Table of Content

20 June 2007, Volume 13 Issue 2

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Article

Tectonic Attribution of Continental Margins of China and Super Accumulation of Metals

PEI Rong-fu, LI Jin-wen, MEI Yan-xiong, WANG Yong-lei, LI Li, WANG Hao-lin

2007, 13(2):  137-147. 

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The crust was split into continental crust and oceanic crust by inhomogeneous differentiation. Continental margins with different tectonic features were formed by continent splitting, oceanic crust subduction or continentcontinent collision. Five marginal tectonic belts, thirteen secondary tectonic regions and fifty-three spacetime distribution structures are subdivided according to the distribution of three continent-continent collision belts. The probabilities for discovering deposits in tectonic regions are calculated based on existing deposits.Continental margins are divided into divergence type, convergence type, collision type and transform type, and their metallogenetic series specialization is summarized. It’s very important for metallogeny that advantageous metallogenetic multi-factors are coupled. Super accumulation of metals is controlled by exceptional metallogenesis and special geological events are key to make metallogenesis exceptional.

A Preliminary Discussion on Large Igneous Provinces and Associated Ore Deposits

XIAO Long1, PIRAJNO Franco2, HE Qi1

2007, 13(2):  148-160. 

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Large igneous provinces (LIPs) are grouped into mafic (MLIPs) and silicic (SLIPs), based on their dominant components, mafic or silicic volcanic and intrusive rocks, respectively. The formation of these LIPs requires extremely high heat flow from mantle, which results in extensive melting of mantle, subcontinental lithospheric mantle and lower crust. The typically massive magmatism of LIPs also results in extensive energy and materials exchange thereby causing the formation of a range of important ore systems. MLIPs and SLIPs have different magma sources, compositions, temperatures, pressures, fluids and oxygen fugacities, and experience distinct magmatic evolutionary histories. MLIPs tend to form magmatic Cr-Cu-Ni-PGE sulfide and V-Ti-Fe oxide deposits, some hydrothermal Cu-Pb-Zn-Au-Ag deposits and distal epithermal systems. SLIPs, on the other hand, tend to form metasomatic and/or hydrothermal Cu-Pb-Zn-Au-Ag, W-Sn, U-Th-REE, As-Sb and low-sulfidation epithermal ore systems. Detailed studies of LIPs and associated ore deposits will enable better constrain the metallogenic and ore genesis models, which in turn will help in the discovery of giant ore deposits.

Intraplate Extensional Magmatism of North China Craton and Break-up of Three Supercontinents and Their Deep Dynamics

YAN Guo-han1, CAI Jian-hui1,2, REN Kang-xu1,3, HE Guo-qi1, MU Bao-lei1,Xu Bao-liang1, LI Feng-tang1, Yang Bin1

2007, 13(2):  161-174. 

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Based on authers' twenty years research on isotope geochronology and spatial distribution of intraplate extensional magmatic rocks (such as alkaline intrusives etc. ) in North China Craton, as well as the existing references, this paper suggests that intraplate extensional magmatisms of North China Craton were concentrated in three stages: the latest Paleoproterozoic-early Mesoproterozoic era（1 850~1 600 Ma）, the middle-late Neoproterozoic era（900~600 Ma）and the latest Palaeozoic-Cenozoic era（250 Ma~now）, respectively, as named the first, the second and the third extensional stage. The appearance frequency and the spatial distribution of the three stages extensional magmatisms are distinctly different from each other. The third stage has the highest appearance frequency and widest distribution, and the first stage takes second place. It must be especially emphasized that the isochron ages of the three stages magmatic rocks are basically coincident with the break-up timings of Columbia, Rodinia and Pangaea supercontinents,respectively, showing different degrees of response of North China Craton to the break-up of related supercontinents. The activities of super mantle hot plumes are popularly considered as the deep geodynamics of dispersal of supercontinents. So it is speculated that the distance from North China Craton to the center of super mantle hot plume activities in the third extensional stage was the nearest, and that in the second stage was the farthest.

History of Volcanic Activity, Magma Evolution and Eruptive Mechanisms of the Changbai Volcanic Province

FAN Qi-cheng1, SUI Jian-li2, WANG Tuan-hua1, LI Ni1, SUN Qian1

2007, 13(2):  175-190. 

