Mineral evolution of pyroxenes of garnet mafic granulites in northwestern Hebei province.

On the basis of petrographic evidence, the pyroxenes in mafic garnet granul ites can be divided into three generations. The first stage diopside (Cpx(1 )) and hypersthene (Opx(1)) was inclosed in garnet porphyroblasts as relict minerals, and formed at temperature about 750 similar to 830 degrees C and pressure 1. 0-1. 26GPa. Al2O3 and Na2O contents in Cpx(1) are respectively 4. 60% and 0. 50%. The mineral assemblage of the peak facies of granulite metamorphism characterized by garnet porphyroblast and Cpx(2)+ Opx(2)+Pl(2) , juncting with equal-angle (120 degrees) triple texture each others in mat rix. It means that the metamorphic reaction at the stage came up to more st able equilibrium at the conditions of T = 780 similar to 860% and P=0. 83 s imilar to 0. 92 GPa. Al2O3 and Na2O contents in Cpx(2) decreased respective ly to 3.04% and 0.42%. The characteristics of the third stage mineral assem blage are the vermicular symplectites of orthopyroxene (Opx(3)), hornblande , plagioclase (Pl(3)) and clinopyroxene (Cpx(3)) surrounding garnet porphyr oblasts. The corona texture represents a rapid decompression reaction durin g post-peak retrograde metamorphism at T = 720 similar to 770 degrees C and P = 0. 55 similar to 0. 68GPa. The contents of Al2O3 and Na2O in Cpx(3) de creased to 1. 96% and 0. 21% respectively. The chemical composition variation in clinopyroxenes and the results of tem perature and pressure calculation indicate that the pyroxenes in the garnet mafic granulites underwent a metamorphic evolution with near-isothermal de compression path. The slower decompressional cooling path (M1-->M2) may be controlled by magma heating and cooling. At the peak stage of metamorphism, the stable equilibrium texture occurred widespreadly in the matrix, it mea ns that the garnet mafic granulite stayed in middle-lower crust for longer time. Then the isothermal decompression path (M2-->M3) representes a rapid uplifting of the crust.