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2005 Vol. 16, No. 4

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Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet
Geng Quanru; Pan Guitang; Jin Zhenmin; Wang Liquan; Liao Zhongli
2005, 7(4)
Abstract:
The Early Jurassic bimodal volcanic rocks in the Yeba Formation , situated between Lhasa ,Dagz ê and Maizhokunggar, composed of metabasalt , basaltic ignimbrite , dacite, silicic tuff and volcanic breccia , are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys. Based on systematic field investigations , we carried out geochemical studies on representative rock samples. Major and trace element compositions were analyzed for these rock samples by XRF and ICP - MS respectively , and an isotope analysis of Rb - Sr and Sm - Nd was carried out by a MAT 262 mass spectrograph. The results show that the SiO2 contents in lava rocks are 41%- 50.4% and 64 %- 69 %, belonging to calc - alkaline basalt and dacite. One notable feature of the basalt is its low TiO2 content , 0. 66 %- 1. 01 %, much lower than those of continental tholeiite. The ?REE contents of basalt and dacite are 60. 3 - 135 μ g/g and 126. 4 - 167. 9 μ g /g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb* =0. 54 - 1. 17 averaging 0. 84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb * =0. 74 - 1. 06 averaging 0. 86.Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone The spinal lherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages. The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.
Petrogenesis of Mesozoic High-Mg Diorites in Western Shandong: Evidence from Chronology and Petro-geochemistry
Yang Chenghai; Xu Wenliang; Yang Debin; Liu Changchun; Liu Xiaoming; Hu Zhaochu
2005, 7(4)
Abstract:
The high - Mg diorites, widely occurring in western Shandong , have important implications in the study of the relationship between the Mesozoic magmatism and the nature of the lithosphere and its thinning period in the eastern North China craton (NCC ). The Tietonggou and Jinling intrusions are typically representatives. LA - ICP - MS zircon U - Pb dating , major and trace element and Sr - Nd isotopic compositions of the Mesozoic intrusive rocks from western Shandong , eastern China , were analyzed. The weighted mean 206 Pb/238 U ages from LA - ICP - MS zircon U - Pb dating results for early norite - diorite , late pyroxene - diorite from the Tietonggou intrusion and biotite - diorite from the Jinling intrusion are (131. 4 ±4. 9) Ma (n =15 ) , (134. 5 ±2. 3 ) Ma (n =13), and (132. 8 ±4. 2 ) Ma (n =12), respectively ,implying that they were formed in the Early Cretaceous. The weighted mean 207 Pb/206 Pb ages for round zircons from late pyroxene - diorite in the Tietonggou intrusion is(2 513 ±54 ) Ma (n =8 ) , suggesting that the basement of the North China craton should exist in the research area. The high - Mg diorites are characterized by enrichment in MgO , Na2O , and light rare earth elements (LREE ) , and they are poor in heavy rare earth elements (HREE) and high field strength elements (HFSE), being similar to adakite.The existence of the mantle peridotite xenoliths with a high - Mg feature for these intrusive rocks implies that their primary magma should be derived from the upper mantle. However, Sr - Nd isotopic compositions (Isr :0. 704 75 - 0. 707 15 ; ε Nd(130 Ma ) values:- 3. 95 to - 13. 30) , depletion in HFSE , and the occurrence of Archean inherited zircons suggest that crustal materials could be involved in the derivation of the primary magma. The compositional difference between the diorites from the Tietonggou and Jinling intrusions could be attributed to those of magma sources and degrees of partial melting. The Early Cretaceous high - Mg diorites are considered to have been formed by the mixed melting of the delaminated lithosphere (lithospheric mantle + crust ) and asthenosphere, based on their geochemistry , the mantle-derived xenoliths, and the Early Mesozoic lithospheric evolutionary history of the eastern North China craton.
