Geological Journal of China Universities

Approaching the Age Problem for Some Metamorphosed Precambrian Basement Rocks and Phanerozoic Granitic Bodies in the Wuyishan Area:The Application of EMP Monazite Age Dating

CHEN Cheng-hong

2008, 14(1): 1-15.

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We use EMP monazite, along with LA-ICPMS U-Pb zircon, dating methods to assert on 5 metamorphic rocks from Wuyisahn Precambrian basement and 5 adjacent Phanerozoic granitic rocks. Results reveal that monazites were largely grown in the Caledonian stage and probably overprinted by a younger age at the Hercynian. Among them, 533.0±44.4 Ma and 451.0±17.7 Ma (no age group from zircon but all >800 Ma) are obtained for a migmatite of Wanquan Group, 422.0±13.7 Ma and 360.0±14.9 Ma (436.4±4.7 Ma from zircon) for a migmatitic leucosome from Badu Group, and 443.3±11.3 Ma (no age group from zircon but concentrated at 450-350 Ma) for a mica quartz schist from Tienjinping Formation. No monazite was found in gneissic rocks from Mayuan Group and granulite from Wanquan Group, but an age of 859.1±9.2 Ma is obtained from zircon.For Phanerozoic granites, 162.3±4.4 Ma is obtained for Hetien body and 215.3±4.5 Ma for Fuchen body, and 418.7±16.4 Ma and 360.6±15.3 Ma for Weipu, 441.8±12.3 Ma and 350±45 Ma for Ninghua and 429.3±15.6 Ma for Zengfengding bodies.Accordingly, we conclude that (1) Wuyishan basement rocks have been strongly influenced by the Caledonian orogeny leading to not only the granitic intrusion but also the strong modification on these rocks; (2) Hercynian event is only reflected by the high-U monazite but not the zircon ages, indicating that the influence of this orogenesis may be limited if it really existed; (3) the fact that the Archean zircons are mostly of inherited origin seems not support the existence of such an Archean basement in Wuyishan; (4) the provenance for Paleoproterozoic monazites that are abundant in W Taiwan sediments has not yet been proven to be correlated to the Wuyishan area as suggested earlier.

Magma Mingling: Evidence from Enclaves

Wang De-zi

2008, 14(1): 16-21.

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The magma mingling between mafic magma and felsic magma is the primary mechanism responsible for the granitoids of crust-mantle mixed source. The dark colored, microgranular and igneous-texture enclaves provide reliable evidence that indicates the existence of magma mingling. These enclaves have the following characteristics: (1) the form of enclaves usually presents an equi-axed shape, which shows that enclave magma existed in the host magma with a globule liquid state; (2) since the temperature of basic magma is higher than acidic magma (temperature excess about 300℃), this type of enclaves often has quenched margin; (3) the enclaves sometimes contain back veins; (4) feldspar xenocrysts captured from the host magma can be seen in the enclaves. Three typical granitic complexes (Pingtan, Putuoshan, Huashan-Guposhan) containing dark colored microgranular enclaves are exemplified in this paper. The formation of dark colored microgranular enclaves can be explained by the injection of the basaltic magma from deep seated magma chamber toward hypabyssal acidic magma champer.

SHRIMP Zircon U-Pb Chronology and Tectonic Significance of the Latest Neoarchean Mengshan Diorite, Western Shandong Province

HOU Gui-ting

2008, 14(1): 22-28.

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The TTG（tonalite-trondhjemite-granodiorite）in the western area and gneissoid adamellite unit in the eastern area of the Western Shandong Province were identified in the eastern block of the North China Craton. These two areas were amalgamated together at the latest Neoarchean along the NNW-trending paleo-structural belt with intrusion of some plutonic diorite bodies.The U-Pb SHRIMP results of zircons from the Mengshan diorite pluton in the southern segment of the paleo-structural belt give the upper interception age of 2 534±77 Ma based on the fine discordant line and give the close concordia age of 2 536±26 Ma that represents the age of diorite pluton emplacement. The diorites along the NNW-trending paleo-structural belt represent the plutonic magmatic activity and the cratonization during the juxtaposion of two Archean microblocks in the Western Shandong.

