Xiao-Lei Wang

Geochronology and geochemistry of Neoproterozoic mafic rocks from western Hunan, South China: implications for petrogenesis and post-orogenic extension

The Neoproterozoic mafic rocks in western Hunan, South China, form a NNE-striking mafic rock belt for which outcrops are found predominantly in Guzhang, Qianyang and Tongdao. Samples from Qianyang and Tongdao yielded ion microprobe U–Pb zircon ages of 747 ± 18 Ma and 772 ± 11 Ma, respectively. The mafic rocks are geochemically divided into two subtypes. Ultramafic rocks from Tongdao are depleted in Nb and Ti, with decoupled Nd–Hf isotopes, and geochemical features similar to the c. 761 Ma mafic–ultramafic rocks from Longsheng, northern Guangxi. Their e Nd (t) value is −2.91, implying an enriched mantle source. Alkaline mafic rocks from Qianyang and Guzhang have high values of TiO 2 , total alkali, some high field strength elements and (La/Yb) N , and low Zr/Nb, La/Nb, Sm/Nd and 143 Nd/ 144 Nd ratios as well as coupled Nd–Hf isotopes. They are geochemically similar to ocean island basalts and show fractional crystallization of Fe–Ti oxides, olivine and pyroxene in the mafic magma. The c. 760 Ma mafic rocks in western Hunan may be the products of post-orogenic magmatism. After the Jinningian (Sibao) orogenic process, the upwelling of the deep asthenospheric mantle caused by the break-off and detachment of the subducted oceanic slab led to extension in the area. The extension might have taken place earlier in the Tongdao and Longsheng areas, which led to the partial melting of the lithospheric mantle that had been metasomatized during early oceanic subduction to generate a relatively large amount of sub-alkaline rocks. However, the less alkaline mafic rocks in Qianyang and Guzhang might have been generated in the relatively later stage of the extension, and may have resulted from a small degree of partial melting of the asthenospheric mantle.

Hf contents and Zr/Hf ratios in granitic zircons

We present a statistical analysis of 2201 electron microprobe determinations of Hf contents and Zr/Hf ratios in zircon from 65 samples of granitic rocks. The modal Hf content and modal Zr/Hf ratio of zircon taken from the data are 1.43 wt% HfO2 and 38.5 respectively. Estimated partition coefficients are 2520 for DZr and 2420 for DHf between zircon and granitic magma. The large and similar Zr and Hf partition coefficients indicate that the crystallization of zircon would deplete the melt in both Zr and Hf, but would not cause significant fractionation of Zr from Hf. On the basis of the geochemical behaviour of Hf, granitic zircons can be divided into three types (early, late and hydrothermal zircon). Compared to the early zircon in granitic rocks, late-crystallizing zircon and especially hydrothermal zircon are characterized by much higher Hf contents and lower Zr/Hf, which can be explained by a change in DHf with decreasing crystallization temperature. The behavior of Hf in granitic zircon may be used as geochemical indicator to track the origins and crystallization environments of the host magma.

Morphology and geochemistry of zircons from late Mesozoic igneous complexes in coastal SE China: implications for petrogenesis

Abstract The Pingtan and Tonglu igneous complexes in SE China are typical of the calc-alkaline series developed at active continental margins. These two complexes are dominated by felsic rocks, temporally and spatially associated with minor mafic rocks. Morphological and trace-element studies of zircon populations in rocks from each of these complexes show that the zircon populations may be divided into 3−4 distinct growth stages, characterized by different distributions of morphological indices (Ipr, Ipy and Iel), and different contents of the substituting elements (Hf, U, Th, Y and P). The four growth stages recognized in the zircons are believed to have formed successively in the magma chamber, during the emplacement, and in the early and later stages of magma consolidation, respectively. All four stages are recognized in the plutonic Pingtan complex, whereas the stages 3 and 4 are less developed in the volcanic/subvolcanic Tonglu complex. Based on the chemistry and morphology of the different zircon populations of the Pingtan and Tonglu complexes, it is suggested that basaltic magmas underplating at the boundary between crust and mantle caused partial melting of the mid−lower crust and produced granitoid magmas. Subsequently, mixing between magmas was important.

