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 80 0 degrees C during a 2500-2400 Ma tectonic event. Nearly all structure s in the grantilites developed during the retrograde exhumation histor y, 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 struc tural domain' that is characterized by a planar gneissic foliation, up right folds and a constant, shallowly SW plunging, lineation pattern. During exhumation rocks probably passed from the 'lower' into the 'upp er structural domain'. Domes, recumbent folds and transposition fabric s resulted from a dynamic interplay between vertical (advective) flow and horizontal flattening. The 'lower structural domain' preserves str uctures reflecting the dominance of vertical flow while the 'upper str uctural domain' preserves structures that resulted from flattening and lateral flow. Horizontal flattening and lateral flow of domal structu res led to total destruction of the domal geometries by transposition in a younger, horizontal gneissic layering. The process of doming, fla ttening and transposition repeated itself as advective exhumation of t he high-pressure rocks progressed. Horizontal fabrics appear the more stable geometry and domes progressively degenerated into horizontal le nsoidal shapes, probably as a result of the low viscosities of the gra nulites. Exhumation of lower crustal material via solid-state advectiv e flow implies that vertical crustal movements of at least part of the crust occurred independently of isostatic readjustments. P-T paths, c haracterized by isothermal decompression over a large pressure range, can therefore be interpreted to result from processes that are indepen dent of crustal thickening, erosion and tectonic denudation. (C) 1997 Elsevier Science Ltd.