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Journal of the Geological Societv. London, Vol. 153, 1996, pp. 409-417.

Palaeozoic pre- and post-collision calc-alkaline magmatism in the Qinling orogenic belt, central China, as documented by zircon ages on granitoid rocks

F. XUE'.*,',A. KRONER', T. REISCHMANN' & F. LERCH' 'Institut fiir Geowissenschaften, Universitat Mainz, 55099 Mainz, Germany 'Department of Geology, Northwest University, 710069 Xi'an, China .'Present address: Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA

Abstract: Basedon large-scale reconnaissance mapping, we identifiedtwo calc-alkaline plutonic assemblagesfrom the northern Qinling orogenic belt.central China. The older assemblage of intrusions. closely associated and deformed coevally with their host sequences, seems to represent the fractionation product of basaltic arc . It therefore predates the collision of the North China Block withthe Central Qinling island-arc system that developed in a SW Pacific-type oceanic domain south of the North China Block. Single-zircon zo7Pb/2'"Pb evaporation dating yielded early to middle Ordovician ages for this assemblage. with a relatively small range from 487.2 f 1.1 to 470.2 f 1.3 Ma. Intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of theolder assemblage. They are interpreted as post-collisional calc-alkaline granitoids.Single zircon dating provided an age of 401.8 f 0.8 Mafor the younger assemblage. consistentwith earlier work that defines an age range from c. 420 to 395Ma. Our datafavour a tectonicmodel involving formation and amalgamation of islandarc and microcontinent terranes between ca. 490 and 470 Ma ago to create the Central Qinling Zone which subsequently collided with theNorth China Block prior to c. 400Maago. A latePrecambrian age of 762.0 +0.7 Ma for a granitoid gneiss at the northern margin of the Yangtze Block supports a Gondwana affinity for this large continental block.

Keywords: Qinling. , calc-alkaline composition, zircon, absolute age.

The Qinling orogenic belt separates the North China Block evaporation method. We use these age data, combined with in the north from the Yangtze Block or South China Block othergeological lines of evidence,to limit theperiod of in the south and is one of the major orogenic beltsin eastern Qinlingocean closure, terrane amalgamation and collision Asia. Many models have been proposed for the evolution of betweenthe Central Qinling Zoneand the North China this belt (Klimetz 1983; Zhang et al. 1984: Mattauer et al. Block andthus provide new constraints on the tectonic 1985; Hsii et al. 1987: Zhang et al. 1989; Reischmann et al. evolution of the Qinling belt. 1990; Kroner et al. 1993: Xue et al. 1995), and most of these agreethat major continental collision occurred in late Triassic time, following closure of a long-lived and, judging Geological setting and tectonics frompalaeomagnetic data, rather wide palaeo-ocean The Qinling Mountains can be divided, from north to south, (Palaeotethys) between the North China and Yangtze Blocks. into four tectonic zones, namely the Lesser, North, Central Prior to this ocean closure in the Triassic, a number of and South Qinling Zones (Fig. 1). The Lesser Qinling Zone continental and arc terranes were amalgamated in the early has a crystalline basement consisting of Archaean to lower Palaeozoic and constituted what is now the Central Qinling Proterozoicrocks, overlain by a middleProterozoic Zone (see Fig. l). During this process the so-called Qinling continentalbasin sedimentary sequence (Sun et al. 1981) ocean, a smallocean basin, or severalbasins, extending and a succeedingupper Proterozoic to middle Ordovician betweenthe North China Block and the terranes now passive continentalmargin sequence (Wang 1991). The makingup the Central Qinling Zone, was consumed, and NorthQinling Zone consists of metasedimentaryand theCentral Qinling Zone finally collidedwith the North metavolcanic rocks. Besidesgeochonological work on China Block, probably in the late Ordovician (Reischmann detritalzircons that limits thedepositional age of the et al. 1990: Kroner et al. 1993; Lerch et al. 1995a, b; Xue et metasediments in this zone between 638f 7 and 484 f 7 Ma al. 1995). Calc-alkaline magmatism has been reported from (Lerch 1993), fossil data and sedimentologic work in a least the axial part of the Qinling belt (Liu et al. 1989: Zhang & deformedand metamorphosed domain shows that these An 1990; Xue et al. 1993), but a precise age assessment of sediments are late Proterozoic to early Palaeozoicin age and this magmatic event over the entire Qinling belt has been so were deposited in continental shelf and slope environments farlacking although rare fossils placesome constraints on (Zhou 1991). Theyprobably constituted passive a the age of this magmatism (Zhang & Tang 1983; Xiao et al. continentalmargin along the southern edge of theNorth 1988),and some geochronological work was performed China Block, together with coeval sediments in the Lesser locally (Li et al. 1989; Lerch 1993). Basedon large-scale Qinling Zone (Wang 1991). This passive continental margin geological mapping, we determined the isotopic ages of a set underwent deformation and metamorphism when it collided of granitoidsamples from various localities along the with theisland arcs and continental blocks of theCentral CentralQinling Zone, using thesingle-zircon Pb-Pb Qinling Zone. 409

