Table of Content

20 September 2008, Volume 14 Issue 3

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Article

Several Problems Worthy to be Noticed in the Research of Granites and Volcanic Rocks in SE China

XU Xi-sheng

2008, 14(3):  283-294. 

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Compared with those in other regions worldwide, the granites and volcanic rocks in southeast China show distinct characteristics of episodical occurrences, complex genetic types and formation processes, and close affinity with metallogeny. In the past 50 years, the diversities of granites and their temporal and spatial distributions have been gradually revealed. The diversities of the granites are due to different source rocks of crustal basement, different geodynamic settings and different degrees of crustmantle interactions. Because of the speciality and complication, the source rocks for the granites, tectonic settings of magmatism and related basic goelogical problems have not been clarified. The weakness of previous research on the granites and volcanics in SE China includes their genetic constraints affected by deeper geology and tectonic geodynamics around. The suggested research subjects related with Mesozoic-Cenozoic geology in SE China in this paper include: (1) A-type granites in three lower Nd model age belts; (2) distribution of four volcanic belts and crust-mantle interactions; (3) whole-rock Nd isotopes and zircon Hf isotope tracing; (4)granite structure.

Petrogenesis of the Xiaozhangjiakou Mafic-Ultramafic Complex, North Hebei:Constraints from Petrological, Geochemical and Nd-Sr Isotopic Data

CHEN Bin，TIAN Wei，LIU An-kun

2008, 14(3):  295-303. 

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The Xiaozhangjiakou mafic-ultramafic complex, north Hebei, is composed mainly of pyroxenites and subordinate dunite. Lenses of harzburgites can be seen in the complex. Pertological and geochemical data suggest a cumulate origin for the pyroxenites and dunites, while harzburgites represent mantle xenoliths (mantle residues after extraction of basaltic melts). The pyroxenites and dunites show significantly enriched Nd-Sr isotopic compositions, with εNd (t )=-2~-10 and I Sr=0.7045~0.7081,suggesting that their parental magma was derived from partial melting of enriched mantle portions. Geochemical and isotopic data consistently demonstrate that variable proportions of lower continental crustal components were incorporated in the maficultramafic complex during the processes of olivine- and pyroxene-dominated fractional crystallization. The pyroxenites experienced the most significant crustal contamination, and cannot be representative of genuine isotopic features of the mantle source. The dunites, however, was the first cumulate crystallized from magma and was contaminated only slightly by crustal components.Therefore, the Nd isotopic composition (εNd(t )=-1.9) of the dunites could be close to that of the mantle source. Formation of theXiozhangjiakou complex was probably related to the closure of the Ancient Asian Ocean at about 240~250 Ma.

Origin and Tectonic Setting of the Early Mesozoic Granitoids in Qinling Orogenic Belt

ZHANG Cheng-li1, WANG Tao2, WANG Xiao-xia3

2008, 14(3):  304-316. 

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The Early Mesozoic granitoids in Qinling Mts are characterized by the metaluminous to peraluminous and moderateto high-K calc-alkaline rocks with enrichment in LILE and LREE and poorness in HFSE, as well as relative depletion in Nb and Ta, showing the geochemical features similar to those of post-collisional granitoids. Some of them are adakite-like and rapakivitextured granitoids. The adakite-like granitoids display high Sr and low Y contents, and rapakivi-textured granitoids show transitional geochemical characters from I- to A-type granites. The structure and geochemistry of these granitic intrusions, together with the bimodal features of the coeval lamprophyres and mafic dykes, suggest that the Early Mesozoic granitoids in Qinling Mts region were formed in a post-collisional setting. The early adakite-like granitoids were formed under the thickened crust during the early post-collision period. The large numbers of normal massive granitoids were produced under the crustal extension environment after delamination in Qinling Mts region, and the occurrence of rapakivi-textured and high differentiated granitoids indicates that Qinling region had evolved into the late stage of post-collisional environment.

Age and Origin of the Dadongshan Granite from the Nanling Range: SHRIMP U-Pb Zircon Age, Geochemistry and Sr-Nd-Hf Isotopes

HUANG Hui-qing1,2, LI Xian-hua3,1*, LI Wu-xian1, LIU Ying1

2008, 14(3):  317-333. 

