Subduction, collision and exhumation in the ultrahigh-pressure Qinling - Dabie orogen
BRADLEYR. HACKERI, LOTHAR RATSCHBACHER2& J. G. LIOU3 IGeological Sciences,University of California, SantaBarbara, CA 93106, USA 2lnstitutfiir Geologie,Technische Universitiit BergakademieFreiberg, Freiberg, D-09599, Germany 3Geologicaland EnvironmentalSciences, Stanford University, Stanford,CA 94305, USA
c':. Abstract: High-pressure metamorphism and ophiolite emplacement (Songshugou ophio- lite) attendedsuturing of the Yangtze craton to Rodinia during the c.l.O Ga Grenvillian oro- . geny. The Qinling microcontinent then rifted from the Yangtze craton at c.750Ma. The .;~, microcontinentErlangping intraoceanic in the Ordovician-Silurian, arc formed in the Early and Ordovician, then both units was wereemplaced accreted onto tothe the Qinling Sino- Korea craton before being stitched together by the c.400 Ma Andean-style Qinling arc. Subsequent subduction beneath the Qinling-Sino-Korean plate created a Devonian- Triassic accretionary wedge that includes eclogites, and formed a coeval volcano-plutonic arc that stretchesfrom the Longmen Shan to Korea. In the Late Permian-Early Triassic, the northern edge of the South China Block was subductedto > 150 km depth, creating the diamond- and coesite-bearing eclogites of the Dabie and Sulu areas. Exhumation from the mantle by lithosphere-scaleextension occurred between 245 and 195 Ma during clock- wise rotation of the craton. The Yangtze-Sino-Korea suture locally lies tens of km north of the exhumed UHP- HP part of the South China Block, implying perhapsthat the very tip of the South China Block was not subducted,or that the UHP-HP rocks rose as a wedge that peeledthe upper crust of the unsubductedSouth China Block from the lower crust. The Tan- Lu fault is an Early Cretaceousto Cenozoic feature. The apparent offset of the Dabie and Sulu UHP terranes by the Tan-Lu fault is a result of this Cretaceousto Cenozoic faulting combined with post-collisional extension north of Dabie.
The Sino-Korean craton and the Qinling micro- basementoutcrops can be assignedeither a Sino- 3. continent colli~ed in the Ordovician-Silurian. Kore;an or Yangtze crat.on affini~y, where~s the ~,., The North China Block (NCB) and the South affinIty of areally extensIvedomaIns of sedImen-
~ China Block (SCB) collided in the Permo-Triassic. tary strata or of volcano-plutonic complexes can 1,4 The orogenic belt associatedwith thesecollisions be ambiguous. 'North China Block' and 'South "f);' and intervening tectonism extends c.2000km China Block' are used to refer to the continental west-east through the Qinling, Tongbai, Dabie blocks north and south of the Qinling-Dabie- Shan and Sulu areasand into Korea (Fig. 1). It is Sulu suture. of specialinterest because it containshigh-pressure The Sino- Korean craton is subdivided into (HP) rocks of four different ages: Grenvillian, three units: the western, eastern and central Devonian, Carboniferousand Triassic. This paper blocks. The western and eastern blocks are aims to provide a review of the collision zone as Archean cratons linked by a central suture belt both a summary of what conclusions have been that formed during collision at c.1.8 Ga (Zhao drawn where and with which data, and as a guide et at. 2000). The U-Pb zircon ages from the to future reseal\;h. Sino-Korean craton cluster at c.3.8, 3.3, 3.0, 2.5 and 1.7-1.8Ga (Song et at. 1996; Yu et at. . 1996; Zhao et at. 2000). The craton consists of Smo-Korean craton a basement of Archaean to Proterozoic meta- In our usage, the Sino-Korean and Yangtze cra- morphic rock overlain by a relatively uninter- tons comprise Precambrian basement and cover rupted 4-8 km thick superjacent section of as describedbelow; in principle, all Precambrian Sinian (Late Proterozoic to Early Cambrian) to
From: MALPAS, J., FLETCHER,C. J. N., ALl, J. R. & AITCHISON, J. C. (eds) 2004. Aspects of the Tectonic Evolution of Chiua. Geological Society, London, Special Publications, 226, 157-175. 0305-8719/04/$15 ((;) The Geological Society of London 2004.
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K22025 m2029 26 26 Geochronology Explanation Foziling 9 Z667,739 M242 zircon core (if present) m225 unit names are italicized b204, 2239 H742,77026 coesite eclogite 2 location reference Explanation Z391 M21825 26 (Hu et al.,1995) K205[c. 870]* K180 post-Triassic rocks m2139 Z75729 age (Ma) Sm23136 Sm2448 Z87025 K: 40Ar/39Ar K-feldspar Sm21048 40 39 Triassic sedimentary rocks 36 5 B: Ar/ Ar biotite Sm228 z480 M: 40Ar/39Ar K-white mica Sm23842 11 40 39 46 Rbwr402 Ar4218 H: Ar/ Ar hornblende Sino-Korean craton cover and basement; letter shows age of youngest rocks Z229[707] Sm20848 2 b: K/Ar biotite 4 KAr394 26 38 Rb486 m: K/Ar K-white mica contact, depositional or intrusive H280 2 Sm400 3 9 h: K/Ar hornblende Cretaceous plutons KAr402 8 m354 Rbwr452 2 o inferred contact 27 Rb383 2 KAr380 113 Z: U/Pb zircon M185,H214 Rb6704 2 KAr403 9 9 35o m4203 KAr3702 KAr401 2 Rbwr424 Rbwr991 Rm: Rb/Sr whole-rock-K-white mica fault Sm218-210 x6, 4 20 KAr480 Triassic plutons Northern Z76825 30 Rb444 Z437 Rbwr7949 Rb: Rb/Sr whole-rock-biotite thrust fault Rm 223-198 Rbwr9739 