Ductile Extension and Uplift of Granuiites in the Datong-Huaian Area, North China

摘要

Archaean granulites in Datong-Huaian area were strongly ductile deformed in response to an extensional detach-ment reacted in lower crust in about 2500-2400 Ma following the crustal thickening. Three litho-tectonic domains can be recognised by their distinct deformation history and kinematic structural patterns formed during the detach-ment process. The lower structural domain comprise more mafic TTG gneiss, dominated on mapping scale by domal structures. The intermediate structural domain consist predominately of biotite-rich gneiss and is character-ised by a strongly foliated tectonic melange which acted as a major macro-scale decollement. The upper domain comprises Khondalites characterised by lower peak-metamorphic pressures and later granulite grade structures. In this rocks, a syn-tectonic S-type granites are widespread. The granulites are characterised by a gneissic fabrics. Foliation and lineations within the lower domain were strongly recrystallized and are now random in orientation, whilst mineral elongation and fold axis are co-lineated and consistently reoriented in the intermediate and upper domains, parallel to the extension direction all with - 25°plunges. Sense of shear indicators across the entire terrain indicate that the hanging walls of the detachment were displaced downward to SW in the lower and intermediate domains, and to WSW in the upper domain. Microlithons of garnet-bearing high-pressure mafic granulites are structurally widespread in the lower and intermediate domains. Development of symplectite textures in these rocks, produced by a near isothermal decompression process, is varying in relationship with their parent mineral assem-blages and deformation stages through the uplifting history. Some of the symplictite textures were elongated paral-lel to the extensional direction.