Rapid eclogitisation of the Dabie–Sulu UHP terrane: Constraints from Lu–Hf garnet geochronology

Abstract

Rapid eclogitisation of the Dabie–Sulu UHP terrane: Constraints from Lu–Hfgarnet geochronologyAlexander Schmidta,⁎, Stefan Weyera, Klaus Mezgerb, Erik E. Schererb, Yilin Xiaoc,d,Jochen Hoefsd, Gerhard P. BreyaaInstitut für Geowissenschaften, Facheinheit Mineralogie, JWG-Universität, Frankfurt, Altenhöferallee 1, 60438 Frankfurt, GermanybZentrallabor für Geochronologie, Institut für Mineralogie, Universität Münster, Corrensstr. 24, 48149 Münster, GermanycCAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth, and Space Sciences, University of Science and Technology of China, Hefei, 230026 ChinadGeowissenschaftliches Zentrum Göttingen, Universität Göttingen, Goldschmidtstr. 26 1-3, 37077 Göttingen, GermanyABSTRACTARTICLE INFOArticle history:Received 8 February 2008Received in revised form 18 June 2008Accepted 20 June 2008Available online 4 July 2008Editor: R.W. CarlsonKeywords:Lu–Hf geochronologySm–Nd geochronologyeclogiteultrahigh-pressureeclogitisationThe Qinling–Dabie–Sulu orogenic belt in eastern China is one of the largest ultrahigh-pressure (UHP)terranes worldwide. Mineral Sm–Nd- and zircon U–Pb dating has been widely used to reveal themetamorphic history of this collisional orogen. However, the exact timing of the UHP metamorphic event(s)remains controversial and ages ranging from 245 Ma to 220 Ma have been suggested. We present highprecision garnet–cpx Lu–Hf ages for six eclogites from the Dabie and Sulu areas. All ages fall in a narrowrange between 219.6 and 224.4 Ma. Five samples define a mean age of 223.0±0.9 Ma and one sample yields aslightly younger age of 219.6±1.4 Ma. This very tight age range is particularly remarkable considering thelarge regional distribution of sample localities (on the order of 100 km at the time of UHP metamorphism)and the wide variety of garnet and eclogite chemical compositions represented. Two samples yield Sm–Ndages that are indistinguishable from their Lu–Hf ages, albeit with larger uncertainties.The identical ages of eclogites from both the Dabie and the Sulu region emphasize their close geneticrelationship and similar metamorphic histories. The Lu–Hf results appear to date a punctuated event ofgarnet growth. Alternatively, the Lu–Hf garnet ages may represent the onset of rapid, contemporaneousuplift and subsequent cooling. However, trace element zoning of Lu and Hf is still preserved in garnetporphyroblasts, even in those with a homogeneous major element distribution. Thus, complete re-equilibration of the Lu–Hf system during peak-temperature conditions probably did not occur. The garnetforming event can be placed toward thefinal stage of the UHP metamorphism, in agreement with somepublished U–Pb zircon ages. A possible trigger for this short-lived and widespread mineral growth episodemay have been afluid that became available at that stage of the metamorphic history. Although HREE-depleted patterns of older zircon grains may indicate the presence of an older generation of garnet, completeeclogitisation may have been inhibited during the major part of the prograde P–T path due to dry conditionsduring most of the UHP metamorphism. The uniform Lu–Hf (and Sm–Nd) ages of all investigated Dabie andSulu eclogites suggest that garnet growth and thus possiblyfluid availability were limited to a short timeinterval over a remarkably large regional scale.