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Fig. 1. Tectonic sketch map of the Qinling Orogenic Belt, modified from Xue er al. (1995). with rectangles showing position of Figs 2-4. Also shown is location of sample Q89/5. Inset delineates major crustal blocks in China and location of Fig. 1.

The central Qinling zone, the most complex amongst the continental block in the south (Figs 1 & 2). Deformation is four zones. has been variously described as the central part relativelyweak away from the Zhongping thrust, the of a Palaeozoic collision orogen (Mattauer er al. 1985) and southernboundary of thevolcanic arc (Fig. 2). as a tectonicmelange (Hsii et al. 1987). Newdata suggest comprise a major part of the volcano-sedimentary sequence, that it maybe appropriate to subdivide this zoneinto but , dacite and chert are also observed, especially separate island-arcs and continental block or terranes rather in the upper part of the sequence. Turbidites with occasional than consider it as one geological entity (Xue et al. 199.5). In daciticlayers occur near the top of thesequence. Trace the study area, these include the Qinling continental block elementgeochemistry suggests that these basalts were andthree volcanic arc terranes (Fig. 1). The Qinling erupted in anisland arc setting. Island-arc tholeiites, continentalblock. commonly designated as the Qinling characterized by flat REE patterns and selective enrichment Complex or Group in theChinese literature (Xiao et al. of LIL elements, occur as interlayers in calc-alkaline basalts 1988; Zhang et al. 1988), consists of various felsic gneisses that are LREE-enriched and display an element distribution andamphibolites and slices of ultramaficrocks. A pattern typical of calc-alkalinebasalts (Xue et al. 1995). tectonothermal event of late Proterozoic age is documented Numerous intrusions of tonalitic, trondhjemitic and doleritic throughregional metamorphism, granitic intrusion and the compositionare closely associated with thesequence (Fig. tectonic incorporation of an ultramafic slice into the block 2).Trondhjemites contain 40-50 vol. Yo plagioclase (You et al. 1991: Li et al. 1991). The three volcanic arcs are (An,, IJ), 20-30 vol. Yo quartz, 2-5 vol. 'X0 biotite and rare described below. K-feldspar.Tonalites have 40-45 vol. % (AnxdO), 15-25 TheSouth Qinling Zone,situated along the northern vol. To hornblende, 10-15 vol. YO quartz and 5 vol. '/O biotite margin of the Yangtze Block, is bounded in the north by the and opaque minerals (Lu 1988). These tonalites are usually Shang Dan Fault Zone (Fig. 1). A thickpile of uppermost porphyritic with phenocrysts of plagioclase and hornblende Proterozoicmiddleto Triassic sediments covers the and a matrix of finer-grained plagioclase and quartz. Based pre-latest Proterozoic crystalline basement. Early Palaeozoic onpetrological features and trace element contents, Lu alkalinemagamatism and late Palaeozoic Triassicto (1988) and Liu et al. (1989) suggested that these granitoids differentialdepression (Shaanxi Geological Bureau 1989) constitutethe products of fractionalcrystallization of indicate that this zone was severely attenuated prior to its basalticmagma represented by thehost basalts. This is in deformation.Both the cover sequence and the basement accordance with the observation that these intrusions occur were strongly shortened during the late Triassic to Jurassic closely associated with thebasalts and were deformed (Mattauer er al. 1985: Matte & Xu 1988: Shaanxi Geological together with the sequence. Bureau 1989). Pre- and post-collisional calc-alkaline magmatism in The Heihe volcanic arc the Central Qinling Zone This is locatedsouth of ZhouzhiCounty town, and is separated by theChenhe breccia fault zone from the The Erlangping volcanic arc meta-volcanosedimentarysequence of theNorth Qinling This is sandwichedbetween the metasediments of the Zone to its north and bounded by the Shang Dan Fault in Lesser-Northern Qinling zones in the north and the Qinling thesouth, a majorsinistral strike-slip fault zone (Fig.3).

