Geoscience

Attention Focused on Mantle-derived Magmatism

Zhang Xiaohui, Asso. Professor of the State Key Laboratory of Lithospheric Evolution, the Institute of Geology and Geophysics, CAS, in cooperation with his colleagues, published a thesis entitled mantle-derived magmatism with the background of post-orogenic extension happended in Middle Triassic at North China craton and that at Xing-Meng Orogenic Belt in Early Devonian Late Silurian to Early Devonian in Geological Magazine and International Geology Review, respectively. The paper published in Geological Magazine（Zhang et al. Geochemistry of the Middle Triassic gabbros from the northern Liaoning, North China: origin and tectonic implications. Geological Magazine, 2009, 146(4): 540-551）, reported that through detailed field survey, Zircon U–Pb SHRIMP dating and systematic geochemical research, Prof. Zhang Xiaohui demonstrated that in Faku dome of northern Liaoning, located at the transitional area from North China craton to Xing-Meng Orogenic Belt, Xiaofangshen gabbros which is taken as emplacement in Early Precambrian, was formed in Triassic（241Ma）. They concentrated large-ion-lithophile-elements and are featured by high-field-strength-element depletions. They show a relatively narrow range of isotopic compositions with medium level radiogenic Sr-Nd and high radiogenic zircon Hf. These geochemical features suggest that they might have been derived from partial melting of a subduction-related metasomatized lithospheric mantle source. The fact that emplacement in Early Precambrian is actually granitic intrusions from the Faku dome from late Paleozoic to early Mesozoic shows that no large scale craton Pre-Cambrian basement exist here. These gabbros, together with alkaline granites, A-type granites and other rocks in other areas of North China Craton of the same time, constitute the post-orogenic magmatic suite that represents the orogenic ending.
In the paper published in International Geology Review （Zhang et al. Geochemistry of hornblende gabbros from Sonidzuoqi, Inner Mongolia, North China: implication for magmatism during the final stage of suprasubduction-zone ophiolite formation. International Geology Review, 2009, 51(4): 345-373）, chronological and geochemical study showed that mafic-ultramafic rocks in Sonidzuoqi, Inner Mongolia was formed in Late Silurian to Early Devonian. They are mainly hornblende-rich gabbroic rocks, featured by both MORB and islandarc magmaticrocks, postulating that the melt formed through the coupling of MORB-type mantle upwelling with aqueous fluid influx derived from slab devolatilization. This petrogenetic scenario suggests that an active spreading centre entered the trench during ridge subduction, bringing to a close an episode of suprasubduction-zone ophiolite formation. The Siluro-Devonian hornblende gabbros, together with a pre-490 Ma ophiolitic me′lange of MORB-OIB affinity, 483–471 Ma arc intrusions, 498–461 Ma trondhjemite-tonalite-granodiorite plutons, and 427–423 Ma calc-alkaline granites from the same area, provide documentation of multistage crustal generation processes during the life cycle of this suprasubduction-zone ophiolite.

More Precise on Modeling of Seismic Wave

The magazine Geophysical Prospecting recently published a thesis entitled Lax-Wendroff and Nystrom methods for seismic modeling (Chen et al. Geophysical Prospecting, 2009, 57: 931-941) written by Chen Jingbo, Associate Research Fellow of the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, CAS. Chen Jingbo applied structure-preserving algorithm in the algorithm design of seismic wave numeral modeling, revealed the structure-preserving nature of current method and developed the new method mentioned above. He meticulously verified and analyzed the relationship between the stability and frequency dispersion of the algorithm, so that the algorithm developed has much higher precision while maintaining the efficiency. It provides an effective numerical modeling method for high-precision seismic technology.