Li Xianhua

SHRIMP U-Pb zircon age,geochemistry and Nd-Hf isotope of Neoproterozoic mafic dyke swarms in western Sichuan:Petrogenesis and tectonic significance

Neoproterozoic magmatic rocks are widespread in the western margin of the Yangtze block, and their origin and genesis have significant implications for understanding the evolution of the Rodinia super-continent. However, there are currently two opposing interpretations for their petrogenesis and tectonic setting: mantle plume-related and island arc origin. To further verify these two competing models, SHRIMP U-Pb zircon age determinations and geochemical and Nd-Hf isotopic analyses are conducted on the mafic dykes in the Kangdian Rift, western Sichuan. U-Pb dating suggests that these mafic dykes were emplaced at 780–760 Ma, spatially and temporally coeval with the Kangding granitoid complex. The parental magmas of these dykes were derived from a depleted asthenosphere mantle source likely triggered by an anomalously-hot mantle plume. Despite some arc-geochemical features caused by variable degrees of contamination of young island arc crust during magma ascending and emplacement, they show general geochemical and Nd-Hf isotopic features similar to those of the intraplate basalts. Our results support the reconstruction model of Rodinia in which the South China block was located between Australia and Laurentia.

Provenance of a prominent sediment drift on the northern slope of the South China Sea

The sedimentation rate of the sediment drift in the southeast of Dongsha Islands is as high as 49 cm/ka in the last 1.05 Ma. Although the sedimentation rate changes with time, the contents of rare elements of the sediments remain almost the same, indicating that the source area of the sediments has been constant with time. On the triangular diagrams of La-Th-Sc and Th-Sc-Zr/10, the samples from the southeast of Dongsha Islands fall within the continental island arc field, overlapping the samples from Taiwan, while the samples from the Pearl River, those from the west of the Philippines that contain volcanic material, are separated from them. This indicates that the sediments from the southeast of Dongsha Islands have a close relationship to those from Taiwan in terms of provenance. In fact, the sediments on the northern slope of the South China Sea were derived from northeast direction, as well as from Taiwan, most probably transported from Taiwan through Penghu channel into the South China Sea. The terrigenous part of the deep-sea sediments in the north of the South China Sea was provided by different sources.

SHRIMP ion microprobe zircon U-Pb Age and Sm-Nd isotopic characteristics of the NE Jiangxi ophiolite and its tectonic implications

The new SHRIMP zircon U-Pb isotopic study suggests that the crystallization age of a highly fractionated magma in the NE Jiangxi ophiolite suite is 968±23 Ma. Re-calculated Sm-Nd isochron age of 955±44 Ma is within analytical errors. consistent with the zircon U-Pb age. With the exception of two anomalous Sm-Nd data, the remaining 15 analyses so far obtained for the ophiolite gaveεNd (T) values falling into a limited range from +4.3 to +6.7, indicating that the ophiolite was derived from a relatively strongly depleted mantle source. Sm-Nd isotopic systematics in some samples may have been strongly affected by post-magmatic events, such as alteration, deformation and metamorphism, resulting in anomalousεNd (T) values. Combined with published40Ar39Ar age data, it can be concluded that the collision between the Yangtze and South China Blocks occurred during 0.97-0.80 Ga.

Geochemistry of granitoid rocks from Zhejiang Province and crustal evolution—I. Phanerozoic granitoid rocks

The Phanerozoic granitoid rocks include the Caledonian, Indosinian and Yenshanian granitoid rocks. The existence of Caledonian and Indosinian granites was evidenced by zircon U-Pb ages. The study of the characteristics of major, trace and rare-earth elements, isotopic composition and petrogenesis for the granitoid rocks has been made. The Caledonian and Indosinian granites were derived from partial melting of the Proterozoic basement rocks and the two tectonic activities were weak. The Yenshanian granitoid rocks were derived from mixing of mantle and crustal materials. It implies that the crustal accretion took place in Mesozoic time.

The Sr, Nd isotopic composition of lamprophyres in Laowangzhai gold orefield, Yunnan Province

Lamprophyres are widely spread over the Laowangzhai gold orefield in northern Mt. Ailao structure zone, Yunnan Province. Lampmphyres in the orefield are temporally and spatially related to gold mineralization. Lampmphyren in the orefield have anomalous Sr and Nd isotopic compositions, e.g. the87Sr/86Sr ratios (0.706 665–0.707 960, the87Sr/86Sr ratio of sample YLW-44 is 0.709 041) are higher than the modern value of the original mantle (0.704 5), the143Nd/144Nd ration (0.512 463–0.512 551) are lower than the modern value of the original mantle (0.512 638), and the calculated values εSr > 0 (28.1–63.7) and εNd< 0 (- 1.34- -3.07). The results of investigation inete that lampmphyres in the Laowangzhai gold orefield with anomalous Sr and Nd isotopic compositions would not be produced as a result of contamination of the primary magma with the isotopic features of MORB by the crust materials high87Sr/86Sr ratios and low143Nd/144Nd ratios in the ploeaws of rising or in the magma chamber, but the result of metasomatism of source mantle.

Geochemistry of granitoid rocks and crustal evolution, Zhejiang Province, China— II. Proterozoic granitoid rocks

Proterozoic granitoid rocks in Zhejiang Province were formed in the Shengongian period (1.8–1.9 Ga) and the Late Jinningian period (0.6–0.9 Ga), respectively. Petrogenetic problems are discussed based on chemical (major, trace elements and REEs) and Nd-Sr isotopic compositions. The Shengongian granites resulted from partial melting of the Badu Group and the Late Jinningian granites are of mantle derivation with or without contamination of crustal material. The crust in Zhejiang had undergone three major periods of growth during 2.6–2.7 Ga, 0.8–1.1 Ga and 0.10–0.12 Ga after it was generated in Archean time. Compositional fractionation in the process of crust evolution is not evident. The presence of Late Jinningian granites of mantle and mantle-crust-derivation along the Jiangshan-Shaoxing Fault is indicative of crust subduction at that time.