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\\\\++++++++++c.

Fig. 2. Geological map of the Erlangp- ing volcanic arc in the Zhongping- Xizhuanghe area, modified from Lu (1988).

Slightly metamorphosed monotonous turbidites of unknown with voluminousgraywackes; it mayhave formed in an origin were thrust upon the Shang Dan Fault and the Heihe islandarc tectonic setting. This is alsosupported by the volcanicarc (Fig. 3). The Heihe arc itself forms a large geochemical features of the basalts in the sequence (Lerch syncline (Inset 2 in Fig. 3). The stratigraphic profile restored 1993; Lerch et a/. 199.5) which are rather similar to those of along the syncline axis is shown in Inset 1 of Fig. 3, which the Erlangping arc volcanics (Xue et al. 199.5). Compositions shows sharp horizontal and vertical lithologic changes. From of the clasts in the conglomerates and enclaves in show thebottom to thetop, pillow basaltsare covered by theymay be derived from the underlying rocks, hence greywackes,andesitic to daciticpyroclastic rocks that excluding the possibility of the Qinling complex serving as laterally change into and greywacke, a thick layer of the basement of these volcanic rocks. limestone, conglomerates composed of various volcanic and sedimentary clasts, turbidites and andesitic pyroclastic rocks thathave intercalated marly beds and laterally change to The Danfeng volcanic arc greywackes.The sequence consists principally of basaltic, Thisconstitutes an elongated lens extending from west of andesitesand dacitic agglomerates and tuffs. interlayered Shangxianto Shangnan (Fig. 1). Itwas stitched to the

I I Fig. 3. Geological map in the Heihe area. Inset 1 gives stratigraphic E-W profile of the Heihe volcanic arc. restored along the synclinal axis. Inset 2 shows cross-section along line A-B.

