Geological Journal of China Universities

DONG Shen-bao，TIAN Wei

2007, 13(3): 353-361.

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The granite petrogenesis evidenced as the growth and the recycling of the Earth’s crust has been known as one of the far-reaching problem related to the geodynamics of mantle vs. crust and the geochemistry of the liquid in filtration within the solid state of the Earth as well. During its development, the origin of granite may be accounted as follows：（1）Migmatitic granite vs. magmatic granite. It signifies the beginning era geological observations of the metamorphic origin of granite on the one hand, and the elementary thermodynamic treatment of granite on the other hand. A bitter controversy has been argued nearly about one hundred years.（2）The stage of anatexis. A stage of laboratory experiment modelling the natural melting of crustal rocks into granite has been initiated.（3）The stage of prevailing of the anatexis. It comprises the major and minor elements all over the anatexis in order to verify the consistencies between geological observation, and those of the thermodynamic treatment, in aiming at the interconnection between geological observation, of the natural granite system and that of the thermodynamic system.（4）The origin of granite related to tectonics. It represents a beginning of the advancing stage toward the geodynamic prospective through the role of paragenesis, which is in connection with the thermodynamic principle in order to form a global pattern of granite petrogenesis. Various themes have been proposed and argued to their true aims it attained. Finally, some vital problems concerning the application of equilibrium thermodynamics, the role of fluid in the mechanism of granite petrogenesis, and some problems concerning the origin of granite with respect to tectonics as well as the heat dissipation through secular change have been discussed.

On Granitic Tectono-Magmatic Assemblages

WANG De-zi，SHU Liang-shu

2007, 13(3): 362-370.

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Granitic tectono-magmatic assemblages mainly reflect the genetic connections between the magma types of granites and tectonic environments. Viewing from the global scope, particularly looking at the situation in China,the granitic tectono-magmatic assemblages could be divided into five major types: (1) oceanic crust subduction type, such as continental margins on both sides of the Pacific Ocean; (2) continent continent collision type, e.g., the Himalaya-Gangdise collisional belt; (3) extensional continental margin type, such as the Southeast China extensional margin and the Basin and Range Province, USA; (4) inland faulted depression type, such as the Lower Yangtze faulted depression and the Qiantan-Xinjiang faulted depression; (5) rift type, such as the Eastern African Rift and the Panxi Rift of China. By calculating the calc-alkali index (CA) and alumina saturation index (ASI), the magma types of the granites can be roughly determined. The temporal and spatial evolution of orogenic granitic belt shows certain kind of regularities: subduction type granites → collision type granites → extensional type granites. Or, it may be described as: precollision granites → syncollision granites → postcollision granites. However, instead of giving general assumptions, we should carefully examine and analyze actual cases and conditions before reaching a conclusion.

Discussion on Genetic Classification of Metamorphic Ore Deposits

SHEN Qi-han

2007, 13(3): 371-382.

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This paper discusses the definition of the metamorphic ore deposits. On the basis of the preceding study, a new genetic classification scheme is tentatively put forward, in which 5 types and 13 subtypes are discriminated, and representative ore deposits examples are given for each type and subtype. The new classification is shown as follows. (1)Metamorphosed ore deposits, which are subdivided into five subtypes: the metamorphosed banded iron formation (BIF) sedimentary subtype, the metamorphosed BIF volcano–sedimentary subtype, the other metamorphosed sedimentary–volcano–sedimentary formation subtype, the metamorphosed igneous alteration ore deposits subtype and the metamorphosed spilite–keratophyric volcano–eruptive sedimentary ore deposits subtype; (2) Metamorphic ore deposits of regional metamorphism. Three subtypes are included: the metamorphic recrystallization subtype, the components recombination by metamorphic chemical reactions subtype and the metamorphic hydrothermal ore deposits subtype; (3)Metamorphic ore deposits of local metamorphism. Three subtypes are determined: the skarn ore deposits of contact–metasomatism subtype, the local thermal contact–metamorphic subtype and the local dynamo–metamorphic subtype;(4) Metamorphosed sedimentary–volcano–sedimentary ore deposits superimposed and transformed by metamorphic hydrothermal solutions; (5) Ore deposits by migmatism. Two subtypes are included: the migmatitic–metasomatic subtype and the later stage migmatitic–hydrothermal subtype.

Global Distribution of Ultrahigh-Pressure Metamorphic Belts and Its Geotectonic Significance

YOU Zhen-dong

2007, 13(3): 383-391.