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Early Yanshanian (Jurassic) granites, which are dominated by weakly peraluminous biotite monzogranite and K-feldspar granite, are ubiquitous in Nanling Range in Southeast China. However, the petrogenesis of the granites remains controversial.The Dadongshan pluton consists of biotite monzogranite and biotite-bearing K-feldspar granite. SHRIMP U-Pb zircon dating of two samples yield ages of 165 ± 2 Ma and 159 ± 2 Ma, consistent with the major formation time of the“ Nanling-series” biotite granites in the neighboring region. The granites are silica-rich (SiO2 > 72%), potassic (K2O/Na2O > 1.6), weakly peraluminous (most have ASI=1.00 ~ 1.11) and enriched in alkalies (K2O+Na2O = 7.4% ~ 9.3%). Samples are invariably enriched in Rb, Th and LREE, yet depleted in Ba, Nd, Sr, P and Ti, with distinct negative europium anomaly (δEu=0.06 ~ 0.34). Low contents of Zr+Ce+Nb+Y (< 350 ppm) and low 10000 × Ga/Al rule out their A-type affinity. Isotopically, samples show high and variable initial Sr ratios (0.7123 ~ 0.7193), yet low and constantεNd(t) (-9.3 ~ -11.5), corresponding to T2DM age of 1.70 ~ 1.89 Ga.However, their zircon Hf isotopes show large variations, withεHf(t) values between -3.5 and -11.8. Mineralogical and geochemical features suggest that the Dadongshan granites are highly fractionated I-type granites. They were probably derived from partial melting of the Proterozoic igneous protoliths with minor addition of juvenile crust or newly mantle-derived magmas, accompanied by assimilation and fractional crystallization.

Petrological and Geochemical Characteristics and Petrogenesis of the Early Cretaceous Taohuashan-Xiaomoshan Granites in Northeastern Hunan Province

WANG Lian-xun1, MA Chang-qian1,2*, ZHANG Jin-yang3, CHEN Ling1, ZHANG Chao1

2008, 14(3):  334-349. 

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The Taohuashan and Xiaomoshan granites occur in Huarong county，Northeastern Hunan Province, and intruded into the Mesoproterozoic Lengjiaxi Group. Using the method of single zircon LA-ICP-MS U-Pb dating, the emplacement ages of 129 ± 1 Ma and 117 ± 1 Ma were yielded for these two plutons. The main rock type of Taohuashan pluton is (gneissic) two-mica monzonitic granite，with SiO2 = 71.75~73.81%，K2O/Na2O = 0.84~1.11，and belonging to weak peraluminous high K calc-alkaline series. It is obvious that the Taohuashan rocks are enriched in LILE and depleted in HFSE; with moderately negatively Eu anomalies, relatively high ΣREE contents, Rb/Sr = 0.9~2.7，（La/Yb)N = 26.92~86.02, high ISr (0.714~0.723），low εNd (-9.76 ~ -10.6）, and high t2DM (1.72~1.79Ga）. The rock type of Xiaomoshan pluton is biotite monzonitic granite. SiO2=69.64~72.73%，K2O/Na2O = 0.62~0.7，metaluminous to weakly peraluminous，Rb/Sr = 0.26~0.88，（La/Yb)N = 11.97~12.67,low ISr (0.707~0.709），high εNd (-6.38~-6.73）， and low t2DM (1.43~1.46 Ga）. The analysis shows that the Taohuashan two mica monzonitic granite is crustal-derived CPG. Its source rocks are the Palaeo-Proterozoic basement of south China. The Xiaomoshan biotite monzonitic granite is similar with CPG type, with low maturity metagreywackes as its source rocks. The Taohuashan and Xiaomoshan plutons were formed in a local compressional environment under the extensional geological setting of South China. The thrust of Jiangnan Fault caused a small scope of crust thickening in Huarong region, and provided a fluid accumulation channel for the wet anatexis of Taohuashan source rocks. The formation of Xiaomoshan pluton is related to magmatic underplating. The hot mantle magma not only provided heat for the partial melting of crust, but also caused magma mixing with the crustalderived melts.

The Metalliferous Mineralization Associatied with Adakitic Rocks

WANG Qiang, TANG Gong-jian, JIA Xiao-hui, ZI Feng, JIANG Zi-qi, XU Ji-feng, ZHAO Zhen-hua

2008, 14(3):  350-364. 