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Qinlingcontinental block by Mesozoicgranites and is bounded in the south by the Shang Dan Fault (Fig. 4). The rockswere metamorphosed to amphibolite facies in the easternpart (Xue 1988) butonly experienced greenschist facies metamorphism in the western part (Xiao er al. 1988). Figure 4 is a sketch map of the volcanic arc in the Taohuapu area. On this map the southern part is underlain mainly by metavolcanic rocks and interlayered chert beds. One of the chert beds was mapped as a continuous layer for more than 6 km. More than 30 YO of the volcanic sequence was derived fromintermediate to felsic volcanicprecursors, while the P other part may originally have been basaltic volcanics (Xue 1988). Thenorthern part mainly includes schists, para- amphibolites and impure marble. Similar to basalts from the Erlangpingand Heihe volcanic arcs. thebasalts from the Danfengarc complex show geochemical features charac- teristic of an environment (Xue ef al. 1995). These volcanicand sedimentary rocks are intruded by numerous trondhjemitic sills, tonalites.gabbros and rare pyroxenites. These intrusions are closely associated with, and underwent thesame deformational and metamorphic history as, the volcano-sedimentary sequence. It is likely, on this basis, that both the metavolcanic and plutonic rocks are derived from P the same mafic source through fractional crystallization. An Fig. 5. QAP diagrams for older (Sa) and younger (5b) intrusive exception to this is a monzogabbroic intrusion 3 km north of assemblages in the Central Qinling Zone. Shadcd area and curves 1 Taohuapu village, whichcuts the foliation of the Danfeng to 8 are from Lameyre L? Bowden (1982). Curves show median arc complex (Fig. 4) and is clearly undeformed. It is grouped trends in various plutonic suites: 1. tholeiitic, Troodos and with ayounger generation of granitoidplutons described Skaergaard: 2. calc-alkaline trondjemitic. SW Finland; 3. 4, S and 6. below. calc-alkaline granodioritic (medium-K). Chile, Peru and Sierra A closeA spatial association of volcanicrocks with Nevada: 7. calc-alkaline monzonitic (high-K), Vosges and Corsica; granitoid and mafic intrusions is commonly observed in the 8. alkaline silica oversaturated trends for Niger. Nigeria, Oslo. volcanicarcs described above. In this paper. we referto Corsica and Kerguelen Islands: 8'. aluminous trend in Nigeria- theseintrusions as the 'older intrusive assemblage'. This Niger. Shaded area is for granitic rocks of crustal origin. assemblage, when plotted on a QAP diagram (Fig. Sa), falls intothe tholeiitic and calc-alkaline granitoid series of Lameyre & Bowden (1982). Thesetholeiitic and calc- alkalinetraits are similar to those of theirhost volcanic discovered the widespread occurrence of a younger plutonic sequencesand support the view thatthe plutonic and assemblage.These intrusions, largely undeformed and volcanic associations are genetically related. unmetamorphosed,truncate earlier deformation and meta- Alongtheentire Central Qinling Zone we also morphicfeatures that were imprinted on the volcanic sequences and the older intrusive assemblage. Additionally, they are quite different from the Mesozoic leuco-granites in petrologicalcharacteristics (Fig. 1). In theHeihe area, where this younger plutonic assemblage is well developed, it definesa compositional spectrum ranging from norite. via gabbro,diorite, tonalite, togranodiorite (Fig. 3). The Lajimiaonorite-gabbro intrusion in thearea southwest of Shangxian (Fig. 1) cuts the foliation of the Danfeng volcanic arccomplex and is clearlyamember of thisintrusive assemblage(Zhang & An 1990). Themonzogabbro in the Taohuapu area (Fig. 4) may also belong to this assemblage. Harris et al. (1986) recognized four groups of granitoid intrusionsfrom various tectonic settings in collisionbelts. which are:(i)pre-collision calc-alkaline arc intrusions (Group I); (ii) syn-collision peraluminousleucogranites ;::;:"W 0;::;:"W (group 11): (iii) late to post-collision calc-alkaline intrusions (Group 111) and(iv) post-collision alkalineintrusions Brp5P (Group IV). With only some slight difference in a few trace Mesowr granite elementratios, group 1 and 111 areextremely difficult to - chen honzon distinguish, using only petrological or geochemical data (as monzogabh m m+-pyroxenlte / lauh inFig. 5). However,a careful correlation of thetime of Fig. 4. Sketch map of the Danfeng volcanic arc in the Taohuapu collision and age of granite intrusion may help to assess the area. 15 km east of the Danfeng county town. modified from Xue tectonic settings of the calc-alkaline granitoids in the Qinling (1988). Belt. The older assemblage of granitoids intruded prior to

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Table 1. Pb isofopic data for single grain zircon evaporation

Sample Zircon colour & Mass Grain Evaporation Mean ro7/Pb/2'"Pb Zo7Pb/Z'KPb age (Ma) no. morphology scans* temp. in "C ratio? and 2cr(m) and 24m) error error