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On the basis of geological studies of some 28 localities of Ultrahigh-Pressure Metamorphic (UHPM) rocks all over the world it is not difficult to find that most UHPM belts are developed in close relationship with the island arc-continent and the continent-continent collisional orogenic belts. Majority of them are developed within the active continental margins where cold continental materials can be subducted deeply under another relatively stable or non-subducted plate, and the subduction depth of which can exceed 90~120 km. Consequently, under the deep-seated high-pressure (>2.5 Gpa) and high-temperature (c. 600℃ or higher) in combination with other favorable metamorphic conditions the UHP typomorphic minerals，such as coesite and diamond, can be formed. According to present data UHPM rocks are spatially concentrated within Eurasia and at its margins. However, there is still lack of information for North America and Australia. Temporally the UHPM rocks are usually Phaenerozoic in age. The Precambrian UHPM belts are few, only two of them are Neoproterozoic in age (Pan-African stage). It is suggested that the collisional orogenic process with UHP metamorphism be a tectonic regime when accretion of the continental crust reaches considerable thickneses. The statistics of temporal distribution of UHPM belts shows two maxima at Paleozoic and Mesozoic-Cenozoic, respectively. It indicates that the break-up of Rodinia and Pangea caused the lowering of geothermal gradient of the continental crust which is favorable for formation of the UHPM belts.

Igneous Petrotectonic Assemblages and Tectonic Settings: A Discussion

DENG Jin-fu1, 2，XIAO Qing-hui 1, 2，SU Shang-guo1, 2，LIU Cui1，ZHAO Guo-chun1，WU Zong-xu1，LIU Yong1，2

2007, 13(3): 392-402.

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This paper discusses igneous petrotectonic assemblages of various tectonic settings: 1. mid–ocean ridge spreading; 2. oceanic island; 3. island arc; 4. MORS–type and SSZ–type ophiolites; 5. active coutineutal marginal arc; 6.compositional polarity of subduction–related magmatic arcs; 7. continental collision; 8. continental rift; 9. stable craton or platform, etc. It is emphasized that the tholeiitic ( TH ), calc–alkaline ( CA ) and alkaline ( A ) series defined from various parameters by different authors have different scientific meanings. In this paper it is indicated that the same written terms of TH, CA and A but with various meanings could easily result in abuse of these terms, especially for the recognition of the tectonic settings. The difference between MORS – and SSZ –type ophiolites in terms of igneous petrotectonic assemblages is discussed. The lowest MgOwt% of the HMA is suggested ( Table 2 ), based on the data of petrological experiments. The terms of magnesian andesite–dacite and adakite are discussed, and in turn, the Kay's magnesian andesite–dacite of higher Sr/Y, lower FeO/MgO, and higher MgOwt% as well as higher Ni, Cr contents, are comprehensively used, which can be more satisfactory to define the slab–melt, rather than the single parameter of high Sr/Y. The various schemes of classification for the continental collision are indicated, and it is necessary to be careful for using these schemes. The post–orogenic and the continental rifting A type granites are suggested to be associated with or without the CA/r, respectively, and the meaning of A and CA, here, is determined by the Peacock's alkali–lime index.

Granitoids and Crustal Growth in the East-Kunlun Orogenic Belt

MO Xuan-xue1, LUO Zhao-hua1, DENG Jin-fu1, YU Xue-hui1, LIU Cheng-dong2,CHEN Hong-wei1, YUAN Wan-ming1, LIU Yun-hua3

2007, 13(3): 403-414.

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The East-Kunlun orogenic belt (abbrev. EKOB) is one of major tectono-magmatic belts in the Tibetan plateau. Four stages of granitoid plutonism took place in Precambrian (the Proterozoic), early Paleozoic (∈-D3), late Paleozoic-early Mesozoic (D3-T3), and late Mesozoic-Cenozoic (after early Jurassic), respectively. Among them, the late Paleozoic-early Mesozoic, especially Triassic granitoids are predominant. The basement of EKOB formed in late Paleo-Proterozoic. The early Paleozoic tectono-magmatic event sequence is comparable to that in the North-Qilian orogenic belt, and became a part of the Qilian-East-Kunlun Caledonian orogenic system. However, EKOB was involved in the Paleo-Tethyan tectonic regime during late Paleozoic-early Mesozoic period, and yielded an integrated Variscian-Indosinian orogenic cycle, similar to the Sanjiang Paleo-Tethyan orogeny. The south-Kunlun suture zone during that time was the main boundary between the north and the south China continents. Subsequently, EKOB was involved in the Tibetan collisional orogenic system in the Cenozoic since Indo-Eurasia collision. 　　While the continental crust of EKOB mainly formed in late Paleo-Proterozoic, juvenile crust was also generated in the Phanerozoic time, similar to the Xing,anling-Mongolia, the Gangdese and the Andes orogenic belts. There are abundant conclusive evidences of underplating and magma mixing in the East-Kunlun granitoid belt. Initial values of 87Sr/86Sr of the East-Kunlun granitoids are mostly less than 0.710 and εNd (t) values of them range from－9.2 to + 3.6. These features imply that inputting of mantle materials and mixing between mantle- and crust- derived materials played an important role in crustal generation and evolution in EKOB during the Phanerozoic time. According to SHRIMP U-Pb dating of zircons from granitoid host rocks, mafic microgranular enclaves (MMEs) and associated gabbros, two major events of underplating and magma mixing took place in EKOB in the early-middle Devonian (394-403 Ma) and the middle Triassic (239-242 Ma), corresponding to the stage from the ending of subduction to the initiation of collision during the Caledonian and Variscian-Indosinian orogeny, respectively.

