Three textural types of amphibole in amphibolites from the Luliang Complex in the Trans-North China Orogen and their tectonic implications

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. Amphibolites contain three textural types of amphibole: inclusion-type, matrix-type and symplectite-type. The inclusion-type amphibole is associated with plagioclase, quartz and ilmenite, preserved as mineral inclusions within garnet porphyroblasts, representing the early prograde metamorphism (M1). The THERMOCALC program yields the maximum P-T conditions of 7.1-6.1 kbar and 668-655°C for the M1 assemblage, based on the core compositions of garnet porphyroblast and inclusion-type hornblende and plagioclase. The matrix-type amphibole coexists with the garnet porphyroblast and other matrix minerals of plagioclase + clinopyroxene ± quartz ± rutile, representing the peak metamorphism (M2). The P-T conditions of the M2 metamorphism were estimated at 9.2-8.3 kbar and 753-748°C, based on the rim compositions of garnet porphyroblast and matrix plagioclase, amphibole and clinopyroxene. The symplectite-type amphibole is cummingtonite that intergrows with symplectite-type plagioclase surrounding embayed garnet grains, representing the post-peak metamorphism (M3), with the maximum P-T conditions of 5.1-4.1 kbar and 737-697°C, estimated 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 defines 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 southern part of the orogen also underwent initial crustal thickening (M1 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.