A DISCUSSION ON THE PETROGENETIC CLASSIFICATION OF GRANITIC ROCKS IN THE NANLING REGION

According to the temporal and spatial relationships,the granitic rocks in the Nanling region may be petrographically classified into four associations, namely, the association consisting mainly of biotite granite, the plagioclase granite-granodiorite association, the granite porphyry—quartz porph yry association, and the association consisting mainly of miarolitic K-feldspar granite. The problem discussed here is whether or not these four associations represent four genetically related series of granitic rocks respectively. The association consisting mainly of biotite granite is the most widespread, covering more than 98％ of the total outcrop area of the Nanling granites, and of the longest duration, occurring from late Precambrian up to Cretaceous times. Based on the geological and petrological characteristics, this association is probably derived from anatexis or granitization of the crustal materials. It may be correlated to the granite of the sedimentary type of B. W. Chappell and A. J. R. White. The plagioclase granite-granodiorite association is the oldest granitic rocks in the Nanling region. It has a Rb-Sr age of 1063 ±95m. y. and an initial Sr⁸⁷/Sr⁸⁶ ratio of 0.7001±0.0039. It is intimately associated with a suite of ultrabasic to intermediate rocks. It is therefore probably an end product of differentiation of a basic magma. The occurrence of the granite porphyry-quartz porphyry association is quite complicated, Some of them occur as dikes cutting the biotite granite association and others as small isolated bodies sometimes mineralized with Mo, W, Sn or Cu and still others as members of the Jurassic volcanic series. Different occurrences may suggest different origins. In order to elucidate the petrogenesis of this association, much work has to be done. The association consisting mainly of miarolitic K-feldspar granite occurs only in late Cretaceous times. The miarolitic K-feldspar granite is composed mainly of microcline and orthoclase（65—75％）and quartz（25—35％）.Its Na₂O content is 4.22％ in average. The Na-bearing minerals such as riebeckite and aegirine are chiefly found in the cavities. Although igneous rocks with higher sodium content are generally regarded as being derived from deeper source such as the lower part of the crust or the upper part of the mantle, the origin of this association could not be ascertained at present for lack of necessary geochemical and isotopic data.