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Adakitic rocks have important implications for the metalliferous mineralization. This paper introduces the nomenclature of“ adakitic rocks”, the distribution and tectonic settings of the adakitic rocks associated with metalliferous mineralization. The factors that the adakitic rocks are favorable for the metalliferous mineralization are discussed in detail at last. Similar to a subducted oceanic crust-derived adakite, the“adakitic rocks”are characterized by the following geochemical characteristics: SiO2≥56wt%, Al2O3≥15 wt%, depleted Y (≤18×10-6 ) and heavy rare earth element contents ( Yb≤1.9×10-6 ), high Sr (rarely＜400×10-6 ), negligibly positive Eu and Sr anomalies, and depleted high field strength element contents. They can be generated in different tectonic settings and by different mechanisms. The favorable tectonic settings for the metalliferous mineralization include arc, within-continent extension, and active collisional orogenic belt between different continents. Many large and giant are closely associated with adakitic rocks, indicating that they have a huge metalliferous potential. In the arc and within-continent extension settings, the interaction between subducted basaltic oceanic crust, or sediments, or delaminated continental crust-derived melts, or released super critical fluids with mantle causes contamination of melts by mantle peridotites on the one side, and metasomatism of mantle peridotites by the melts or fluids with high Fe2O3 on the other side. Thus, the raised oxygen fugacity in the mantle causes metal sulfides to be decomposed, which is favorable for the Cu-Au mineralization.

Petrology and Geochemistry of the Xinchang Composite Igneous Complexes,Zhejiang and Their Geological Implication

DONG Chuan-wan1,2，YANG Shu-feng1，TANG Li-mei1，DU Zhen-yong1

2008, 14(3):  365-376. 

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The Xinchang composite igneous complexes include several composite lava flows and composite intrusive massifs.The composite lava flows consist of basalt and rhyolites/rhyolitic ignimbrite, and may accommodate partly a small amount of intermediate lavas and enclaves. The composite intrusive massifs consist of diabase and granite, and contain dioritic enclaves. Geochemically, these rocks are of calc-alkaline series, enriched in LREE, LILE (e.g. K, Rb and Ba), but depleted in HFS elements (e.g. Ta, Nb, Zr and Hf), and with similar REE patterns and trace elements spidergrams. They have limited range of ISr values from 0.7069 to 0.7079 and εNd(t ) values from -2.3 to -5.3, implying that a full exchange of trace elements and isotopic compositions took place among them. The overall elemental and Sr-Nd isotopic characteristics of the Xinchang composite igneous complexes are similar to those of the basalt-rhyolite composite lava flows and intrusions in the coastal area of Zhejiang and Fujian provinces.It is proposed that the Xinchang composite igneous complexes were formed under a extension tectonic setting and derived from the basaltic magma produced by partial melting of sub-continental lithospheric mantle which could have been influenced by early subduction processes, and underplated and interacted with deep crust melted granitic magma. This research, based on the composite igneous complexes, provides a typical case to better understand the evolution of Late Mesozoic geodynamic environmentand tectono-magmatic activities in the coastal area of southeastern China.

Earthquake Source and Hazard Analysis:the May 12, 2008, Ms 8.0 Wenchuan Earthquake, Sichuan, China

CHEN Sheng-zao

2008, 14(3):  377-386. 

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The May 12, Ms 8.0 Wenchuan earthquake is tectonically due to the convergence of the India plate against the Eurasia plate at a rate of ~50 mm/yr, which resulted in the crustal movement toward the eastern Sichuan Basin and the southeastern China continent from the uplifted Tibetan Plateau. The epicentral area is located in the active Longmenshan centralized seismic zone along which the aftershocks are concentrated within a ~350 km long and ~100 km wide active belt of horizontal projection.Hypocentral depths range generally from ~10 to ~20 km within the crustal seismogenic zone above the brittle-ductile transition boundary, characterized typically by shallow tectonic earthquakes. Focal mechanism of the Wenchuan earthquake bears a close correlation with the Longmenshan tectonic movement indicated by the particular deep structural anomalies of the rapid crustal transition deepened westward, gravity gradient, high Vp/Vs ratio of average 2.2, and geological features of thrust faulting style with strike slip history. Response spectra for rock and soil sites are, respectively, estimated and compared for different tectonic regimes, based on the study of source, path and site effects, emphasizing soil site amplification effect. Also studied are safety concerned issues for prevention of future damaging earthquakes including geological hazards, seismic design, earthquake early warning systems (EEWS), and strategies of long term to short term earthquake forecasting. A sample of design spectra is presented on weighted average rock-site attenuation models. Topic on seismotectonics and earthquake dynamics in the areas of high seismic risk could be well incorporated with engineering geology and environmental engineering. Study on performances of the newly designed and constructed engineering structures should provide most valuable input to improving seismic design codes in China and throughout the world. It is a challenging task to predict earthquakes; however, effort should never be given up. Earthquake disaster reduction and prevention is a realistic measure for safeguard of future damaging earthquakes.

