Petrology and P-T history of the Wutai amphibolites: Implications for tectonic evolution of the Wutai Complex, China

Abstract

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M1 assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M2 stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P-T conditions for this assemblage have been estimated using the program TWEEQU at 10-12 kbar and 600-650°C. The M3 assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yeilds P-T conditions of 6.0-7.0 kbar and 600-650°C. M4 is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400-500°C and relatively lower pressures. Taken together, the qualitative P-T estimates from M1 and M4 and the quantitative P-T estimates from M2 and M3 define a clockwise P-T path for the Wutai amphibolites. The estimated P-T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M1 + M2), subsequent isothermal exhumation (M3), and finally cooling and retrogression (M4). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.