The Presence of "Zircon Zone" along a Cretaceous Gtanodiorite-Granite Contact in North Kyushu

Abstract

In North Kyushu there are two groups of granitic rocks: the Older (Lower Triassic) and the Younger (Later Cretaceous)., The former consists of the Asakura and Itoshima Granodiorites together with other minor intrusions, and the latter of the Kurate, Masaki, Hirao, Kaho, Kobukuro, and Sawara Intrusions., Of the Younger granitic rocks, the third and the fourth are dealt with in this paper., Field investigation has revealed that the Hirao Granodiorite (GD) is older than the Kaho Granite (Gr)., The microscopic characters of these rocks and the xenolithic inclusions of GD in Gr have been described in detail., Besides the usual petrographic work, a special study of the accessory zircons has been done with the results as follows: (1) The various characters of the accessory zircons (Table 2), especially color and the features of crystal habit (Fig., 7), are practically invariant throughout a respective granitic body., The zircon, therefore, has been used as a leading mineral for discerning the granitic bodies., (2) The different kinds of crystal habits of zircons in the granitic rocks investigated are classified into three fundamental types: Habit-S, -D, and -C (Fig., 8)., The percentage of these three types statistically characterizes each granite (Fig., 9)., Thus it is certain that any granite body can be represented by its own "habit-ratio " of zircons., (3) The "habit-ratio" of zircons for the hybridized xenolith of GD in Gr lies between those of the two granitic rocks (Fig., 9), and the xenolith is superior to its original GD in the amount of zircon (Table 3)., This fact, namely the mixing of two kinds of zircons in one rock, may well be interpreted as indicating an addition of zirconium from the granitic magma (Gr) to the xenolith during the hybridization, though the mechanism of crystallization of the zircon in the xenolith has not yet been clearly elucidated., (4) The same fact is observed within the GD-Gr contact zone, about 500 m., wide (the "zircon zone"; Fig., 1)., It is naturally suggested that the presence of the "zircon zone" should serve as a good criterion not only in detecting the existence of two kinds of granitic rocks but also in deciding the mixing of and the order of intrusion in the two, if no xenolith is found in the field., Since it is an easier and more exact practice to find out a "zircon zone" than to verify by the ordinary petrographic method the hybrid phenomenon on a wide scale, the means of investigation here presented may further extend our knowledge on the granite geology.,