大别山北部石榴二辉麻粒岩变质结构和水活度及其演化特征

METAMORPHIC STRUCTURE, WATER ACTIVITIES AND THEIR EVOLUTIONARY FEATURES OF PYRIGARNITE, NORTH OF DABIE MOUNTAINS, EAST OF CHINA

通过对大别山北部石榴二辉麻粒岩岩相学、矿物地质温压计和热力学计算 ,获得 4个主要的变质演化阶段的矿物共生组合、温压条件和相应的水活度条件 :(1)榴辉岩相阶段(M1) ,以Cpx(含Jd) +Q +Ru +Gt组合为代表的残留矿物 ,并呈包体的形式产于石榴石中 ,t=6 12～ 75 0℃ ;(2 )麻粒岩相阶段 (M2 ) ,以Opx +Cpx +Gt+Q +Ti+Mt矿物组合为特征 ,其相应的t =837～ 887℃ ,p =1.0 3～ 1.2 5GPa ,水活度为 0 .718～ 0 .799;(3)角闪岩相阶段(M3 ) ,矿物组合为Cpx +Gt+Amp +Pl+Mt ,t=5 30～ 6 6 0℃ ,p =0 .85～ 0 .95GPa ,其相应的水活度为 0 .2 3～ 0 .2 4;和 (4)低角闪岩相阶段 (M4 ) ,其形成的温压条件为t=495℃ ,p =0 .5 6～ 0 .70GPa ,相应的水活度为 0 .11～ 0 .13.石榴二辉麻粒岩变质反应、变质结构、矿物组合及其演化 ,不仅受控于形成时的温压条件 ,而且与形成时体系中水活度的演化有着密切的成因关系 .水活度的演化特征表明 ,变质流体在变质作用过程中 ,对变质反应温度起着一定的缓冲作用 .

The lithofacies research, the calculation of the mineral and geological temperature and pressure, and the thermodynamic calculation are all used in this paper to obtain the mineral association assemblies?, the temperature and pressure conditions and the corresponding water activity conditions that occurred in the four major metamorphic evolutionary stages of the pyrigarnite in the Dabie Mountains: (1) The ecologite facies stage (M 1) is characterized by the residual minerals represented by Cpx (jadeite_bearing) + Q + Ru + Gt that occur as inclusions in the garnets at t =612-750?℃. (2) The granulite facies stage (M 2) is featured by the mineral assembly consisting of Opx + Cpx + Amp + Gt + Q + Ti + Mt at t =837-887?℃, p =1.03-1.25?GPa. The water activity in this stage ranges between 0.718-0.799. (3) The amphibolite facies (M 3) is characterized by the mineral assembly consisting of Cpx + Gt + Amp+ Pl + Mt at t =530-660?℃ and p =0.89-0.95?GPa. The water activity in this stage was reduced to 0.23-0.24. (4) The low amphibolite facies (M 4) occurred at t =495?℃ and p =0.56-0.70?GPa. The water activity in this stage was 0.11-0.13. In conclusion, the metamorphic reaction, the metamorphic structure, the mineral assembly and evolution of the pyrigarnite are not only controlled by the temperature and pressure conditions, but also closely related in cause and effect to the evolution of water activity during its formation system. The evolutionary feature of the water activity shows that the metamorphic fluids may have played a buffering role in the metamorphic reaction temperature during the metamorphic stages.