THE STRUCTURAL STATE OF K-FELDSPARS FROM GRANITIC ROCKS OF VARIOUS ORIGIN IN SOUTHEAST CHINA
Xue Jiyue Zhang Gendi(Department of Geology, Nanjing University)
In this paper the method proposed by Camentsev et al. (1977) is adopted to deduce the Al-occupancies in K-feldspars. More than 200 samples collected from 53 granitic bodies of various localities in SE China have been studied and their t_1o, t_1m, t_2o, and t_2m have also been calculated. Meanwhile a t_1o —t_1m—(t_2o+t_2m) triangular diagram and the histogram of t_1 for K-feldspars are also constructed.The values of t_1 for all these K-feldspars range from 0.60 to 1.00; microcline with t_10.9 and orthoclase with t_1=0.80±0.10 are dominant in granitic rocks.There is a distinct relationship between the origin of granitic rocks and the Al/Si ordering of K-feldspars. The Al-occupancies in two T_1 sites of those from granitic rocks of metasomatic origin are0.9 with t_1o0.00 as well. Those crystallized from granodiorite quartz monzonite, tonalite plutons are orthoclases with t_1 values within the range 0.70—0.80, while those from hypabyssal—abyssal granite plutons are either monoclinie or triclinic with t_1= 0.85 —0.90. Sometimes both of them may coexist even in a single grain. In such case there will be a 131 reflection peak of orthoclase appearing between the 131 and 11 reflection peaks of microcline on a X-ray powder diffraction chart.The relationship between various varieties of K-feldspars and the origin of granitic rocks is also discussed. It is believed that the maximum microcline(t_1o0.90) is formed from hydrothermal solution or melt under 500℃, so that it occurs commonly in metasomatic granite or crystallizes from magma rich in volatile components under lower temperatures. Monoclinic K-feldspars occur in magmatic granitic rocks and their t_1 values increase with decreasing temperatures of rock formation. This variance of t_1 indicates continuous ordering of Al/Si in monoelinie K-feldspars. The discontinuous transformation between monoclinic orthoclase and triclinic microcline is supported by(1) a zoned K-feldspar, discovered in the Xiuning pluton, Anhui Province, is made up of orthoclase as its core and microcline as its rim without any transition variety with median Al/Si ordering configuration between them and(2) there is a gap between monoclinic and triclinic K-feldspars as shown in Fig.1.