Geochemistry of Huheengeer Complex, Bayan Obo Region, Inner Mongolia and It，s Tectonic Implications

ZHAO Lei

2008, 14(1): 29-38.

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Huheengeer mafic-ultramafic complex is found in Bayan Obo region, Inner Mongolia. Petrographically, the complex shows a concentric zoning in rock distribution: hornblende-peridotite, hornblende-gabbro and diorite from core to margin successively. Characteristics of structure and chemistry of the pluton suggest that the rocks were derived from a gabbroicdioritic magma by crystal differentiation. The hornblende-peridotite and hornblende-gabbro were formed by cumulation of early crystallized mafic minerals, and the other rocks crystallized from a magma formed after cumulative process by eutectic crystallization. The parental magmas were compositionaeey similar to high-Al basalt and (or) basaltic andesite. The K-Ar apparent ages of the rocks are between 242-287 Ma. The major rock-forming minerals in the Huheengeer Complex are olivine, pyroxene,plagioclase and hornblende. Based on electron microprobe analysis (EMPA), pyroxene is diopside, and plagioclase is bytownite (An90.6) in amphibole gabbros. The complex also shows geochemical characteristics of enrichment in LREE, LILE (K，Rb，Sr，Ba), and depletion in HREE, HFSE (Nb，Ta，Zr，Hf). According to the AlZ-TiO2, An-Mg,, Hf/3-Th-Ta and Hf/3-Th-Nb/16 diagrams, the Huheengeer complex is suggested to be generated at an active continental margin. All the combined information indicates that these plutons are mafic-ultramafic complex of peritotite-diorite type and they should be a record of Paleoasian Ocean plate subduction to the northern side of North China plate in the Permian.

Geochemistry and Origin of Siliceous Rocks from the Gufeng Formation of Middle Permian in the Pingdingshan Area,Chaohu Region, Anhui Province

YANG Shui-yuan

2008, 14(1): 39-48.

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The bedded siliceous rocks of this study occur in the Gufeng Formation of the Middle Permian in the Pingdingshan area, Chaohu region, Anhui Province. The origin and depositional environments for the siliceous rocks have been debated for a long time. In this paper, we report the detailed field and petrographic observation, Combined with the data of major elements,rare earth elements and oxygen isotopic compositions of these rocks. our results show that in the Al-Fe-Mn ternary, and the SiO2-(K2O+Na2O)，SiO2-Al2O3，SiO2-MgO，Al2O3-(K2O+Na2O) binary discriminant diagrams, these rocks fall in the biochemicallydeposited area. These siliceous rocks show an average K2O/Na2O ratio of 1.32, slightly negative Ce anomaly, and a comparatively enrichment of LREE, which indicate a biochemically-deposited origin for these rocks. The siliceous rocks have the total REE contents of 19.87~63.95 μg/g, with an average of 44.59μg/g. Their δCe values are 0.72~0.97, with an average of 0.84, and their Ceanom values are -0.174~-0.011, with an average of -0.088. The average (La/Yb)N and (La/Ce)N values are 1.01 and 1.16,respectively. All these geochemical characteristics suggest that these siliceous rocks were deposited in a continental margin setting where the sea water during deposition was under an anoxic condition. We found volcanic hairs in the mudstones interbedded with the siliceous rocks, which indicates that some of the silicon in the siliceous rocks might come from the volcanic rocks.

Cenozoic Extensional Rejuvenation of Salt Structures in the Paradox Basin, Utah and Colorado, USA

GE Hongxing

2008, 14(1): 49-63.

