The Formation Of Granite

As the iconic rock of continents, granite forms the foundation of the upper continental crust, and its formation process is usually closely related to continental tectonics, metamorphism, and mineralization. Since the 18th century, when geological science was still in its infancy, the origin of granite has been the subject of numerous debates. Discussions on the origin of granite can be found in the works of Gilluly (1948), Pitcher (1993), and Young (2003), among others, which will not be listed here. It is worth mentioning that since the advent of plate tectonics theory in the 1960s, many explanations regarding the origin of granite have been re-examined within the theoretical framework of plate tectonics. In many cases, understanding seems to converge, but the actual debate continues.

Bowen's (1914, 1922, 1948) theory of basaltic magma fractional crystallization had a misconception: it combined the order of mineral crystallization with the rock sequence from basic to acidic in magmatic rocks. Experimental results have shown that the fractional crystallization of basaltic magma can only produce a very small amount of residual granitic melt, which clearly contradicts the fact that numerous granites exist in the field (Holmes, 1926; Read, 1957). The mineral reaction series can actually be applied to magmatic systems of different compositions. In other words, the rocks initially crystallized from the magmatic system are not necessarily basic rocks, and the rocks finally formed are not necessarily felsic (acidic) rocks, because the nature of the rocks crystallized from the melt depends on the composition of the melt, not the order of mineral crystallization (Kennedy, 1933). Walton (1960) commented on Bowen's understanding as follows: "There is nothing wrong with Bowen's chemical theory or its application to the differentiation of basaltic magma; it remains a fundamental principle of petrology. However, it is somewhat speculative to rigidly confine igneous theory to a single model, assuming that the evolution of most igneous rocks is due to the intrusion, cooling, crystallization, and differentiation of basaltic magma into the crust. The same chemical theory can be applied to other models."

The debate between the "metamorphicists," represented by H. H. Read, and the "magmatists," represented by N. L. Bowen, in the 1940s (Gilluly, 1948), ended with more and more scholars agreeing on the magmatic origin of granite. However, where did the magma that forms granite bodies come from? In Bowen's words: "Whence the granites?"

The overwhelming consensus on this question is that granite is formed by the partial melting and solidification of various rocks with different compositions in the Earth's crust. This view combines two different early understandings of the origin of granite: magmatism (which holds that granite comes from the crystallization of magma) and metamorphism (which holds that granite is formed from silica- and alumina-rich sedimentary rocks transformed by granitization under dry or wet conditions). The idea that granite is the result of ultra-metamorphism (anatexis) of crustal rocks is of great significance for studying the origin and chemical differentiation of the Earth's crust, as it relates to the thermal state of the crust and the composition of the protolith during specific periods, including how much granitic magma can be produced, the temperature of granite formation, the water content and source, tectonic setting, and plate tectonic processes.