Q89/ 15 Short-prismatic. 1 66 1560 0.056695 f 30 479.7 f 1.2 idiomorphic. pink 2 66 1S80 0.056700 f 19 479.9 f 0.7 light to dark brown 3 88 1S90 0.056700 f 26 479.9 f 1.0 4 87 1598 0.056697 f 5 1 479.7 f 2.0 5 95 1598 0.056696 f 50 479.7 f 2.0 6 116 1596 0.056696 f 39 479.7 f 1.5 Mean 1-6 0.056697 f 16 479.8 f 0.6

Q89/26 Short. stubby. idiomorphic. 1 63 1 598 0.056462 f 76 470.5 f 3.0 prismatic. dark red 2 62 1 S96 0.056459 f 75 470.4 f 2.9 brown 3 88 1595 0.056443 f 50 469.8 f 1.9 4 58 1S96 0.056451 f 63 470.1 f 2.5 Mean 1-4 219 0.056454 f 33 470.2 f 1.3

089/1 Short. stubby, idiomorphic. 1 110 1575 0.056918 f 54 488.3 f 2.1 red brown 2 84 1584 0.056990 f 43 487.5 f 1.7 Mean 1&2 194 0.05691 f 25 488.0 f 1.4

CH 1557 Long-prismatic, 1 95 1S80 0.056898 f 42 487.5 i 1.6 idiomorphic. clear to 2 66 1587 0.056872 f 41 486.6 f 1.6 yellowish 3 40 1 585 0.056897 f 71 487.5 * 2.7 Mean 1-3 201 0.056889 f 28 487.2 f 1.1

Q89/25 .ong-prismatic, 1 136 1595 0.054738 f 28 ,401.5 f 1.1 idiomorphic. clear 2 120 1604 0.054761 f 33 402.4 f 1.4 3 86 1602 0.054736 f 39 401.4 f 1.6 Mean 1-3 342 0.054745 f 19 401.8 f 0.8 Q89/5 .ong-prismatic. 1 80 1580 0.064653 f 32 763.1 f 1.1 idiomorphic. dark red- 2 66 1575 0.064645 f 37 762.8 f 1.2 brown 3 66 1580 0.064559 f 37 760.0 f 1.2 Mean 1-3 212 0.064621 f 20 762.0 f 0.7

* Number of 2"7Pb/Z'HPb ratios evaluated for age assessment. t Observed mean ratio corrected for non-radiogenic Pb where necessary. Errors based on uncertainties in counting statistics.

the deformation related to collision, and these rocks are, as a massive granodiorite that intruded into the gneissic trondhjemite discussedabove, very possibly differentiation products of Q89/26(Fig. 3). An additional sample, Q89/5, aleucogranitic the arc volcanic volcanics. Therefore, they are classified as gneiss,was taken from the Yaolinghe Complex southeast of Group I pre-collisional calc-alkaline arc intrusions in terms Shangnanand represents the basement of theYangtze Block of Harris et al. (1986). On theother hand, the younger (Henan Geological Bureau 1989) (Fig. 1). assemblage intrusions are post-collisional and may belong to theGroup-I11 calc-alkaline plutons of Harris et al. (1986). Single zircon evaporation We also notice that a few of the samples from the younger Kober (1986) has shown that the Pb components with the highest assemblage are slightly more potassic, providing supportive activationenergy normally reside in theundamaged crystalline evidence for a small degree of crustal involvement (Fig. Sb). phase of zirconthat shows no post-crystallization Pb-loss and thereforeyields concordant z'J7Pb/2"Pb ages. Pb-phases due to radiation damage (metamict zones) have low activation energy and Samples for zircon dating areremoved during low-temperature evaporation. The method involvesrepeated evaporation and deposition of Pb-isotopes from Sampleswere collected from various parts of the Central Qinling chemicallyuntreated single zircons in adouble-filament arrange- Zone in order to determinethe ages of thismagmatism. Four of ment(Kober 1987), and our laboratoryprocedures as well as them,089/l5, Q89/26. Q89/l and CH1557, are from the older comparisonswith conventional and ion-microprobe zircon dating intrusiveassemblage. Q89/15, collected near Xizhuanghe (Fig. 2). are published elsewhere (Kroner & Todt 1988; Kroner et al. 1991). north of Xixia (Fig. l), is a syntectonic trondhjemite intruded into Isotopicmeasurements were carried out ona Finnigan-MAT 261 basalts, but the trondhjemite and its host basalt clearly underwent mass spectrometer at the Max-Planck-Institut fur Chemie in Mainz. the main deformation together. Q89/26, taken four km southwest of Nocorrection was made for mass fractionation which is lessthan Chenhe (Fig. 3), is a gneissic trondhjemite in the Heihe area. Q89/1 onepermille (Kober 1987), significantly less than the relative and CH 1557, respectively, are a tonalite stock and a trondhjemite standard deviation of the measured Z07Pb/2"hPb ratios (see Table 1) dyke in thebasalts of theDanfeng volcanic arc complex (Fig. 4). andinsignificant atthe age range considered in thisstudy. The younger intrusive assemblage is represented by sample Q89/25. Measurement of achip of CurtinUniversity SHRIMP I1 zircon

