Zhang Benren

Geochemical and Pb-Sr-Nd isotopic compositions of granitoids from western Qinling belt: Constraints on basement nature and tectonic affinity

Geochemical and Pb-Sr-Nd isotopic compositions of five Indosinian granitoid intrusions from the western Qinling belt provide insights into basement nature and tectonic affinity. The results show that the western Qinling granitoids incline towards basic in their bulk chemical composition. The granitoids belong to high-K to shoshonitic series with K2O/Na2O=1.04–1.86 and are dominantly metaluminous with A/CNK=0.90–1.05 (most samples have A/CNK of <1.0). They have similar trace elemental compositional patterns. In Sr-Nd isotopic compositions, they display some extent heterogeneity with ISr=0.70682–0.70845, ɛNd(t)=−4.85 to −9.17 and TDM=1.26–1.66 Ga. They are characterized by high radiogenic Pb isotopic compositions. Their initial Pb isotopic ratios are 206Pb/204Pb=17.996–18.468, 207Pb/204Pb=15.565–15.677 and 208Pb/204Pb=38.082–38.587. Geochemical and Sr-Nd isotopic compositions reveal that magma for the granitoids was derived from partial melting of high-K (Rb) basaltic rocks, which might be formed in 900–1400 Ma. It is suggested that a large amount of the Proterozoic high-K (Rb) basaltic rocks, which underlie the Phanerozoic sedimentary cover, constitute the crustal basement of the western Qinling belt. Pb-Sr-Nd isotopic compositional comparison between the eastern Qinling and the western Qinling Indosinian granitoids indicates that the crustal basement of the western Qinling is distinct from that of the eastern Qinling. The Baoji-Chengdu railway close to south-north orientation can be taken as an approximate boundary between both basements. The Pb-Nd isotopic compositional characteristics of the western Qinling granitoids suggest that the basement of the western Qinling belt has an affinity with the Yangtze block.

Geochemical evidence for Proterozoic continental arc and continental-margin rift magmatism along the northern margin of the Yangtze Craton, South China

Abstract Middle to Late Proterozoic volcanic-intrusive magmatism of continental island-arc and continental-margin rift nature is well developed in the Xixiang-Beiba area, Shaanxi Province, along the northern margin of the Yangtze Craton. In the Xixiang area, volcanism evolved temporally from submarine low-K arc tholeiite through calc-alkaline to subaerial alkali or shoshonitic. This was accompanied by gradational enrichment of LREE and Zr, an increasing negative Eu anomaly, as well as strong depletion of compatible elements Cr, Co, V and Sc. The associated Hannan Intrusive Complex evolved from gabbro through tonalite-trondhjemite-granodiorite to granite with the distribution of basic to felsic plutons suggesting arc polarity. There is also a systematic lateral increase in K 2 O content, K 2 O/Na 2 O ratio, and La N /Yb N ratio towards the interior of the Yangtze Craton in these rocks. This spatial framework and compositional variation resembles that found in Circum-Pacific island arcs or active continental margins. In contrast, both volcanic and intrusive rocks in the Beiba area are characterized by alkaline compositions and a bimodal habit; they are composed of tholeiite (gabbro) and alkali rhyodacite-rhyolite (alkali granite), and lack rocks of intermediate composition. They may have formed in a continental-margin rift setting. Geological and geochemical evidence further suggests that the Xixiang paleo-arc was a mature continental arc built on older sialic basement. The emplacement of voluminous tonalites and trondhjemites thickened the crust and may have marked an important continent-building event at the northern margin of the Yangtze Craton in the Middle to Late Proterozoic.

Indian Ocean-MORB-type isotopic signature of Yushigou ophiolite in North Qilian Mountains and its implications

In order to explore the disputed issue concerning the tectonic affinity of the ancient ocean mantle of North Qilian Mountains (NQM), geochemical and Sr, Nd, Pb isotopic compositions of pillow basalts of the Yushigou Ophiolite (YSGO) suite from NQM have been analyzed systematically. The pillow basalts exhibit tholeiitic characteristics, with flat chondrite-normalized REE patterns ((La/Yb)N = 0.98–1.27). They display no Nb, Ta, Zr, Hf negative anomalies, and show MORB features in 2Nb-Zr/4-Y and Ti/100-Zr-Y × 3 tectonic discrimination diagrams. These results indicate that the Yushigou ophiolite is most likely to be formed in a mid-ocean ridge or mature back-arc basin. Their isotopic data show a relatively broad and enriched 87Sr/86Sr (0.70509–0.70700), restricted 143Nd/144Nd (0.512955–0.512978). Pb isotopes are in the range of 206Pb/204Pb (18.054–20.562), 207Pb/204Pb (15.537–15.743) and 208Pb/204Pb (38.068–38.530). These isotopic data imply that the basalts originated from the depleted mantle (DMM), with the involvement of enriched mantle components (mainly EMII). Geochemical comparisons between the basalts in YSGO and the MORB-type basalts of ophiolite suites occurring in the known ancient Tethyan tectonic domain indicate that the ancient oceanic mantle represented by YSGO suite forming in early Paleozoic in the North Qilian Mountains is very similar to the Tethyan mantle in both trace elements and isotopic compositions. The North Qilian Mountains should be a part of the Tethyan tectonic domain in early Paleozoic. This further implies that the Tethyan tectonic domain can be deduced to early Paleozoic in the study area, which will be helpful to discussing the tectonic affinity and evolution of the North Qilian Mountains.

