Mineralogy and alkali metasomatism of Tieshan complex body in Zhenghe County, Fujian Province, southeastern China

Tieshan complex body (TCB) is located in Zhenghe County on the border of Zhejiang and Fujian, covering an area of about 5.7 km2. Tieshan complex body (TCB) is the most developed metasomatised area in the southeast of China, which was misunderstood as a syenite intrusion. Much Study has carried on at Shizigang in the southwest of the TCB. The alkali metasomatites are composed of feldspar, pyroxene, amphibole, biotite and melanite et al. Potassic feldspars in Complex body have four kinds of forms and can be separated into three stages: (1) one in facial alkali alteration and one in alkali alteration with melanites, (2) one in light alkali feldspar veins, and (3) one in dark alkali feldspar veins and. Pyroxenes had three kinds of forms: some in breccias, some in feldspar and some accompanied with amphiboles. Amphiboles had two kinds of forms: in breccias and scatter in feldspars in the veins, its mineral compositions are composed of basaltic hornblende, ferrous pargasite, actinolite and ferroactinolite, all belong to calcic amphibole subgroup. Biotites are scattered in breccias and in the veins or which is the product of metasomatised pyroxenes. The compositions of biotites show low-Ti and Fe, but high-Mg, which were the thermal products of a higher oxygen fugacity. Melanites occurred as layers and belts, which were metasomatised by feldspars and epidotes and amphiboles and carbonates and sericites and diopsides, their ingredients are solid solution series between andradite and Grossular and reflect the melanites were formed in the relative oxidative, alkaline environment. X-ray powder diffraction and infrared spectrum analyses of K-feldspar from early alkali feldspar provide degrees of ordering of 0.71–0.80, structural parameters of 1.27–1.61, triclinicities of 0.23–0.29, T1 (o) + T1 (m) of 0.86–0.90 degrees of ordering of 0.67–0.88 from late K-feldspar, its structural parameters of 1.10–1.91, triclinicities of 0.30–0.70, T1 (o) + T1(m) of 0.84–0.93. Both two kinds of alkali feldspars are microcline, most adularia. Their structure temperatures vary from 200 to 370 °C which are the highest temperatures at which K-feldspars were formed. Thus, it is concluded the K-feldspars in Tieshan is originated from thermal fluid, and the fluid was potassium-rich and a little rich in sodium at the later stage. The 39Ar-40Ar age of feldspars shows 118.68 ± 7.70 Ma and 114.61 ± 6.74 Ma, which represent the ages of early stage facial alkali alteration and late stage dark alkali feldspar veins respectively. It is obvious that an original granites were formed at about 250 to 255 Ma, but all the rocks were considered to be strongly metasomatised by alkali fluid later, most occurred during 108–126 Ma in Yanshanism. Metasomatism of the TCB reflected that complex history of the region. The metasomatised rocks include fenite and amphibole feldsparite. Fenite, especially apatite-rich fenite is associated with REE mineralization, which is helpful to understand the mechanism of a large scale of rare earth deposits in the region.