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Volcanic rocks cover large area of the Changbai Mountain Range, Northeast China. As the largest Quaternary volcanic province in China, the Changbaishan volcanic province can be divided into four parts, including the Tianchi volcanic province (TVP), the Wangtian,e volcanic province (WVP), the Longang volcanic provicne (LVP) and the Tumen River volcanic province (TRVP). Volcanic activity in TRVP and WVP began in Pliocene, dominated in Pliocene-Mid-Pleistocene (5.5 ~0.19 Ma) and Pliocene-Early-Pleistocene (4.77 ~2.12 Ma), respectively. TVP and LVP activated in Quaternary, from Early-Pleistocene to Holocene. The volcanic activities and products in these four volcanic provinces are different. In TRPV, it is tholeiite from flood eruption; while in TVP, WVP and LVP, the volcanic eruptions formed lava shields of potassic trachybasalt in the early stage, and then different magma evolution paths occurred. In TVP and WVP, potassic trachyebasalt emplaced into the crustal chamber, stored and experienced strong fractional crystallization. The evolved magma was mainly alkaline rhyolitic and pantelleritic, highly enriched in silicon, alkalies and volatiles. Its eruptions formed two huge stratovolcanoes. Activity of the potassic trachybasalt beneath the TVP controlled two kinds of eruptions: the millennium eruption of Tiachi Volcano, which was triggered by refluxing of the mantle trachybasalt magma to the crustal chamber and subsequent magma mixing; and the centric eruption of the potassic trachybasalt, which formed the big cone of Tianchi Volcano and many other small cones in the vicinity. In the LVP, on the lava shield occurred over 160 volcanic cones, The rock type of these eruptions was unique potassic trachybasalt. Geochemical data of trace elements and Sr-Nd-Pb isotopes indicate that the mantle sources were mixtures of asthenosphere and enriched lithospheric mantle. As to the LVP, the volcanics were dominated by depleted mantle of asthenosphere, and with less entertainment of enriched lithosphere. After the late Pliocene, geodynamics of the Changbai Mountain Range was controlled by the subduction of the western Pacific Plate and the extension of the Northeastern Asian back arc. In this special active continental margin, large scale of potassic trachybasalt activity occurred again.

Petrogenesis and Tectonic Setting of the Paleoproterozoic Xiong'er Group in the Southern Part of the North China Craton: a Review

ZHAO Tai-ping1, XU Yong-hang1,2, ZHAI Ming-guo3

2007, 13(2):  191-206. 

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The Xiong'er Group formed in 1.80-1.75 Ga of Paleo-Proterozoic and contains predominantly volcanic lavas with minor sedimentary rocks and volcanic clastic rocks (4.3% in thickness). It is the largest magmatism in scale after the formation of the crystalline basement of the North China Craton，which vary from 3 km to 7 km in thickness，and crops out over an area of 60 000 km2. The volcanic lavas are composed predomiantly of basaltic andesites and andesites, and minor dacitic-rhyolitic rocks. The volcanic lavas are lack of the rocks with SiO2 = 62% ±and therefore appear as a bimodal association. The mineral assemblage of mafic-intermediate lavas mainly contains pyroxene and plagioclase, and rarely with amphibole and bitotite. The rocks are characteristic of high Fe and K and low Al, Ca and Mg contents. All the volcanic rocks are enriched in large-ion lithophile elements (LILE, eg, Rb, Ba) and light REEs (LREE) and are depleted in high field strength elements (HFSE, e.g., Nb, Ta and Ti), suggesting an arc-related setting. The geochemical and isotopic compositions of the lavas indicate derivation from a mantle source previously contaminated by subducted material and minor crustal contamination during the upwelling of the magmas. The sedimentary rocks of the Xiong'er Group formed the Dagushi and Majiahe Formations. The rock assemblage and geochemisty of the sedimentary rocks indicate that the Xiong'er Group formed in a passive continental margin and it was mainly a marine facies when lava erupted. Therefore the paleogeographic environment of the Xiong'er Group evolved from continental facies to marine facies then to continental facies again. It is suggested that the Xiong'er Group was originated from a failed trident continental rift environment. The arc-like geochemisty resulted from an enriched subcontinental lithospheric mantle previously contaminated by subducted slab. The volcanic rocks of the Xiong'er Group is also widespread in the Baoji and Lvliang area and also appear as 1.75 Ga-old mafic dyke swarms, A-type granites, later anorthosite and rapakivi granites (1.75-1.70 Ga), and alkaline granitoids (~1.65 Ga). These intrusive rocks are assumed to have genetic link to the volcanic rocks of the Xiong'er Group. These intrusive rocks resulted from the breakup of the North China Craton, suggesting that the North China Craton was under an extention environment at~1.80 Ga. However, a detailed work is worthwhile to figure out whether a post-collision magmatism or a mantle plume that triggered the emplacement of the intrusive rocks.