Diversity in early crustal evolution: 4100 Ma zircons in the Cathaysia Block of southern China

Zircons are crucial to understanding the first 500 Myr of crustal evolution of Earth. Very few zircons of this age (>4050 Ma) have been found other than from a ~300 km diameter domain of the Yilgarn Craton, Western Australia. Here we report SIMS U-Pb and O isotope ratios and trace element analyses for two ~4100 Ma detrital zircons from a Paleozoic quartzite at the Longquan area of the Cathaysia Block. One zircon (207Pb/206Pb age of 4127 ± 4 Ma) shows normal oscillatory zonation and constant oxygen isotope ratios (δ18O = 5.8 to 6.0‰). The other zircon grain has a ~4100 Ma magmatic core surrounded by a ~4070 Ma metamorphic mantle. The magmatic core has elevated δ18O (7.2 ± 0.2‰), high titanium concentration (53 ± 3.4 ppm) and a positive cerium anomaly, yielding anomalously high calculated oxygen fugacity (FMQ + 5) and a high crystallization temperature (910°C). These results are unique among Hadean zircons and suggest a granitoid source generated from dry remelting of partly oxidizing supracrustal sediments altered by surface waters. The ~4100 Ma dry melting and subsequent ~4070 Ma metamorphism provide new evidence for the diversity of the Earths earliest crust.

Spatiotemporal reconstruction of Late Mesozoic silicic large igneous province and related epithermal mineralization in South China: Insights from the Zhilingtou volcanic-intrusive complex

Silicic large igneous provinces (SLIPs) generally reflect large-scale melting of lower crustal materials and represent significant metal reservoirs. The South China Block-Coastal Region (SCB-CR) SLIP hosts several large epithermal deposits. To better understand these deposits, we document the spatio-temporal framework of the host SLIP across the SCB-CR. Using zircon U-Pb dating and geochemical and isotopic analysis, we identify four stages of emplacement. Stage 1 felsophyre (ca. 149 Ma) shows a chemical affinity to highly fractionated I-type granites. Stages 2 and 3 of low-Mg felsic volcanics (ca. 128 to 111 Ma) and stage 4 felsite (ca. 100 Ma) have higher eHf(t) and eNd(t) values than stage 1 felsophyre, suggesting a significant contribution of newly underplated juvenile crust to the magma sources. Stage 4 diabase (ca. 101 Ma) was likely produced by melting of subduction˗metasomatized asthenospheric mantle. Together with reliable published data, we build a new spatio-temporal framework of volcanics and infer that the majority of the SCB-CR SLIP was related to the gradual northwestward subduction of the Izanagi plate beneath South China in a continental arc setting during ca. 170 to 110 Ma, and minor contribution was from the eastward retreat of the subducting slab in a back-arc setting during ca. 110 to 90 Ma. We conclude that the large scale epithermal mineralization was generated by melting of the metal-rich, thin (30-40 km), newly underplated hydrous juvenile crust during the tectonic transition from arc to back-arc settings.

Neoproterozoic backarc basin on the southeastern margin of the Yangtze block during Rodinia assembly: New evidence from provenance of detrital zircons and geochemistry of mafic rocks

Several tectonic models have been proposed for the Neoproterozoic amalgamation of the South China block during the assembly of Rodinia. However, the timing of the end of arc magmatism between the two subblocks in the South China block (i.e., the Yangtze and Cathaysia blocks) remains controversial because it is unclear whether the 860−820 Ma magmatic rocks and coeval sedimentary basins in this area are related to subduction or plume activity. Here, we present new detrital zircon U-Pb and Hf isotopic data for the sediments directly overlying early Neoproterozoic arc volcanic rocks in this region. These data reveal a rhythmic change in source coincident with a progressive increase in the amount of juvenile and old crustal detritus within these sediments. This result, combined with the presence of a fining-upward grain-size trend and horizontal bedding within these sediments, provides evidence of bidirectional sources that are consistent with a backarc setting. The juvenile crustal material within these sediments was sourced from adjacent arc terranes to the east, whereas the old crustal detritus was derived from the Yangtze block to the west. In addition, sensitive high-resolution ion microprobe zircon U-Pb dating of mafic rocks within equivalent sedimentary sequences yielded ages of ca. 860−840 Ma, and these mafic rocks have arc- or mid-ocean-ridge basalt−like geochemical features that indicate the initiation of backarc spreading associated with Neoproterozoic NW-directed subduction. The data from the sediments and mafic rocks suggest the presence of a backarc basin system at ca. 860−820 Ma within the southeastern margin of the Yangtze block. This in turn indicates that Rodinia assembly was not completed until ca. 820 Ma, with the South China block possibly acting as a connection between a Neoproterozoic Andean-type active continental margin and Grenvillian belts on the paleo−western margin of the Rodinia supercontinent.

Annular crustal accretion and reworking around the Yangtze nuclei during Neoproterozoic: a link to the initiation and termination of annular subduction of oceanic crust

This journal supplement has title: Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt ConferenceThis journal supplement has title: Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt Conference