I. Classification of genetic types of granite
In addition to the above-mentioned rock types classified according to mineral composition, granite is also classified from the perspective of its genesis. The genetic classification of granite mainly considers the source rock characteristics of magma and its tectonic environment.
1. Classification of granite types according to source rocks
Chapel & White (1974, 1977) put forward the concepts of S-type granite and I-type granite by studying the granite in the lachlan fold zone in Australia. S-type granite slurry is mainly formed by partial melting of weathered sedimentary rocks (mainly argillaceous rocks) or metamorphic sedimentary rocks, while I-type granite slurry is formed by partial melting of weathered igneous rocks. From then on, Loiselleetal. (1979) has separated type A granite from type I granite, mainly referring to alkaline granite. No matter S type or I type, source rocks are crustal rocks. If the granite magma comes from the mantle source, it is called M-type granite. Castro Tal. (199 1) It is considered that I-type granite is formed by mixing magma from both ends of mantle source (M-type) and crust source (S-type), and it is called H-type granite. I, S, M and A classification of granite is a common classification scheme at present, and its main features are shown in Table 5- 1.
◎ type I granite: the chemical composition is high in Na2O and CaO, Na2O/K2O > 1, aluminum saturation index A/CNK=Al2O3/(CaO+Na2O+K2O) (molecular ratio) < 1. 1, and corundum does not appear in CIPW standard minerals, or its content is less than 60. There are no aluminum-rich minerals such as muscovite, garnet and cordierite, only amphibole and magnetite.
◎S-type granite: The chemical composition is characterized by rich aluminum, and the aluminum saturation index is A/CNK > 1. 1. Corundum with content > 1% appears in CIPW standard minerals, with low CaO content, and the initial ratio of Na2O/K2O < 65438+87Sr/86Sr is > 0.708. Aluminum-rich minerals, such as garnet, muscovite, cordierite, andalusite, etc. , appears in the actual minerals, and amphibole does not appear.
◎M-type granite: mainly refers to the differentiation of mantle-derived magma. Some people classify the granite produced by melting the oceanic crust under the subduction ocean island arc as M-type. A/CNK < 1. 1, Na2O/K2O are high, and the transition group elements such as Cr, Co, Ni and V are high in rocks, and the initial ratio of 87Sr/86Sr is very low, less than 0.705.
◎A-type granite: The original meaning refers to a set of granite rich in alkali and volatile matter, and alkaline granite is its typical representative. Now it refers to alkali-rich, anhydrous and non-genetic granites, including some aluminous granites.
Table 5- 1I, S, M and A Granite Characteristics Comparison
2. Granite types classified according to magmatic tectonic environment.
The study shows that the characteristics and rock assemblage of granite formed under different tectonic environment conditions are different, and the classification of granite tectonic environment is put forward. There are many such classification schemes, such as ridge granite, volcanic arc granite, intraplate granite, collision granite and post-orogenic granite proposed by Pearceetal. (1984,1987). Maniar & Piccoli (1989) classified granites into orogenic granites and non-orogenic granites. Orogenic granite can be divided into four types: island arc granite, continental arc granite, continental collision granite and post-collision granite. Non-orogenic granites can be divided into granites related to rifting, granites related to continental orogeny and oceanic plagiogranite. Barbarin( 1990) classifies granitoids into seven types according to their mineral assemblages, outcrops, lithology, location characteristics, geochemistry and isotope characteristics, and different types of granitoids correspond to different geodynamic environments and source regions (mantle source region, crust source region and crust-mantle mixed source region) (Table 5-2). Winter(200 1) summarized the genesis and characteristics of granite and related rocks in different tectonic environments based on the study of granite by Pitcher( 1983, 1997) and Barbarin( 1990) (Table 5-3).
