Abstract:

The origin of granite is both an ancient and a frontier scientific problem. One hundred years ago debate on the origin of granite ended with the prevailing view that granite is igneous rather than metamorphic in origin. However, over the past century researchers have shown that the igneous theory is not perfect and the mechanism of basalt crystallization differentiation into granite had been severely challenged. Today it is considered an indisputable fact that granite originated from partial melting of the lower crust, which indicates the source of granite is metamorphic rock. There are many theories on the formation of granites. After many years of testing, the four-stage (melting, melt segregation and ascent, and magma emplacement) theory of granite formation is considered more plausible. Based on detailed study of this theory, this paper proposes a new four-stage theory which divides the granite formation process into two main parts: melt generation and formation (melting and melt aggregation), a heating process, and melt ascent and magma emplacement, a cooling process. The core of this theory is the conjecture of a “lower crustal magma chamber”, which refers to the giant space formed by melt aggregation. This conjecture, first, solves the space problem of a magma chamber in the lower crust. As in situ partial melting of the lower crust only changes the material composition of its products (melt plus remnant), with no space issue involved, the total volume of the lower crust is basically unchanged. And, as there is continuous mantle heating, a lower crustal magma chamber can grow gradually and become very large. Second, we consider the driving force behind magma uplift is not the buoyancy of magma itself. Rather, as the lower crustal magma chamber overflows along the fault zone, the formation pressure from tens of kilometers of strata beneath the magma chamber may transform into great force, driving the magma upward. Therefore, theoretically, granite can rise very quickly, almost instantaneously on the geologic time scale. Third, this conjecture reasonably explains the ancient problem of granite emplacement. It is precisely because the lower crustal magma chamber moves out and ascents, the space it occupied is immediately compacted and filled by the overlying strata, and the subsequence collapse of the underlying strata directly affects the fragile upper crust. A void is then created in the weak part of the upper crustal structure to provide space for the rising magma to complete its emplacement process. Apparently, a space displacement is realized in the disappearing of a magma chamber in the lower crust and magma emplacement in the upper crust. It seems that the concept of a “lower crustal magma chamber” can better resolve many traditional controversies regarding granite formation. The conjecture needs to be verified. Finally, to further study the above issues we suggest using two interdisciplinary approaches-metamorphic-igeous petrology and physical geology.

Key words: granite, four-stage theory, lower crust magma chamber, collapse mode, metamorphic-igeous petrology, physical geology