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The famous breached salt anticlines and walls in the northeastern Paradox Basin have previously been considered to be preexisting diapirs amplified by late Cretaceous to Paleocene Laramide regional compression. Similarly, the spectacular arrays of normal faults along the valley sides have been attributed to dissolution of underlying salt. We present evidence that both folding and normal faulting were a response to NE-directed regional extension during late Eocene to Oligocene, which reactivated the preexisting NW-trending salt diapirs. Salt diapirs, which grew mainly by sedimentary differential loading during late Paleozoic and Mesozoic times, localized the Cenozoic extension within the weak diapirs and their thin roofs, resulting in normal faults above the diapir crests. Faults are en echelon because extension was oblique to the trend of the preexisting salt structures. Extension created fault-slip and rollover anticlines separated by listric normal faults. Where the source layer was thick, diapirs reactively rose below the normal faults. Elsewhere, where the source layer was depleted, diapirs subsided as they widened during regional extension.The flanks of tall diapirs subsided, whereas low diapirs entirely subsided. Crestal grabens sank into the widening, subsiding diapirs, creating cusps of salt. Salt locally extruded over extensionally foundered grabens.

Is the Jiangnan Orogenic Belt a Grenvillian Orogenic Belt:Some Problems about the Precambrian Geology of South China

Zhou Jin-cheng

2008, 14(1): 64-72.

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This discussion is made on the basis of age data reported in recent years of 21 century, which have been obtained by SHRIMP, LA-ICP-MS zircon U-Pb and other new dating methods. The assembly of the Yangtze and Cathysia blocks took place between 860 and 820 Ma. The continent-continent collision event forming Grenvillian orogenic belt spanned a period from 1 190 to 980 Ma. Therefore, the collision between the Yangtze and Cathysia blocks occurred about 320~160 Ma later. The basement strata in the Jiangnan orogenic belt underwent thoroughly greenschist facies metamorphism. High grade metamorphism, such as granular facies existed widely in the Grenvillian orogenic belt, has never been found in the metamorphic sedimentary-igneous rocks in the Jiangnan orogenic belt. Because of above two reasons, the Jiangnan orogenic belt should not be considered as a Grenvillian orogenic belt. It is proposed that the Jiangnan orogenic belt might be a product of collision between two neighborhood blocks during the transformation from the Rodinia to Gondwana supercontinent. The Sibao, Lengjiaxi and Fanjingshan groups in the western part of the Jiangnan orogenic belt, as basement strata, were previously believed to be Mesoproterozoic. Recent dating results of detrital zircons from the sandstones of the basement strata indicate that these strata have the maximum depositional ages of 870~860 Ma, i.e. Neoproterozoic. The S-type granites distributed along the Jiangnan orogenic belt may be considered as a petrological record of the continent-continent collisional event between the Yangtze and Cathaysia blocks rather than the products of plume magmatism. Some Neoproterozoic mafic volcanic-intrusive rocks (~760 Ma) from the western part of Jiangnan orogenic belt and the mafic rocks from the bimodal volcanic rocks (803~818 Ma) distributed around or within the Yangtze block or within Cathaysia block show some geochemical signatures of OIB. However, their exposed volumes are very small. They might have been generated in the post-orogenic extensional stage due to local upwelling of asthenospheric mantle. It is believed that they shouldn't be considered as petrological evidence of breakup of the Rodinia.

Characterisics and Origin of the Stylolites in Deep Burial Volcanic Rocks from the Northern Songliao Basin

ZHANG Feng-qi

2008, 14(1): 73-81.

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Stylolite structure is widely distributed in deep burial acidic volcanic rocks in the northern Songliao Basin.Macroscopeically, stylolites can be divided into two groups. One group often develops along the interfaces of different rocks or fluxion planes and its shape and dip are controlled by these interfaces. The other group is related with the tectonic fissures. The results of observation by scan electronic microscope show that the stylolites have a complex spatial shape and their column-sides are characterized by micro- striations. Analysis by electronic probe indicates that the stylolites in volcanic rocks are composed of iron oxides, kaolinite and leopoldites, partially with a small amount of carbonates. Through comprehensive analysis, a conclusion can be drawn that the stylolites in volcanic rocks developed in the post-diagenetic period and are related with the dissolution of quartz and alteration of feldspar. The generation of stylolites may be controlled by the lithologic characters, ground temperature,fluid regime and ground stress.