Extension of the Sulu UHP Belt to the Korean Peninsula:Evidence From Orogenic Belts, Precambrian Basements,and Paleozoic Sedimentary Basins

ZHAI Ming-guo1,2, GUO Jing-hui1,2, LI Zhong1, CHEN Dai-zhao1, PENG Peng1,2, LI Tie-sheng1,2，ZHANG Yan-bin1,2, HOU Quan-lin3, FAN QI-cheng4, HU Bo1

2007, 13(3): 415-428.

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Considerable debate on whether and how the Sulu Orogenic Belt extends eastward to the Korean Peninsula has been lasting over the past decade. New results reported here include the following: (1) an eclogite and retrograded eclogite-bearing complex (Hongseong Complex) was discovered in South Korea, in which the eclogite occurs as lenses in ca. 810~820 Ma granitic gneiss. SHRIMP zircon dating of the eclogite yielded～230 Ma for the metamorphic age and 880 Ma for the protolith age; (2) The basement of the Rangnim, Gyeonggi and Yeongnam massifs have affinities to the basement of the North China Block (NCB). However, the Gyeonggi Massif encloses a minor amount of large or small slabs of the Hongseong Complex which is similar to the rocks of the Sulu Belt; (3) Two main Paleozoic basins within the Rangnim and Gyeonggi massifs have similar Paleozoic tectono-stratigraphy to the NCB; (4) The Imjingang and Ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts. Based on these facts, we propose a crustal-detachment and thrust model and suggest that the collision belt between the Yangtze Block (YB) and NCB (Sino-Korea Craton) is preserved along the western margin of the Korean Peninsula. The lower part of the UHP metamorphosed lithosphere of the YB was subducted under the Korean Peninsula and not exposed on the surface. The lower crust of the YB (the Hongseong Complex) was detached from the subducted lithosphere and thrust over the Korean Peninsula, and inserted into the basement rocks of the Gyeonggi Massif. The upper crust of the YB possibly was detached from the lower crust and overthrusted along the Honam and Chugaryong shear zones. The Imjingang and Ogcheon belts possibly represent the detached upper crust of YB and their present occurrences are controlled by a Mesozoic strike slip shear structure. All these detached lower and upper crustal slabs were strongly deformed during the Late Jurassic and Early Cretaceous tectonic event leading to their present geological distribution and characteristics.

Age of Crystalline Basement in Western Margin of Yangtze Terrane

GENG Yuan-sheng, YANG Chong-hui, WANG Xin-she, REN Liu-dong, DU Li-lin, ZHOU Xi-wen

2007, 13(3): 429-441.

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Based on the study of zircon SHRIMP U-Pb dating on metamorphic strata in crystalline basement,and supracrustal enclave in orthogneiss in western margin of Yangtze terrane, the ages of 816±8.6 Ma and 826±13 Ma for a mylonitic felsic schist and a biotite amphibolite in the gabbro-diorite gneisses in Kangding area were determined, which are considered as the eruption ages of felsic volcanics and volcanic tuff; The ages of 816±9 Ma and 818±8Ma for a felsic mylonite and an amphibolite in metamorphic strata of Luding area, respectively, were obtained which are also suggested to be the eruption ages of intermediate and basic volcanics; The ages of 830±7 Ma and 827±10 Ma were yieled for an amphibolite and a hornblende biotite plagioclase gneiss in Cida area, which indicate the timing of volcanic-sedimentary association. The above SHRIMP zircon U-Pb chronological results show that the so-called crystalline basement in the western Yangzi plateform is chiefly constructed by Neoproterozoic magmatic rocks corresponding to the Yanbian group and Yanjing group in age, and their difference is only expressed in the metamorphic grade. Therefore, our new chronological evidence do not support the existence of an Archean to Paleoproterozoic crystalline basement in the western margin of Yangtze terrane.

