (Figure 3.5b).

In a word, Hongmen diorite was formed in the uplift and extension stage after collision orogeny, which is the product of differential crystallization, assimilation and contamination of underplating mantle-derived magma.

potash granite

It consists of Sihaishan granite and Motianling granite, the former is syenite granite and the latter is light granite.

Geological characteristics and rock characteristics

Sihaishan granite is mainly distributed in the north and south sides of Aolaishan magmatic active zone, invading Aolaishan granite, and its exposed area accounts for about 2.6% of the area of early Precambrian intrusive rocks in western Shandong. The rocks are mainly composed of potash feldspar (5 1% ~ 64%), syenite (22% ~ 33%), plagioclase (5% ~ 25%) and a small amount of biotite (1% ~ 5%). From the early intrusive body to the late intrusive body, the rock structure changed from medium coarse-grained structure to fine-grained structure, and the mineral particle size changed from coarse to fine. On the whole, the material composition shows a trend of gradually decreasing dark minerals and gradually increasing bright minerals. In the actually measured mineral QAP map (Figure 3. 15), it is located in the granite area of continental orogenic uplift.

Motianling granite is scattered in Ningyang-Yinan area in the middle of Aolaishan magmatic active belt, and intrudes into Aolaishan granite and Hongmen granodiorite. The exposed area accounts for about 65438 0.2% of the area of early Precambrian intrusive rocks in western Shandong. The rocks are mainly composed of plagioclase (22% ~ 44%), potash feldspar (16% ~ 27%) and quartz (25% ~ 5 1%), with a small amount of dark minerals (biotite, amphibole and epidote), with a total content of about 2% ~/kloc. From the early intrusive body to the late intrusive body, the rock structure is from medium to fine, and the mineral composition tends to evolve in the direction of potash feldspar. In the measured mineral QAP map, the projection points are on the upper and lower sides of the granite area after collision (Figure 3. 15).

Geochemical characteristics of 3.5.2.2.

The chemical compositions of the main elements of Sihaishan granite and Motianling granite (Table 3. 1) are rich in iron and poor in magnesium, with FeO * (total iron) being 0.97% ~ 3.34% (average 2.04%), MgO being 0. 16% ~ 0.92%, FeO */(. The alkali content is high, and the ALK (K2O+Na2O) of most samples is 9.23% ~1.40%, and the alkalinity index [a.r = (Al2O3+Cao+ALK)/(Al2O3+Cao-ALK)] is 2.64 ~ 4.40. On the K2O-SiO2 _ 2 diagram, it is located in the coarse rock area of Olive 'an (Figure 3.18a); On the Na2O-K2O diagram (Figure 3. 18b), it is located in the A-type granite area; In the R 1-R2 diagram (Figure 3. 17), it is located in the granite area in the late orogenic period.

Geochemical characteristics of rare earth elements (Table 3. 1) are: high total amount of rare earth, obvious fractionation of light rare earth, weak fractionation of heavy rare earth and negative europium anomaly (Figure 3. 19). It shows the general characteristics of A-type granite.

Trace elements (Table 3. 1) are rich in Rb, Th and ce, but poor in high-field elements such as Sr, Ba, Nb, P, Ti, Cr, Y, Zr and Hf. On the normalized spider web diagram relative to mid-ocean ridge basalt, Sr, P and Ti show obvious V-shaped valleys (Figure 3. 19), indicating that plagioclase, apatite and ilmenite are separated and crystallized during diagenesis.

Table 3. 1 chemical analysis results of main elements (%), trace elements (μg/g) and rare earth elements (μg/g) in the whole granite of Sihaishan and Motianling Table 3. 1 geochemical analysts of granite of Sihaishan and Motianling, including main elements (wt .%), trace elements and rare earth elements (μ g/g).

sequential

Analysis and testing unit: National Geological Experimental Testing Center; Analysis methods: main element X-ray fluorescence spectrometer (3080E), trace element plasma spectrometer (IRIS), plasma mass spectrometry (X series), X-ray fluorescence spectrometer (2 100) and rare earth element plasma mass spectrometry (X series).