Survey and Analysis of Secondary Geological Hazards after Wenchuan Earthquake

SHI Bin, WANG Bao-jun, ZHANG Wei, XU Jie

2008, 14(3):  387-394. 

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Based on the field investigation and data gathering, the genesis of earthquake disasters and secondary geological hazards is analyzed in this paper. The importance and major contents of engineering geological survey in post-disaster reconstruction are introduced, which are useful in reconstruction of Wenchuan earthquake area.

Mixing of Siliciclastic-Carbonate Sediments within Systems Tracts of Depositional Sequences and Its Controlling Factors

LI Xiang-hui

2008, 14(3):  395-404. 

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Mixing of siliciclastic and carbonate sediment is widespread in variable marine sedimentary environments through the Phanerozoic. Terminology of mixing and its classification are introduced in the paper, and the controlling factors upon mixing within systems tracts of depositional sequences are discussed. In general, the punctuated mixing is the most ordinary sedimentation in all systems tracts of mixed depositional sequences, and the source mixing associated with a little facies and punctuated mixing occurs within lowstand systems tract (LST); the facies and punctuated mixing becomes the main sedimentation in the shelf marginal systems tract (SMT) and the late highstand systems tract (HST); and the transgressive systems tract (TST) is characterized by the punctuated mixing with a little facies mixing. It is proposed that the mixing within systems tracts of depositional sequences is chiefly constrained by the patterns of the sea-level changes, i.e., the mixing within LST, SMT and late HST could be induced by amplitude, rate, and time duration of sea-level changes, and the mixing within TST could be related to the source supply. The mixing could be more or less influenced by the former topography and carbonate production in moderate paleoclimate. Fuether more, the regional tectonic activities might be partially responsible for mixing, which means that the cycles of mixing procession within systems tracts of depositional sequences in a sequence set or supersequence could be responses of vpulse regional tectonic activities.

Distribution Characteristics and Tectonic Significance of Cenozoic Remnant Formation, Bohai Bay Basin

DING Zeng-yong1, WANG Liang-shu1, ZHONG Kai2, ZHANG Peng1, LIU Shao-wen3

2008, 14(3):  405-413. 

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Remnant thickness distribution maps of all main Cenozoic formations and members of Bohai Bay Basin were obtained by conducting division, comparison and depth validation of seismic profile data for all secondary tectonic units of Bohai Bay Basin,as well as by using geological data of real drilling. These maps clearly reflect the complete sedimentary framework of the basin and the distribution characteristics of the Cenozoic remnant formations. Through deep revealment of the geological information,new evidence was obtained for a correct recognition of the Cenozoic forming and evolution mechanism of Bohai Bay Basin. Theresults of omprehensive analysis and comparison of spacial distribution, sedimentary center and long axis strike of remnant formations of different sequences indicate that the whole basin was under the control of back-arc extensional stress produced by the subduction of the West Pacific Plate beneath the Eurasia Plate from the Kongdian to the Dongying sedimentary periods.However, the influence and interaction of dextral shear stress superimposed continuously in the entire process of Cenozoic forming and evolution of this basin. This stress field is supposed to be related to the combination of the dextral slip activity of Tanlu fault with the far-distance effect of the India-Eurasia collision.

Desorption and Carbon Isotope Fractionation of Coalbed Methane in Jincheng Area

DUAN Li-jiang1, TANG Shu-heng1, LIU Hong-lin2, LI Gui-zhong2, WANG Bo2

2008, 14(3):  414-418. 

