Chen Peirong

Single zircon LA-ICPMS U-Pb dating of Weishan granite (Hunan, South China) and its petrogenetic significance

A detailed microstructure analysis and LA-ICPMS U-Pb dating have been done for zircons of Weishan granite from Hunan Province, South China. The results indicate that the Weishan granite is a multistage batholith formed during the late Indosinian-early Yanshanian time. The intruded time of the late Indosinian granite is 211.0±1.6Ma and 215.7±1.9Ma (two samples), whereas that of the early Yanshanian granite is 187.4±3.5Ma and 184.5±5.1Ma (two samples). In combination with other geochronological data for Indosinian rocks of South China and the adjacent region, it is inferred that the late Indosinian granites of South China (especially Hunan Province) are probably formed under extension regime as a consequence of post-collision stress relaxation, which is a spontaneous response to intracontinental thickening attributed to the collision and extrusion of two Indosinian seams, namely Qinling-Dabie and Song Ma. Moreover, it is also deduced that the early Yanshanian granites of Hunan Province could not be directly related with the subduction of Paleo-Pacific plate towards Eurasian continent, and they are most likely derivation of the mid-or lower-crustal materials because of decompressional melting under the continuous extension setting.

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.

Geodynamic setting of Mesozoic magmatism and its relationship to uranium metallogenesis in southeastern China

In southeastern China, Mesozoic magmatism is wide-spread and formed abundant rare metal and nonferrous metal deposits.

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.

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.

Some high-P-subtype and low-P-subtype F-rich granites in South China

The F-rich granites in South China could be distinguished as the high-P subtype and the low-P subtype according to their P2O5 contents. There are obvious differences in chemical composition of these two subtypes. The high-P subtype is strongly peraluminous and characterized by low silica and very low REE contents, while the low-P subtype is weakly peraluminous and characterized by high silica and very high REE contents. There are also some differences in chemical compositions of feldspars and micas for these two subtypes. The phosphorus of the high-P subtype mainly appears to be in the feldspar structure as PAISi−2 substitution or subordinately in amblygonite as an accessory mineral, while the phosphorus of the low-P subtype is mainly in apatite and other phosphate minerals.

The Tin-Bearing Topaz-Quartz Porphyry, Yangbin Area, Taishun County, Zhejiang Province A Fluid Inclusion Study

The tin-bearing topaz-quartz porphyry in the Yanbin area is subvolcanic in origin, derived from granitic residual magma through strong crystallization differentiation. The rock contains various types of inclusions (hydroxyl-melt, vapor-rich, halite-bearing multiphase, and liquid-rich), which permits us to trace the evolutionary path of the fluid. It is suggested that immiscibility took place in a pulsating manner between melts and fluids during the rock-forming process with the homogenization temperatures ranging from 580°C to 180°C and the salinities varying from low through high to low (5.1–10. 5»6. 9-21.4»30-48»16-22»2-10 in wt% NaCl). The hydroxyl-melt inclusions are considered as evidence of magma-hydrothermal transition.