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

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphose d ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, a long with lesser amounts of siliciclastic and carbonate rocks and banded ir on formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M-1 assemblage is composed of pla gioclase + quartz + actinolite + chlorite + epidote + biotite + rutile, pre served as mineral inclusions in garnet porphyroblasts. The metamorphic cond itions for this assemblage cannot be quantitatively estimated. The M-2 stag e is represented by garnet porphyroblasts in a matrix of quartz, plagioclas e, amphibole, biotite, rutile and ilmenite. P-T conditions for this assembl age have been estimated using the program TWEEQU at 10-12 kbar and 600-650 degrees C. The M-1 assemblage is shown by amphibole + plagioclase +/- ilmen ite symplectic coronas around embayed garnets and yields P-T conditions of 6.0-7.0 kbar and 600-650 degrees C. M-4 is represented by chlorite and epid ote rimming garnet, chlorite rimming amphibole and epidote replacing plagio clase under greenschist-facies conditions of 400-500 degrees C and relative ly lower pressures. Taken together, the qualitative P-T estimates from M-1 and M-4 and the quantitative P-T estimates from M-2 and M-3 define a clockw ise 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 (M-2 + M-2), subsequent is othermal exhumation (M-3), and finally cooling and retrogression (M-4). Thi s tectonothermal path, along with those of the Fuping and Hengshan complexe s, which bound the southeast and northwest margins, respectively, of the Wu tai Complex, is considered to record the early Paleoproterozoic collision b etween the eastern and western segments of the North China craton. (C) 1999 Elsevier Science B.V. All rights reserved.