Eastern Qinling Research Articles

Geochemical Characteristics and LA‐ICP‐MS Zircon U‐Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

Abstract Geochemical studies on the amphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The amphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N = 0.33–0.55, (La/Sm)N = 0.45–0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N‐MORB. The second group of rocks have (La/Yb)N = 0.63–0.95, (La/Sm)N = 0.69–0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82, 0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N‐MORB and E‐MORB but closer to the former. The two groups of rocks both exhibit flat patterns from Th to Yb in the highly incompatible elements spider diagram, but the first group of rocks have lower element abundances than the modern N‐MORB, indicating a derivation of their mantle source from more depleted mantle source than the present N‐MORB. The abundances of Th, Ta, Nb, La and Ce in the second group of rocks are slightly higher than those of the present N‐MORB, and other elements, such as Hf, Zr, Sm, Ti, Y and Yb, are close to those of the N‐MORB, indicating that the original magma was derived from depleted mantle but mixed with the enriched mantle. These characteristics, combined with the regional geology and previous studies, provide further evidence that the mafic‐ultramafic rocks have the features of a typical ophiolite.Zircon grains from the amphibolite are generally rounded, and in most of them a distinguishable core‐mantle texture is preserved as shown in the cathodoluminescence (CL) images. The core or core‐mantle parts of the zircon grains are also rounded, same as those in basalts from other regions of the world. The LA‐ICP‐MS trace element and U‐Pb isotopic analyses show that the zircon grains from the amphibolites are similar to the typical magmatic zircon in terms of their very low U and Th contents (62.36–0.10 μg/g and 78.47–0.003 μg/g, respectively). Seven pits from the core and core‐mantle parts of the zircon grains yielded an average weighted 206Pb/238U age of 973±35 (2s̀) Ma with the Th/U ratios range from 0.01 to 8.38 and mostly greater than 0.23. This age is consistent within the error range with the whole‐rock Sm‐Nd isochron age of 1030±46 Ma for the same kind of rocks reported by Dong et al. (1997a). In a combined analysis with the zircon positions on the CL images and the corresponding Th/U ratios, the age of 973±35 Ma is probably the formation age of tholeiite, the protolith of the Songshugou amphibolite. The geochronological determination gives further evidence that the Songshugou ophiolite was formed during the Neoproterozoic. In addition, there is one pit from the rim of a zircon grain giving a 206Pb/238U age of 572±199 (1s̀) Ma with a Th/U ratio of 0.08. It may represent the age of the accretionary zircon in the amphibolite‐facies metamorphism.

U‐Pb Zircon Geochronology and Geochemistry of Granitoids in the Douling Group in the Eastern Qinling

Abstract LA‐ICPMS U‐Pb zircon dating of the Sanpinggou, Gangou and Fengzishan granitoids in the Douling Group of the Eastern Qinling yields ages of 760–685 Ma, which represents a strong tectono‐magmatic event in the southern Qinling during the late Neoproterozoic. Geochemical data show that these intrusions have wide compositions ranging from minor gabbros through diorites to granodiorites. They are relatively enriched in LILE, poor in HFSE and strongly depleted in Nb and Ta, displaying affinities of I‐type granites formed in an active continental margin with oceanic subduction. In contrast to granitoids, gabbros and enclaves in the granitoids have higher REE abundances, relatively flat REE patterns, lower LILE, slightly higher HFSE and more depletion in Nb and Ta. All these suggest that the gabbros were formed by partial melting of the upper mantle above the subduction zone and the granitoids by the partial melting of the lower crust. Combined with regional geological data, the subduction‐related granitoids in the Douling Group, together with the Tuwushan A‐type granite with an age of 725 Ma and contemporaneous basic dikes in the Wudang Block, provide evidence for local subduction of oceanic basins between different blocks during the rifting in the Southern Qinling in the Neoproterozoic. Thus, the coexistences of various magmatic rocks formed in different tectonic environments indicate a complicated tectonic evolution and variety of tectonic frameworks in the Qinling area in the Neoproterozoic.

Determination of the Neoproterozoic Shicaogou Syn‐collisional Granite in the Eastern Qinling Mountains and Its Geological Implications

