变质作用温度与压力极限值的估算方法

Estimation of minimum or maximum pressure or temperature conditions in metamorphic rocks

变质矿物共生组合中缺少某种矿物,或者变质矿物的原始成分被不同程度地破坏,无法直接应用压力计或温度计算出具体的温度或压力数值时,可以根据实际情况,估算P-T的极限值。估算方法包括如下几类:(1)根据纯相矿物的相变条件,可以确定温度或压力的极限值。例如,根据Al2SiO5矿物相图可知,与红柱石平衡的矿物组合,稳定存在的压力不超过Al2SiO5矿物三相点的压力条件(0.375±0.025GPa);与夕线石平衡的矿物组合,稳定存在的温度不低于Al2SiO5矿物三相点的温度条件(504±20℃);(2)某些特殊矿物组合,其稳定域本身就具有温度或压力极限值的指示意义。例如,高温或超高温变质岩中的紫苏辉石+夕线石+石英组合,稳定存在的压力不低于1.05GPa;假蓝宝石+石英组合稳定存在的温度不低于1050℃;(3)特殊矿物消失的反应线,可以限定温度极大值。例如,白云母脱水分解的反应,在中等压力条件下,不超过650℃,即与白云母+石英平衡共生的矿物组合一般不会高于650℃;(4)对于矿物固溶体发生出溶的情况,根据新生出溶页片和残留基体矿物,采用溶线温度计(二长石温度计、二辉石温度计、方解石-白云石温度计)计算出的温度数值,代表原先成分均匀的矿物固溶体出溶之前的温度极小值;(5)如果岩石中发生了退变质反应,要恢复变质作用高峰期的P-T条件,需要尽可能恢复高峰期的矿物成分,或者采用接近高峰期的矿物成分;(6)根据压力计模式反应中位于高压侧或低压侧的某种纯相矿物,可以确定压力极限值。例如,金红石位于GRAIL压力计模式反应的高压一侧。当岩石中缺少金红石时,可以人为假定岩石中存在金红石,计算出的压力为真实压力的极大值。由于压力计模式反应大多为纯转变反应,因此根据实际矿物组合和压力计模式反应,一般可以估算变质作用压力的极小值或极大值。估算变质作用P-T的极限值,方法并不限于本文所述的例子。

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 ＋ H20 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.