ELSEVIER Earth and Planetary Science Letters 161 (1998) 215–230U=Pb zircon ages constrain the architecture of the ultrahigh-pressureQinling–Dabie Orogen, ChinaBradley R. Hackera,Ł, Lothar Ratschbacherb,LauraWebbc, Trevor Irelandd, Doug Walkere,Dong ShuwenfaDepartment of Geology, University of California, Santa Barbara, CA 93106-9630, USAbInstitut fu¨r Geologie, St. Norbert Str. 6, Universita¨t Wu¨rzburg, D-97299 Zell am Main, Pleicherwall 1, D-97070 Wu¨rzburg, GermanycGeological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USAdGeological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USAe120 Lindley Hall, Geology Department, University of Kansas, Lawrence, KS 66045, USAfChinese Academy of Geological Sciences, 534 East Zongshan Rd, Nanjing 210016, ChinaReceived 3 March 1998; revised version received 26 June 1998; accepted 3 July 1998AbstractNew SHRIMP and TIMS zircon ages,40Ar=39Ar ages, and eclogite locations contribute significantly to our understand-ing of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze craton that was subducted to ultrahighpressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex,intruded over a 10-Myr interval between 137 and 126 Ma, that accommodated ¾100% N–S stretching of the pre-existingcollisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishablezircon populations. The population of Triassic zircon ages ranges from ¾219 to ¾245 Ma, leading us to question theprevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect lateretrogression at crustal pressures following the bulk of exhumation.  1998 Elsevier Science B.V. All rights reserved.Keywords: Dabie Mountains; zircon; absolute age; Sino–Korean platform; high pressure; orogenic belt1. IntroductionThe Dabie–Hong’an–Tongbai–Qinling are a2000-km long ultrahigh-pressure (UHP) orogenformed chiefly in the Triassic by attemptedN-directed subduction of the Yangtze craton ora microcontinent beneath the Sino–Korean craton(Fig. 1) [1,2]. Investigation of UHP tectonics hasfocused on the Dabie Shan because of the wideŁCorresponding author. Tel.: C1 805 893 7952; Fax: C1 805893 2314; E-mail: [email protected] of continental crustal rocks that were meta-morphosed under a complete range of low to ultra-high pressures and temperatures. From S to N, themain rock units are a fold-and-thrust belt, blueschist,high-pressure amphibolite, quartz eclogite, coesiteeclogite, the Northern Orthogneiss unit (NOU),the Luzhenguang Group, and the Foziling Group(Fig. 2). All are intruded by voluminous Cretaceousplutons, and units on the margins of the moun-tains are overlain by Cretaceous and younger allu-vium (references in Fig. 1). The blueschist througheclogite units constitute a prograde metamorphic se-0012-821X/98/$19.00  1998 Elsevier Science B.V. All rights reserved.PII S0012-821X(98)00152-6216 B.R. Hacker et al. / Earth and Planetary Science Letters 161 (1998) 215–230Fig. 1. Collisional orogen in central China, comprising the Wudang, Qinling, Tongbai, Hong’an, Dabie Shan, and Su-Lu areas.quence, with the coesite-bearing eclogites indicatingsubduction of continental crust to >120 km [3,4].Hacker et al. [5] interpreted these units as a singletranspressionally (dextral) shortened crustal segmentexhumed by extrusion over the Sino–Korean cratoncombined with erosion. The NOU is a magmatic-structural dome formed during Cretaceous NW–SEsubhorizontal extension [5] that overprints an earliergranulite-facies basement sequence. The Luzhen-guang and Foziling Groups are less well knownunits that may represent continental basement andcover, respectively.In the Hong’an area (Figs. 1 and 2), blueschist-facies rocks are better developed and distinctgreenschist, prograde amphibolite, and eclogite-ret-rogressed-to-amphibolite units have been mapped(e.g., [6]). Also, a wider variety of metamorphicrocks not generally grouped with the ultrahigh- andhigh-pressure units crop out in E–W trending fault-bounded units at the northern limit of the mountainrange (i.e., the Sujiahe through Erlangping Groupsin the NW corner of Fig. 2). Many of these units arewell known in the Qinling area (Fig. 1; e.g., [7]).This paper summarizes new geochronological=petrological=structural data that address significantquestions about the architecture of this orogen: (1)Where is the suture between the collided Sino–Korean and Yangtze cratons? (2) What is the spa-tial extent of the UHP metamorphism? Were, forinstance, the Northern Orthogneiss, Luzhenguang, orFoziling units subjected to UHP? (3) Were all rocktypes in the UHP units metamorphosed at UHP orare some of them lower pressure rocks that werejuxtaposed at a later date? (4) What is the temporalrelationship between the UHP metamorphism andthe granulite-facies metamorphism of the NOU?2. Qinling–Dabie orogenic architecture and UHPmetamorphismThe Triassic suture between the Sino–Korean andYangtze cratons has been proposed to be locatedeither in the central Dabie Shan (Fig. 2) betweenthe NOU and the coesite eclogite unit (e.g., [8]), oralong the Xiaotian–Mozitang Fault near the north-ern edge of the Dabie Shan (e.g., [9]). Neither ofthese is a primary feature; the former is a Cretaceousnormal fault, and the latter is a Cretaceous sinistraltranstensional fault [5]. The location of the sutureconstrains the size and composition of the materialthat was subducted to UHP depths and is centralto understanding the scale and mechanism of ex-humation. We report a new location for the suturebased on U=Pb zircon ages,40Ar=39Ar ages, and thedescription of a new eclogite locality.B.R. Hacker et al. / Earth and Planetary Science Letters 161 (1998) 215–230 217Fig. 2. Map of the Dabie Shan and Hong’an areas drawn from our mapping in 1993–1997 and Regional Geological Survey of Anhui [30], Regional Geological Survey ofHenan [29], Regional Geological Survey of Hubei [45], Okay et al. [9].218 B.R. Hacker et al. / Earth and Planetary Science Letters 161 (1998) 215–230Interpretation of the eclogite units as a coherentUHP terrane (e.g., [10]), has come under increasedscrutiny. ‘Eclogite’ units exposed in the Dabie andHong’an blocks are composed of micaceous parag-neiss, granitic orthogneiss, and ¾2 vol%