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The canister desorption experiments of the coalbed methane（ CBM） from the coal samples of the Jincheng area were carried out, and the CBM desorption rate was calculated as 65.0% ~ 96.2%. The estimated recovery of coalbed gas well should be higher in this area. The CBM adsorption time is 0.33~8 d, which indicates that the CBM-producing well should reach a peak productivity in a short time. The CBM desorption process can be divided into two stages, the gas desorption rate is higher at the first stage and lower at the second stage. In the desorption process, the CBM carbon isotope tends to become heavier, but the tendency seems to be faster at the first stage and slower at the second stage. The CBM carbon isotopic fractionation during the gas desorption from coal reservoir might result in a fluatuation of δ13C1 value in well head in a narrow range. A longer period of fluctuation might mean a better exploitation stability and a higher production. Through the correlation analysis of the desorption amount and the carbon isotope value, the total gas desorption predicting equation was regressed, by which the producible reserves of the coalbed gas well can be predicted.

Sedimentary Microfacies and Heterogeneity of the Turbidite Sand Bodies:A Case Study in the Middle 3rd Member of the Shahejie Formation in the T74 District of Shengtuo Oilfield, Dongying

CAI Chuan-qiang1,2, YAN Ke3*, YANG Shao-chun3, REN Huai-qiang3,CHEN Ning-ning3

2008, 14(3):  419-425. 

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Sedimentary microfacies and reservoir heterogeneity of the turbidite sand bodies in the middle 3rd Member of the Shahejie Formation in the T74 district of Shengtuo oilfield were studied systematically based on the core observation, electrofacies analysis, well log interpretation and image analysis. The results show that the reservoir in this member is composed of three stages of turbidite sand bodies, which can be divided into two kinds of microfacies, the central microfacies and the marginal microfacies.The reservoir heterogeneity of the turbidite sand bodies is very significant, and the inlayer heterogeneity is characterized by indistinct rhythmicity and high permeability contrast. The interlayer heterogeneity and areal heterogeneity are controlled by the sedimentary microfacies, and the physical properties of sandbodies in central microfacies are better than those in marginal microfacies. The microscopic pore structure in these two kinds of microfacies is also different. The moderate pores and throats occur typically in the central microfacies, but the minute pores and throats are mainly developed in the marginal microfacies.

Study on Infrared Spectra and Infrared Radiation Characteristics of Tourmaline

LI Wen-wen1, WU Rui-hua1, DONG-Ying2

2008, 14(3):  426-432. 

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This paper studies the infrared spectra (IS) characteristics of different tourmaline species, and the infrared radiation (IR) characteristics after heat treatment on tourmaline species at different temperatures. The results show that the high IR ratio of tourmaline is due to the infrared activity of vibration in its crystal structure. The IR ratio of Fe tourmaline is close to that of Mg tourmaline, and both are higher than that of Li tourmaline. The IR ratio of tourmaline has some relationship with heat treatment temperature: with increasing heat treatment temperature, the IR ratio decreases. When heated to 800℃, the IR ratio reaches a maximum; while heated to higher than 900℃, tourmaline begins to decompose and its infrared radiation ratio drops down. At the room temperature, the largest monochromatic radiation wavelength of tourmaline is between 9μm and 10μm, which matches the wavelength (9.72μm) of the absolute blackbody very well. It demonstrates that tourmaline is a good material for infrared absorption and radiation.

Stable Isotopic Characteristics of Xiliugou Polymetallic Deposit in the Western Part of the North Qilian Mountains

KANG Hong-jie1 , SUN Bai-nian1*, LIU Xiao-huang1,2, LI Xiang-chuan, ZHU Duo-lu1, ZHANG Wei-xin2

2008, 14(3):  433-441. 

