PARAGENETIC ASSEMBLAGES OF BERYLLIUM SILICATES AND PHOSPHATES FROM THE NANPING No. 31 GRANITIC PEGMATITE DYKE, FUJIAN PROVINCE, SOUTHEASTERN CHINA

摘要

Silicates and phosphates are two principal groups of the more than 100 beryllium minerals, but they are rarely documented to cocrystallize as magmatic phases. Here, we describe complex paragenetic assemblages of various species of Be minerals in five zones from the outer contact inward in the Nanping No. 31 pegmatite dyke, in southern China. Three types of magmatic paragenetic assemblages of Be minerals are distinguished. The first type consists primarily of Be silicates (beryl + phenakite) in the outer zones (zones I–II), which likely results from Al-poor melts. The second type is typically represented by the complex association of Be phosphates + silicates, typically interstitial to saccharoidal albite in zone II. This type shows a general sequence: phenakite → hydroxylherderite or hurlbutite → fluorapatite → beryl, which is suggestive of the competitive predominance of phosphorus over silicon during the crystallization of Be minerals. The third is uniquely described by Cs-rich beryl in zones III and IV. This type of beryl is easily altered by hydrothermal fluids, leading to secondary Cs-poor beryl and nanpingite. Our results indicate that crystallization and the sequential assemblages of Be minerals are clearly related to the environment and evolution of pegmatite crystallization.