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 volcanic arc sequences, seems to represent the fractionation product of basaltic arc magma. It therefore predates the collision of the North China Block with the 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. tectonics, 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 the Central CentralQinling Zone, using thesingle-zircon Pb-Pb Qinling Zone. 409 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/3/409/4901078/gsjgs.153.3.0409.pdf by guest on 27 September 2021 410 F. XUE ET AL. 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).Basalts as a tectonicmelange (Hsii et al. 1987). New data suggest comprise a major part of the volcano-sedimentary sequence, that it maybe appropriate to subdivide this zoneinto but andesite, 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 through regional metamorphism, granitic intrusion and the composition are closely associated with the sequence (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