Fig. 3. 18 K2O- silica and Na2O-K2O diagrams

Fig. 3. Identification diagram of18k2o-silica and Na2O-K2O.

Fig. 3. 19 standardized types of rare earth chondrites and trace element N-MORB in sihaishan and motianling granites.

Fig. 3. 19 standardized REE model and N-MORB standardized trace element spider diagram of granite chondrites in Sihaishan and Motianling.

The diagenetic age of 3.5.2.3

The main isotopic ages of Sihaishan granite and Motianling granite in the past were 247 1 ~ 233 1 Ma and 2457 ~ 2206 Ma [10], respectively. Zircon SHRIMP isotopic age of Sihaishan granite was analyzed. The test results are shown in Table 3.2, and the isotopic age value is 2533±8Ma (Figure 3.20).

Table 3.2 Zircon U-Pb Isotopic Data of Sihaishan Granite

Note: The data test was completed by Wan Yusheng of SHRIMP, Beijing Ion Probe Center. The error is 2σ. Pbc and Pb? They are ordinary lead and radioactive lead.

Figure 3.20 Isotopic Age Map of Sihaishan Granite

Fig. 3.20 Zircon U-Pb Isotope Consistency Map of Sihaishan Granite

Genesis and tectonic magmatic activity background analysis of granite in 3.5.2.4.

The above geochemical characteristics of Sihaishan and Motianling, especially their alkali-rich and high-iron-magnesium ratios; The contents of high field strength elements Zr, Hf, Nb and Y are high. MagmaElemental is rich in rubidium, uranium and thorium, but low in barium and strontium. The transition elements Cr and Ni are seriously depleted, which shows the geochemical characteristics of A-type granite. In the geochemical composition discrimination diagram, the sample points are cast near the A-type granite area (Figure 3. 18) and the A2-type granite area (Figure 3.25438+0). The rock is composed of adamellite and syenite, and the dark minerals contain a small amount of biotite, but do not contain alkaline dark minerals such as aegirine and sodalite, which have the characteristics of Al-A granite (A2 granite). Therefore, it is considered that Sihaishan and Motianling granites belong to A2 type granites. In the Nb-Y diagram, it is located in the volcanic arc+collision granite area (Figure 3.20), which shows the geochemical information of "arc" magmatism.

Fig. 3.2 1 genetic discrimination map of granite in sihaishan and motianling.

Fig. 3.2 1 discrimination of tectonic environment of sihaishan and motianling granites

The potash granite in the study area is similar to A2 granite defined by Eby[56]. It is generally believed that this kind of granite is related to land-land collision or island arc magmatism, formed in the lithosphere stretching and thinning environment in the late orogeny, and closely related to mantle upwelling and underplating. Why can the geotectonic environment of crustal stretching and thinning meet the physical and chemical conditions such as low pressure, relative water shortage and high temperature necessary for the formation of A-type granite? The upwelling and underplating of mantle materials may be the bridge and link closely related to the magmatic activity of A-type granite structure. The research shows that mantle materials can directly or indirectly participate in the formation of A-type granite, and there are three main ways [57]: (1) The mantle basaltic magma invading the crustal extension environment directly derives A-type granite through high crystallization or liquid immiscibility; ② During the emplacement of mantle magma, A-type granite saturated with SiO2 _ 2 was produced through the interaction between crust and mantle (such as mixing with crustal magma or being mixed with felsic crust); ③ The high-temperature mafic magma that underplates the mantle provides the necessary energy for the deep melting of the overlying crustal materials to form A-type granite magma. In different extensional tectonic units, the activities of mantle materials are also different. The extension and thinning of continental and oceanic plates are mainly related to mantle plume or hot spot activities. Post-collision extension of orogenic belt is mainly related to gravity collapse and settlement. At the plate edge, apart from subsidence, upwelling and undercutting of asthenosphere mantle material, subduction plate detachment is also an important way to lead to asthenosphere mantle material activity.

