Zhang Hongfei

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.

U-Pb zircon dating constraints on formation time of Qilian high-grade metamorphic rock and its tectonic implications

In order to constrain the formation time of high-grade metamorphic rocks in the Qilian Mountains, U-Pb zircon dating was carried out by using LA-ICPMS technique for a paragneiss of the Hualong Group in the Qilian Mountains basement series and a weakly foliated granite that intruds into the Hualong Group. Zircons from the paragneiss consist dominantly of detrital magma zircons with round or sub-round shape. They have 207Pb/206Pb ages mostly ranging from 880 to 900 Ma, with a weighted mean age of 891 ±9 Ma, which is interpreted as the magma crystallization age of its igneous provenance and can be taken as a lower age limit for the Hualong Group. Magma crystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upper age limit for the Hualong Group. Accordingly, the formation time of the Hualong Group is constrained at sometime between 875 and 891 Ma. A few zircons from both paragneiss and weak-foliated granite display old inherited ages of 1000 to 1700 Ma and young metamorphic ages of Early Paleozoic. The zircon age distribution pattern confirms that the Qilian Mountains and the northern margin of Qaidam Basin had a united basement, with geotectonic affinity to the Yangtze Block. The results also reveal that sediments of the Hualong Group formed by rapid accumulation due to rapid crustal uplift-erosion. This process may result from intensive Neoproterozoic orogenesis due to assembly of the suppercontinent Rodinia.

Geochemical and Pb-Sr-Nd isotopic compositions of Indosinian granitoids from the Bikou block, northwest of the Yangtze plate: Constraints on petrogenesis, nature of deep crust and geodynamics

This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma), Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3: 14.56%–16.48%) and Sr (352 μg/g–1047 μg/g), and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g), resulting in high Sr/Y (36.3–150) and (La/Yb)N (7.8–36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752, εNd(t) values from −3.1 to −8.5, and initial Pb isotopic ratios 206Pb/204Pb=17.891−18.250, 207Pb/204Pb=15.494−15.575, and 208Pb/204Pb=37.788−38.335. Their geochemical signatures indicate that the granitoids are adakitic. However, they are distinct from some adakites, generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust, because they have high K (K2O: 1.49%–3.84%) and evolved Nd isotopic compositions, with older Nd isotopic model ages (TDM=1.06–1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thickened basaltic lower crust. Combined with regional analyses, a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand, based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks, it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type.

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.

Tectonic Evolution of the Dabie-Sulu UHP and HP Metamorphic Belts, East-Central China: Structural Record in UHP Rocks

The complex structural history of the Dabie-Sulu terrane is deduced from various scales of structural features studied in UHP metamorphic units combined with metamorphic and thermal data. Excluding pre-UHP events, the following sequence of distinct tectonometamorphic stages is suggested: (1) The first deformation, D1, produced weak foliation and lineation in massive eclogite in the P-T stability field of coesite/diamond under low differential stress. (2) The D2 event is mainly inferred from a dominant coesite eclogite-facies texture characterized by a stretching mineral lineation, mesoscale sheath-like folds, and a network of ductile shear zones. (3) D3 structures and fabrics developed shortly after the formation of granulite/amphibolite—facies symplectites. These structures are characterized by a regional, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern, leading to tectonic juxtaposition or nappe-like structures defining shear zone-bounded crustal slices. The D3 deformation event was associated with decompressional partial melting and intense retrogressive metamorphism. (4) Post-collisional ductile crustal thinning and extension affected the D3 foliation and compositional layering, producing a regional, gently dipping D4 main amphibolite-facies foliation and stretching lineation, dome- and arc-shaped structures, and low-angle and extensional detachments typified by different stretching directions. All of these structures formed prior to intrusion of Late Mesozoic plutons, faulting, basin development, and tectonic unroofing at shallow crustal levels (D5). The first two stages of ductile structures (D1 and D2) were related to subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>27 kbar) and HP metamorphic conditions at 230-250 Ma. In contrast, D3 and D4 stages of Late Triassic to Jurassic (~230-170 Ma) ductile deformation accompanied exhumation of UHP and HP metamorphic rocks to crustal levels, initially driven by compression and later by extension. D4 structures dominate the map pattern of most UHP and HP metamorphic rocks in the orogen. The final D5 deformation largely controls the present-day geomorphology of the Dabie-Sulu region. A modified tectonic evolution model for the UHP and HP metamorphic belts is proposed. It involves continental subduction and collision between the Yangtze and Sino-Korean cratons, and subsequent polyphase exhumation of the UHP and HP metamorphic rocks.