EXTENSIONAL COLLAPSE AND UPLIFT IN A POLYMETAMORPHIC GRANULITE TERRAIN IN THE ARCHEAN AND PALAEOPROTEROZOIC OF NORTH CHINA

The Archaean granulite terrain in the Datong-Huai An area, north China , comprises a basement complex of felsic and mafic granulite, overlain by a sedimentary sequence dominated by metapelite and metapsammite. B oth lithological associations are separated by a tectonic contact, and structural, textural and stratigraphic data indicate that the terrain was affected by two prominent tectono-thermal cycles (C(A) and C(B)), associated with two granulite facies events, MI and M2, which probabl y occurred approximately 2500 Ma and 1800 Ma ago, respectively. Of the se, M1 events were dominant and associated with all major structures i n the area. In the basement gneiss, MI reached peak metamorphic condit ions of 12-14 kbar and 800-900-degrees-C during a foliation-forming ev ent, D2Ba, which transposed an earlier S1Ba foliation of unknown age a nd origin. In the allochthonous cover sediments M1 peak-conditions of about 8 kbar and 800-degrees-C were reached after a prograde thickenin g event, D1co, associated with the formation of a bedding-parallel fol iation, while large amounts of S-type granites were emplaced. The loca lised D2Ba and D1Co structures are interpreted as spatially distinct r esponses at different crustal levels to one metamorphic event. Juxtapo sition of basement and allochthonous cover along a large, low-angle no rmal shear zone occurred during D3. This detachment zone accommodated extension after crustal thickening, and allowed unloading of the footw all resulting in a rise of the basement rocks through 4-6 kbar as dedu ced from syn-D3 decompression textures. Coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertic al displacements occurred in the hanging wall of the detachment. Durin g C(B), M2 peak-conditions of about 4-6 kbar and 650-700-degrees-C wer e reached coeval with the development of discrete, left-lateral strike -slip zones (D4) that transect both basement and allochthonous cover, and accommodated some uplift. The deduced C(A) and C(B) P-T paths have equivalents in many Precambrian granulites. In general decompressiona l paths like the one recorded in the basement rocks, are linked to pro cesses involving crustal thickening followed by extensional collapse. In the Datong-Huai An area it can be shown that low-angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the Archaean-Proterozoic boundary.