The energy provided by the high-temperature mafic magma of the underplating mantle makes the overlying crustal material deeply melt to form A-type granite magma; The extensive development of collisional granite-Oleshan granite proves that the activity mode of mantle material is related to gravity collapse and settlement caused by post-collision extension of orogenic belt. It is considered that there was an Archean ocean basin on the west side of Jiaoliao landmass, and there were Qianhuai landmass and Fuping landmass on the west side of the ocean basin [19]. At the end of NeoArchean, the island arc of western Shandong collided with the western landmass, and the lithosphere was stretched and thinned in the post-collision orogeny period. Mantle materials upwelling and undercutting, and mixed with crustal materials to form diorite-dominated intrusive rocks, and the energy provided by mantle magma caused overlying crustal materials. Sihaishan granite and Motianling granite are the earliest A-type granites in Shandong Province. The appearance of A-type granite indicates that the crust of western Shandong has evolved into a mature rigid crust similar to modern continent.

3.5.3 tectonic deformation of the aolaishan magmatic active zone

Proterozoic deformation structure in western Shandong is characterized by large-scale ductile shear deformation, and two stages of ductile deformation zones and corresponding foliation structures are developed. The first phase of ductile deformation zone is developed in the Aolaishan magmatic active zone, with a huge scale, the longest is 85 kilometers and the widest is several kilometers. Strike northwest to northwest, showing strong dextral strike-slip nature, basically inheriting the deformation characteristics of Archean, but its structural level is shallow, with obvious mylonite zone and high greenschist facies metamorphism. The main ductile deformation zones are shown in Table 3.3.

Table 3.3 List of ductile deformation zones in the first stage of Aolaishan magmatic active zone Table 3.3 Early pipeline shear zone of Aolaishan magmatic rock zone

The second stage of ductile deformation is mainly developed in the eastern edge of the Aolaishan magmatic active zone, that is, the Yishu fault zone, which is characterized by the NNE ductile deformation zone. They vary in size and characteristics. The main active time of ductile deformation in this period is about 1800Ma, and the deformation period is later than the emplacement time of Hongmen diorite and earlier than the Mesoproterozoic Niulan diabase. Ar-Ar isotopic dating of polycrystalline muscovite in the deformation zone shows that the plateau age is/kloc-0 1852.02±20.96Ma and the isochron age is1867.16 49.02 Ma [49]. Mylonite and dolomite schist, which are widely developed in ductile deformation zone, belong to middle-shallow structural level, and the temperature and pressure are low during deformation. The characteristics of each ductile deformation zone are shown in Table 3.4.

Table 3.4 List of Phase II Ductile Deformation Zones in Aolaishan Magmatic Belt Table 3.4 Late Aolaishan Shear Zone

3.5.4 Paleoproterozoic continental crust evolution in western Shandong.

Proterozoic is a period when the North China Craton was formed by the structural combination of two mature landmasses (the eastern landmass and the western landmass) with the scale of microcontinent. During this period, the crustal evolution in western Shandong can be roughly divided into three stages.

(1) arc-land collision stage

From the end of NeoArchean to the beginning of Proterozoic, the island arc of Luxi block collided with the western block, and a large number of granites of the same collision continental crust remelting type were emplaced. The granite type is G 1G2, and the Archean basement is folded and deformed. The western continent is uplifted to the island arc zone.

(2) Post-collision stretching stage

After the collision orogeny, the extension of lithosphere became thinner, mantle material upwelling and underplating, and A-type granite produced by the interaction between crust and mantle emplaced along the crustal extension zone. Potash granite is an important event of continental cratonization and crustal differentiation.

(3) Rotation after collision

When two adjacent landmasses collide, due to the differences in the size of landmasses, the material composition at the edge of landmasses and the different forces on both sides during the collision, large-scale intense shearing action and rotational movement are formed during the collision, resulting in a large number of ductile shear deformation zones. The movement direction of the ductile shear zone in western Shandong mainly reflects the characteristics of right strike slip, and most of them have the characteristics of steep foliation, gentle lineation, long extension, strong deformation, bedding shear slip and retaining two-stage deformation fabric. Generally speaking, the occurrence and development of these ductile shear deformation zones are controlled by the tectonic background and inevitable result of Proterozoic arc-land collision. Paleoproterozoic Luxi continental crust experienced a relatively complete collision-extensional cracking evolution process, and completed the second cratonization of Shandong block basement.