Qingli Zeng

Magnetic study of the UHP eclogites from the Chinese Continental Scientific Drilling (CCSD) Project

Received 7 July 2008; revised 4 December 2008; accepted 2 January 2009; published 24 February 2009. [1] To establish the relationships between rock magnetism of eclogites and the corresponding retrograded metamorphic processes, this study integrated both rock magnetism and metamorphic petrology of 171 eclogite samples with different degree of retrograded metamorphism at the depth range of 100–2050 m of the main hole of the Chinese Continental Scientific Drilling (CCSD) Project, located in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China. Results show that the average density is a suitable indicator for the degree of the retrograded metamorphism. Opaque minerals in variable eclogites are mainly magnetite, ilmenite, pyrite, and hematite. With increasing the metamorphic retrogression, the concentration of opaque minerals generally increases; however, magnetic properties of eclogites first increase from well-preserved eclogite to partially retrogressed eclogites because of the neoformation of coarse grained magnetite particles, and then decrease for the completely retrogressed eclogites because of the consumption of the magnetite particles and the formation of weakly magnetic minerals (e.g., hematite). These processes can be summarized as garnet + omphacite + rutile + SiO2 +H 2O ! amphibole + plagioclase + magnetite + ilmenite ! amphibole + epidote + hematite + ilmenite. The retrograded metamorphic eclogites with elevated magnetic properties take up 25% of all rocks and thus are one of major magnetic rocks in the CCSD main hole located in the Sulu UHP area. Therefore the significant volumes of retrograded eclogites may account for the magnetic anomalies flanking the northeastern part of the Sulu UHP metamorphic belt.

Combining Genetic Algorithm and Generalized Least Squares for Geophysical Potential Field Data Optimized Inversion

A genetic algorithm (GA) and generalized least squares (GLS)-based approach, hereafter called GA-GLS, is proposed to solve geophysical optimized inversion. In this method, GA is exploited to initialize nonlinear parameter estimation, and GLS is used for accurate local search. Here, we compare the results from GA, GLS, and proposed GA-GLS to invert the synthesized potential field. The results show that GA-GLS outperforms GA in terms of accuracy, as well as GLS, which needs given initial parameters. The real data are taken to verify the feasibility of implementing it in practice.

Petromagnetic properties of granulite-facies rocks from the northern North China Craton: Implications for magnetic and evolution of the continental lower crust

This paper studies magnetic properties and composition of granulite-facies rocks of both the Neogene and Archean continental lower crust in the Neogene xenolith-bearing Hannuoba(汉诺坝) alkaline basalt and the exposed lower crustal section in the Archean Huai’an(淮安) terrain (Wayaokou (瓦窑口)-Manjinggou(蔓菁沟 profile), the northern North China Craton. It provides a unique opportunity for a comparative study of magnetic properties and composition of both the Archean and Neogene continental lower crust. We measure magnetic parameters (susceptibility κ and magnetic hysteresis parameters, such as saturation magnetization Js, saturation isothermal remanent magnetization Jrs, and intrinsic coercivity Hc) of eleven Hannuoba lower crustal xenoliths and nine terrain granulites from the Archean Huai’an terrain. Results indicate that the average values of κ, Js and Jrs of Archean granulites are 4 122×10−6 SI, 523.1 A/m and 74.9 A/m, respectively, which are generally higher than those of granulite-facies xenoliths (1 657×10−6 SI, 163.9 A/m and 41.9 A/m, respectively). These two types of granulites contain ilmenite, (titano) magnetite, minor hematite and some “magnetic silicates” (clinopyroxene, plagioclase and biotite). The Mg-rich ilmenite in granulite-facies xenolith is relatively higher than that in terrain granulites. We observe a more evolved character as higher magnetic as well as lower Sr/Nd, Cr/Nd, Ni/Nd, Co/Nd and V/Nd ratios in terrain granulites. These differences in magnetic characteristics reflect their different origins and evolutions. The high magnetization of granulites in the Huai’an terrain represents magnetic properties of the Archean continental lower crust, and low magnetization of granulite-facies xenoliths represents magnetic properties of the Cenozoic lower crusts in the northern North China Craton.