西大别石榴黑云斜长片麻岩中金红石的退变质演化

以取自大别山西段七里坪的石榴黑云斜长片麻岩为例,使用矿物热力学数据库和THER-MOCALC程序（3.33版）,计算了0.3~1.5GPa和450~650°C条件下的P-T视剖面相图,讨论了该类岩石中金红石、钛铁矿与榍石矿物稳定的P-T条件,分析了该岩石从高压条件近等温降压过程中金红石的退变质作用。研究表明,金红石具有较高的稳定压力,一般大于0.75GPa,压力低时稳定范围仅局限于较低温的小区域内;榍石具有小于565°C的较低温度和0.35~1.40GPa的压力范围,榍石可与金红石共生,但其P-T条件处于榍石稳定域边缘较狭窄的范围内;在温度大于475°C、压力低于0.75GPa的区域,金红石可与钛铁矿共生,钛铁矿则稳定于温度大于570°C、压力低于0.55GPa的范围。根据金红石向钛铁矿的转变及体系含水量,约束该片麻岩角闪岩相退变质P-T条件为压力0.55~0.75GPa且温度大于570°C。

Retrograde metamorphism of rutile in garnet-biotite-plagioclase gneiss from the western Dabie Mountain

With the example of garnet-biotite-plagioclase-gneiss from Qiliping,western Dabie Mountain,the P-T pseudosections were calculated in range of 0.3～1.5GPa and 450～650°C by using the latest thermodynamic data set and the software THERMOCALC version 3.33.The stability of rutile,ilmenite and sphene in such gneiss were discussed,and the retrograde metamorphism from high-pressure of rutile in this sample was analysed.The results show that rutile is generally stable in higher pressure than 0.75 GPa,and in lower pressure within a narrow temperature range;sphene is stable in conditions of 0.35～1.40GPa and lower than 565°C,and the coexistence of sphene with rutile is stable in a narrow annular range around the sphene stable domain;the rutile can be stable with ilmenite together in a large P-T range of more than 475°C in temperature and less than 0.75GPa in pressure,but the ilmenite is only stable in domain of more than 570°C in temperature and less than 0.55 GPa in pressure.The transformation of rutile into ilmenite in this gneiss and the water content in the rock system may constrain the retrograde metamorphic condition in amphibolite facies as 0.55～0.75 GPa in pressure and more than 570°C in temperature.