Ling Hongfei

Crust evolution in Southeast China: evidence from Nd model ages of granitoids

Nd isotopic compositions of 58 granitoids in South China have been reported in this paper. These data together with other published data reveal that granites with Nd model ages ( t DM ) greater than 1.8 Ga are distributed mainly in three areas: southwestern Zhejiang-northwestern Fujian, two sides of the Wuyi Mountain and Wanyangshan-Zhuguangshan. These granites are believed to be derived from partial melting of old crust in these areas. The Mesozoic granites with t DM 1.6 Ga are distributed in three zones: the Gangang structural zone, Nanling latitudinal structural zone and Fujian-Zhejiang coastal zone. These zones may have been an extensional tectonic setting and mantle-derived components or magmas may have been involved to different extents in the granite formation. Based on Nd model ages of granites and published chronological data of mafic and ultramafic rocks, it is believed that the crust in South China experienced episodic accretions, among which the early-middle Proterozoic is the most important period of crustal accretion.Nd isotopic compositions of 58 granitoids in South China have been reported in this paper. These data together with other published data reveal that granites with Nd model ages(tDM) greater than 1.8 Ga are distributed mainly in three areas: southwestern Zhejiang-northwestern Fujian, two sides of the Wuyi Mountain and Wanyangshan-Zhuguangshan. These granites are believed to be derived from partial melting of old crust in these areas. The Mesozoic granites withtDM < 1.6 Ga are distributed in three zones: the Gangang structural zone, Nanling latitudinal structural zone and Fujian-Zhejiang coastal zone. These zones may have been an extensional tectonic setting and mantle-derived components or magmas may have been involved to different extents in the granite formation. Based on Nd model ages of granites and published chronological data of mafic and ultramafic rocks, it is believed that the crust in South China experienced episodic accretions, among which the early-middle Proterozoic is the most important period of crustal accretion.

Estimate of influence of U-Th-K radiogenic heat on cooling process of granitic melt and its geological implications

The U-Th-40K concentrations of granite are on 1–2 orders of magnitude greater than those of basaltic-ultrabasic rocks. Radiogenic heat of a granitic melt has significant influence on the cooling-crystallization period of the melt. In this paper we derived a formula to calculate prolongation period (tA) of cooling-crystallization of a granitic melt caused by radiogenic heat. Calculation using this formula and radioactive element concentrations (U=5.31×10−6; Th=23.1×10−6; K=4.55%) for the biotite adamellite of the Jinjiling batholith shows that the tA of the adamellite is 1.4 times of the cooling period of the granitic melt without considering radiogenic heat from the initial temperature (Tm=960°C) to crystallization temperature (Tc=600°C) of the melt. It has been demonstrated that the radiogenic heat produced in a granitic melt is a key factor influencing the cooling-crystallization process of the granitic melt, and is likely one of the reasons for inconsistence between emplacement ages and crystallization ages of many Meso-Cenozoic granitoids.

Nd isotopic composition and material source of pre- and post-Sinian sedimentary rocks in Xiushui area, Jiangxi Province

Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area, Jiangxi Province. Significant differences are noticed between them in their Nd isotopic compositions. As for the pre-Sinian lightly metamorphozed sedimentary rocks,143Nd/144Nd=0.512000 ∼ 0.512214, ɛNd (T) = −2.57 ∼ 3.91, and TDM=1563 ∼ 1859 Ma;143Nd144Nd=0.511710 ∼ 0.511902, ɛNd(T)= −8.04 ∼ −9.99, and TDM= 1833 ∼ 2426Ma are suggested for the Sinian-Cambrian sedimentary rocks. These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period. Mantle material appears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area, Jiangxi Province. Significant differences are noticed between them in their Nd isotopic compositions. As for the pre-Sinian lightly metamorphozed sedimentary rocks,143Nd/144Nd=0.512000 ∼ 0.512214, ɛNd (T) = −2.57 ∼ 3.91, and TDM=1563 ∼ 1859 Ma;143Nd144Nd=0.511710 ∼ 0.511902, ɛNd(T)= −8.04 ∼ −9.99, and TDM= 1833 ∼ 2426Ma are suggested for the Sinian-Cambrian sedimentary rocks. These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period. Mantle material appears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.