Abstract The Shicaogou granite has been identified as a magnesian (Fe‐number=0.71–0.76), calcic to calc‐alkalic (MALI=3.84–5.76) and peraluminous (ASI=1.06–1.13) granite of the syn‐collisional S‐type, with high SiO2 (>71%), Al2O3 (>13%) and Na2O+K2O (6.28%–7.33%, equal for NaO2 and K2O). Trace element and REE analyses show that the granite is rich in LILE such as of Rb, Sr, Ba and Th, and poor in HFSE like Yb, Y, Zr and Hf. Its Rb/Sr ratio is greater than 1; the contents of Nb and Ta, and the ratio of Nb/Ta as well as the REE geochemical features (e.g. REE abundance, visible fractionation of LREE and HREE and medium to pronounced negative Eu anomalies) are all similar to those of crust‐origin, continent‐continent syn‐collisional granite. Moreover, the granite exhibits almost the same pattern as that of the typical continent‐continent syn‐collisional granite on the spider diagram and all samples fall within the syn‐collisional granite field.The cathodoluminescence (CL) investigations have revealed that the zircon from the Shicaogou granite represents a typical magmatic product characterized by its colorless, transparent and euhedral crystals, and distinct zoning of oscillatory bands. Residual cores of irregular zircon can be found in a few enhedral grains. Trace element studies of the zircon grains, with high contents of P, Y, Hf, Th, U and REE and high ratios of Th/U, obviously positive Ce anomalies and HREE enrichment compared to LREE, also result in the same conclusion.The LA‐ICP‐MS U‐Pb isotopic data from 24 spots of 21 zircon grains demonstrate that 20 spots in the oscillatory zone yield an average weighted 206Pb/238U age of 925±11 Ma, indicating that the Shicaogou granite was formed in the Neoproterozoic. Combined with other Neoproterozoic syn‐collisional granites found in the study area, the present geochronological determination can further reveal that collision‐amalgamation events could have occurred among some continental blocks in the Qinling orogenic belt during the Neoproterozoic. This in turn provides an accurate chronological constraint on the Neoproterozoic break‐up and convergence in the belt.

Affinity of Dabie Block and its Relation to the Tectonic Architecture in Eastern Qinling Orogen, China

Affinity of Dabie Block and its Relation to the Tectonic Architecture in Eastern Qinling Orogen, China

Meso- and Neoproterozoic Tectonothermal Events and Structural Evolution in Eastern Qinling Orogenic Belt, China

Meso- and Neoproterozoic Tectonothermal Events and Structural Evolution in Eastern Qinling Orogenic Belt, China

Organic-walled microfossils of Neoproterozoic Dongjia Formation, Lushan County, Henan Province, North China

The northern slope of the Eastern Qinling Ranges is situated in a transitional region between the Qinling Fold Belt and the North China Platform. The Neoproterozoic Dongjia Formation of the northern slope of the East Qinling Ranges sits immediately beneath the glaciogenic Luoquan Formation. Abundant and moderately diverse organic-walled microfossils have been obtained by palynological preparation from grey–green sandy shale and grey carbonaceous shale with intercalated sandstone in the lower part of the Dongjia Formation. By considering known stratigraphic distributions of acritarchs, and abundant radial fibrous fabrics in microdigitate stromatolites and a narrow range of δ 13C values (ca. 0±1‰ vs. PDB) from the underlying carbonate successions, the age of the Dongjia Formation is estimated to be early Neoproterozoic. In addition, many specimens of small Micrhystridium-like spinose acritarchs have been found in the glaciogenic Luoquan Formation. This supports the presence of a `Luoquan Glacial Period' after the Varangian glaciation. Nine previously described acritarch species have been identified, and one new combined species, three uncertain species, one undetermined form and fragment of an algal colony are described.

The genetic type and tectonic setting of ore associated mesozoic porphyries in the Eastern Qinling Mountains

The genetic type and tectonic setting of ore associated mesozoic porphyries in the Eastern Qinling Mountains

Mineral chemistry and boron isotopic composition of tourmaline from the Devonian metallogenic district of Shanyang-Zhashui, eastern Qinling

Toumaline is widespread in the host strata of strata-bound base metal sulphide deposits in the Devonian metallogenic district around Shanyang-Zhashui in eastern Qinling. As a member of the schorl-dravite series, the tourmaline is characterized by Mg > Fe and Na > Ca, showing apparent chemical zonation which records the geochemistry during its formation and subsequent regional metamorphism and hydrothermal overprint. The close similarity in chemical and isotopic constitutions between the tourmaline of the main metallogenic epoch in this district [FeO/(FeO + MgO)=0.34 − 0.39 and δ11B=−7.6‰ − − 8.8‰] and those related to massive sulphide deposits typical of submarine (exhalative) hydrothermal sedimentation may add further support to a similar mechanism of mineralization for the strata-bound deposits in the district.