Geochemical constraints on the evolution of North China and Yangtze blocks

Abstract The Qinling orogenic belt represents the convergence zone between North China and Yangtze (South China) blocks. It is inferred from the contrasting geochemical characteristics and evolutionary histories that the two blocks evolved separately in their early history, rather than being two parts split from a common Archean basement. Here, we present systematic geochemical studies of ultramafic and mafic intrusives, ophiolite suites, granites, and volcanic and sedimentary rocks, as well as amphibolite- and granulite-facies rocks, aimed at determining their tectonic evolution. Synthesis of these data indicates that the convergence and suturing of the two blocks had a long, complicated history including middle-late Proterozoic southward and early Paleozoic northward subduction of oceanic crust and a final Silurian convergence of their continental margins with the subsequent continent-continent collision principally in the late Paleozoic.

Tectonic subdivision of Dabie orogenic belt, central China: Evidence from Pb isotope geochemistry of late Mesozoic basalts

It has long been debated that the Dabie orogenic belt belongs to the North China or Yangtze craton. In recent years, eastern China has been suggested, based on the Pb isotopic compositions of Phanerozoic ore and Mesozoic granitoid K-feldspar (revealing the crust Pb) in combination with Meso-Cenozoic basalts (revealing the mantle Pb), being divided into the North China and Yangtze Pb isotopic provinces, where the crust and mantle of the Yangtze craton are characterized by more radiogenic Pb. In this sense, previous researchers suggested that the pro-EW-trending Dabie crogenic belt with less radiogenic Pb in the crust was part of the North China craton. In this paper, however, the Late Cretaceous basalts in the central and southern parts of the Dabie orogenic belt are characterized by some more radiogenic Pb (206Pb/204Pb=17.936−18.349,207Pb/204Pb=15.500−15.688,208Pb/204Pb=38.399−38.775) and a unique U-Th-Pb trace element system similar to those of the Yangtze craton, showing that the Mesozoic mantle is of the Yangtze type. In addition, the decoupled Pb isotopic compositions between crust and mantle were considerably derived from their rheological inhomogeneity, implying a complicated evolution of the Dabie orogenic belt.

Crust-Mantle Interaction in Dabie Orogenic Belt, Central China: Geochemical Evidence from Late Cretaceous Basalts

It has been suggested that eclogites in the Dabie orogenic belt are exhumation products, which had subducted into the deep-seated mantle and undergone ultra-high pressure metamorphism during the Triassic. But no direct evidence supports this process except the calculatedp-T conditions from mineral thermobarometers. The Late Cretaceous basalts studied in the present paper, however, have provided some geochemical evidence for crust-mantle interaction in the area. These basalts are distributed in Mesozoic faulted basins in central and southern Dabie orogenic belt. Since little obvious contamination from continental crust and differentiation-crystallization were observed, it is suggested, based on a study of trace elements, that the basalts are alkaline and resultant from batch partial melting of the regional mantle rocks, and share the same or similar geochemical features with respect to their magma source. In the spider diagram normalized by the primitive mantle, trace element geochemistry data show that their mantle sources are enriched in certain elements concentrated in the continental crust, such as Pb, K, Rb and Ba, and slightly depleted in some HFSE such as Hf, P and Nb. Pb-Sr-Nd isotopic compositions further suggest the mantle is the mixture of depleted mantle (DM) and enriched one (EMI + EMII). This interaction can explain the trace element characteristics of basaltic magmas, i. e., the enrichment of Pb and the depletion of Hf, P and Nb in basalts can be interpreted by the blending of the eclogites in DOB (enriched in Pb and depleted in Hf, P and Nd) with the East China depleted mantle (As compared to the primitive mantle, it is neither enriched in Pb nor depleted in Hf, P and Nb). It is also indicated that the eclogites in the Dabie orogenic belt were surely derived from the exhumation materials, which had delaminated into the deep-seated mantle. Moreover, the process subsequently resulted in compositional variation of the mantle (especially in trace elements and isotopes), as revealed by the late mantle-derived basalts in the Dabie orogenic belt.

Crustal growth rate of the North China platform: A method of estimating mass equilibrium between crust and mantle

AbstractThe following equation is proposed in this paper to estimate the crustal growth rate of the North China Platform on the basis of mass equilibrium between the crust and the mantle:

Geochemical subdivision and evolution of the lithosphere in East Qinling and adjacent regions——Implications for tectonics

x = \frac{{C_{PM}^1 - (C^1 /C^2 )_{PM} . C_{PM}^1 }}{{C_{CC}^2 - (C^1 /C^2 )_{RM} . C_{CC}^2 }}