Table 5-2 Granite Types, Magma Sources and Their Relationship with Tectonic Environment
Second, the genesis of granite
There are two main views on the formation of granite magma: magma differentiation and partial melting of crustal rocks. Bowen (1928) thought that granite magma was formed by the differentiation and evolution of basaltic magma, but later studies confirmed that a large area of granite was mainly exposed in the continental crust and rarely exposed in the ocean, which was inconsistent with the widespread existence of basalt in the mainland and the ocean, and there were few contemporary basalts or gabbro exposed in the extensive exposed areas of continental granite. This shows that granite magma should have an independent origin and is closely related to the crust. The view that granite is mainly deep melting of the crust has been widely recognized. Mantle granite is extremely rare. According to the existing research, even the oceanic plagiogranite in ophiolite suite is not entirely derived from the differentiation of basaltic magma, but partly from the melting of oceanic gabbro.
Experimental petrological studies show that partial melting of mantle peridotite can not directly produce acidic magma, but only alkaline magma. However, crustal rocks can be melted to different degrees, resulting in granite magma with different compositions. The most typical experiment is Winkler's (1976) melting experiment of hard sandstone under the condition of PH2O=2× 108Pa. When heated, the hard sandstone is transformed into gneiss, and the mineral combination is: syenite (36%)+ plagioclase (33%, An 19. When heated to 687 10℃, gneiss began to melt, and the melt composition (4 1%, albite 28%, potash feldspar 3 1%) was equivalent to adamellite. Alkali feldspar completely melted at 700℃, and the melting amount reached 30%. At 740℃, plagioclase completely melts into the melt, and the melt content reaches 75%, and the melt composition is equivalent to granodiorite. This experiment fully shows that the partial melting of continental crust materials can produce granitic magma, and the melting degree increases with the increase of temperature, and the composition of the melt is constantly changing. Therefore, source rocks with the same composition can be melted at different temperatures to form granites with different compositions. S-type granite is formed by magma emplacement produced by melting of this weathered continental crust sedimentary rock or metamorphic sedimentary rock. Not only the melting of continental silicon-aluminum crust can produce granitic magma, but also the partial melting of the lower crust of basic igneous rocks can form I-type granite, such as Phanerozoic granite in Central Asian orogenic belt (Wu Fuyuan et al., 2007).
Table 5-3 Granites and Related Rock Assemblies Formed in Different Tectonic Environments
An important problem of massive granite formed by crustal melting is heat source. At present, there are two main understandings: one is that orogeny causes crustal thickening, which leads to the increase of geothermal gradient and partial melting of rocks; Second, the heat source mainly comes from the mantle, and the mantle-derived basic magma gathers at the bottom of the crust in the form of underplating. The huge heat brought by this high-temperature magma caused large-scale metamorphism and partial melting of the lower crust, forming granite magma. The basic magma from the mantle not only provides the heat needed for the formation of granite magma, but also partially mixes with the acidic magma (Pitcher, 1997) produced by the melting of the lower crust to form different types of granitic rocks, forming a series of rock assemblages with continuous transition in composition. This is also the mainstream view about granite slurry mixing at present.
In addition to temperature, the formation of granite slurry is also controlled by adding water and reducing pressure. Campbell & Taylor (1983) once pointed out that "there is no granite without water, and there is no continent without ocean", which profoundly explained the important role of water in the formation of granite, because the addition of water can greatly reduce the melting temperature of rocks. The decrease of pressure reduces the melting point of rock, which is beneficial to the melting of rock. Thompson (1999) even thinks that decompression is an important mechanism for the formation of granite slurry. The extension of the crust is a decompression environment, which makes the crust thinner, which is beneficial to the upwelling of asthenosphere material and the underplating of mantle-derived magma, leading to the increase of crust temperature and the melting of crust material. Due to the above reasons, a large number of granites were formed in subduction zone and post-orogenic extensional tectonic environment.
Think about a problem
1. Briefly describe the similarities and differences among granite, granite porphyry and rhyolite.
2. Briefly describe the genetic types and characteristics of granite provenance.
3. Briefly describe the structural genetic classification and characteristics of granite.
4. Briefly describe the genetic viewpoint and main basis of granite slurry.