Orthogneiss Rbb194,194,197,198,198 3 47 m425 45 3 Sm983 antiform M205,206,206,212 H~400K220 b432 Paleozoic plutons 29 45 Z1.3 Z2.129 B201,202,211,21035 K23045 z488,4875 H420 synform Kuanping unit homocline Z21422Sm246, Songshugou structural trend Z73926 Rb24023H22524Z21829 Z223[1.8]41 Erlanping unit 26 coesite eclogite M179 23 24 45 14 Dengfeng Rm236 M190 K176, H³386K270 Pb2.5 Z79825 M183,19026Z238[1.9] Dengfeng Qinling unit quartz eclogite mz223,20934Z233[2.5] northern Liuling greenstone T3 Sm23135Sm226x2, 22 22 31 37 =Qinling(?) Rb2.82 Songshugou ophiolite eclogite → amphibolite Z218 Z638 772 Rbb169,171,173,174,181 S i n o - K r e a c t ( N C B ) B22724 22 25 Z218-240,K203, o 2 → Z674 Z781 B199,216, 111 UPb2.4 Liuling unit blueschist greenschist 24 2 coesite M195,H195(?) Lu Shan f Rb2.6 o ault eclogite 31 36 22 35 Douling, Sujiahe (incl. Huwan), Luzhenguang unit Z78126 Z772 Sm221 Z227,B190 25 M200B19024,B195,K19526 Z224 9 29 Triassic high-pressure and ultrahigh-pressure rocks o b182,216,231 Z218 115 M19128 Z230,mz20934 P o M1787 2 Yangtze craton cover and basement; 33 M196 K20424
Songxian a n g t z e c r o ( S C B ) tz eclogite 14 letter shows age of youngest rocks quar 2.8 Y Taihua gneiss Lu Shan M20624 M1951 2 2 KAr127 2 26 KAr100 Z2.6 M210,198 B~197,M20826M2161 K100-105 M22124 m2109 2 2 Z1.5 2 Xiong'er KAr370 KAr103 3 2 49 b430 K55 KAr141 Foziling sed: Pb 2.5, 1.9-1.8 Ga, 1.0-0.7 Ga, 455 Ma Kuanping 49 Lushi 2 Queshan Foziling granites: Z719-766 3 T KAr1402 KAr131 49 b121 b3893 Z3912 3 Meishan Fm. Luzh sed: Pb734-1.9 O 8 Rb105 45 b1243 Guandaokou H402K~200 Xiong'er KAr3842 2 Ar4218 Z2.0 C . T3J1 7 Lo-Nan F 45 Erlangping H434 B101 KAr3982 3 m2159 b135 45 G 7 Luzhenguang N 9 K95 Srwr991 H433 Kua 9 Foziling 3 nping m450,433 b341 I F. 26 H<498 K100, 60 Songshugou Qinling z0.8,2.613 KAr4072 n fau 3 1 L Shang-Dan → 45 HT4047 ita lt b140 B327 M348 K~1000 220 KAr4852 oz 8 13 O Xinyang M Sm224 1 2 Xinyang z470, 435 → 17 e m186, 2289 n- 25 B314 8 KAr795 N M220 256 r zon 9 tia Z771 4 Shangnan KAr718 4 ea m196,203 o Xi'an Rb143 N M301,247 5 2 16 Xinyang sh ia Z725 13 n X Danfeng → 45 z762 Z747 Douling z1.7Sm1.9Sr422H833 12 G H401 wa ous Northern Rb40210 45 B297,264 K~1000 250 z746, 776 7 7 Hu ace t t H152 or K? K205±5 45 2 BH316 H135 33 re Orthogneiss 45 H~1000 KAr880 Suixian & M205 Z404,780,1.0,1.5 13 C ul 4 45 M243,266 a Liuling M806->750 Yaolinghe A f I Ar214 H400→222M22713 coesite 3 e 2 I D A B I E b135 Lu Heih Z656 eclogite Ar2174 rieb2161 B12013 n- 1 Shanyang M232 Wudang & Ta F. Yaolinghe H O N G 'A N he F.Q Nanyang en u Z226,85422 44 Ch go Basin Z~750 a g F. M18028 Sh Shan-Fen Z22732 coesite Rb4184 Z77826 eclogite Z2004 Xiaomoling K17133 25 Z761 quartz eclogite Shiyan blueschist P2 M235,K20526 M225, 22218 H23745 S H A N lu F. M232→15045 Gong T S 33 Wudang Shan zone M234 J hear 3 J n s amphibolite 1 Huwa Z660,73526 17 Sm44643 M230 coesite M19017 M~400x217Z312[~400]21 M23323 15 2 21 Z745 F. Z307 Z311,307[433] M26317 mT2079 ang 39 40 1 Wuhan P Fangxia Z400H399? Sm422 m201,2059 blueschist Wud 21 Z232 M196,22233 33 J2 o M22433 M231 M19518 eclogite 108 o 33 J1 T 18 qua 313 M230 rtz J3 M212 M2473324917 eclogite Z77926 M23617 M23433 M20533 33 eclo M207 Zhuqi F. gite b2269 m2329 M21417 N Shennong →am M20118M19833 ph ~2.9, 2.9-3.3 (19) iboli 0.9(15) te m1979 M19533 32¡ 33 0.9(15) Huangling M19633 M237 K16933 J 109¡ M21318 2 31¡ J2 100 km 111¡ 30o 113o North South North China Block South China Block
Sino-Korean craton Kuanping: Sino-Korean Erlangping- Qinling: Sino-Korean Northern Liuling: Southern Liuling: Douling: Yangtze Yangtze craton (southern margin) Pr oterozoic-Silurian Danfeng-Heihe cover, ophiolite and = Qinling(?) Paleozoic basement (northern margin) passive margin/ intraoceanic Yangtze basement flysch and molasse(?) +crustal derived accretionary wedge(?) arc Mesozoic evolution same Mesozoic Jurassic volcanics as Sino-Korean craton thru Early continental amalgamated by 400 Ma Upper Triassic Triassic Devonian- clastics continental Permian(?) marble, lacustrine to alluvial amphibolite, siliclastic & orthogneiss, Lower- paragneiss, marble, gneiss volcaniclastic acid to Middle Triassic Permian shallow marine-lacustrine amphibolite ~400Ma rocks dominantly evaporite, unstable shelf thru middle Carboniferous- Ti-rich carbonate & Devonian =Kuanping(?) contact angtze riftingangtze
Y pillow basalt Early Permian andesites Qinling Upper ~310 & breccia Permian metamorphism suture thru 400 Ma(?) Middle =Qinling(?) eclogite South China Block tectonic Lower Carboniferous Carboniferous hiatus Devonian-Permian(?) to
4Ð8 km flysch and molasse Upper Ordovician ~12 km Ordovician thru pre-Early platform amphibolite- Sinian Silurian Lower Triassic greenschist suture paragneiss Grenville (~1 Ga) coal and ~435 Ma paragneiss& basement formation of phosphorite Xiong'er & metamorphism amphibolite Guandaokou 2.1-2.6 Ga Rodinia Devonian Groups 670-990 Ma clastics (partly Upper Sinian basement suture 400 Ma deep marine) to Lower Triassic
oceanic Qinling Lower coesite Songshugou Dengfeng greenstone rifting Yangtze platform carbonates 2.5-2.8Ga ~638Ma sediments eclogite ophiolite Silurian Taihua gneiss detrital zircon up to =Yangtze craton(?) relict eclogite volcanism on passive margin ~420 Ma ~1 Ga pre-~435Ma pillowed accretionary wedge volcanics & subduction zone subduction zone craton or passive margin sheeted dikes older than basement ~480 Ma 0.7-0.8 Ga
