Ar-40/Ar-39 thermochronology from the northwestern Dabie Shan: Constraintson the evolution of Qinling-Dabie orogenic belt, east-central China

Seven structural units with distinct kinematics have been recognized in the northwestern Dabie Shan (114 degrees E-115 degrees E and 31 degrees N-32 d egrees N), east-central China. They are, from north to south: (1) the north -verging Mafan fold belt (MFB) involving early Paleozoic metamorphic rocks; (2) the Liangting melange belt (LMB), which is separated from the MFB by a south-dipping thrust and consists of gneissic and marble blocks surrounded by highly sheared greenschist; (3) the north-verging Laoshan ductile fold belt (LFB), which was thrust northward over the Liangting melange belt and involves Proterozoic schists; (4) the Suhe thrust sheet (STS), which consis ts of phyllite and slate above a folded thrust fault dipping both to the no rth and south; (5) the 5-8-km-wide Huwan shear belt (HSB) in the footwall o f the folded thrust, consisting of mylonitic gneisses and sheared ecologite ; (6) the Kafang eclogite-bearing gneiss dome (KGD), which is defined by br oadly warped foliations; and (7) the 10-km-wide Pendian ductile shear belt (PSB) characterized by dextral strike-slip deformation. Nine phengite and muscovite samples in eclogites, mylonitic shear zones, an d metamorphic gneisses from four of the seven structural units have been da ted by the Ar-40/Ar-39 step-heating method. Phengites from deformed eclogit es in the HSB yield complex age spectra that we interpret as most likely in dicating closure with respect to Ar loss between 350 and 430 Ma. The rock s amples collected from the eclogite-bearing gneiss display a significant sta tic recrystallization of quartz, which suggests temperatures in excess of c a 350-400 degrees C. The ages of 350-430 Ma from samples 9669B and 9669C ar e, therefore, interpreted as retrograde metamorphic ages for the eclogites. Six additional phengites sampled from mylonitic rocks yield two groups of a ges, 263-249 and 236-214 Ma. The older group samples (96-5, 9667 and 9692) were collected from mylonitic rocks with top-to-north shear sense in the LM B, HSB and KGD, respectively. 9667 and 9692 exhibit a strong static recryst allization, which means that ductile deformation took place at a temperatur e in excess of 350-400 degrees C. We therefore adopt the age of 263-249 Ma as the cooling ages of white micas. Considering sample 96-5 shows microfabr ics formed at lower temperature around or slight below the closure temperat ure of white mica, there is a strong possibility that the age of 254 +/- 1 Ma represents a crystallization age rather than a cooling age. Because thes e samples are taken from the top-to-north shear zones, we also interpret th e 263-249 Ma event as a result of thrusting-induced denudation. The younger group samples (96106, 96124 and 96113) collected from dextral shear belts in the HSB and PSB exhibit a strong static recrystallization at a temperatu re in excess of 350-400 degrees C. Their ages (236-214 Ma) are therefore in terpreted as the cooling ages related to later dextral strike-slip faulting . Sample 96101 collected from weakly foliated rock without penetrative deform ation on the north limb of the KGD shows poorly recovered quartz microfabri cs at the temperature around, or slightly below, the closure temperature of white mica. Its age of 187 +/- 1 Ma is, therefore, interpreted as crystall ization age during the downdip detachment event related to doming. The Ar-40/Ar-39 data from the northwestern Dabie Shan suggest four distinct ive cooling events. On the basis of the local structural framework and regi onal tectonic relationships, we suggest that the 410-350 Ma cooling event w as related to either subduction of an oceanic plate beneath the Sino-Korean plate or collision between the Sino-Korean plate and a microcontinental bl ock in the middle Paleozoic. The three younger episodes of cooling between the late Permian and the late Triassic may all have been related to the col lision between the Sino-Korean and Yangtze Flares. Specifically, the 263-24 9 Ma event could have been a result of thrusting-induced denudation, wherea s the 236-214 Ma cooling event is likely related to strike-slip faulting. T he younger ages of 195-187 Ma obtained from the gneiss dome are interpreted to be a result of regional denudation related to doming during the late st age of collision between the Sino-Korean and Yangtze plates. (C) 2000 Elsev ier Science B.V. All rights reserved.