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Age in Ma Age in Ma 32014?0 4:5 450 4?5 5yO 475 500 525 550 80 Meanage: 488.0 f 1.4 Ma Meanage: 479.8 f 0.6Ma g 60 $n...... I... : :.. a a ._ ...... Grain 1, c 66 ratios m Q8911 L :.: Grain 2, 66 ratios Grain 1, 110 ratios

T 0.055 0.056 0.057

400 425 450 475 500 460 480 500 80 I

Meanage: Meanage: 487.2 f 1.1Ma 470.2 f 1.3Ma P Q89126 .-c Grain 1, 63 ratios E CH1557 n Grain 2, 50 ratios (D =Grain 1, 95 0 1 Grain 3, 62 ratios 2 3oI Grain 2, 66 )c Grain 4, 44 ratio 0 - 0 Grain 3, 40 ratios 01- 0 b a 5 Z 15-

0.055 0.056 0.057 0.0560 0.0565 0.0570 (207Pb@06Pb)* (207Pb/206Pb)* Fig. 6. Histograms showing distribution of radiogenic lead isotope Fig. 7. Histograms showing distribution of radiogenic lead isotope radios derived from cvaporation of single zircons from granitoid radios derived from evaporation of single zircons from granitoid samples belonging to the older intrusive assemblage. (a) Spectrum samples belonging to the older intrusive assemblage. (a) Spectrum for six zircon grains from the trondjemitic sample Q89/15. for two grains from tonalitic sample Q8Y/1, Taogou, Danfeng, Xizhuanghe. Xixia, integrated from 518 ratios. (b) Spectrum for integrated from 194 ratios. (b) Spectrum for three fractions of two four grains from trondhjemitic gneiss sample Q89/26, Heihe area, small grains each from foliated trondjcmitic dyke sample CH1557, integratcd from 219 ratios. For sample locations see Figs 2 & 3. Taohuapu area, Danfeng, integrated from 201 ratios. For sample locations see Fig. 4.

standard C/Z3yielded a 2"7Pb/2"hPb ageof 564.8 f 1.4 Ma, identical to the adopted age of 564 for this standard (Pidgeon er al. 1994). observed. Only data from the high-temperature runs or those with In our experiments evaporation tempcratures weregradually no changes in thePb-isotope ratios were considered for increased in20-30°C steps during repeatcd evaporation-deposit geochronologic evaluation after testing for statistical outliers (Dixon cyclesuntil no further changes in the2"7Pb/2'KPb ratios were 1950). The calculated 2'17Pb/2'KPb ratiosand their 2u (mean) errors

