Exhumation of high-pressure granulites and the role of lower crustal advection in the North China Craton near Datong

Granulites in the Datong-Huai'an area of North China are characterized by high P-T assemblages (14 -16 kbar, similar to 900 degrees C) that underwent decompression cooling to similar to 7 kbar and similar to 800 degrees C during a 2500-2400 Ma tectonic event. Nearly all structures in the grantilites developed during the retrograde exhumation history, and can be subdivided into: (I) the stratigraphically lower,'lower structural domain' that is characterized by complex folding with 55-10 km wide domes surrounded by concentric troughs, preserving concentric lineation patterns; and (2) the stratigraphically higher 'upper structural domain' that is characterized by a planar gneissic foliation, upright folds and a constant, shallowly SW plunging, lineation pattern. During exhumation rocks probably passed from the 'lower' into the 'upper structural domain'. Domes, recumbent folds and transposition fabrics resulted from a dynamic interplay between vertical (advective) flow and horizontal flattening. The 'lower structural domain' preserves structures reflecting the dominance of vertical flow while the 'upper structural domain' preserves structures that resulted from flattening and lateral flow. Horizontal flattening and lateral flow of domal structures led to total destruction of the domal geometries by transposition in a younger, horizontal gneissic layering. The process of doming, flattening and transposition repeated itself as advective exhumation of the high-pressure rocks progressed. Horizontal fabrics appear the more stable geometry and domes progressively degenerated into horizontal lensoidal shapes, probably as a result of the low viscosities of the granulites. Exhumation of lower crustal material via solid-state advective flow implies that vertical crustal movements of at least part of the crust occurred independently of isostatic readjustments. P-T paths, characterized by isothermal decompression over a large pressure range, can therefore be interpreted to result from processes that are independent of crustal thickening, erosion and tectonic denudation. (C) 1997 Elsevier Science Ltd.