Ocean-Continent Subduction within the Paleotethyan Archiopeligic Ocean from Mineralogical Evidence of Muztag Ophiolite, Eastern Kunlun Mountain, China
Lan Chaoli;Li Jiliang;He Shunli
2005, 7(4)
Abstract:
Former studies show that the Muztag ophiolite , outcropped in the East Kunlun area of Xinjiang , formed in a supra - subduction zone environment. This study is to gain more information about the type of subduction zone. Through field work, thin section observation and microprobe analysis, petrological and mineralogical characteristics of the metamorphic peridotites of this ophiolite are obtained.Although the olivines of metamorphic peridotites appear in three textures of metamorphic relict , meta -morphic recrystallizations and orthopyroxene - melting crystallizations by thin - section observations, they have stable and low Fo range of 87. 8 - 89. 5 by microprobe analysis. The orthopyroxenes show metamor -phic relict and melting relict textures, with a low En of 88 - 90 and a wide range of Al2O3 content , from 2. 90 wt% to 5. 13 wt %. The spinels develop anhedral - subhedral textures, with Cr # ( =Cr/(Cr +Al ) ) focusing on two ranges of 0. 508 - 0. 723 and 0. 100 - 0. 118 , respectively. Based on these petrological and mineralogical observations, and combined with the era and tectonic setting for the Muztag ophiolite ,it can be concluded that the ophiolite formed in a supra - subduction zone where the oceanic crust subduc - ted down to the continental arc with a thick continental crust, and resulted from ocean - continent subduction within the Paleotethyan archiopelagic ocean in the East Kunlun area of Xinjiang.
LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite, Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance
Chen Nengsong ; Liu Rong;Sun Min; Li Huimin;He Lei ; Wang Qinyan; Zhang Hongfei
2005, 7(4)
Abstract:
Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed.LA - ICP - MS U - Pb dating was conducted on three textural domains in zircon from a high - temperature ,high - pressure felsic granulite in the Huangtuling area , North Dabieshan , Central China. The metamorphic growth - derived detrital zircon domain yields a 207 Pb/206 Pb age in the range of (2 493 ±54) -(2 500 ±180 ) Ma. The magmatic genesis - derived detrital zircon domain gives a 207 Pb/206 Pb age ranging from 2 628 Ma to 2 690 Ma , with an oldest 206 Pb/238 U age of (2 790 ±150) Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a dicsordia with an upper intercept age of (2 044. 7 ±29. 3 ) Ma. Compositions of the mineral assemblage, major element geochemistry , and es-pecially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history , i. e. ~ 2. 8 Ga magmatism and ~ 2. 5 Ga metamorphism , and was deposited in a basin not earlier than 2. 5 Ga. The high - temperature and high - pressure granulite - facies metamorphic age was precisely constrained at (2. 04 ±0. 03 ) Ga , which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT (ultrahigh temperature ) metamorphosed slab.
Metallogenic Age of Dapinggou Gold Deposit in Northern Altun Area, Northwestern China
Chen Bailin; Wang Xiaofeng; Yang Yi; Chen Xuanhua
2005, 7(4)
Abstract:
Studying the metallogenic age of Dapinggou gold deposit may provide important material not only for the study of metallization and the further prospecting of gold deposits but also for research into tectonic evolution in the Altun area. In this study , we used Rb -Sr isotopic dating method of fluid inclusion in the Au - bearing quartz , combining field investigation , REE, sulfur isotopes, lead isotopes ,strontium isotopes and hydrogen - oxygen isotope , to determine the metallogenic age of the Dapinggou gold deposit. The only newly - discovered medium - sized gold deposit in the northern Altun area , Dapinggou gold deposit , is controlled by a ductile shear zone. It is characterized by mainly altered mylonite - types with a little K -feldspar quartz vein - types. Data from REE , sulfur isotopes , lead isotopes and strontium isotopes show that the source materials of the deposit were derived mainly from the metamorphic rocks.The data from the inclusion and hydrogen - oxygen isotope indicate that the ore - forming fluid came mainly from dynamic metamorphic water and partly from remelting magmatic water , with a little influence from meteoric water. The density of the ore - forming fluid is 0. 791 - 0. 971 g /cm3 and it contains 2. 36 %- 5. 5 % salinity. The gold deposit was formed at 1. 61 - 2. 68 km below the surface at a temperature of 198 - 290 ℃ and a pressure of (420 - 700)×105 Pa. The isotopic age from the Rb - Sr isochron of fluid inclusion in quartz is 487 Ma , which indicates that the mineralization occurred in the early Caledonian epoch. The study on the metallogenic age of Dapinggou gold deposit may provide important material not only for the study of metallization and the further prospecting of gold deposits but also for research into tectonic evolution in the Altun area.
