Tieshan complex in Zhenghe, Fujian province has long been regarded as syenite, which is Indosinian alkali intrusive rock uncommonly in South China. This study conducted an investigation for the Shizigang rocks in the south-western area of the Tieshan complex. The results show that their lithology is not common intrusive rock but a metasomatic rock due to strong potassic fluid. The metasomatic rock hosts some residues of protolith, including Indosinian grantic bodies, sandy conglomerate of Early Jurassic Lishan Formation, and volcanic rocks of Early Cretaceous Nanyuan Formation. Therefore, it can be inferred that the metasomatic rock formed in the Cretaceous. K-feldspar of the metasomatic rock occurs in four forms, surfacial potassic alteration, early light colour potassic veins, late dark potassic veins, and melanitepyroxene K-feldspar rock in origin. The first surfacial potassic alteration contains Or of 77.9%~94.3%, Ab of 5.2%~21.3% and An of 0%~0.8%. The second contains Or of 81.5%~91.3%, Ab of 8.2%~17.4%, and An of 0.5%~4.1%. The third contains Or of 91.8%~96.8%, Ab of 3.2%~9.3%, and An of 0.1%~0.4%. The last contains Or of 85.2%~90.0%, Ab of 10.8%~14.6%, and An of 0.2%~0.6%. Potassic metasom atism can be divided into early stage and late stage. X-ray powder diffraction and infrared spectrum analyses show that the early stage metasomatism formed the first two forms of K-feldspar with or dering degree of of 0.71~0.80, structural parameters of 1.27~1.61, triclinicities of 0.23~0.29, and T1(o)+T1(m) of 0.86~0.90; while the late formed the last two forms of K-feldspar with ordering degree of 0.67~0.88, structural parameters of 1.10~1.91, and triclinicities of 0.30~0.70, T1(o)+T1(m) of 0.84~0.93. The first two K-feldspars consist mainly of microclines, especially highly microclines, with crystallization temperatures varying from 200 to 370℃. The types of metasomatic rocks include quartz K-feldspar rock, pyroxene K-feldspar rock and dark garnet K-feldspar rock.