Zhang Bangtong

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.

Elemental geochemistry and Nd isotopic characteristics of the metasedimentary rocks from the metamorphic belt in central Jiangxi: Provenance and tectonically environmental constraints

The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-are-naceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [high Th/Sc (0.57−3.59), La/Sc (1.46−12.4), La/Yb (5.84−19.0)] and variable Th/U ratios, with ΣREE=129−296µg/g, δEu=0.51−0.86, and (La/Yb)N=3.95−12.9. The Nd isotopic model agestDM of these rocks vary from 1597 to 2124 Ma. Their143Nd/144Nd values are low [ɛNd(0)=−11.4 to −15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and K-rich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199±26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model agetDM (1597−2124 Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic (1100−1600 Ma).The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-are-naceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [high Th/Sc (0.57−3.59), La/Sc (1.46−12.4), La/Yb (5.84−19.0)] and variable Th/U ratios, with ΣREE=129−296µg/g, δEu=0.51−0.86, and (La/Yb)N=3.95−12.9. The Nd isotopic model agest DM of these rocks vary from 1597 to 2124 Ma. Their143Nd/144Nd values are low [ɛNd(0)=−11.4 to −15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and K-rich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199±26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model aget DM (1597−2124 Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic (1100−1600 Ma).

Petrology, chronology and isotope geochemistry of the proterozoic amphibolites from Xiangshan, central Jiangxi province, China

On the basis of a comprehensive study on the petrology, trace elements and isotopic geochemistry of the Xiangshan amphibolites, we suggest that the protoliths of the amphibolites were basalts formed in an island-arc tectonic setting. The basaltic magma was derived from a slightly depleted mantle source with a small amount of crustal contamination. Assemblage of the rock-forming minerals indicates that these amphibolites underwent a low-grade metamorphism of amphibolite facies. According to the formation age (1113 Ma) and subsequent metamorphic age (726.6 Ma) of the basalts as well as the geological and geochemical features of these amphibolites, a tectonic model of Proterozoic oceanic island-arc setting is proposed for central Jiangxi.

Geochemical evidence for contribution of ore-forming materials from peraluminous granite basement —— Taking Fucheng pluton and No. 6722 uranium deposit in southern Jiangxi Province as examples

Using the induced fission-track method, mobile uranium leaching and lead isotope analysis, this work obtianed geochemical features of the peraluminous Fucheng granite basement and the host rock (shoshonite) of the No. 6722 uranium deposit in southern Jiangxi Province, (i) Uranium contents of the leucocratic rock-forming minerals (0.18 μg/g for quartz, 0.36 μg/g for feldspar) are lower than the uranium content of the whole rock (4.6 μg/g). Biotite and some accessory mineral inclusions (zircon, monazite and uraninite) are the main uranium carriers of the Fucheng granite pluton. The fissure uranium in altered minerals (hydromica and chlorite) increased evidently, (ii) Leachable rate of mobile uranium in the biotite granite is 10.4⨴; while that in the altered granite increased to 31· · (iii) Caculation based on lead isotopes shows that during alteration the Fucheng granite lost uranium (AU = −37%–−65· · ), whereas the Caotaobei shoshonite gained uranium (AU = +37· ·–+58 · ·). These features suggest that the ore-forming material of the No. 6722 uranium deposit was mainly derived from the altered peraluminous granite basement of Fucheng pluton.

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.

Speciation and precipitation of uranium complexes in hydrothermal solutions related to granite-type uranium deposits

Uranium bearing hydrothermal solutions during the stage of ore deposition are weakly alkaline and of the Ca2+ - Na+ /HCO3− - F− type. UO2(CO3)22− and UO2F4−, are dominant in the hydrothermal solutions with respect to their activity. Wall-rock hydrothermal alterations, temperature and pressure drop and the reducing capability of rock assemblage (ΔEh) led to a decrease in Eh of the hydrothermal solutions and an increase in Eh at which uranium began precipitating. Therefore, the mechanism of uranium precipitation is essentially the reduction of uranium complexes.The granite-type uranium deposits are the most important type of uranium resources in China. Discussions will be made in this paper concerning the hydrothermal speciation and precipitation mechanisms of uranium complexes in the light of fluid inclusion and geological data from some major deposits of this type in South China.

The use of bond dissociation energy to analyze the geochemical behavior of uranium in the process of granitic magmatism

It is estimated that the uranium-oxygen bond dissociation energy (DU4−0+=1465.38kJ/mol) is higher than silicon-and aluminium-oxygen bond dissociation energies (DSi4−0+=1264.41 kJ/mol and DAl3−0+=1105.32 kJ/mol). During the process of magmatic differentiation with increasing degree of polymerization for silicon-oxygen complex ion, uranium in granitic melts tends to combine with oxygen to form the coordination polyhedron [UOx2x−n] and to occur in the form of nuclei and crystals of uraninite, as has been demonstrated by the induced fission-track study of quartz syenite from Huangmeijian.

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.