Metamorphism of the Lüliang amphibolite: Implications for the tectonic evolution of the North China Craton

Abstract

The Lüliang Complex is located in the southern sector of the Trans-North China Orogen, a continent-continent collisional belt along which the discrete Archean Eastern and Western blocks amalgamated to form the basement of the North China Craton. The complex consists of Paleoproterozoic supracrustal rocks and granitoid plutons of which the former include amphibolites and pelitic schists/gneisses. Petrological evidence from the Lüliang amphibolites indicates three distinct metamorphic mineral assemblages. The early inclusion assemblage (M1) is composed of hornblende + plagioclase + quartz + ilmenite, preserved as mineral inclusions within garnet porphyroblasts. The THERMOCALC program yielded maximum P-T conditions of 7.1 to 6.1 kbar and 668 to 655 °C for the M1 assemblage, based on the core compositions of garnet porphyroblasts and inclusion-type hornblende and plagioclase. The peak porphyroblast-matrix assemblage (M2) is garnet + hornblende + plagioclase + clinopyroxene ± quartz ± rutile, representing the growth of garnet porphyroblasts and matrix minerals. The P-T conditions of the M2 assemblage were estimated at 9.2 to 8.3 kbar and 753 to 748 °C, based on the rim compositions of garnet porphyroblasts and matrix plagioclase, hornblende and clinopyroxene. The post-peak symplectite assemblage (M3) is represented by cummingtonite + plagioclase symplectite surrounding embayed garnet grains, with the maximum P-T conditions of 5.1 to 4.1 kbar and 737 to 697 °C, based on the rim compositions of garnet and symplectic cummingtonite and plagioclase. A combination of petrographic textures, reaction relations, mineral compositions and P-T data define a clockwise P-T path involving near-isothermal decompression for the Lüliang amphibolites. This suggests that like those metamorphic complexes in the northern part of the Trans-North China Orogen, the Lüliang Complex in the central part of the orogen also underwent initial crustal thickening (M 1 and M2), followed by erosional and/or extensional exhumation (M3), which is in accordance with collision between the Eastern and Western Blocks to form the coherent basement of the North China Craton at ∼1.85 Ga.