Geochemistry of the Baishitouquan Topaz-bearing Amazonite Granite: Zoning and Magma Evolution

GU Lian-xing，WU Chang-zhi, ZHANG Zun-zhong, GOU Xiao-qin,LIU Si-hai, ZHENG Yuan-chuan and ZHANG Guang-hui

2007, 13(2):  207-223. 

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The Mesozoic Baishitouquan (BST) topaz-bearing amazonite granite pluton has been dated at 209.6±9.6 Ma by the Rb-Sr isochron method. This pluton exhibits five lithological zones, which, gradational from the lowest level upwards, are leucogranite (zone a), amazonite-bearing granite (zone b), amazonite granite (zone c), topaz-bearing amazonite granite (zone d) and topaz albite granite (zone e). Geochemically, The rocks are characterised by higher F (>2%) and Rb (500~1087×10-6), lower P2O5 (≤0.06 %), Na2O>K2O, A/NKC=1.00~1.11, Σ14 REE=28.6~231.9×10-6 with gull wing-shaped distribution patterns (LaN/LuN=0.11~0.68) and strong Eu depletion (Eu/Eu* =0.0005~0.0110). δ18O=9.75~7.32 ‰, εNd (t )= -4.4~-4.9. The magma for this pluton was derived by partial melting of a mica gneiss in the middle crust. The pluton exhibits the following geochemical transition from zone a to zone e: 1) Increasing F, Al2O3 and Na2O, and decreasing SiO2, (Fe2O3+FeO+MgO+MnO) and K2O. Plots of normative compositions on the Qz-Ab-Or diagram move gradually towards the Ab apex. 2) Overall, contents of Cr, Ni, Co, V, W,Nb, Zr, U, Th and Y decrease, while contents of F, Li, Rb, Hf, Ta, Sn, Sc, Ga and Zn increase. 3）K/Rb, Al/Ga, Nb/Ta and Zr/Hf decrease, and K/Cs, Th/U, and (La/Lu)N increase; 4) Whole-rock δ18O decreases from 9.25~9.75‰ in zone a to 7.32‰ in zone e. It is interpreted that crystallisation of the magma started from zone a and proceeded upwards to zone e, and the vertical zoning was produced by fractional crystallisation accompanied by fluid fractionation. There is a compositional gap on the transition trend between zone d and e. This gap is manifested in mineralogy by sharp increase in topaz, albite and muscovite and decrease in K-feldspar and amazonite; in major elements by sharp increase in Na2O and CaO and decrease in SiO2 and K2O; in trace elements by sharp increase in F, Ga, Sr and Ba, and sudden decrease in Li, Rb, Sc, Zn and Sn; in REE behaviour by sudden increase in Eu/Eu* and （La/Lu）N, and decrease in ΣREE; and in isotope composition by sharp decrease inδ18O. Such a gap might have been constrained not only by fractional crystallization and fluid fractionation, but also by involvement of meteoric water, contamination of wallrock-derived components, and leaching and dispersion due to subsolidus metasomatism.

Derivation of Adakitic Magma by Partial Melting of Subducted Continental Crust

ZHANG Hong-fei1, WANG Jing1, XU Wang-chun1, YUAN Hong-lin2

2007, 13(2):  224-234. 

