Lithospheric thermal structure and rheology of the eastern China



The temperature distributions of the lithosphere underneath the mainland of China were estimated by applying local isostatic equilibrium-constrained geothermal calculations. Maps of the lateral temperature variation at depths of 40, 70, and 100 km are presented for the whole Chinese continent, with the thermal thickness of the lithosphere is calculated. Lithospheric roots of 160–200 km thickness underlie Tarim and the Upper Yangtze platform, but are absent beneath the entire Sino-Korean platform. In general, the Tibetan plateau and fold belts to the north have warm but thick lithospheres, whereas thinner thermal lithospheres have been identified in northern Tibet and central Tian Shan around Issyk-Kul Lake. The warm and soft lithosphere in the Tibetan plateau and Tian Shan are caused by uniform north-south shortening, which may represent a snapshot of the early stage of convective thinning of the convergent lithosphere. However, the lithospheric thinning beneath northeastern China might be related to volatile infiltration by dehydration of the deeply subducting Pacific slab during the Cenozoic. Dry and wet upper mantle rheology display “jelly sandwich” and “creme brulee” pictures, respectively, demonstrating the mechanical behaviour of the Chinese lithosphere outside the Tibetan plateau. Considering a more geologically evident wet-mantle rheology, the “creme brulee” model can approximate the lithospheric rheology for the most earthquake-prone regions on the Chinese mainland.

Thermal State and Strength of the Lithosphere Beneath the Chinese Mainland

An isotopic study was systemically carried out on the granitic complex,diorite-porphyrite, ores and ore minerals of the 103 Ma Xiaoxinancha gold-rich copper deposit in Jilin province to determine the geodynamic model of diagenesis and metallogenesis.Results show that the initial Nd and Sr isotopic compositions of the granitic complex are in the range of 0.70425-0.70505 for(~(87)Sr/~(86)Sr)_i, 0.51243-0.51264 for I_(Nd),and -1.31 to +2.64 fore_(Nd)(t);those of the diorite-porphyrite are in the range from 0.70438-0.70448 for(~(87)Sr/~(86)Sr),,0.51259-0.51261 for I_(Nd),and +1.56 to +2.09 fore_(Nd)(t).For ores and sulfides,the(~(87)Sr/~(86)Sr)_i,7_(Nd),ande_(Nd)(t) values are in the range from 0.70440-0.70805,0.51259- 0.51279 and +1.72 to +5.56,respectively.The Pb isotopic ratios of the granitic complex range from 18.2992-18.6636 for ~(206)Pb/~(204)Pb,from 15.5343-15.5660 for ~(207)Pb/~(204)Pb,and from 38.1640-38.5657 for ~(208)Pb/~(204)Pb For diorite-porphyrite,the isotopic ratios of ~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb are 18.3919,15.5794 and 38.3566,respectively,whereas those of the ores and ore sulfides vary from 18.2275-18.3770 for ~(206)Pb/~(204)Pb,from 15.5555-15.5934 for ~(207)Pb/~(204)Pb and from 38.1318-38.3131 for ~(208)Pb/~(204)Pb.The results indicate that the mineralization was correlated to the formation and evolution of the granitic complex and the diorite-porphyrite.Combining with the reported data in petrologic characteristics,elemental geochemistry and chronology,conclusions can be drawn that the geodynamic settings of diagenesis and metallogenesis of this deposit were consistent with the subduction of the Izanagi oceanic plate during the Early Cretaceous.The diorite-porphyrite was formed by the emplacement of the adakitic magma triggered by partial melting of the enriched mantle,which originated from the derivative continental lithospheric mantle metasomatized by dehydration fluids from the subducting oceanic crust.The granitic complex was produced by fractional crystallization of the mixture between the adakitic magma and the high-K calc-alkaline acidic magma,which were generated by the remelting of the lower crust in the course of intraplate upwelling of the adakitic magma.The ore-bearing fluid reservoir convened in a late stage of the evolution of the mixed magma chamber.

An Isotopic (Sr, Nd and Pb) Tracer Study on the Xiaoxinancha Gold-rich Copper Deposit in Yanbian, China: Implication for the Geodynamic Model of Diagenesis and Metallogenesis



Chemical and isotopic compositions have been measured for N2-He-rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.1–1.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium is high in the northern island (12%–16% of total He) and gradually decreases towards southern coast (l%–3% of total He). Such a distribution pattern is most likely controlled by the Pleocene-Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although δ13C values of CO2 (–20‰ to −27‰) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and δ13CCO2 suggests a two end-member mixing of mantle and crustal components with CO2/3He ratios of 2×109 and 8×1011, respectively. However, the low CO2/3He ratios (1–22×106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas-water-rock interaction.

Helium, Argon and Carbon Isotopic Compositions of Spring Gases in the Hainan Island, China

https://link.springer.com/content/pdf/10.1007%2Fs11589-012-0865-z.pdf

A preliminary study on the lithospheric thermal structure and rheology of the Tibetan plateau



Based on conservation of energy principle and heat flow data in China continent, the upper limit of 1.3 μW/m3 heat production is obtained for continental crust in China. Furthermore, using the data of heat flow and helium isotope ratio of underground fluid, the heat productions of different tectonic units in China continent are estimated in range of 0.58–1.12 μW/m3 with a median of 0.85 μW/m3. Accordingly, the contents of U, Th and K2O in China crust are in ranges of 0.83–1.76 μg/g, 3.16–6.69 μg/g, and 1.0%–2.12%, respectively. These results indicate that the abundance of radioactive elements in the crust of China continent is much higher than that of Archean crust; and this fact implies China's continental crust is much evolved in chemical composition. Meanwhile, significant lateral variation of crustal composition is also exhibited among different tectonic units in China continent. The crust of eastern China is much enriched in incompatible elements such as U, Th and K than that of western China; and the crust of orogenic belts is more enriched than that of platform regions. It can also be inferred that the crusts of eastern China and orogenic belts are much felsic than those of western China and platform regions, respectively, derived from the positive correlation between the heat production and SiO2 content of bulk crust. This deduction is consistent with the results derived from the crustal seismic velocity data in China. According to the facts of the lower seismic velocity of China than the average value of global crust, and the higher heat production of China continent compared with global crust composition models published by previous studies, it is deduced that the average composition models of global continent crust by Rudnick and Fountain (1995), Rudnick and Gao (2003), Weaver and Tarney (1984), Shaw et al. (1986), and Wedepohl (1995) overestimate the abundance of incompatible elements such as U, Th and K of continental crust.

Zhiming Sun

Papers

Crustal Composition of China Continent Constrained from Heat Flow Data and Helium Isotope Ratio of Underground Fluid