Deformation history of the Paleoproterozoic Liaohe assemblage in the Eastern block of the North China Craton

Abstract

The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D 1 to D 3) and four episodes of metamorphism (M 1 to M 4). The earliest greenschist facies event (M 1) is recognized in undeformed or weakly deformed domains wrapped by the S 1 schistosity, suggesting that M 1 occurred before D 1. The D 1 deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F 1), an associated penetrative axial planar schistosity (S 1), a mineral stretching lineation (L 1) and regional-scale ductile shear zones. Concurrent with D 1 was M 2 metamorphism, which occurred before D 2 and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F 1 folds and other sense-of-shear indicators, together with the radial distribution of the L 1 lineation surrounding the Liaoji granitoids, imply that D 1 represents an extensional event. The D 2 deformation produced open to tight F 2 folds of varying scales, S 2 axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW-SE compression. Following D 2 was M 3 metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D 3) formed NW-WNW-trending folds (F 3), axial planar kink bands, spaced cleavages (S 3), and strike-slip and thrust faults, which deflect the earlier D 1 and D 2 structures. D 3 occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M 4). The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D 1) and the M 1 and M 2 metamorphism, and the final closing of which was associated with the D 2 and D 3 phases of deformation and M 3 and M 4 metamorphism. © 2004 Elsevier Ltd. All rights reserved.