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Present geological models for adakitic magma generation assume that most adakitic magmas were produced by partial melting of subducted oceanic slabs. However, it is also possible that the adakitic magma may be generated by partial melting of subducted continental crust in continental collisional settings. Here, we present a case study of the Guanshan granite (229 Ma) from the southeastern part of the Qilian Orogenic Belt. The Guanshan granite is characterized by high K (K2O: 4.12~5.16 %; K2O/Na2O: 0.97~1.64) and Sr/Y (13.6~84.1), low Y (6.8×10-6~15.7×10-6) and HREE (eg. Yb: 0.62×10-6~1.31×10-6), strongly fractionated REE patterns with (La/Yb)N =17.5~41.6 and evolved Sr~Nd isotopic compositions (87Sr/86Sr)229Ma = 0.70587~0.70714, εNd(229 Ma) = -10.9~-5.16,tDM = 1.10~1.49 Ga). These geochemical signatures show that the Guanshan granite has an affinity with continenttype adakitic rocks, but are distinct from the adakites, produced by partial melting of subducted oceanic slabs and/or underplated basaltic lower crust in arc settings. Pb-Sr-Nd isotopic compositions of the Guanshan granite are quite distinct from those of the Precambrian basement rocks, the Early Paleozoic volcanic rocks and the Early Paleozoic granitoids in the Qilian belt of the southern margin of the North China plate, but are similar to those of the lower crust of the northern margin of the Yangtze (South China) plate, as probed by the Early Mesozoic granites of the northern margin of the Yangtze plate. It is proposed that the magma for the Guanshan granite was derived from partial melting of the subducted Yangtze continental crust during the Triassic continental collision between the North China and Yangtze plates. Our result provides a new model that adakitic magma can be generated by partial melting of subducted continental crust in continental collisional settings.

The Geochemical Characteristics of the Early-Cretaceous Volcanics in Luzhong Region and Their Source Significances

XIE Zhi, LI Quan-zhong, CHEN Jiang-feng, GAO Tian-shan

2007, 13(2):  235-249. 

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The sub-continental lithospheric mantle (SCLM) under the eastern part of North China was thinned and the geochemical characteristics were replaced during Mesozoic and Cenozoic. The Mesozoic SCLM under eastern Yangtze block shows the similar procedure as that in North China craton. This paper reports the geochemical characteristics of Mesozoic volcanics, which will be helpful for understanding the characteristics of Yangtze SCLM and the timespace relationship of the replacement characteristics from Mesozoic to Cenozoic. The Luzhong volcanics, occurring in eastern part of China and belonging to Yangtze block, are a set of Late-Mesozoic alkali-rich shoshonitic rocks, which include trachybasalts-basaltic trachyandesite-trachyte. The Suangmiao group, part of the Luzhong set, was studied in detail in this paper. The volcanics were slightly contaminated by crust materials during the formation, and the elemental and isotopic compositions can well reflect their mantle source. The basaltic rocks are characterized by enriched Rb, K, Sr, Th and LREE, and depleted HFSEs. They also have enriched Sr-Nd isotopic compositions with (87Sr/86Sr)i = 0.7060~0.7063，εNd(t ) = -3.9~-6.2，(206Pb/204Pb)i = 17.788~18.125，(207Pb/204Pb)i = 15.511~5.546, (208Pb/204Pb)i = 37.735~38.184. These data suggest that the magma was partially melted from enriched mantle source, which was effected by lower crust materials within the source, and followed by crystal fractionation. The magma composition and mantle source characteristics of Luzhong volcanics imply the extension of lithospheric mantle and asthenosphere upwelling in the Late Mesozoic.

Genesis of Compositional Zoning of Clinopyroxene Phenocrysts in the Wozhong Late Eocene High-Mg Ultrapotassic Lavas, Western Yunnan, China: Magma Replenishment-Mixing Process

HUANG Xiao-long, XU Yi-gang, YANG Qi-jun and CHEN Lin-li

2007, 13(2):  250-260. 

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Normal, reverse and oscillatory zonings are common in clinopyroxene phenocrysts in the Wozhong late Eocene high-Mg ultrapotassic lavas, western Yunnan province. Both the mantle of reverse zoning and the cores of normal zoning phenocrysts are all characterized by high Mg# values (0.83 ~ 0.90), low TiO2 (0.13 ~ 0.29 %), Al2O3 (0.73 ~ 1.68 %) and Na2O contents (0.22 ~ 0.42 %). They were equilibrious crystallization products from the host ultrapotassic lavas. The cores of reverse zoning phenocrysts are relatively low in Mg# (0.77 ~ 0.84), but have similar Ti/Al ratios (0.06 ~ 0.16) as the mantle of reversed zoning and the cores of normal zoning phenocrysts. The compositions of oscillatory zoning phenocrysts vary in the ranges of those for normal and reverse zoning phenoxcrysts. Consequently, it is suggested that these zoning phenocrysts were cognate affinity with the host ultrapotassic lavas. The reversed zonation discovers magma mixing process in the chamber in which a partially crystallized magma was replenished by a mafic magma, and oscillatory zoning denotes several episodes of such mixing process. The greencore clinopyroxene phenocrysts are very rare, in some samples are obviously low in Mg# (0.50 ~ 0.74) , relatively high in Al2O3 (1.66 ~ 3.63 %) and Na2O (0.87 ~ 2.17 %), very low in Ti/Al ratios (< 0.05) and high in AlVI/AlIV ratios (0.38~ 0.76). The green-core phenocrysts are interpreted as wall-rock xenocrysts from the lower crust, indicating the crustal contamination during magma evolution.

