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• W96608059 abstract "The Archaean granulite terrain in the Datong-Huaian area, north China, comprises a basementcomplex of fe lsic and mafic granulite (TTG gneiss), overlain by a sedimentary sequence dominated bymetapelite and metapsammite (khondalite series). Both lithological associations are separated by atectonic contact. Structural, textural and tectono-lithostratigraphic data indicate that the terrain wasaffected by two prominent tectono-thermal cycles (CA and CB), associated with two granulite faciesevents, M I and M2. Of these, the M l event was dominant and associated with all major structures inthe area. The granulites were subsequently exhumed, as a result of gravity unloading/uplifting anddenudation. This process probably occurred during the early Palaeoproterozoic in response to crustalthinning after thickening at the end of the Archaean (2400-2500 Ma), although final exposure may haveresulted from further exhumation during a later granulite event (M2) around 1900 Ma. From theMesoproterozoic ( 1800 Ma) onward the granulite terrain was covered unconformably by sediments.During cycle CA, the granulites underwent strong ductile deformation in response to anextensional event involving a low-angle detachment, which resulted in the exhumation of high-Pgranulites. On the basis of the distinct deformation histories and kinematic-structural patterns, threelitho-tectonic domains can be recognised that formed at different structural levels during thedetachment process. The lower litho-tectonic domain is mainly composed of TTG gneiss, and ischaracterised by kilometre-scale domal structures and associated recumbent folds. These domesresemble solid state diapirs or mantled gneiss domes. The intermediate litho-tectonic domain consistspredominantly of biotite-rich felsic gneiss with lenses or microlithons of the TTG gneiss and/orkhondalite series rocks derived from both the lower and upper structural domains. The intermediatedomain is characterised by a strongly foliated tectonic melange containing km-scale sheath folds, andacted as a macro-scale decollement. The upper domain comprises sillimanite-rich metapelites orkhondalites with a dominant foliation and is characterised by lower peak-metamorphic pressure andlater granulite grade structures. It is extensively intruded by S-type granites that were emplaced duringthe second granulite event at 1900 Ma. The intermediate and upper structural domains may have beenjuxtaposed across the low angle decollement zone.Granulites in all three domains are characterised by well-developed gneissic fabrics. Foliationtraces and various lineations within the lower domain underwent strong recrystallisation and as a resultof doming, display a wide range of orientations. In contrast, lineations in the intermediate domain arewell preserved and constant in orientation, parallel to the axes of several generations of closed to openfolds. The lineations plunge shallowly (10-30°) to the SW (between 200-230° ) and delineate theextensional 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 tothe SW or WSW, normal sense of movement.Microlithons of garnet-bearing, high-pressure mafic granulites that record pressures between12-16 kbars, and which formed originally in the lowermost crust, are widespread in the lower andintermediate domains. Development of symplectite textures in these rocks, due to a near isothermaldecompression process, vary in response to different parent mineral assemblages and the stage offormation in the uplift history. The symplectites are variably recrystallised and elongated parallel to themain regional extensional lineation, indicating that the structural domains developed duringexhumation of the terrain. Some late brittle normal faults present in the eastern part of the central domemay represent continuous uplift at a brittle upper crustal level. These faults did not significantlyinfluence the basement rocks.Petrological studies show that in the basement, MI reached peak metamorphic conditions of12-14 kbar and 800-900 °C during the foliation-forming event, D26a, which transposed an earlierS 1 Ba foliation of unknown age and origin. In the allochtonous cover sediments, M I peak-conditionsof about 8 kbar and 800 °C were reached after a prograde thickening event, D1 c0 , forming a beddingparallelfoliation and during which large amounts of S-type granites were emplaced. The localisedD28a and D1 c0 structures are interpreted as spatially distinct responses at different crustal levels tothe 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 aftercrustal thickening, and allowed unloading of the footwall resulting in a ri se of the basement rocks to4-6 kbars conditions as deduced from syn-D3 decompression textures. Coeval cooling textures in theallochthonous cover sequence indicate that lateral rather than vertical di splacements occurred in thehanging 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), thattransect both basement and allochthonous cover, and accommodate some uplift.The deduced CA and C6 P-T paths have equivalents in many Precambrian granulites. Ingeneral decompressional paths like the one recorded in the basement rocks, are linked to processesinvolving crustal thickening followed by extensional collapse.Investigation of palaeomagnetisation in the granulites show that the magnetic fabric isdominated by the 03 foliation and extensional lineation formed during extensional collapse anduplift, whilst the magnetic remanence was acquired during later stages of uplift and cooling when thetemperature fell below the Curie point. The pole positions define a continuous swathe plottingbetween 51 ° E, 20 ° S and 170 ° E, 35° S summarised by group mean poles. They are assigned to theinterval ca. 2000 - 1840 Ma and extend the apparent polar wander record for the North China Shieldbackwards in time from the record derived from Mesoproterozoic supracrustal successions.The present structural and petrological data in combination with the results frompalaeomagnetic investigations show that the Datong-Huai an granulites have experienced ductileextensional detachment and uplifting processes, which occurred in the lower crust after crustalthickening. It is concluded that a low-angle detachment may have playc::d an important role in theexhumation history of the granulites as long ago as the Archaean-Proterozoic boundary." @default.
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• W96608059 date "2001-01-22" @default.
• W96608059 modified "2023-09-22" @default.
• W96608059 title "Structural and Metamorphic Evolution of the Archaean High-pressure Granulite in Datong-Huaian Area, North China" @default.
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