Estimation of Minimum or Maximum Pressure or Temperature Conditions in Metamorphic Rocks
详细信息
- 责任者：Wu Chunming (College of Earth Science ; University of Chinese Academy of Sciences ; Beijing 100049 ; China) ; Chen Hongxu
- 刊名：Acta Petrologica Sinica
- 出版年：2013
- 卷期：29(5)
- 关键词：变质作用 ; metamorphism ; P—T—t轨迹 ; P-T-t trace
- 页码：p.1499-1510
- 图表：illus., 68 refs.
- 分类号：0802
- issnNo：ISSN1000-0569
- isbnNo：CN11-1922/P
摘要
When a mineral necessary in computing pressure ( P) or temperature ( T) conditions is absent,or compositions of the relevant mineral( s) were changed due to retrograde reactions in a metamorphic assemblage,and thus the true P and / or T cannot be determined,maximum or minimum P and / or T may be estimated and some examples are given in this work. ( 1) Extreme P or T conditions can be deciphered using phase diagrams of pure phases. For example,T of an assemblage containing sillimanite cannot be lower than 504 ± 20℃ ,whereas P of an assemblage containing andalusite cannot be higher than 0. 375 ± 0. 025GPa; ( 2 ) Some assemblages themselves indicate extreme P or T conditions. For example,the hypersthene + sillimanite + quartz assemblage can only be stable at P higher than 1. 05GPa and the sapphirine + quartz assemblage cannot be stable lower than 1050℃ ,in high- and ultra-high temperature rocks; ( 3) The equilibrium conditions of some reaction-out curves of mineral( s) also imply the limit of P or T conditions. For example,the reaction muscovite + quartz → K-feldspar + kyanite/sillimanite + H2O suggests that an assemblage containing muscovite cannot be stable at T higher than ca. 650℃ at moderate P; ( 4) T yielded from solvus thermometry stands for the lower T limit of a unified solid solution phase when deriving the T conditions through combining the newly formed exsolution lamella and the relic matrix phase; ( 5) Metamorphic peak P-T conditions can only be determined on the basis of retrieving the chemical compositions of peak minerals when later retrograde reactions occurred and the peak minerals were destroyed partially; and ( 6 ) Maximum or minimum P conditions can be deciphered using model reactions of barometers when one phase is absent. For example,when rutile is absent in a metapelitic assemblage,maximum P can be estimated by considering that rutile is present because rutile lies on the higher pressure side of the GRAIL model reaction. Extreme P values can be anticipated because most of the barometers are built on net transfer reactions. Other similar methods in deciphering extreme P or T conditions can also be conducted,albeit not completely described in this work.