Experimental Study on Kinetics of Water-Rock Interaction of the Hucun Skarn Copper Deposit in Tongling, Anhui Province

LI Jin-wen, PEI Rong-fu, ZHANG Rong-hua, MEI Yan-xiong, HU Shu-min, WANG Jun

2007, 13(2):  261-271. 

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An experiment of water-rock interaction in the packed bed reactor between granodiorite and pure water under the condition of stable pressure of 22 MPa and continuous current has been performed to study the formation process of the Hucun skarn copper deposit in Tongling, Anhui. The experimental results show that the rates of waterrock interaction are inversely correlated to the velocity of water flow and positively correlatied to the residence time of the water in the system. These results may explain distribution of the copper, molybdenum etc. mineralizations around the pluton. It is inferred that the copper in the ore-forming fluid might be transferred in a supercritical state and be separated out under the critical state. The leaching of copper from the rock is difficult at low temperatures.The experiment indicats that the dissolution rates of copper, zinc and other ore-forming elements are different under the same temperatures, which might be one of reasons for zoning of the ore-forming elements.

Comparison between the Zircon U-Pb Ages and the Whole-Rock Rb-Sr Ages of Granite Plutons and Its Geochemical Implication

WU Jun-qi, ZHANG Bang-tong, LIN Hong-fei and CHEN Pei-rong

2007, 13(2):  272-281. 

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The frequency analysis of age difference (Δt) between the results of zircon U-Pb dating and whole rock Rb-Sr dating for 32 granite plutons shows that the Δt displays a symmetrical normal distribution (skewness C sk=0.36; kurtosis C ku=2.99) in the histogram. The Δt is both positive and negative, with a mean of 2.08 Ma, and the percentage of age difference is less than 5% (absolute value). Using least squares regression procedure, a best regression equation with high correlation coefficient (r=0.998) and regression coefficient close to 1 (a=1.003) is fitted to the points of the zircon U-Pb ages and the whole rock Rb-Sr ages for 32 granite plutons. These statistic characteristics indicate that the zircon U-Pb dating ages are corresponding with the whole-rock Rb-Sr isochron ages for granites as a whole. The whole-rock Rb-Sr isochron dating method for granites is mature and reliable. the zircon U-Pb age cannot represent the emplacement ages of granites, but the crystallization age.

Zircon U-Pb Age and Isotope Geochemistry of Neoproterozoic Jingtan Volcanics in South Anhui

WU Rong-xin1, 2, ZHENG Yong-fei1, 3, WU Yuan-bao1

2007, 13(2):  282-296. 

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Zircon U-Pb ages, whole-rock elements, Sr-Nd isotopes, and mineral O isotopes were determined for Neoproterozoic Jingtan vocanics in South Anhui. Zircon U-Pb dating indicates: the Jingtan volcanics consist of two episodes of volcanic rocks with the formation ages of 820±16 Ma and 776±10 Ma, respectively. The Jingtan volcanics show high contents of mafic components (ΣTiO2+ Fe2O3T+MgO) of 4.7% to 16.36%, LREE enrichment,moderate negative Eu anomalies, strong enrichment in large ion lithospheric elements (LILE) of K, Rb, Ba, Th and U,but pronounced negative anomalies of Sr and high field elements (HFSE) of Nb, Ta and Ti relative to the neighboring elements. Neutral whole-rock εNd(t ) values of -2.79 to -1.71 imply their derivation from relatively juvenile crust. High A/CNK ratios of 1.37 to 1.61 and high zircon δ18O values of 6.52 to 8.98‰ point to a supracrustal orign characteristic of S-type magmatic rocks. Quartz, plagioclase and K-feldspar show O isotope disequilibria when paired with zircon, suffered different degrees of post-magmatic alteration at subsolidus temperatures. On the basis of the element and isotope geochemistry, 776±10 Ma Jingtan volcanics and Shi’ershan granites are coeval. The Jingtan volcanics were directly derived from the melting of the sediments of juvenile crust, but the main source rocks for the Shi’ershan granites are about 825 Ma magmatic rocks, indirectly derived from the resetting of early Neoproterozoic juvenile crust. It is the juvenile crust in the Jiangnan orogen that was repeatedly worked due to extensional collapse of arc-continent collision orogen during the Rodinia breakup, resulting in this episode of volcanic rocks. They are neither the product of mantle superplume activity, nor the direct product of island arc magmatism, but a product of plate-rift magmatism.