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Age in Ma Results and discussion 325 350 375 400 425400 375 350 325 Thefour samples of theolder assemblage give similar Ordovicianages with arelatively small rangebetween 488 Mean age: and 470 Ma. The zircons from trondhjemite sample Q89/15 aredark red-brown. long-prismatic and idiomorphic. Six grainswere evaporated individually andyielded identical '"'Pb/'"hPb ratios which combine to a mean '"'Pb/""hPb age g 120 Q89125 ll of 479.8 f 0.6 Ma (Table l, Fig. 6a). Four small, light brown, stubbyand idiomorphic zircons from sample Q89/26. the Grain 1, trondhjemiticgneiss from the Heihe area, were also isotopically homogeneous and their mean ""Pb/"hPb age is Grain 2, 470.2 f 1.3Ma (Table I. Fig. 6b).Two idiomorphic, short 0 Grain 3, prismatic and transparent zircon grains. pink to light brown, of tonalitic sample Q89/1 from the Danfeng area combine to

c a mean ""'Pb/""Pb age of 488.0 f 1.4 Ma (Fig. 7a). Lastly. 0 B three zircon fractions consisting of two long-prismatic, clear D to yellowish, idiomorphicgrains, each from foliated trondhjemite dyke sample CH1557, have a mean ""Pb/'"hPb Z 40 age of 487.2 f 1.1 Ma(Table 1, Fig. 7b).identical, within error, to the age of zircons from sample Q89/1. We consider all theseages to reflectcrystallization of thehost rocks of thezircons and to represent a late stage of theisland arc history.Some fossils reportedfrom thevolcanic arc ... 0.053 0.d54 0.055' sequences are in concordance with our age data; Zhang &L Tang (1983) found radiolaria in the Erlangping arc sequence that gave a Cambrian to Ordovician age, while Xiao et d. 750 760 770 780770 760 750 (1988) reported the Palaeozoic fossil Cyclostylidac. 8 Threeclear. long-prismatic. idiomorphic zircons from granite sample Q89/2S of the younger intrusive assemblage in the Heihe area yielded a "'7Pb/'"hPb age of 401.8 f 0.8 Ma (Table 1, Fig. Sa). Thisconfirms Lerch PI d.'s (199.5~) 0 Grain 3, 66 ratios chronologicaldata for granitoids from the same area that gave ages ranging between 422 f 14 and 395 f 6 Ma and is Mean age: 762.0 f 0.7 Ma also in agreement with the 403 f 17 Ma Sm-Nd isochron age forthe Lajimiao norites in theShangxian area (Li e/ al. 1989. 1993). These ages have a small range between 422 and 395 Maand indicate that the post-collisional calc-alkaline magmatismprincipally occurred between late Silurian and early Devonian. Three dark red-brown, idiomorphic zircons from sample Q89/5.a leucogranitc from the basement of theYangtze Block. providea mean ""Pb/"'"Pb age of 762.0 f 0.7 Ma (Table 1, Fig. Xb). Kroner et al. (1993) also reported zircon ages of 776 f 8 and 746 f 10 Ma for granitoid intrusions in theTongbai Complex. a high-grade basement unit in the South Qinling Zone. in theeastern Qinlingbelt (Henan GeologicalBureau 1989). Theseages, together with data fromother sources (Ma & Wu 1981: Sun & Lu 19x5). 0.0640 0.06450.0640 0.0650 indicate that the basement along the northern margin of the (207Pb/206Pb)' YangtzeBlock, or the South Qinling Zone.formed as a result of a late Proterozoic tectonothermal event. Fig. 8. Histograms showing distribution of radiogenic lead isotope radios derived from evaporation of zircon grains from granodiorite Theabove results constrain the collision of thearc of the younger intrusive assemblagc and a leucogranitie from the system with theNorth China Block atbetween the late basement of the South Qinling Zone. (a) Spectrum for three grains Ordovicianand middle Silurian. The extensivc marine from granodiorite sample Q80/25, Heihe area. integrated from 342 regression on the North China Block in late Ordovician time ratios. (b) Spectrum for three grains from leucogranite samplc (Wang 1985) mayfurther limit this collision to late Q89/5, Laotan. southeast of Shangan. integrated from 212 ratios. Ordovician.Thus, the late assemblage of calc-alkaline For sample locations see Figs 1 & 3. granitoids intruded c. 30-50 Ma after cessation of collision. Similar time intervals of c. S0 Ma have also been noted in the Variscan belt of Europe (Harris et al. 1986) and in the arebased on all measurementsevaluated and are presented in Tibetan Plateau and adjacent areas, e.g. theIndia-Eurasia Table I. The ""Pb/""Pb spectraare shown in histogramsthat collision at c. 45Ma is post-dated by activecalc-alkaline pcrmit visualassessment of thedata distribution from which the volcanoes in Tengchong, southwestern Yunnan of China (Li ages are derived. 1983) and other areas (Xiang 1986).