Using lithologic Characteristics and Strata Thicknesses in Karst Region to Estimate Laterite-forming Capacity: A Case Study for Triassic-Jurassic Sediments in Jinyang District, Guiyang City, Guizhou Province

YANG Rui-dong

2008, 14(1): 82-89.

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The laterite-forming capacity of carbonate rocks is an important problem in investigating development and evolution history of the karst morphology. This paper studies the carbonate rocks sections，their laterite formation by weathering, and distribution of underlain Permian-Triassic-Jurassic sediments in Jinyang District, Guiyang City, Guizhou Province. Combining with development features, thicknesses and lithologies of the strata, and the empirical values of laterite formation for carbonate rocks, the thickness of laterite in the research district is estimated. By comparison of the theoretical laterite thickness with the practical preserved laterite thickness, it is deduced that the present day preserved laterite layer is the weathering product of the third peneplanation stage since the Early Pleistocene. Its formation age is about 400 ~1 300 ka. Our results show that the present day preserved laterite layer overlying carbonate rocks is not only weathering product of the carbonate rocks, but also of their clastic rocks interlayers which make considerable part of the laterite soil. Moreover, the Jurassic (containing no carbonates) clastic sediments were also important original rocks for laterite formation. Consequently, the sources of laterite formation in the study district were multiple and complex, but not the carbonates solely.

Two Neogene Machilus (Lauraceae) Fossils Leaves from Tengchong,Yunnan Province and Its Paleoenvironmental Significance

WU Jing-yu

2008, 14(1): 90-98.

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A fossil leaf of Machilus leptophylla and a fossil leaf of Machilus cf. longipedicellata were collected from the Pliocene Mangbang Formation in Tengchong, Yunnan Province. Their architectural and epidermal characteristics were investigated by laboratory observation and cuticular analysis, and were compared with their living relative species. These two fossil leaves are oblong, coriaceous. Their primary veins are straight and moderately thick, and their secondary veins are pinnate. However,epidermal structure of the Lauraceae is very typical, the subsidiary cells are brachyparacytic and the guard cells are located underthe two lateral subsidiary cells. The trichome bases of these two fossil leaves are unisetose. Therefore, the cuticles of two fossil leaves are considered to belong to the Machilus based on the detailed comparison of leaf architecture and epidermal structure.The results show that both leaf architectural and epidermal characteristics of the fossils are very similar to the living features and thus the combined usage of leaf architecture and cuticle in fossil classification and identification can yield more reliable results than solely used. The convincing identification has undoubtedly enriched the fossil records of Lauraceae in China. In addition,the stomatal density (SD) and the stomatal index (SI) have been calculated for these fossils and are compared with their living species. The results imply that leaves of fossil Machilus leptophylla and fossil Machilus cf. longipedicellata may have the potential to be used as indicators of palaeoatmospheric CO2 concentration. Through comparing the stomatal ratios of the modern Machilus leptophylla and Machilus longipedicellata (the living relative species of these two fossil leaves) to the fossil species, indicate that the CO2 concentration in the Pliocene is much higher than that of today. Our analysis suggests that the Neogene climate in western Yunnan might be warmer and more humid than the current condition.

The Patterns of Tectonic Slope-Break Zones and Characteristics of Sequence Stratigraphy in the East of Southeastern Hainan Basin

GAN Hua-jun

2008, 14(1): 99-105.

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Based on the detailed seismic and well log data, utilizing work station and seismic interpretion software, taking Cenozoic strata in the east of Southeastern Hainan basin for an example, this paper studies the tectonic framework and sequence stratigraphy. Three types of tectonic slope-break zones were developed in the study area: the fault break-zone, the single slopebreak zone and the combined slope-break zone. Considerable amount of geological bodies in lowstand system tracts (LST) controlled by different tectonic framework were distinguished, including incised channel, basin floor fans, slope fans, low standwedge, which implies that their soure rocks were mainly derived from the east of the continental shelf in the basin. The characteristics and distribution of LST in the area correspond to the patterns of sequence stratigraphy framework, and sandbodies in LST could be available reservoirs. Directed by the theory of slope-break zones, this paper indicates the relationship between tectonic slope-break zones and reservoir distribution, and predicts the potential prospecting targets.

