Structural and Metamorphic Evolution of the Archaean High-pressure Granulite in Datong-Huaian Area, North China

The Archaean granulite terrain in the Datong-Huaian area, north China, comprises a basement complex of fe lsic and mafic granulite (TTG gneiss), overlain by a sedimentary sequence dominated by metapelite and metapsammite (khondalite series). Both lithological associations are separated by a tectonic contact. Structural, textural and tectono-lithostratigraphic data indicate that the terrain was affected by two prominent tectono-thermal cycles (CA and CB), associated with two granulite facies events, M I and M2. Of these, the M l event was dominant and associated with all major structures in the area. The granulites were subsequently exhumed, as a result of gravity unloading/uplifting and denudation. This process probably occurred during the early Palaeoproterozoic in response to crustal thinning after thickening at the end of the Archaean (2400-2500 Ma), although final exposure may have resulted from further exhumation during a later granulite event (M2) around 1900 Ma. From the Mesoproterozoic ( 1800 Ma) onward the granulite terrain was covered unconformably by sediments. During cycle CA, the granulites underwent strong ductile deformation in response to an extensional event involving a low-angle detachment, which resulted in the exhumation of high-P granulites. On the basis of the distinct deformation histories and kinematic-structural patterns, three litho-tectonic domains can be recognised that formed at different structural levels during the detachment process. The lower litho-tectonic domain is mainly composed of TTG gneiss, and is characterised by kilometre-scale domal structures and associated recumbent folds. These domes resemble solid state diapirs or mantled gneiss domes. The intermediate litho-tectonic domain consists predominantly of biotite-rich felsic gneiss with lenses or microlithons of the TTG gneiss and/or khondalite series rocks derived from both the lower and upper structural domains. The intermediate domain is characterised by a strongly foliated tectonic melange containing km-scale sheath folds, and acted as a macro-scale decollement. The upper domain comprises sillimanite-rich metapelites or khondalites with a dominant foliation and is characterised by lower peak-metamorphic pressure and later granulite grade structures. It is extensively intruded by S-type granites that were emplaced during the second granulite event at 1900 Ma. The intermediate and upper structural domains may have been juxtaposed across the low angle decollement zone. Granulites in all three domains are characterised by well-developed gneissic fabrics. Foliation traces and various lineations within the lower domain underwent strong recrystallisation and as a result of doming, display a wide range of orientations. In contrast, lineations in the intermediate domain are well preserved and constant in orientation, parallel to the axes of several generations of closed to open folds. The lineations plunge shallowly (10-30°) to the SW (between 200-230° ) and delineate the extensional direction. Meanwhile, the lineations in upper domain plunge shallowly to the WSW (250- 260\ Sense of shear indicators in the intermediate and upper domains predominantly indicate a top to the SW or WSW, normal sense of movement. Microlithons of garnet-bearing, high-pressure mafic granulites that record pressures between 12-16 kbars, and which formed originally in the lowermost crust, are widespread in the lower and intermediate domains. Development of symplectite textures in these rocks, due to a near isothermal decompression process, vary in response to different parent mineral assemblages and the stage of formation in the uplift history. The symplectites are variably recrystallised and elongated parallel to the main regional extensional lineation, indicating that the structural domains developed during exhumation of the terrain. Some late brittle normal faults present in the eastern part of the central dome may represent continuous uplift at a brittle upper crustal level. These faults did not significantly influence the basement rocks. Petrological studies show that in the basement, MI reached peak metamorphic conditions of 12-14 kbar and 800-900 °C during the foliation-forming event, D26a, which transposed an earlier S 1 Ba foliation of unknown age and origin. In the allochtonous cover sediments, M I peak-conditions of about 8 kbar and 800 °C were reached after a prograde thickening event, D1 c0 , forming a beddingparallel foliation and during which large amounts of S-type granites were emplaced. The localised D28a and D1 c0 structures are interpreted as spatially distinct responses at different crustal levels to the Ml metamorphic event. Juxtaposition of basement and allochthonous cover occurred during D3, along a large, low-angle normal shear zone. This detachment zone accommodated extension after crustal thickening, and allowed unloading of the footwall resulting in a ri se of the basement rocks to 4-6 kbars conditions as deduced from syn-D3 decompression textures. Coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertical di splacements occurred in the hanging wall of the detachment. During cycle C6 , M2 peak-conditions of about 4-6 kbar and 650-700 °C were reached coeval with the development of discrete, left-lateral strike-slip zones (D4/M2), that transect both basement and allochthonous cover, and accommodate some uplift. The deduced CA and C6 P-T paths have equivalents in many Precambrian granulites. In general decompressional paths like the one recorded in the basement rocks, are linked to processes involving crustal thickening followed by extensional collapse. Investigation of palaeomagnetisation in the granulites show that the magnetic fabric is dominated by the 03 foliation and extensional lineation formed during extensional collapse and uplift, whilst the magnetic remanence was acquired during later stages of uplift and cooling when the temperature fell below the Curie point. The pole positions define a continuous swathe plotting between 51 ° E, 20 ° S and 170 ° E, 35° S summarised by group mean poles. They are assigned to the interval ca. 2000 - 1840 Ma and extend the apparent polar wander record for the North China Shield backwards in time from the record derived from Mesoproterozoic supracrustal successions. The present structural and petrological data in combination with the results from palaeomagnetic investigations show that the Datong-Huai an granulites have experienced ductile extensional detachment and uplifting processes, which occurred in the lower crust after crustal thickening. It is concluded that a low-angle detachment may have playc::d an important role in the exhumation history of the granulites as long ago as the Archaean-Proterozoic boundary