Meta-Sedimentary Rocks and Tectonic Division of the North China Craton

WU Chang-hua

2007, 13(3): 442-457.

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The basement of the North China craton, like many other old cratons in the world, consists mainly of tonalitetrondhjemite-granodiorite gneisses (TTG), greenstone belts and a small amount of Precambrian meta-sedimentary rocks. Although the meta-sedimentary rocks are less distributed, their evolution regularities can be reflected by TTG, because a sedimentation in Precambrian always followed a cycle of granitoid continental crust. According to incomplete statistics of the Precambrian granitoid ages from the North China craton, it is showed that the craton experienced multiple phases of evolution from a nucleus to a mature continent through the cycle events at 3.8 Ga, 3.3 Ga, 2.9 Ga, 2.5 Ga and 1.8-1.9 Ga, which are exhibited by peaks on the histograms of granitoid ages and by ca. 500 Ma as a period. Correspondingly, the amount of sedimentary rocks increased along with evolution of granitic crust, but the deposition was temporally different from the granitoid events, i.e. deposition took place between two cycles of granitoids crust with a period of ca. 500 Ma. This period is similar to the interval in a Phanerozoic supercontinental cycle from breakup to assembly, indicating similar continent evolution regularities in both Precambrian and Phanerozoic. Therefore, it is possible for using meta-sedimentary rocks to interprete the craton evolution. However, the Precambrian (especially the Archean) sedimentary rocks are so minor that they cannot reveal the shape of the old blocks. Meanwhile, the Paleoproterozoic khondalite series was deposited in a continent-marginal setting and is better conserved and distributed, it can be used to model the assembly of craton. Generally speaking the tectonic division based on khondalite should be the best among many other tectonic models for the North China craton, because the marginal sedimentary rock was the boundary of block. Thus, based on khondalite and combined with magmatism, sedimentology, metamorphism, structural deformation, and geochronology in a variety of Archean terranes, a new tectonic subdivision model for North China craton is presented in the paper. The tectonic units are succesively: from west to east, Ordos block / Central (Jin-Meng) assembly belt / Ji-Lu-Yu block / (Tan-Lu fault) / Seashore (Jiao-Liao) block group.Because the protolith ages of the khondalite are Paleaoproterozoic, the final assembly occurred at ~1.8-1.85 Ga, i.e. the famous Lüliang Movement in China .

Discovery of the Precambrian Nb-Enriched Basalts in the Northern Hebei Province

WANG Ren-min1，FENG Yong-gang1， CHENG Su-hua2

2007, 13(3): 458-462.

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There are a series of ancient oceanic crust relics along the fracture belt between the Archean granulite belt and the Paleoproterozoic Hongqiyingzi group in the North Hebei Province. Recently we have found widespread Nb-enriched basalt (Nb > 7×10-6 and Nb/Th+Nb/La > 0.6 ) in this area, which is very rare in other regions. The discovery will provide powerful proof for distinguishing ancient oceanic crust and will also supply important information for the Precambrian geological and tectonic evolution.

Igneous Carbonatites in North China Craton: The Temporal and Spatial Distribution, Sr and Nd Isotopic Charateristics and Their Geological Significance

YAN Guo-han1, MU Bao-lei1, ZENG Yi-shan1, CAI Jian-hui2, REN Kang-xu3, LI Feng-tang1

2007, 13(3): 463-473.

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According to the research of some predecessors and the present authors, the ages of the igneous carbonatites in North China Craton are focused in three stages: the latest Paleoproterozoic-early Mesoproterozoicera, the early Mesozoic era and the late Mesozoic era, which means that these three stages are obviously extensional periods in evolution history of North China Craton. The igneous carbonatites generated in the first and the second periods are distributed in northern and southern margins of the North China Craton, and the third one is recognized in the eastern and central parts of this craton. The isotopic geochemistry of these carbonatites is characteristic of negative εNd(t ) , decreasing with formation ages. On the εNd(t )-I Sr correlation diagram, most of the samples fall within the third and the forth quadrants, near the enriched mantle evolution trendline except for those located in Laiwu-Zibo area, where the enrichment degrees become higher and higher with their ages. This paper also suggests that the deep geodynamic mechanics of these carbonatites probably be related to the hot mantle plume activities or the subduction of the blocks around the North China Block (NCB), resulting in extensional setting of NCB. With the pressure reduction, the alkaline ultramafic-mafic or intermediate magmas were generated by partial melting of the mantle, and subsequent upwelling along the regional deep faults and formation of the complexes by fractional crystallization. The igneous carbonatites are mainly the late products of above-mentioned magmatic evolution. In addition, some carbonatites are argued most likely to be the direct crystallization products of the carbonatite magma derived from the low degree partial melting of the upper mantle.

