Abstract
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
... read more
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.
show less