Feldspar Lead Isotopic Composition of Granitoids from the Eastern Qinling Orogenic Belt and Their Tectonic Significance

AbstractThis paper reports 48 feldspar lead isotope analyses from 27 granitic intrusions, which formed from the Late Proterozic to Mesozoic within the Eastern Qinling oregenic belt. It is found that the granitic rocks of South Qinling are characterized by a strong block–effect and depletion in U–Pb and Th–Pb, showing that these rocks came from the same lead isotope tectono–geochemical province, while those of North Qinling are characterized by higher U–Pb and Th–Pb for Late Proterozoic to Early Paleozoic ones and lower U–Pb and Th–Pb for Late—Palaeozoic and younger ones in their feldspar lead isotopic composition. In the North Qinling block, lead isotopic signatures reflect that the source of granitic magma had changed since the Late Palaeozoic. Comparison of feldspar lead isotopic composition between South Qinling and North Qinling shows that there is marked difference in lead isotopic composition for pre–Palaeozoic granitoids, indicating that the South Qinling and the North Qinling blocks belong to different tectonic units, but the similarities in lead isotopic composition are quite clear, which indicates that the South Qinling block had been welded with the North Qinling block and that the magma sources of both blocks were identical. The analysis provides direct evidence for underplating of the continental crust of South Qinling beneath the North Qinling block in the continent–continent interaction stage of the Eastern Qinling oregenic belt.

Early Palaeozoic Evolution of the Zhen'an‐ Xichuan Block and the Small Qinling Multi‐Island Ocean Basin

Abstract Based on studies of palaeogeography, palaeobiogeography, palaeomagnetism, geochemistry and volcanism, this paper proposes that the Zhen'an ‐Xichuan area was a small Early Palaeozoic block rifted away from South Qinling and suturing onto North Qinling earlier than the other parts of South Qinling. In the Early Palaeozoic Qinling was a small archipelagic ocean basin with 5 rows of islands including the Zhen'an ‐Xichuan block. The drifting of the Yangtze and North China plates and the islands between them in the same direction at different speeds caused their suturing process to be different from the classic plate collision, which is the major feature of the suturing of the multi‐island Tethys ocean basin. This also explains the problem that the Caledonian collision did not result in orogeny in eastern Qinling.

Chemical compositions of tourmaline in the Yindongzi-Tongmugou Pb-Zn deposits, Qinling, China: Implications for hydrothermal ore-forming processes

A microprobe study has been carried out on the chemical composition of tourmaline from the Yindongzi and Tongmugou stratabound Pb-Zn ore deposits, eastern Qinling, China. Tourmaline was analysed from a variety of rock types representative of its various occurrences associated with the ore bodies. All the tourmalines studied here belong to the schorl-dravite series. Most are of hydrothermal origin with Mg > Fe and Na > Ca. Some detrital cores of tourmaline have been recognized from their geometry and chemistry, with Fe > Mg. The chemical trends from core to rim in zoned grains suggest a multi-stage model for the growth of tourmaline and genesis of the ore bodies. The first stage was represented by a more Mg-rich hydrothermal fluid in the submarine hydrothermal system, producing Mg-rich tourmalines by selective replacement of clay-rich sediments close to the sediment-water interface. The second stage was dominated by Fe-rich hydrothermal fluid and resulted in overgrowth of Fe-rich tourmaline rims. This stage also led to the nucleation and growth of new tourmaline crystals and was responsible for the formation of the main massive sulphide orebodies. Finally, a further period of hydrothermal activity or a metamorphic event led to the formation of an additional rim of Mg-rich tourmaline.

Late Paleozoic sedimentary and tectonic evolution of the Shangdan suture zone, Eastern Qinling, China

The Qinling orogen separating the North China plate from the Yangtze plate is a key area for exploring the timing and process of integration between the two plates. A number of models have been developed for this orogen in terms of plate tectonics, but none of them can satisfactorily reconcile the contradictions of data from sedimentary records, ages of ophiolitic assemblages, paleomagnetic data, deformational and metamorphic history, and geochemical features of detrital sediments. Recent studies on sedimentary sequences of the Shangdan suture zone and reevaluation of existing relevant data show that intense orogeny in the Qinling did not take place immediately after the closure of the early Paleozoic Qinling ocean. There did not exist the so-called Devonian “molasse” and the Triassic “melange”. The new discoveries presented in this paper certainly put the constraints on interpretations of the Qinling tectonic evolution. Actually the integrating process was quite complex and took a long time of evolution from end of the early Paleozoic to the late Triassic.

Metamorphic history and tectonic evolution of the Qinling Complex, eastern Qinling Mountains, China

Abstract The metamorphic history and tectonic evolution of the Qinling Complex is divided into formation and modification stages. During the Proterozoic formation stage, three deformational sequences are recognized. Andalusite–muscovite, sillimanite–muscovite and sillimanite–K‐feldspar zones of amphibolite facies regional metamorphism are earlier than, or synchronous with the first or second phase of folding. Ductile shear zones were formed and Caledonian granites were emplaced during the modification stage. The granites superimposed contact aureoles (garnet–K‐feldspar zone) on the regional metamorphic fabric.Metamorphic reactions, P–T conditions of metamorphism and P–T–t paths were estimated by analysis of mineral textures and standard thermobarometric techniques. The P–T–t path of the Proterozoic tectonometamorphic cycle shows prominent clockwise decompression. The P–T–t path of the Caledonian tectonometamorphic cycle is characterized by an early rise of pressure and temperature, followed by isothermal decompression (rapid uplift) and finally with isobaric cooling.The P–T–t paths of the two tectonometamorphic cycles reflect two major stages of collision and uplift in the evolution of the Qinling orogenic belt during the Proterozoic and Caledonian–Hercynian periods, respectively.