Deep Mechanical Background for the Cenozoic Volcanism in the Tibetan Plateau
Xiong Xiong;Wang Jiye;Teng Jiwen
2005, 7(4)
Abstract:
The principle prerequisite for the formation of a volcano is the generation of a channel for magma transportation. There is little research on the deep mechanical mechanism for the formation of a magma transportation channel in the Tibetan plateau. Based on the subcrustal mantle convectiongenerated stress field inversed by gravity anomalies, together with its relationship to the Cenozoic volcanism in the plateau, and the mechanism of crustal fracture formation, as well as the numerical results of the evolution of mantle convection beneath the plateau, this paper investigates the deep mechanical mechanism for the formation of a magma transportation channel in the Tibetan plateau. There are two significant extensional convection- generated stress zones beneath the plateau, in which the volcanic rocks in the central and northern parts of the plateau are distributed. The Linzizong volcanism in southern Tibet correlates the upwelling mantle flow prior to the India- Asia collision or during the early stage of the collision. The magnitude of the stress is ~ 100 MPa, which is the same order of force that causes crustal fractures. The evidence implies that the mantle convection- generated stress is one of the principle causes of crustal fractures, and furthermore, the formation of the magma transportation channel in the Tibetan plateau.
Effect of Initial Principal Stress Direction on the Dynamic Characteristics of Carbonate Sand
Yu Haizhen ;Zhao Wenguang; Wang Ren; Li Jianguo; He Yang
2005, 7(4)
Abstract:
The dynamic characteristics of carbonate sand under wave loads are very important for constructions on the ocean floor. The initial principal stress direction has been known to exert some influence on the dynamic characteristics of sand during cyclic loading. In an effort to investigate this aspect of the problem , several series of cyclic undrained tests were carried out on a saturated and loose sample of carbonate sand using a geotechnical static and dynamic universal triaxial shear apparatus. In this testapparatus, a hollow cylindrical sand specimen is subjected to a simultaneous application of both triaxial and torsional modes of shear stresses, which brings about the continuous rotation of principal stress axes. The test results indicated that the initial principal stress direction has a considerable influence on the dynamic strength of loose carbonate sand and with the increase of initial orientation of principal stress, dynamic strength will be reduced, the cyclic pore pressure increased , but the residual pore pressure reduced.
Anti-rebound Cushion Device for Hydraulic Breaker
Zhao Hongqiang
2005, 7(4)
Abstract:
This paper analyzes the phenomenon of rebound impact and its negative influence on the present hydraulic breaker. To get over its shortcomings, a new anti- rebound cushion device has been designed to prevent the phenomenon of rebound impact. A hydraulic cushion is used to absorb the rebound impact energy , which can be released for the next stroke of the hydraulic breaker.Thus, there is little loss of energy , and the efficiency of the impact system can be increased by 5%. The absorption effect of the hydraulic anti- rebound cushion increases the service life of breaker components by up to twice as long as in the current breaker. A dynamic model and a motion equation of the anti- rebound cushion device are presented, and the optimum frequency and damping ratio are obtained, providing optimum design parameters for the anti- rebound cushion device.