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Conclusions basis,that the Qinling Complexmay have been derived Ourresults support a tectonicscenario in which from the Yangtze Block rather than the North China Block, consumption of theQinling Ocean along a south-dipping possibly in latest Precambrian to early Cambrian time, and subduction zone (Fig. sa), or zones, led to collision of the thendrifted northwards toamalgamate with the late North China Block with the amalgamated terranes making Cambrian to Ordovician arc terranes and to collidewith the up the Central Qinling Zone in the late Ordovician to early North China Block. Xue et al. (1995) havesuggested that considerable Silurian (Fig. 9b)(Zhang et al. 1989; Reischmann et al. 1990; shortening occurred in the Lesser Qinling Zone, the North Kroner et al. 1993; Lerch et al. 1995~; Xue et al. 1995). South-dipping subduction, as inferred above, resulted in the Qinling Zone and the Central Qinling Zone during the late formation of complexintra-oceanic island-arc system in Ordovician to middle Silurian collision event. This collision Cambrian (?) to early and middle Ordovician time south of probablytransformed the basin into which rocks of the theNorth China Block (Fig. 9a) which includedthe SouthQinling Zone were deposited into an intraplate Erlangping,Heihe and Danfeng volcanic arcs, continental remnantbasin thatremained undeformed until the terraneslike the Qinling block, and volcanic arcs with mid-Triassic (Fig. 9b & c). It is feasible that this basin was continentalbasement like the Xieyuguan Group volcanics closed,beginning in thelate Triassic, by the collision of overlyingProterozoic metamorphic basement west of the continentalblocks at the southern margin of theYangtze Heihearc (Shaanxi Geological Bureau 1989). Inthe late Block (Xue et al. 1995). Final collision of the North China Ordovician to middle Silurian, convergence led to closure of Block with the Yangtze Block was completed in the Jurassic theQinling Ocean and amalgamation and collision of this (Fig. 9d). island-arcsystem with the North China Block (Fig. 9b). Post-collisionalcalc-alkaline magmatism occurred in late Thisstudy is part of ajoint project between the Department of Silurian and early Devonian times. Geology,Northwest University, Xi'an, and the Department of The late Proterozoic age reported here for a rock from Geosciences, IJniversity of Mainz, and was funded by the German thebasement along the northern margin of theYangtze Volkswagen Foundation and the Chinese National Natural Science Blockmay be significant insofaras a lateProterozoic Foundation. A.K. acknowledges the use of analytical facilities in the tectonothermalevent also characterizes theQinling Max-Planck-Institut fur Chemie in Mainz.Discussions with D. B. Complex (You et al. 1991: Li et al. 1991) but is absent in the Rowley and rewiews by D. Davis and W. Fitches helped to improve an earlier version of this manuscript. Correspondence to A. Kroner North China Block and its margins. We speculate, on this (email: KROENER @ mzdmza. zdv. uni-mainz.de).

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Received 15 Feburary 1994; revised typescript accepted 29 September 1995 Scientific editing by Jane Evans.

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