Pb-Sr-Nd Isotopic Characteristics of the Gabbros from Jinan and Zouping and the Contribution of the Lower Crust to the Magma Source

LI Quan-zhong1,2，XIE Zhi1，CHEN Jiang-feng1，GAO Tian-shan2, YU Gang1，QIAN Hui1

2007, 13(2):  297-310. 

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Pb-Sr-Nd isotopic ratios and element compositions of the gabbros from Jinan and Zouping have been studied. We obtain that (87Sr/86Sr)i＝0.7041~0.7056，εNd(t )＝-6.0~-13.0，( 206Pb/204Pb)i = 16.545~16.998，(207Pb/204Pb)i = 15.242~15.350，(208Pb/204Pb)i = 36.488~36.944 (except for SD792 possessing highly radiogenic Pb isotopic characteristics) . Trace element characters are not indicative of the source of the gabbros because of their cumulate nature. A comparison with typical EMⅠtype oceanic and continental basalts suggests that the mantle source of the Jinan and Zouping gabbros shows an isotopic characteristics similar to that of the EMⅠmantle，but with a significant contribution of the lower crust based on Pb isotopic diagrams. The addition of the lower crust into the mantle may be related to the subduction of the Pacific Ocean Plate, which resulted in the lower crust delamination.

Hainan Mantle Plume and the Formation and Evolution of the South China Sea

YAN Quan-shu1,2 and SHI Xue-fa1

2007, 13(2):  311-322. 

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Seismic tomographic images obtained from the mantle under the southeast Asia region indicate there may exist a mantle plume beneath and around the Hainan island. A sub-vertical low-velocity column is imaged beneath the Hainan and the South China Sea, and extends from shallow depths to 660-km seismic discontinuity (i.e., the interface between upper mantle and lower mantle), and continuously to a depht of 1900 km. There is a large quantity of Cenozoic alkali basalts distributed in the South China Sea and its adjacent areas which include Leiqiong Peninsula, Hainan Island, Beibuwan Basin, Weizhou Island in Guangxi province and Indochina block. The geochemical data for these basalts show the characteristics of OIB-type basalt and DUPAL-like isotopic anomaly, and imply its deepseated origin. In addition, the average value of Tp (mantle potential temperature) for the South China Sea inferred from olivine-fluid equilibrium, is 1661℃, which is higher than that of MORB and lies between the corresponding values of Hawaii hotspot and Iceland hotspot. Based on evidences mentioned above, combined with numerical model experimental data, it shows that there does exist a mantle plume beneath the Hainan Island and adjacent areas. Until recently, scholars have developed many models about the formation and evolution of the South China Sea, and the debating issue is the geodynamic source. We suggest that the Hainan plume may be a significant geodynamical source for the formation and evolution of the South China Sea. Here the Hainan plume is introduced into our preliminary model about the formation and evolution of the South China Sea. The model is as follows: (1) 50-32 Ma, Integrated effects of collision between thd Indian Ocean plate and Euro-Asian plate resulted in retrogression of Pacific plate, created a extensional tectonic setting, and provided a channel for ascent of the mantle plume; (2) 32-21 Ma. When the head of mantle plume arrived at asthenosphere, it immediately interacted with the spreading center of the South China Sea by lateral material flow, which enhanced spreading spead. During 26-24 Ma, there took place a ridge jump, which adjusted the spreading center from nearby 18。N (i.e., present-day center of NW sub-basin) to nearby 15.5。N (i.e., present-day center of East sub-basin); (3) 21-15.5 Ma. With the mantle plume effect gradually enhancing, the hotspot-spreading center interaction became more and more intensive, and at about 21 Ma, there took place a ridge jump again, and induced the opening of SW sub-basin; (4) 15.5-0 Ma. Due to collision between the Indo-Australian plate and the Sunda continent, the spreading stopped. Subsequently, the earlier formed oceanic crust subducted along Nansha trench and Manila trench. However, the mantle plume still existed up to now. An actual evidence is: since the Pliocene a large amount of alkali basalt erupted in the South China Sea and its adjacent areas.