plutons subduction 490-470 Ma (~490-470 Ma) 435-470 Ma ~490-470 Ma subduction and collision(?) intraoceanic arc Triassic HP-UHP metamorphism subduction ³400 Ma (245-220 Ma) no collision
~440-390 Ma Qinling arc and arc metamorphism
Figure 3: Geologic units, tectono-thermal/sedimentary events, facies interpretation, radiometric ages, and tectonic interpretation of the Paleozoic-early Mesozoic Qinling orogenic belt. Formation of the intra-oceanic Erlangping arc between ~490-470 Ma was followed by accretion of the lower Qinling micro-continent to the intra-oceanic arc, and to the Sino- Korean craton. Subduction underneath the northernmost Liuling unit imprinted the ~400 Ma Qinling arc on the Sino-Korean + Qinling collage. A subduction signature is again evident during the mid-Carboniferous to Late Permian on the Sino-Korean craton, when the Paleo-Tethys was subducted northward, producing the andesitic magmatism on the Sino-Korean craton. In the Late Permian-Early Triassic, the leading edge of the Yangtze craton was subducted to >150 km and subsequently exhumed by crustal extension (after Ratschbacher et al. in press). N
113o 35o
T3 Sino-Korean craton basement and cover 111o
Lu Shan fau 35o lt T 115o o P2 33
Q i n l i n g Taihua gneiss
XiongÕer T o n g b a i hern lim sout iTt 3of S D a b i e S h a n ino-K O orea Guandaokou n cr XiongÕer hern limit of aton nort Trias C Erlangping Intraoceanic Arc sic m LoÐNan F. eta mor H o n g ' a n southern limit of NCB/nor ph Foziling thern limit o ism Luzhenguang f Yangtze Qinling Microcontinent affini itan faul thern limit of D ty rock oz t sou evonia s M Kuanping F. n tecto ne nÐ ÐDan nism r zo otia Xi'an Shang Douling shea ia t ous X wan ace n Hu ret humatio Xinyang C Liuling iassic ex Tr Lu faul f - it o lim Tan rn he . rt e F . no Nanyang Hong'an h u F Chen go Basin HP-UHP Sha nÐFeng F. Sha Terrane Dabie P2 HP-UHP Terrane Wudang Shan T S Core Complex J3 J1
F. ng T1 Wuhan P Wuda J o 2 108 o 33 J1 T1 J Yangtze craton basement and cover3
Zhuqi F. Lower Yangtze fold-thrust belt
32¡ J 109¡ 2 31¡ J2 111¡ 30o 113o
Figure 4: Major units and unit boundaries of the Qinling-Dabie orogen (see Figure 2 and its caption). . 1
162 B. R. HACKERET AL
North China Block-South China Block suturein NCB (Yin & Nie 1993). This intra-oceanic arc the Hong'an-Dabie areas. can be traced eastward as far as Hong'an, High-pressure metagabbro, amphibolitized where it disappearsbeneath Cenozoic sediments ~clogite and felsic granulite associated with (Figs 2 and 4). No such ophiolite is known from c.l.0-l.2 Ga ophiolitic rocks were emplaced on Sulu or Korea, but there are several candidates tb the lower part of the Qinling unit in the Song- for Palaeozoic ophiolites in the Kunlun-Altyn shugou area, following high-pressure meta- Tagh-Qaidam-Qilian area (Matte et al. 1996; morphism in an oceanic setting (Liu et al. Sobel & Arnaud 1999). 1996; Song et al. 1998; Zhang 1999).The amphi- bolitized eclogite was derived from MORB, is in S"l. E I D . Q. I. fault contact with mantle peridotite and contains I unan- ar y evoman In mg arc 4ecompressiontextures common in many eclo- Following emplacementof the Early Ordovician gites, including symplectites of plagioclase and Erlangping intra-oceanic arc, a continental mar- diopside derived from omphacite estimated to gin arc was built on the Kuanping, Erlangping have formed at pressures ~ 1.5 GPa (Zhang and Qinling units from c.438-395 Ma; this 1999). A Sm-Nd mineral isochron of 983 Ma implies that the Erlangping and Qinling units' is taken to date later cooling. The high-P felsic were amalgamatedwith the Kuanping unit (and granulite includes garnet + kyanite + micro- thus the Sino-Korean craton) prior to c.440 Ma perthite + quartz + rutile assemblages formed (Fig. 3) (Ratschbacheret al. in press). Signifi- ;. at 800-900°C and 1.3-1.6GPa (Liu et al. cantly, none of these Silurian-Early Devonian 1996). plutons crop out within the Liuling unit. How- Palaeoclimatic, palaeobiogeographical and ever, the northern part of the Liuling unit, palaeomagneticdata imply that the North China which Ratschbacheret al. (in press) correlated and South China blocks were close to or part of with the Qinling unit, experienced the regional near-equatorial East Gondwana through the contact metamorphism produced by this batho- Late Devonian, and that rifting of the NCB lith, and Silurian to Lower Devonian strata in t1romGondwana took place after the Devonian the northern Liuling unit received metamorphic ~hao et al. 1996; Huang et al. 2000). detritus (Mattauer et al. 1985) from the Qinling i arc and c.780Ma and c.l.0Ga detrital zircons (see above). We thus interpret the southern Ophiolites and accretionary complexes NCB, and units as far south as the northern Liuling unit, as having been stitched together Early Ordovician Erlangping Ophiolite, the by the 400 Ma magmatic-metamorphic event, Qinling Microcontinent and their and suggest that an Andean-type continental ... margin arc was built along the