New index of ferromanganese crusts reflecting oceanic environmental oxidation

Ferromanganese crusts (hereinafter crusts) form in aerobic environment and the environmental oxidation degree is recorded by the redox sensitive element Co in the crusts. The ages of the layers from the surface to bottom of the crusts are determined, and main element contents at high resolution along the depth sections of three crusts from the Pacific Ocean are analyzed by an electron microprobe. Thus the variations of Co/(Fe+Mn) and Co/(Ni+Cu) with age/depth of the crust layers are obtained. By comparing the ratios of Co/(Fe+Mn) and Co/(Ni+Cu) with the δ18O curves of the Pacific benthic foraminifera, we find that these two ratios can reflect the variation of the environmental oxidation state under which the crust layers deposit. The evolution of the oxidation degree reflected by the two indexes resembles the evolution of temperature since the Oligocene reflected by the δ18O curves of the Pacific benthic foraminifera. This suggests that the crust-forming environment after the Oligocene is controlled mainly by the oxygen-rich bottom water originated from the Antarctic bottom water (AABW). However it is not the case prior to the Oligocene. Furthermore it suggests that the environmental oxidation degree controls the formation of the crusts and the Co contents in the crusts. This explains why the Co contents in the crusts increase with time up to now.

Geochemical characteristics and genesis of topaz-bearing porphyries in Yangbin area of Taishun County, Zhejiang Province

The host rocks of the porphyry tin deposits in the Yangbin area are principally topaz-bearing porphyry dikes about 2 km long and 2–20m wide. Three lithologie types are identified for the dikes: topaz-bearing potassium feldspar granitic porphyry, topaz-bearing monzonitic granitic porphyry and topaz-bearing quartz porphyry. The content of topaz in the rocks ranges from 10 to 20 vol.%. Porphyritic texture is characteristic, with quartz, potassium feldspar and albite as main phenocryst minerals. The phenocryst occupies 10–20 vol% of the rocks. The rock groundmass consists of subhedral topaz, quartz and protolithionite. Topaz has a unit-cell parameter b=8.797 (Å), and F:OH=1.92:0.18, indicating a F-rich variety formed at high temperature. The topaz-bearing porphyries occurring in this area are strongly peraluminous (A/NKC=1.574–12.94), with high ratios of F/Cl (146–303) and Rb/Sr (5–122). They are rich in incompatible elements (Sn, 313 × 10−6–1042 × 10−6; W, 6 × 10−6–218 × 10−6; Nb, 27 × 10−6–54 × 10−6), but poor in compatible elements (Sr, 10 × 10−6–28 × 10−6; Ba, 58 × 10−6–73 × 10−6; V, 3 × 10−6–12 × 10−6, Cl, 150 × 10−6–226 × 10−6). The rocks are also characterized by high total REE amount (281.69 × 10−6–319.76 × 10−6), with strong Eu depletion (δEu=0.01–0.03) and low ratio of LREE/HREE (0.78–0.84). In summary, the authors propose an idea of S-type genesis for the topaz-bearing porphyries with tin mineralization, instead of I-type.

Geochronology and isotope geochemistry of the Xiqiu spilite- keratophyre in Zhejiang Province, China

Spilite- keratophyre is a key member of the Shuangxiwu Group volcanic series in Zhejiang Province. Presented in this paper are the reliable Sm- Nd internal isochron age (1012±28 Ma, ЄNd(T)=4.4±0.1) and whole- rock Rb- Sr isochron age (972±40 Ma,ISr=0.70327±8) obtained for the first time by the authors. From the available Nd, Sr and O isotope data in conjunction with the major and trace elements data it is suggested that the Xiqiu spilite- keratophy re is the product of island- arc volcanism during the late Middle Proterozoic.

The geochemical evolution of the proterozoic continental crust in northwestern Jiangxi and western Zhejiang, China

The petrochemistry and Sm-Nd isotopic compositions and- the geochemical characteristics of REE, U,Th, etc. in the Late Proterozoic and Early Palaeozoic strata in northwestern Jiangxi and western Zhejiang provinces are described in this paper. It seems to be sure that the middle Proterozoic strata of southeastern China are low in the degree of maturation. The strata contain much mantle-derived material. At the end of Late Proterozoic there was an abrupt turn with respect to the crustal geochemical evolution of the eastern part of South China. Since then, the geochemical environment has undergone a change from a simple reducing environment to a complex oxidizing-reducing environment, which would be geochemically beneficial to the formation of Late Sinian to Early Cambrian U-bearing formations.