On the “late” formation of paragonite and its breakdown in pelitic rocks of the southern Damara Orogen (Namibia) |
| |
| Authors: | E Hoffer |
| |
| Institution: | (1) Mineralog.-Petrolog. Inst., Goldschmidtstr. 1, D-3400 Göttingen, Federal Republic of Germany |
| |
| Abstract: | The formation of paragonite at the transition from the low-grade to the medium-grade matamorphism and its breakdown in the presence of quartz in the upper medium grade in common metapelites is investigated.The microprobe work on the white micas from the low and medium-grade rocks yields compositional differences in respect to the celadonite substitutions and the paragonite content. The low-grade white micas are phengites having Si4] 6.25 to 6.44 and Altot 4.89 to 5.20. The paragonite component in solid solution in the phengites ranges from 11 to 17 mole %. In the transition from the low-grade to the medium-grade metamorphism, concomitant with the breakdown of chlorite, the phengites change to muscovites having Si4] 6.07 to 6.16 and Altot 5.36 to 5.56. At the same time, the amount of paragonite in solid solution increases up to 22±2 mole % and paragonite makes its first appearance as a separate mineral. The increase of the percentage of paragonite in solid solution in the muscovites is due to the drastical modal decrease of muscovite in the course of the breakdown of chlorite. The formation of paragonite is readily explained by the muscovite-paragonite solvus. Paragonite forms thin lamellae (1–20  m) interlayered with muscovite lamellae (1–40 m). The average composition is Pg88.5Ms7Mar4.5. Paragonite occurs together with staurolite+biotite, kyanite+biotite, cordierite +biotite, and andalusite+biotite. In the presence of quartz, it breaks down in the lower part of the andalusite zone to andalusite and albite-rich plagioclase. At the same time, the amount of paragonite in solid solution in the muscovites decrease to 11–15 mole %. The basal spacings d(002) of the phengites and muscovites investigated show a clear dependence on the Na+ content and the celadonite substitutions. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