southern margin amalgamanon wIth the SIno-Korean Craton of the NCB at this time; we place the subduction The distribution of ophiolites within the Qinling- zone producing the Silurian-Devonian arc Dabie-Sulu orogen provides important infor- south of the northern Liuling unit (Ratschbacher tpation about the (partial) closure of ocean et al. in press). The local absence of Upper " basins. The oldest Palaeozoic orogenic event in Ordovician (c.445 Ma) through Lower Carbon- ~e Qinling orogen was the formation of the iferous (c.350 Ma) rocks from the NCB in the ~rlangping intra-oceanic arc, which includes Qinling orogen probably reflects uplift related l' the Heihe, Danfeng and Erlangping units to the formation of this Andean.style Qinling arc. (]Fig.3). Sedimentsassociated with the arc contain High-pressure rocks that may be associated Cambrian-Ordovician through Ludlovian- with the Qinling arc have been found by Hu Wenlockian (419-428 Ma) radiolaria, and tron- et al. (1995, 1996) in the northern Qinling area djhemites,tonalites, gabbros,and rare pyroxenites (Fig. 2, offset). Hu et al. reported lenses and with single-zircon PbjPb ages of 470-488 Ma blocks of eclogite, coesite-bearing eclogite and that intrude a volcanic sequence(see summary retrogressed amphibolite within garnet-bearing in Ratschbacheret al. in press). Ratschbacher quartz and phengitic mica schist. While the indi- 8t al. (in press)proposed south-directed emplace- vidual outcrops are no more than a few metres ment of this arc on to the lower Qinling unit (the wide, the belt of eclogites eJ(tendsmore than ~inling microcontinent) between c.470 and 10 km (Fig. 2). The eclogites c~nsist of garnet + 435 Ma, based on the present geographical pos- omphacite+ rutile + quartz + zoisite :t phengite itions of the arc, the upper Qinling unit and the (3.5 Si atoms per formula utrit):t amphibole; lpwer Qinling unit. The Erlangping-Qinling col- many have been extensively retrogressed. lision may havebeen related to the collision of the Inclusions of coesite and its pseudomorphs in Pamir-South Tarim-Qaidam continent with the garnet and omphacite were identified in a few
I i SUBDUCTION,COLUSION AND EXHUMATION 163 samples, based on optical properties and the of California (Ye et al. 1994; Liu et al. 1996). presenceof radial fracturesaround the inclusions; They have yielded SHRIMP U - Pb zircon ages this identification, however, should be confirmed of c.310 Ma (Sun et al. 2002), 400 Ma (Jian by Raman spectroscopy. Pressure-temperature et al. 1997), a Sm/Nd isochron of 422 Ma estimates based on mineral composition range (Li et al. 1995), an amphibole 40Ar /39Ar age of from 1.3 to 1.5GPa and 590 to 758°C; coesite 400Ma (Jian et al. 1997), and a muscovite of course would indicate P> 2.6 GPa. A Sm- 4OAr/39Ar age of 400Ma (Xu et al. 2000a). Nd isochron for gamet, omphacite,rutile, amphi- Sun et al. (2002) directly tied their c.310 Ma bole and whole rock is reportedly 400:t 16 Ma age to high-pressurerecrystallization with laser (although we are unable to reproduce this age ICP-MS trace-element analyses that demon- from the reported isotopic ratios). strated zircon growth while garnet was stable The Qinling continental arc can be traced east- and plagioclase was not stable. ward as far as Hong' an, where it disappears During and after the formation of the Qinling beneath Cenozoic sedimentary rocks (Figs 2 Andean-style volcano-plutonic arc, the southern and 4). No such arc is known from Sulu or margin of the NCB was dissected by sinistral Korea, but possible correlatives exist in the Kun- wrenching along the Lo-Nan and Shang- lun Mountains of northern Tibet (Matte et al. Dan shear systems at 420-380 Ma (Fig. 2) 1996; Yang et al. 1996). (Ratschbacheret al. in press). We assume that oblique subduction imposed these spectacular transDressivewrench zones. Muscovite and bio- Devonian- Carboniferous accretionary wedge tite 4t>Ar /39Ar ages ranging from 348 to 314 Ma A probable fossil accretionarycomplex, also with have been interpreted to reflect a continuation high-pressure rocks, lies south of the Silurian- of this strike-slip motion into the Permian (Mat- Earl~ Devonian arc in the Liuling and Sujiahe tauer et al. 1985). On a larger scale, the NCB unit$ (Fig. 3). The Liuling unit is a mixture of and SCB rifted from Gondwana in the Late siliciclastic and volcaniclastic rocks, amphibolite Devonian-Early Carboniferous (Li & Powell and minor carbonate (You et al. 1993). Correla- 2001). tive rocks include the Xinyang Group in Tongbai- Hong'an and the Foziling unit in Dabie. . .. . pevpnian fossils in the Tongbai area (Du 1986; Devoman-Tnasslc volcano-plutonIc arc Niu' et al. 1993) have been used to infer a It is unclear whether the inferred Devonian-Per- Devpnian age for the entire Liuling; however, mian subduction beneath the NCB produced a t:lu and Meng (1995) showed that several continental margin arc. Younger, Permian and conglomerate-bearing, volcaniclastic deposits, Triassic metaluminous, probably I-type plutons locally containing metabasalts and metacarbo- with low Sri ratios (Xue et al. 1996a) intrude nates, have proven Upper Devonian and Lower the Kuanping, Erlangping, Qinling and