Granulites in the Tongbai Area, Qinling Belt, China: Geochemistry, petrology, single zircon geochronology, and implications for the tectonic evolution of eastern Asia

The Tongbai area of the eastern Qinling belt in China includes granulite‐grade metamorphic assemblages (Qinling Complex) which were previously regarded as Archean to early Proterozoic in age and belonging to the southern margin of the North China plate (craton). Our petrological and geochemical data characterize these rocks as two‐pyroxene granulites and garnet granulites which formed at temperatures of 757°–840°C and pressures of about 9.5 kbar and are now found as xenoliths in granodioritic gneisses. The protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro. The 207Pb/206Pb ratios derived from evaporation of single zircons yield ages of 470±20 and 470±14 Ma, respectively, for the basic granulites which we interpret to reflect the time of protolith emplacement. These are intruded by a 435±14 Ma granodioritic gneiss post‐dating granulite formation. A metaquartzite sample contains detrital zircons as old as 2555±8 Ma. Two samples of granitoid gneiss from the Tongbai Complex S of the Qinling granulites have single‐zircon 207Pb/206Pb evaporation ages of 776±8 and 746±10 Ma, respectively, and document late Proterozoic igneous activity. We suggest that the Qinling granulites document an important and hitherto unknown phase of early Silurian crustal thickening following subduction and continental collision and that both the Qinling and Tongbai Complexes were part of the southern margin of the North China craton prior to this event and record late Proterozoic igneous activity.

Petrological Study on the Ductile Shear Zones in the Core of the Eastern Qinling Orogenic Belt in Western Henan

Abstract A series of ductile shear zones of the overthrust and strike‐slip types and related ductile shear metamorphic rocks, including tectonic melange and mylonites, were formed in the core of the Qinling orogenic belt in the course of the Caledonian‐Indosinian ductile and brittle‐ductile reworking. The study on their petrography, variations in composition and conditions of formation is conducive to revealing the metamorphism‐deformation history of the core of the Qinling orogenic belt and further to understanding the dynamic mechanism of its evolution.

Micropaleontological study of Lower Palaeozoic siliceous sequences of the Yangtze Platform and Eastern Qinling Range

From the siliceous rocks of the Niutitang Formation in Sandu district, Guizhou Province, a Lower Cambrian microfossil assemblage has been procured and studied by means of scanning electron microscope. At the same time, the siliceous Damiao Formation of the Erlongping Group in the main Eastern Qinling ophiolite belt, as now known also contains a microfossil assemblage, similar in some features to the former and calibrated stratigraphically, the rocks underlie marmorized strata of the same formation containing Silurian tabulates. As a result, the early Palaeozoic (rather than Proterozoic as presumed before) age of the main Qinling ophiolite complex can be considered to be finally determined. This will be of great importance for obtaining a better understanding not only of the historical development of the Qinling proper, but also of the original relationship between the South and North China blocks.

Geochemistry and Tectonics of the Xionger Group in the Eastern Qinling Mountains of China—a mid Proterozoic Volcanic arc Related to Plate Subduction

Summary A mid Proterozoic volcanic series, the Xionger Group, in the eastern Qinling Mountains of central China, may be one of the earliest products of plate subduction in eastern Asia. The main rock types of the 1710 Ma Xionger Group are andesites and dacites of the calcalkaline series. The rocks of the Xionger Group have trace element contents and REE patterns which are comparable to those of lavas from modern active continental margins, implying that the Xionger Group volcanics had a similar tectonic setting. It appears that there was an important event in China at 1800–2000 Ma, after which plate tectonics with the Wilson cycle was the main contributor to crustal growth.

Deformation Characteristics and Tectonic Evolution of the Eastern Qinling Orogenic Belt*

Abstract The Qinling Mountains separating the northern from the southern China plate is a key region for the study of structural evolution of eastern Asia. It is composed of the Palaeozoic fold belt in its northern part and the Variscan and Indosinian fold belts in its southern part. The evolution of the former is marked by the closure of a northward subducting oceanic basin in the early stage, followed by southward obduction of ophiolites and intracontinental thrusting during the Variscan; whereas that of the latter is represented by intracontinental, shallow crustal deformation on the basis of a large‐scale detachment structure (with a horizontal slip of at least of 100 km). Since the late Palaeozoic, however, both of the belts have been cut by a series of east‐west sinistral strike‐slip faults.