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The Characteristics and Genetic Mechanism of Igneous Topazites in South China
Author(s) -
Changshi Liu,
Weizhou Shen,
Dezi Wang
Publication year - 1996
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/j.1755-6724.1996.mp9001004.x
Subject(s) - phenocryst , porphyritic , igneous rock , quartz , topaz , geology , geochemistry , mineralogy , silicate , rhyolite , chemistry , volcanic rock , plagioclase , volcano , paleontology , organic chemistry
Igneous topazites found recently in South China are closely associated with F‐rich granites in time and space. They have a typical igneous porphyritic texture. The phenocrysts are topaz and quartz, while the groundmass consists mainly of prismatic or acicular topaz microlites and anhedral quartz. Compared with granites, topazites are rich in SiO 2 (65.06%‐81.12% ) and Al 2 O 3 (13.01% ‐18.09%) but poor in MgO and Na 2 O and strongly péraluminous with A/NKC = 3.204‐37.313. Geochemically, the rocks are evidently depleted in Sr and Ba, but enriched in Sn, W, Nb and F. The concentrations of Sn and partial W in the topazites are 1‐2 orders of magnitude higher than the average of acid rocks. The F‐riched granites related to the topazites in the area usually have much higher I Sr (0.7103‐0.7460) and δ 18 O Q ‰ (9.5‐14.24), but lower ǐ Nd (T) (‐4.8‐ 9.4). It suggests that the topazite is similar to S‐type granites in genesis. Hydroxylated silicate melt inclusions, consisting of several grains of quartz and aqueous fluids, have been found in the quartz phenocrysts. The entrapment temperature of these inclusions is about 520°C.