Liuling Carboniferous and suspected Carboniferous to units as far east as Xi'an (Figs 2 and 4). Coeval Permian ages. The Sujiahe unit mainly contains intrusions pierce the western edge of the Yangtze volcaniclastic rocks and 'melange' (Ratschba- Craton in the Xue Shan, Longmen Shan, the cher et al. in press). Songpan-Ganze flysch and the SCB cover Blocks of eclogitic rocks and retrograde south of the Qinling mountains (Fig. 1, Regional amphibolites occur in a shear zone 1-3 km Geological Survey of Gansu 1989, Regional wide near the southern edge of the Sujiahe unit Geological Survey of Shaanxi 1989). The plutons 5 - 20 km northwest of Tongbai. The eclogites intruding the SCB cover raise an important pro- con~ain gamet, omphacite, quartz, rutile, phen- blem, as they appear to be south of the inferred ~iteand barroisitic amphibole, and have yielded north-dipping subduction zone. East of Xi'an P-T estimates of 480-550 °C and 1.3-1.8 GPa this arc may end, disappearunder Cenozoic sedi- twe~ et al. 1999). Ye et al. (1994) reported Sm- mentary rocks, or be covered by younger thrust Nd tnineral/rock isochrons of 533 :t 13 Ma and slices. Middle Carboniferous to Early Permian $44114 Ma and a 40Ar /39Ar barroisite age of andesitesform a NE-trending belt acrossthe east- 399:it4 Ma. This belt of HP rocks in the Sujiahe em half of the NCB (Fig. 1) (Zhang 1997), but unit extends as far east as the Xiongdian area of the belt is surprisingly far north of and oblique Hong' an, where eclogite lenses or layers crop to the inferred NCB-SCB suture. put jn quartzite and felsic gneiss.These eclogites We suggest that the pre-collisional, Permo- con~ain minor glaucophane and phengite in CarboniferousNE trend of the NCB-SCB suture addition to gamet, omphacite and rutile, and in Tongbai-Hong'an-Dabie and the suture-arc fonned at Po::: 1.3-1.5 GPa, To:::590-680°C, distance were modified by the Triassic, syn- similar to tectonicblocks in the FranciscanComplex collisional clockwise rotation of the SCB
-
SUBDUCTION,COLLISION AND EXHUMATION 165 (Wei et at. 1998; Tsai & Liou 2000; Xu et at. only 400-800 MPa at 350-450 DC(Eide & Liou 2000b; Zhou et at. 2000; Liu et at. 2001); (3) 2000). While most of the evidence of UHP the Zhangbaling and Bengbu areas, a mostly derives from eclogites, the paragneissalso con- blueschist-faciesextensional complex (our obser- tains local unambiguous indications of meta- vations); and (4) the Sulu area (Wallis et at. morphism at similar pressure and temperature, 1999) (Fig. 1). such that the eclogites clearly were metamor- The evidence of Triassic HP and UHP meta- phosed in situ (Liou et at. 1996), the same may morphism comes chiefly from a few per cent of not hold true for ultramafic blocks, some of eclogite and garnet peridotite boudins that are which record recrystallization pressures>4 GPa hosted by paragneiss plus less granodioritic- (Okay 1994; Hacker et at. 1997; Liou & Zhang tonalitic orthogneiss (Fig. 6) (Cong 1996; Liou 1998). The spatial distribution and metamorphic et at. 2000). The highest temperaturesand press- conditions of these: HP through UHP rocks uresattained in the coesite-and, locally, diamond- indicate subduction~zonemetamorphism on a bearing Dabie eclogites were 825-850 DC and regional scale. 3.3-4.0 GPa (Carswell et at. 1997). The coesite- The age of UHP metamorphism in the Dabie ~ free eclogites reached somewhat lower peak Shan is now well constrained to be Middle- conditions of 625-700 DC and 2.2-2.4 GPa in Late Triassic (e.g. Hacker et at, 2000). The the Dabie Shan (Okay 1993; Liou et at. 1996; most recent data indicate that UHP recrystalliza- ~ Carswell et at. 1997),and have been divided tion may have begun as early as 245Ma and into kyanite-bearing and kyanite-absenteclogites extended through c.225-230 Ma (Okay 1993; in Hong'an, with estimatedphysical conditionsof Hacker et at. 1998; Hacker et at. 2000; Li et at. 550-650 DC, 1.6-2.5 GPa and 450-550 DC, 2000; Chavagnac et at. 2001; Ayers et at. 0.8~1.2GPa, respectively (Eide & Liou 2000). 2002). High-pressure rocks in Hong'an (the The amphibolite unit is a hornblende-richorthog- area least affected by Cretaceousreheating) indi- neiss; peak pressure and temperature estimated cate that the regional amphibolite-facies over- from one locality are > 1.0 GPaand ::::650 DC(Liu print at 500-650 DCand 0.8 GPa occurred during & Liou 1995). Blueschists reachedconditions of the 195- 225 Ma time span (40Ar /39Ar phengite
I ! , i i, Eclogites Garnet peridotites 8 7 Dable-5ulu te"ane , Dabie-5uluterrane I. EasternDable , II. CentralDabie " @Mantle-derIVedspinel III.Yangkou Beach, 5ulu, @peridotlte (5) and ' , " IV. Weihai, 5ulu , ' 6 G garnet peridotite (G) , G 5 NQ: North Qinling ,'UHP . Crustal maflc-ultramsfic b HW: Huwan .,. , ' complex ~""c~ ~e j,. , ,%0" ~'(' ~,,- 5 ~" - 4 ov, ,/ ~ - ~, DryEC tV '- Q. ,. . , --,(!) Lw-EC 4 ' , GI' " "", ~ 3 ' " , , m,' " j GI ' ... 3,'- / I Q. , i ,,' ~\e 2 " " 2 .j," I ~<;,-" ~o\\\8
~ II 1
0 0 200 400 600 600 1000 200 400 600 1000 1200 1400 Temperature °C km-1
Fig. 6. Metamorphic pressures and temperatures for eclogites and peridotites of the Qinling-Dabie-Sulu orogen (modified after Liou et ai. 1999; Zhang et ai. 2001).
