The Origin of Granite and Continental Masses in an Expanding Earth
|Title||The Origin of Granite and Continental Masses in an Expanding Earth|
|Author(s)||Lorence G Collins|
|Keywords||granite, continental masses, expanding Earth, thermal breakdown, melting, crystallization|
In the 1940s a major debate raged as to whether some granite bodies of plutonic dimensions were formed from magmas or by replacement processes (Grout, 1941; Read, 1948). Geologists favoring replacement, the "granitizers", suggested that Si and K are introduced into diorites and gabbros as Al, Fe, Mg and Ca are subtracted from the mafic rocks to convert them to felsic granites (Table 1). "Magmatists", geologists supporting a melt-origin, believed that granite magmas are created in "dry" rocks devoid of free water and at temperatures hot enough to cause thermal breakdown of water-bound minerals (Whitney, 1988; Leake, 1990). At these temperatures partial melting of granite compositions occur and leave mafic restites. Experimental work in closed systems on melted natural and artificial granites convinced the magmatists that granites of plutonic dimensions formed from melts derived from mantle or lower crustal sources (Tuttle and Bowen, 1958; Luth et al., 1964). The experimental work showed that granitoid compositions plot near eutectics of phase diagrams. Lowest temperature points in these diagrams represent ultimate conditions for either (1) initial melting or (2) fractional crystallization of silica-saturated magmas. The magmatists concluded that because most granite plot near these low-temperature points, their bulk chemical content must be controlled by the sequence in which minerals first melt or the sequence in which minerals first crystallize and fractionally settle from a melt.