Metallogenic Prediction Model for Spatial Information of Mineral Deposits Based on GIS in Gejiu Granite Depression Zone

WANG Xiong-jun

2008, 14(1): 106-113.

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Using Geographic Information System (GIS), the present authors quantitatively studied the model establishment of the space information related with metallogenic prognosis in Gejiu granite depression zone, Yunnan Province. On the basis of geology of this mining field, at first we divided the information statistics units. Then we determined the geological information variables,which consist of stratigraphic positions, structural features, magmatic bodies configurations, mineralization-alteration types, and geophysical-geochemical-remote sensing anomalies, along with coding and assignment. The method of signature analysis was adopted to ensure the mineralized anomalies favorability model of the space information grid units. Finally, the prospecting targets were delineated from the mineralization prediction units. The results clearly indicate the feasibility of the method combining the mineralization space information grid units and the mathematic signature analysis.

Geological Characteristics and Prospecting Criteria of Xianglushan Tungsten Deposit, Northwestern Jiangxi Province

TIAN Bang-sheng

2008, 14(1): 114-119.

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Xianglushan Tungsten Deposit is a super-large skarn-type scheelite deposit. The major ore bodies of this deposit are found in the contact zone between a Late Yanshanian granite body and the impure limestone of the Middle Cambrian Yangliugang Formation which makes the core of the Xianglushan Anticline. The W contents in the granite are over 50×10-6. The average δS34 value of ore is +3.4 ‰. The ore formation process mainly occurred in the skarn stage, and in a lesser amount in the sulfide stage.The major ore-forming controlling factors are anticline structure with its accompanied interlayer slipping planes, S type granite with higher W concentration, and Middle Cambrian impure limestone country sediments. The major hydrothermal alterations are skarnization, hornfelsization, silicification, greisenization, chloritization and fluoritization. These hydrothermal alterations and pyritization, sheelitization, wolframitization and their oxidation products are direct ore prospecting criteria.

Dissolution Fractionation Model of Natural Gas Components and Its Application

CHENG Fu-qi

2008, 14(1): 120-125.

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Because of dissolving ability difference among different gas components, dissolution itself could cause original components of natural gas to be changed. This is so called dissolution fractionation of gas components. Dissolution fractionation could happen during natural gas migration and dissipation in water. It is necessary to find out dissolution fracionation mechanism and establish fracionation model for tracing origin and distribution of natural gas. Based on the Henry's Law, a dissolution fractionation model of gas components was established. The model calculation shows that, the original gas components in the Sinian reservoir of Weiyuan field are CH4-72.70%, C2H6-0.09%, CO2-22.41% and N2-4.07%, and the dryness coefficient (C1/C1-5) is 99.876%. Because of dissolution the CO2 contents in natural gas decreased greatly while hydrocarbon gases contents increased slightly. Resumption of the original components of natural gas is helpful to genetic discrimination and gas-source correlation.

A Study on Estimating Hydrogeologic Parameters with Improved Parallel Genetic Algorithms

KONG Xiang-long

2008, 14(1): 126-132.

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Simplex Methods are quite efficient, but not very robust. They normally converge into a small number of iterations, but often fall into local minima. On the other hand, Genetic Algorithms are very robust, but unacceptably expensive. On the basis of analysis of the advantage and disadvantage of Simplex Methods and Genetic Algorithms, we present a hybrid method named SPGA aimed at coupling the efficiency of Simplex Methods and the robustness of Genetic Algorithms for solving the groundwater inverse problem. In the paper, we introduce the operators of SPGA and apply it to the inverse simulation of a water source area. As shown by the calculation results, the SPGA has strong point of high precision and fast speed.

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