Rapakivi

Classification：Igneous Rock

Rapakivi belongs to granitic rocks, which are red or gray, often containing dark minerals, amphibole and biotite. Its main characteristic is to have a special texture, i.e. longer rapakivi structure.

Features：
Rapakivi is a kind of granite. Its characteristics are potassium feldspar phenocryst and ovate feldspar. This feldspar phenocryst is formed by metasomatism, while the outer longer stone is formed by assimilated surrounding rock. This kind of rock distribution in North China can be used as decorative stone.

Mineral composition:
The phenocryst of Rapakivi is spherical oval potassium feldspar (feldspar and micro plagioclase), which is surrounded by longer stone rings or sodium longer rings. As with single crystals of feldspar crystal card's twin, or for some irregular shaped, fan-shaped crystal aggregate. The granite matrix minerals are mainly quartz, potash feldspar, biotite, and sometimes amphibole. The accessory minerals are zircons, apatite, and metallic minerals.

Petrochemical characteristics:
The range of SiO2 content was 71.43%~73.25%. With the increase of SiO2 content, the content of Al2O3 and MgO decreased, the changes of CaO and Na2O were not obvious, K2O/Na2O values were all greater than 1, and Al2O3 content was greater than K2O+Na2O+CaO content. The Litman index of rock is 2.68~3.04, with a mean value of 2.82, which belongs to calc alkaline series. The mafic index MF=70.39~74.36, consolidation index SI=2.15 to 8.64, and differentiation index DI=81.07~92.90 indicate that the degree of crystallization of rock mass is poor, and the degree of differentiation is relatively high. In the silicon alkali map, the subalkaline series of rocks fall into the alkaline series, and the AFM diagram falls into the calc alkaline series and is distributed near the AF line near the A end, showing the characteristics of the poor iron and alkali rich.

Output and distribution:
Rapakivi is mainly formed in the Precambrian period. The rocks similar to it have more ringed speckite and longer ringed syenite. They are often symbiotic with other intermediate acid intrusive rocks. In China, it is produced in Miyun, Hebei, Chicheng, Jiangxi Shangrao, Liaoning Huanren, Shaanxi Shang county and Fujian Zhangzhou in Beijing. [1]

Cause:
In recent years, Rapakivi has become one of the focus issues of many rocky scientists. The study shows that it has unique geochemical characteristics and special crustal structure and physical and chemical environment when it is formed. Its appearance reflects the existence of a thickened crust, which is due to the mantle magma under dry and anhydrous thick crust. Under the action of underplating or lithospheric mantle delamination, the lower crust is partly fused by heat. The existing magma records show that in the early Paleozoic era, the Tethys ocean, including the Qun Taigan ocean and the Middle Eastern Kunlun ocean, subducted to the north of the East Kunlun, forming a series of arc basins, and the region was intruded in the subducted granitic rocks in the maglor and Qun Taige magmatic subzones. The S1 D1 ocean basin vanished, arc arc collision arc land collision, orogenic belt formed, and the Early Silurian same collision granites were formed one after another, and after the late Silurian, the collision of magmatism has been completed, and the transformation of the Dhi Te Tis oceanic lithosphere system to the continental lithosphere system has been completed.