Seismic Analysis of Shear-Wall Structures with Vertically Installed Dampers
Yang Jun; Li Li ;Fan Aiwu
2005, 7(4)
Abstract:
Shear - wall structures are quite common in seismic areas because of their successful seismic behavior during severe earthquakes. But shear walls are prone to brittle failure. This study proposes a new method of vertically installed dampers (VID ) to reduce the vibration in shear - wall structures. The motion characteristic of a vertical damping system is that every mass has horizontal and rotational displacements simultaneously. The establishment of dynamic equations should take into account the equilibrium conditions of both horizontal and rotational vibrations. Dynamic equilibrium equations of VID systems are derived from a model of a structure with VID. An example shear - wall structure , with and without VID , is studied. There are some changes in the characteristics of the maximum horizontal displacement response. Without dampers, the relative displacements between different floors in the shear wall increase with height. With dampers, the relative displacements are more uniformly distributed , and lateral displacements at the top and at the bottom are closer. When the damping coefficient is 1 000 kNs/m , the numerical results reveal that the maximum horizontal displacement and the maximum rotational displacement of the top floor have reduced by 59. 3 % and 54. 8 % respectively.
Reinforcing a Dangerous Rock Mass Using the Flexible Network Method
Yang Wendong; Xie Quanmin; Xia Yuanyou; Li Xinping
2005, 7(4)
Abstract:
Because the main failure type of a dangerous rock mass is collapse, the treatment of such a mass should focus on controlling collapse failure. When treating dangerous rock masses, disturbing the mass (e. g. by blasting) needs to be avoided, as this new damage could cause collapse.So the self- bearing capacity of the mountain mass must be used to treat the dangerous rock mass.This article is based on a practical example of the control of a dangerous rock mass at Banyan Mountain, Huangshi, Hubei Province. On the basis of an analysis of damage mechanism and the stability of the dangerous rock mass , a flexible network reinforcement method was designed to prevent the collapse of the rock mass. The deformations of section Ⅱ w of the dangerous rock mass before and after the flexible network reinforcement were calculated using the two- dimensional finite element method. The results show that the maximum deformation reduced by 55 % after the application of the flexible network reinforcement, from 45. 99 to 20. 75 mm, which demonstrates that the flexible network method is effective, and can provide some scientific basis for the treatment of dangerous rock masses.
Numerical Simulation of Moisture Movement in Unsaturated Expansive Soil Slope Suffering Permeation
Chen Shanxiong; Yu Song; Liu Zhiguo ; Xu Haibin
2005, 7(4)
Abstract:
This study develops a way of analyzing moisture movement in unsaturated expansive soil slope. The basic equations and the integrated finite difference method for moisture movement in unsaturated soils are briefly described, and the calculation code MFUS2 has been developed. The moisture movements in unsaturated expansive soil slopes suffering precipitation were simulated numerically. The simulation results show that expansion or contraction must be taken into account in an analysis model. A simplified equivalent model for calculating rainwater infiltration into expansive soil slopes has been developed. The simplified equivalent model divides the soil slope into two layers according to the extent of weathering of the soil mass at depth. Layer Ⅰ is intensively weathered and moisture can be fully evaporated or rapidly absorbed. The moisture movement parameters take into account the greater soil permeability caused by fissures. Layer Ⅱ is unweathered and the soil is basically undisturbed. The moisture movement parameters of homogeneous soils are applicable. The moisture movements in unsaturated expansive soil slopes suffering precipitation were simulated numerically using the simplified equivalent model. The simulation results show that the moisture movement in the expansive soil slope under rainfall permeation mainly takes place in the extensively weathered layerⅠ, which closely simulates the real situation.
Decoupling Conditions for Elasto-plastic Consolidation Question Based on Numerical Modeling Method
Cheng Tao; Wang Jingtao; Dong Bichang
2005, 7(4)
Abstract:
Elasto- plastic consolidation is one of the classic coupling questions in geomechanics. To solve this problem, an elasto - plastic constitutive model is derived based on the numerical modeling method. The model is applied to Biot ’ s consolidation theory. Incremental governing partial differential equations are established using this method. According to the stress path, the decoupling condition of these equations is discussed. Based on these conditions, an incremental diffusion equation and uncoupling governing equations are presented. The method is then applied to numerical analyses of three examples. The results show that(1) the effect of the stress path should be taken into account in the simulation of the soil consolidation question; (2) this decoupling method can predict the evolvement of pore water pressure; (3) the settlement using camclay model is less than that using numerical model because of dilatancy.