Study on the origin and rock-forming simulation experiments of aquamarine-bearing pegmatites of Ailaoshan, Yunnan, China

摘要

The Ailaoshan aquamarine-bearing pegmatites are associated with Proterozoic metamorphic rocks in the southern portion of the Ailaoshan fault-folded complex. The gem-bearing pegmatite mineralization zones of the region occur in areas generally consistent with the regional tectonic trend. The pegmatites are found in metamorphic rocks, migmatites and in the inner/outer contact zones of gneissoid granites. The Rb-Sr isochron drawn for the pegmatites is 26～31 Ma, (I. E. in Himalayan). The homogenization temperatures of melt and liquid inclusions in minerals vary from 185 to 920°C , which are comparable to the inclusions observed in banded migmatites and ptygmatic quartz veins in the surrounding metamorphic rocks. The mineralization fluids of the pegmatite were rich in HCO_3 and CO_2, and their compositional assemblages are comparable to metamorphic fluids. Results of H, O, C, Si etc. isotopic analyses and REE, and Be analyses indicates that the sources of mineralization components that formed the pegmatites are closely associated with metamorphic fluids and the enclosing metamorphic rocks. A pegmatite structure simulation experiment was conducted at high temperature and pressure (840°C and 1,500 × 105Pa. ) , with various metamorphic rock samples in a water-rich and volatile-rich environment. When the liquidus was reached, the temperature was gradually decreased at the rate of 5 ～ 10°C/day over a time period of three months. SEM energy-dispersive spectrum analyses were performed on the experimental products. A series of pegmatoid textures were observed including zonal texture, megacryst texture, drusy cavities, crystal druses, and vesicular texture along with more than ten types of minerals including plagioclase, microcline, quartz and biotite. Different metamorphic rock melts generated different mineral assemblages. Experiment results revealed that the partial melting of metamorphic rocks could form melts similar to pegmatite magmas. Based upon the geological characteristics, geochemistry, and pegmatite texture simulation experimental results, it is concluded that the mineralization components of Ailaoshan aquamarine-bearing pegmatites came from metamorphic rocks. The petrogenetic model for the origin of pegmatites is related to ultrametamorphism and metamorphic anatexis.