Geochronological Sequence of Mesozoic Intrusive Magmatism in Jiaodong Peninsula and Its Tectonic Constraints

ZHANG Tian and ZHANG Yue-qiao

2007, 13(2):  323-336. 

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Jiaodong Peninsula，located in the North China Craton east of the Tanlu fault zone，is one of the most important areas of Mesozoic intrusive rocks population．A geochronological sequence of Mesozoic intrusive rocks has been established through a detailed study of high-resolution isotopic ages of intrusive plutons in this area. Three periods with different magmatisms are distinctive：the Late Triassic mantle-derived granites aged 225~205 Ma， the Late Jurassic crustal-derived S-type granites aged 160~150 Ma, and the Early Cretaceous crust-mantle mixed sourced granites aged 130~105 Ma．A comparison of geochronological framework of the Mesozoic intrusive rocks with those outcropped in Liaodong and Luxi-Xuhuai areas provides key information to constrain tectonic evolution of North China．It points out that the Late Jurassic crustal-derived S-type granites in Jiao-Liao area record an important crustal thickening event, which is considered to be geodynamically related to flat subduction of the Pacific plate beneath the Asia continent. This was followed by lithospheric delamination and attenuation during the Early Cretaceous, which resulted in widespread crust-mantle mixed sourced granitoid magmatism in East China. Crustal thickening and thinning are considered to be main cause for changes in Mesozoic tectonic regime and sequential evolution of magmatism in East China.

Preliminary Study on Re-Os Isotope Geochemistry of Picrites from Lijiang Area，Yunnan Province

CHEN Lei1, ZHI Xia-chen1*, ZHANG Zhao-chong2, SHI Ren-deng1

2007, 13(2):  337-343. 

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Here we present Re-Os abundances and Os isotopic compositions of 12 picrites and 6 associated basalts collected from Daju and Shiman sections, Lijiang area, western part of Emeishan large igneous province (ELIP Zhang et al, 2006). The results show that the picrites and associated basalts have distinctive Re-Os isotopic features. The picrites have higher Os abundances (1.5×10-9~3×10-9), and sligtly lower Re abundance (<0.05×10-9) than associated basalts (<0.5×10-9 and <0.8×10-9, respectively). However, the picrites have unradiogenic 187Os/188Os (0.1233~0.1266), but the basalts have supradiogenic 187Os/188Os (0.1338-0.1577). The Re-Os isotope geochemistry of ELIP lava may shed light on the mantle plume origin of ELIP. The mantle plume was derived from the asthenosphere rather than the deep core-mantle boundary. In other word, the ELIP eruption was controlled by interaction between lithosphere and asthenosphere (Saunders, 2005).

Fluid Inclusion Planes Measurement and Paleostress Field Analysis for Cenozoic Gonggashan Granite

SUN Sheng-si ，JIA Dong，HU Qian-wei，CHEN Zhu-xin， JIA Qiu-peng，LUO Liang，LI Yi-quan，DENG Fei

2007, 13(2):  344-352. 

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Gonggashan granite is a syntectonic granite, emplaced at 12.8±1.4 Ma. It extends parallel to Xianshuihe fault, which is a major active sinistral strike-slip fault in eastern Tibet Plateau. Along Chuanzang Road cross section and Maoniugou to Tagong Grassland cross section, 10 oriented specimen were collected to conduct the fluid inclusion planes (FIP) study. The attitudes of FIP measured on Fedoroff universal stage show that the preferred orientation is EW and subvertical. The FIP results indicate that the Gonggashan granite suffered E-W horizontal compression，which is consistent with the sinistral slip of Xianshuihe fault. Laser Raman Microspectroscopy data reveal that most of fluid inclusions in FIP are of NaCl-H2O composition. Microthermometric analysis of fluid inclusions allows distingction of two general types: Type A is characterized by Tm ice in the range of －9.0 to －8.2℃ (salinity 11.9%~12.8%) , Th varies from 126.0 up to 197.0℃ . Type B is characterized by Tm ice in the range of －4.7 to －0.5℃ (salinity 0.9%~7.4%) , Th varies from 144.0 up to 314.6℃. The FIP of Gonggashan granite reveals two stages of fluid percolation，and both evolved with declined temperature and salinity. Paying more attention to syntectonic Gonggashan granitic magmatism and its relationship with Xianshuihe faults is of very important scientific significance in understanding the deformation mechanism of Tibet Plateau.