, h
L " _Il" ... iI\ill I, 'I I 166 B. R. HACKERET AL I ,: ages). ~ost o.f the ~-f:ldsrar 40Ar/~9 Ar spectra exten.sionalexhumation of the HP- UHP rocks i have mflectIons mdicatmg cooling below remams unclear. I c.200 °C near 170Ma, although some rocks were The HP to UHP rocks of Dabie- Hong' an are this cool as early as 200 Ma. Such P- T paths, mostly a structural homocline with SE-dipping with concomitant decompressionand cooling are foliation, SE-plunging lineation, and overall .. only possibleif exhumationwas rapid or the slab top-to-NWflow that includessignificant coaxial was refrigerated by deeper level subduction stretching (Fig. 7). In northern Hong' an, how- (Hacker & Peacock 1995; Ernst & Peacock ever, the S-oriented structures rollover through 1996). The exhumation rate is only crudely con- horizontal into a 5-km-thick zone of N(W)-dip- strainedto c.3-l5 mm/year. ping foliations, N(W)-plunging lineations, and a " Geochronology shows that the Middle-Late N(W)-directed senseof shear. This, the Huwan Triassic thermal event associatedwith the UHP shear zone (Webb et at. 2001), effectively strad- metamorphism in the eastern Dabie Shan and dIes the Triassic suture. It developed out of the Hong'an areas extended as far north as the high-strain gneisses of typical Yangtze affinity i Erlangping unit in the Qinling area, the Xinyang in northern Hong'an, contains strongly retro- , 6 unit in theTongbai and Hong'an areas, and the gressedeclogite boudins of pre-Triassicage in i .. Foziling and Luzhenguangunits in the Dabie the Sujiahecomplex (see above),and dies out r Shan(Hacker et at. 1.998;Hacker et at. 2000). northwardin the phyllitic quartzitestypical of The absence of HP metamorphism means that the southern Liuling unit. The Huwan shear these northernmost units were not subducted zone can be followed - with interruptions by Cre- during the collision, but the Late Triassic meta- taceousplutons - into northern Dabie, where it is morphism is certainly related to the collision. truncatedby a large Cretaceouspluton. The pseu- dostratigraphyof the Hong'an area also defines a
series of kID-scale NW -trending synforms and t . .. antiforms that are overturned to the north in cen- Co uslonal defonnatlon tral Hong'an, and are upright to south facing in The Triassic NCB-SCB collision produced a southern Hong'an. The coesite-bearing eclogite distinct set of structuresthroughout the Qinling- unit forms the core of the northernmostantiform; Dabie orogen: NW-SE to N-S contraction by the lowest pressurerocks, blueschist, are present folding and thrusting Ithroughout the belt, sub- only on the south limb of this orogen-scalefold. horizontal NW -SE to N -S extension within The westward decrease in peak metamorphic core complexes along the northern margin of pressures reveals that the antiform plunges the South China Block, and dextral transpressive west. This antiform extends eastward into reactivation of existing shearzones in the Qinling Dabie where it is partially overprinted by the area (Fig. 5). dominantly Cretaceousintrusions and structures The UHP rocks in both Dabie-Hong'an of the Northern Orthogneiss (Ratschbacheret at. (Faure et at. 1999; Hacker et at. 2000) and Sulu 2000), and then terminates against the Tan-Lu (Wallis et at. 1999;Faure et at. 2001)form the fault. .. cores of structural d\)mes, exhibit a top-NW The large-scale Wudang Shan dome (Fig. 4) sense of shear and are overlain by extensional that lies west of the UHP Hong'an-Dabie area faults that exhumed die UHP rocks. At least in is an extensional core complex overprinted by Hong'an-Dabie, the entire crystalline core of foreland folding and thrusting. Deformation in " the orogen constitutes a normal-sense shear the Wudang Shan began with an early unre- zone c.15 km thick; *e Huwan shear zone, a solved, but possibly contractional deformation, normal-sense detachment that reactivated the was followed by sub-vertical contraction and plate suture, tops the ~xtensional allochthon in sub-horizontal N -S extension during blues- Hong'an (Hacker et I at. 2000; Webb et at. chist-greenschist-facies metamorphism of the 2001). Associated Triassic metamorphic core basement and basement-cover contact zone; complexes in the northern part of the SCB 40Ar /39Ar dating of syn- to post-kinematic horn- i.clude the Wudang Shan (Ratschbacheret at. blende and muscovite puts the extension at, or in press) and Zhangbaling-Bengbu (our unpub- prior to, 230-235 Ma. Subsequentfolding and lished data); whether the Lu Shan (Lin et at. thrusting of the basement and Palaeozoic cover 2000), Wugong Shan (Faure et at. 1996), sequenceoccurred during N-S contraction. Dongling (Grimmer et at. in press), and Jiuling The northern limit of the Triassic extensional Shan (Lin et at. 2001) basement uplifts within deformation (Fig. 4) is located at different the foreland SCB fold-thrust belt south and east positions along the Qinling orogen. In the of Hong'an-Dabie, which show a Triassic to Qinling mountains, the northern limit of exten- Cretaceousextensional overprint, are related to sion is the Wudang basement; the Douling unit
I I
i
1_- I"if' li8lm'c c I 'I' f Neogene sediments Northern Hong'an Central Eastern Dabie Shan Eocene sediments Northern Dabie Shan Lower Cretaceous sediments Early Cretaceous volcanics 125-137 Ma syenite to tonalite pre-Cretaceous cratonal cover sequence eclogite S1 coesite coesite absent foliation (¥) & lineation ( ) shear bands & shear zones foliation (¥) & lineation ( ) shear bands & shear zones foliation (¥) & lineation ( ) shear bands & shear zones fault/shear zone 115oE synform, antiform Qinling Gp suture Eastern foreland Meishan Gp metamorphic boundaries Sujiahe Gp Luzhenguang Gp lineation, showing sense Foziling Gp Xinyang Gp D287 of motion of upper plate 1 Erlan- t brittle stretch direction ping Gp l lt u Fau a L g F n Qinling Gp ta zi u 2 B Mo L nÐ Ð tia n us Xiao a 3 o T e c ta Sf e c r i C o
z
o
n e D287 D285 coesite eclogite Northern Orthogneiss C 1
3 coesite eclogite 2 quartz eclogite B S0
Yangtze D284 SSWbasement trend blueschist quartz eclogite D285 1
amphibolite S1 3 30oN S0 D284 B 2 L1 D283 Southern L1 Hong'an SE-NW trend Southwestern Dabie Shan Southeastern Dabie Shan D283 1 N fold & thrust belt bblueschistlueschist 3 fold & thrust belt L2 L1 2 Yangtze L1 Yangtze basementbasement
10 km foliation (¥) & lineation ( ) shear bands & shear zones foliation (¥) & lineation ( ) shear bands & shear zones foliation (¥) & lineation ( ) shear bands & shear zones Figure 7: Structural overview of the Dabie-Hong'an area (after Schmid et al. 1999; Hacker et al. 2000; Ratschbacher et al. 2000; Webb et al. 2001). Areas disturbed by Cretaceous plutons are not included in the synoptic stereonets. Stereonets from eastern foreland show dominantly brittle faults: (1), (2), and (3) indicate principal stress directions; B, fold axis; S0, bedding; and Sf, foliation. I 168 B. R. HACKERET AL.