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A polymetallic deposit zone with W, Mo, Cu, Pb, Zn mineralization was found in the western part of the North Qilian Mountains. The Xiliugou polymetallic deposit is located in the western part of this polymetallic deposits zone. In order to discuss the source of ore-forming fluid of this deposit, the authors selected 9 ore samples of the main ore-forming stage from this deposit and measured their H, O, C isotopic compositions. The results show that the DV-SMOW‰ values of the ore-forming fluid are -47‰ ~-72‰ with very small variation. The δ18O values in the main stage vary between 11.9‰ ~14.1‰. The δ18OH2O‰ values of the hydrothermal fluid equilibrated with quartz are -0.79‰ ~3.64‰（ the average homogenization temperature of the fluid inclusions is 190℃）. The δ13C values of fluid inclusions captured in the quartz are -6‰~-4.2‰, also with a small variation range. Microthermometric studies indicate that the homogenization temperatures and salinities of the fluid inclusions in the oreforming stage range from 126 to 384℃ and from 1.05% to 16.45% NaCl equivalent, respectively, with most values in the range of 140~200℃ and 1.74%~6.45% NaCl equivalent, respectively. It is shown that the Xiliugou polymetallic deposit is a shallow, low temperature, low salinity and low density hydrothermal deposit. The magmatic water is the main source of the ore-forming fluid, and the meteoric water is subsidiary.

On the Stress State, the Additional Tectonically-Induced Static Pressure and the Formation Depth of the Rocks and Ores in Earth Crust

Lü Gu-xian1, LIU Rui-xun2, CAI Yong-en2, WANG Fang-zheng3, GUO Tao1, MA Rui3,HU Bao-qun1,5, SHEN Yu-ke1, ZHOU Guo-fa1

2008, 14(3):  442-449. 

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The paper puts forward some questions to the application of the formula: D=P/d, where D, P and d represent depth,pressure and specific density, respectively. This formula stems from a fluid mechanical principle, the Pascal's principle, describing the relationship between pressure and depth in static fluid. This formula is applied to fluid material only. If the material is a solid, and it may undergo shear or differential stress, then this formula can not be applied. All rocks existing from the Earth surface down to the boundary between the mantle and the core show solid character. When a foece is acted on a solid unit, no matter the force is tectonic or gravity, there always exist two stress parts in the solid stress field: the uniform stress and the differential stress.However, in the case of liquid material, the stress is uniform, but not differential. The results of geo-stress measurements show that the horizontal stress acted on the vertical section should be greater than the vertical stress acted on the horizontal section;and that the nearer the distance to the orogen belt or shear zone, the greater the horizontal stress is, thus indicating the leading role of tectonic force in the crust stress field. In fact, the force pushing the plate movement is mainly the horizontal stress, but not vertical stress. The total static pressure at a position in crust is composed of at least two parts: the pressure induced by the tectonic activities and the pressure induced by the gravity. The former pressure is the additional tectonically-induced static pressure. The reasonable method to calculate the depth should be the subtraction of the additional tectonically-induced pressure from the total pressure, and then divided by the rock specific density, i.e. D = (P-Pt)/d, where Pt represents the additional tectonically-induced static pressure.

Preliminary Research on Risk of Land Subsidence

YU Jun1, 2，WU Jian-qiang3，WANG Xiao-mei3，SU Xiao-si4

2008, 14(3):  450-454. 

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Land subsidence, which is induced by over-pumping groundwater continuously, has impeded the sustainable economic development in plain areas. As a new subject in disaster research, risk management demands positive prevention instead of passive management. In this paper, the risk management theory was applied to land subsidence research, and the main objectives and methods were discussed in detail. According to the characters of land subsidence, the author pointed out that the risk assessment should include three aspects: the vulnerability assessment, the destructiveness assessment and the resistibility assessment. Furthermore, a primary indicator system of risk assessment was established, some general mathematical models and spatial analysis techniques were also introduced. Finally, a case study was carried out in Suzhou-Wuxi-Changzhou area.The result shows that the infringement caused by the same level of land subsidence is more serious in developed areas than in underdeveloped ones. Therefore, to strengthen the risk resistibility, enhancing the disaster prevention inputs is suggested.

Three Typical Seepage Curves under Stresses and Their Mechanism Analysis

TANG Hong-xia1，ZHOU Zhi-fang2

2008, 14(3):  455-458. 

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Three typical seepage curves are obtained using fractured seepage tests based on diffirent pressures. The main factors affecting the sample permeability are discussed. The fractures would be deformed under the condition of positive stress and be ruptured and extended under the condition of shear stress. The permeability variation of fractures is also analyzed during fracture deformation, rupturing and extension. The mechanisms of three typical seepage curves, which seem to abnormal but actually normal, are explained under the condition of stress. At the same other conditions, the relationship between permeability and stress of samples is determined by the angles between stresses and fractures.