Trace Element Studies of Haplogranites

Tuttle & Bowen (1958) showed that the relative abundances of the normative components Q, Ab and Or in the most felsic granites (haplogranites) are tightly constrained. Hence the major elements Si, Na, K and Al show limited variations in those rocks. However, other elements, whose abundances are not directly controlled by the phase relations between quartz, feldspars and melt at magmatic temperatures, can vary widely in abundance. Such variations provide valuable tools for considering the genesis of individual haplogranites.

Tuttle & Bowen (1958, p. 124) regarded felsic granites as “the end of the road for fractional crystallization”. Fifty years later we can qualify that statement. It applies only to the major elements since minor elements can be enriched or depleted in the melt as further fractionation occurs. Such magmatic enrichments may be an important precursor to mineralization for some elements such as Sn. As melts evolve by fractional crystallization, cumulative granites must also form and although they were never completely molten may be indistinguishable from the products of direct or equilibrium crystallization of a melt on a Q-Ab-Or diagram. However, the abundances of minor elements, particularly Ca which is a minor element in this case, would discriminate the two groups.

Granite compositions corresponding to mineral-melt equilibria at the lowest magmatic temperatures may also be the result of partial melting of older quartzofeldspathic rocks, either igneous or sedimentary in origin. Tuttle & Bowen admitted that possibility but did not embrace it. Certainly the felsic S-type granites must have formed in that way, and probably also many I-types. When fractional crystallization did occur, the original magma was more generally a partial melt of the crust, rather than basalt as Bowen supposed. Minor element abundances may discriminate all of these origins and may well assist in establishing source rock and primary magma compositions.