is non-mylonitic and lacks significant Triassic sedimentation (Huang & Opdyke 2000) or metamorphism. In northern Hong' an, the unconfonnable deposition of coarse carbonate Huwan shear zone constitutes a lithosphere- breccias (Breitkreuz et al. 1994). Yin & Nie scale reactivation of the Devonian-Triassic (1993) proposed that the transition from marine subduction complex. In Dabie, the Huwan to continental sedimentation was diachronous: detachment cannot be mapped, as Cretaceous Early Permian in Shandong and Korea, Lower/ tectonism has obliterated earlier fabrics, but the Upper :Permian south of Sulu, Lower/Middle northern limit of extension must be south of Triassic SE of Dabie. the Foziling and Luzhenguang units, which Nie ~t al. (1994) and Zhou & Graham (1996) have clear Yangtze craton affinity and were proposed that detritus eroded from the active unaffected by HP metamorphism, very likely Qinling - Dabie mountain range was channeled coinciding with the Early CretaceousXiaotian- westward and deposited to fonn the Songpan- Mozitang crustal-scaleshear zone (Hacker et al. Ganze flysch from the Anisian through the 2000; Ratschbacheret al. 2000). Norian (c.240-2 10 Ma). Bruguier et al. (1997) Much of the area shown in Figure 2 has found detrital zircons with U /Pb ages of 233 WNW-trending folds and craton-directedthrusts and 231 Ma within the Songpan-Ganze flysch . of millimeter to kilometre scalethat fonned during that m~tch the ages of zircons found in Dabie the collision (Mattaueret al. 1985; Regional Geo- (Hacker et al. 2000). logical Surveyof Henan 1989;Regional Geological Post-collisional, Jurassic, continental clastic Survey of Shaanxi 1989; Regional Geological sedimentationon both cratons was accompanied Survey of Hubei 1990). On the NCB, the north- by deposition of up to 5 km of calc-alkaline, crus- directed Lu Shan thrust, placing crystalline base- tal-derived, intennediate-composition volcanic ment over Palaeozoic cover, was active in the rocks: tuff, volcanogenic sandstone,and some Middle Triassic and Late Jurassic (Huang & lava in the Late Jurassic(Regional Geological Sur- Wu 1992). Huang and Wu (1992) also identified vey of Anhui 1987;Regional Geological Surveyof a seriesof Mesozoic S-directedthrusts imbricating Henan J989;Regional Geological Surveyof Hubei the Sino-Korean basement, Kuanping, Qinling, 1990). Gneiss cobbles, presumably derived from Erlangping and Douling units. The fold-thrust the Dabie Mountains, first appearedin the Zhuji belt along the Lower Yangtze river began to Fonnation on the northern slope of the Dabie develop through NW -SE contraction in the Mountains in Middle Jurassic time (Ma 1991). Middle Triassic and continued through the Middle Jurassicsedimentary rocks in the Lower Early Jurassic, by which time the shortening Yangtze fold-thrust belt show clear evidence of direction had rotated to NNE-SSW (Schmid erosionof the Dabie UHP core, including Triassic- et al. 1999). Jurassiqdetrital micas with high-Si contents and The Early Palaeozoiccore of the Qinling oro- zircon grains as young as c.218Ma (Grimmer gen was reactivated during the relatively early et al. ~ press) their oldest mica ages indicate that stages of the NCB-SCB collision by overall exhum~ion in Hong'an-Dabie might have started top-south imbrication during a change from at 240 t 5 Ma. NW -SE (dextral transpression) to NE-SW . shortening. Ratschbacheret al. (in press) docu- . . mented c.200-250 Ma low-grade metamorphism Plate-scale collision model and dextral transpression along the Lo-Nan, While there is still much that we do not know, Shang-Xian and Shang-Dan faults, and N-S some key structural observations constrain the shortening within the Liuling and Douling units. exhum~tion mechanism of the UHP rocks (Hacker et al. 2000): Syn- to post-collisional overlap assemblage (I) Unfolding the 'Hong' an antifonn' - the Changes in sedimentation patterns are often orogen-scalefold trending NW -SE across among the best guides to collisional timing. Hpng'an-Dabie - yields aN-dipping Depositional facies up through the Permian lithospheric slab with coesite eclogite in trend E- W in the southern NCB, and NNE in the north and lower pressurerocks progress- the northern SCB (Han et al. 1989; Sun et al. ively farther south and closer to the interior 1989; Wang et al. 1989; Zhang et al. 1989). of the SCB. The upper boundary of the slab Although collision-related (?) shortening began is: the Huwan shear zone, which encom- on the SCB in the Early to Middle Triassic, p~sses the suture between the NCB and the major sedimentological change occurred t~ SCB, implying that it is essentially a in the Middle Triassic, which was marked plate boundary reactivated as a litho- locally by either a depositional hiatus, continental sphere-scale,nonnal-sense shear zone.
J'C : . : j
I
I 170 B. R. HACKERET An. two plates as the result of Late Pennian-Early ed~e of the SCB. In Dabie, it lies south of Triassic indentation of the NCB by the SCB; th(j Luzhenguang unit. This implies that they rationalized that the active margin of the the Douling, Luzhenguang and other units NCB should have been relatively straight and (i) represent the leading edge of the SCB that the passive margin of the SCB could have but were not subducted;(ii) were originally been relatively complex. Gilder et al. (1999) cri- south of the HP- UHP rocks and ended up ticized this idea becauseof a 'lack of significant in their presentposition as a result of exhu- folding of Phanerozoicrocks. . . north of the Sulu mation of the HP- UHP rocks; or (iii) are belt', and proposed instead that the older NCB part of the NCB (although of Yangtze affi- acted as a rigid indentor to deform the SCB in nity, like the Qinling unit), but are now the Early to Middle Jurassic - leading to the mysteriously south of the Liuling, Xinyang palaeomagnetically recorded bending of the an(l Foziling units and escapedintrusion by Lower Yangtze fold-thrust belt (Gilder et al. the Qinling arc. 1999), formation of the Tan-Lu fault, and 600 (4) A weaknessof the exhumation model out- of relative rotation between the NCB and SCB lined in the previous section is that we (Zhao & Coe 1987). While there is structural have not identified a sole thrust along the and thermochronological evidence of Late Cre- southern and eastern edges of Dabie- . taceous and Cenozoic strike-slip and normal Hong' an. SE-directed extrusion of the faulting (Ratschbacheret al. 2000; e.g. Grimmer w~dge of UHP-HP rocks should have et al. 2002), no well-documented structural data induced equivalent shortening at the tip of demonstratesinistral Triassic or Jurassic motion thcrextruding wedge, but one of the charac- along the Tan-Lu fault. However, Mid-Triassic tetjistics of the Triassic/Jurassic foreland NW -SE contraction and Late Triassic-Early de~rmation eastof Dabie is upright folding Jurassic N -S contraction in the eastern Dabie an a fo~land (Schmi~ et al. 1999) .implies ~at if the c? bination o.f hin~erland-.and foreland- Tan-Lu fault eXIstedat that time then It would di cted thrusting WIth relatively moderate have been sinistral. Note that the age of Triassic shprtening (Schmid et al. 1999; Grimmer metamorphism in the Qinling area is similar to et ial. in press). To explain the apparent that in Dabie; the proposed younging and west- laqk of the mega-thrust and the distinctive ward migration of collision (e.g. Zhao & Coe stljuctural style, we can envision at least 1987; Yin & Nie 1993; Zhang 1997) is not sup- thtee scenarios. (i) The leading edge of ported by extant geochronology (Hacker & th~ UHP-HP rocks forms a wedge that is Wang 1995). buried beneath the cover strata of the fore- land. This implies that the crust of the fore- .. land was detached at middle to upper Problems and possible solutions c~stal levels and thrust northwestward on The preceding sections have outlined a number tol the crystalline core. (ii) The UHP-HP of important problems within the Qinling - rocks were exhumed by buoyancy into !Dabie orogen. the middle/lower crust, and the overlying " ! rocks were removed by extension and ero- 1(1) The boundary between rocks of Sino-Kor- sil1>n.A plate rotation model proposesthat ean and Yangtze affinity appears to be a af(er initial collision in the easternpart of the suture of Ordovician-Silurian age. This ott>gen,continued closure of the 'Songpan- " interpretation could benefit from further G~ze Sea' by subduction of oceanic litho- analysis of the tectonic histories of key spherecaused the SCB to rotate 60 0 clock- units in the Qinling orogen. wise (required by palaeomagnetic data),
1 (2) Devonian-Triassic rocks of the Liuling, pulling parts of the subducted SCB back Xinyang, and Foziling units separate the toward the surface. NCB from the SCB. Theserelatively mono- tonous units thus potentially experienced quite a varied history. Can this history be read from these rocks?
1 (3) The northern boundary of Triassic exhuma- Summary tion is generally south of the northern edge Grenvillian orogeny, involving oceanic subduc- of the SCB. In the Qinling mountains, it lies tion ~ith HP metamorphism and ophiolite south of the Douling unit and south of the emplacpment(Songshugou ophiolite), assembled Yangtze craton cover. In Tongbai and the Ya1)gtzecraton, including the Qinling micro- Hong'an it coincides with the northern contint1lit, into Rodinia. Rifting at c.750 Ma
I '] - SUBDUCTION,COLLISION AND ~ATION 171
separatedthe Qinling microcontinentfrom the CAO,R. ~ ZHU,S. 1995.The Dabieshancoesite-bear- Yangtze craton. Intra-oceanic arc formation ing eqlogiteterrain - a neo-Archeanultra-high (Erlangping-Danfeng-Heihe) between c.490- press~ metamorphicbelt. Acta GeologicaSinica, 470 Ma was followed by the accretion of the 69, 2~2-242. lower Qinling unit to the intra-oceanic arc and CARSWE~,D. A., O'BRIEN,P. J., WILSON:R. N.. & the North China Block. Oceanward (northward ZHAI~M.~997. The~obarometIJ°fphenglte-~ng in present coordinates) subduction beneath the eclogitesm the Dable.MountanIs of centralChina. rth L . 1. ... Jounl41of MetamolphlcGeology, 15, 239-252. no em ill mg urnt. lmpnnted the c.400 Ma CHAVAGNAC,V. & JAHN,B.-M. 1996.Coesite-bearing An~ean-type mag.matlc arc o~ t~ the North eclogitesfrom theBixiling Complex,Dabie Moun- Chma Block. OblIque ~ubductlon Imposed the tains,China; Sm-Nd ages,geochemical character- spectacularEarly Devornan left-lateral transpres- istics£d tectonic implications. Chemical Geology, sive wrench zones. A subduction signature is 133, 9-51. again evident during the mid-Carboniferous to CHAVAGAC, V., JAHN, B.-M. VILLA, I. M., Late Pennian on the North China Block, when WHIT~HOUSE,M. & LIU, D. 2001. Mu1tichrono- the Palaeo-Tethyswas subductednorthward, pro- ~etriq evidencefor an in. situ origin of the ultra- " ducing the andesitic magmatism on the North hig.h-pressure metamorphic terrane of Dabieshan, fhina Bloc~. In the Late Pennian-E~ly Trias- Chinaf Journal of Geology, 109, 633-646. SICthe leadmg edge of the South China Block CHEN,D'i Wu, Y., WANG,Y.-J., ZHI: X., ~A, Q. & was subducted to >150km and subse uentl ~~Nt J' 1998. .Age~, Nd and S~ IS.otOpl~ compo- . . q. y sItton of the JIaozlyan gabbrolc mtruslon from exhu.med by crustal extensIon dunng clockwIse the n rthern Dabie terrain. Scientia Geologica rotatIon of the craton. Sinic ,7,29-35. CHEN,J., E,Z.,LIU, S.,LI,X. &FOLAND,K.A.1995. Thanks to Yongjun Yue, Stanford, for assistancewith COOl~ g age of Dabie orogen, China, determined Chinese publi~ations; Shuwen Dong, Yueqiao Zhang and ?y 40 _39 ~ and fission track techniques. Science manyother Chinese colleagues for field guidanceand discus- m Ch na, SenesB, 38, 749-757. sions;our colleaguesJ. C. Grimmer,L. Franz,R. Schmid, CHEN, ~., RAN, Y., You, Z. & SUN, M. 1992. E. Enkelmann and R. Oberhiinsli for discussions;and Whoi-rock Sm-Nd, Rb-Sr and zircon Ph-Ph C. Blake, P. Robinsonand C. Fletcherfor useful criticism. datin of the metamorphic complex in the inte- Fundedby projects Ra442/14, 19 of the DeutscheFor- rior f Qinling orogenic belt, western Henan, schungsgemeinschaftand NSF grants EAR-9796119, and~eir im.plications with regard to crustal EAR-9725667and EAR- ~ _._~ '--. J' j 1?2 B. R. HACKERET AL. ERNST,W. G. & PEACOCK,S. M. 1996. A thennotec- HACKER,B. 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