Phanerozoic Metamorphic Episodes and Characteristics of Cathaysia Block

YU Jin-hai，WANG Li-juan，WEI Zhen-yang，SUN Tao，SHU Liang-shu

2007, 13(3): 474-483.

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There were several episodes of metamorphism in the Cathaysia Block. The Caledonian metamorphism occurred over the most area, and the highest grade was up to granulite facies. The granulites scatter over the Cathaysia Block with NE direction parallel to orogeny belt. It is proposed that the metamorphism took place in the orogenic setting, which likely resulted from the assembling and colliding of the Yangtze Block to the northern Gondwanaland, leading to deformation and metamorphism of sediments in the passive continental margin of the northern Gondwanaland. According to LA ICP-MS zircon U-Pb dating results of the Meixian gneiss and Xingning migmatite in eastern Guangdong Province and geochronological data of the monazites from nearby metamorphic rocks, the Hercynian metamorphism in the Cathaysia Block mainly took place at 260~280 Ma, younger than the typical Hercynian orogeny event in the central European continent. The Hercynian metamorphism in the Cathaysia Block was locally distributed, and probably occurred under the extension setting. The Indosinian metamorphism was wider in the Cathaysia Block, and the metamorphic grade in the Darongshan-Shiwandashan area at southwestern Cathaysia might reach granulite facies. The metamorphism in other regions was characterized by middle-low pressure faceis series, which might be related with the extension setting in late Indosinian orogeny. LA ICP-MS zircon U-Pb isotope analyses indicate that the Indosinian metamorphism in the central Cathaysia occurred at 231~232 Ma. The Yanshanian metamorphic rocks, which are distributed along the coastal area of southeastern China and Central Range of Taiwan, show the feature of paired metamorphic belts. It probably suggests that the metamorphism be closely related with the subduction of Pacific Ocean under the southeastern China. The directions of metamorphic belt were different from Indosinian to Yanshanian events, indicating that tectonic regime influencing the Cathaysia Block was transformed in these two episodes.

Geology and Zircon U-Pb Isotopic Chronology of Dantazi Complex, Northern Hebei Province

LIU Shu-wen1,3, LU ¨ Yong-jun1, FENG Yong-gang1, ZHANG Chen1,TIAN Wei1,YAN Quan-ren2, LIU Xiao-ming3

2007, 13(3): 484-497.

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The Dantazi complex in northern Hebei province, located in the middle segment of northern margin of the North China Craton, consists of the tonalitic, trondhjemitic and granodioritic gneisses (TTG) metamorphosed in high amphibolite facies to granulite facies, and a supracrustal rock association of mafic two-pyroxene granulites, clinopyroxene amphibolites, garnet plagioclase fine-grained gneisses, garnet sillimanite biotite schists and minor marbles. Detailed studies in SHRIMP and LA-ICPMS zircon U-Pb isotopic chronology reveal that the Dantazi complex experienced seven stages of evolution: (1) 2 600-2 530 Ma, the formation of the country rocks of the TTG gneisses, including some mafic supracrustal rocks that might be sources of the TTG magma; (2) 2 517-2 505 Ma, tonalitic magma emplaced and crystallized; (3) 2 494-2 473 Ma, trondhjemitic magma emplaced and crystallized;(4) 2 427-2 404 Ma, the plutonic and supracrustal lithological associations in the complex were metamorphosed and deformed in high amphibolite facies to granulite facies (earlier stage) and reworked; (5) ~1 859 Ma, mafic magma emplaced, forming some mafic dykes and stocks; (6) ~1 834 Ma to 1 793 Ma, the Datazi complex experienced an intensive regional metamorphism in granulite facies (later stage); (7) ~1 730 Ma, the complex was altered by intensive activation of geological fluids. However, there is no ~300 Ma Paleozoic age record in all the samples analyzed for zircon U-Pb isotopic chronology, suggesting that the Dantazi complex did not experience the Paleozoic metamorphism.

A Comparative Study on the UHP Metamorphic Ophiolitic Rocks in Zermaat-Saas Zone, Western Alps and Western Tianshan, China

ZHANG Li-fei1, LU¨ Zeng1, LI Xu-ping2, Kurt Bucher3

2007, 13(3): 498-506.

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The general geology of ultrahigh pressure（UHP） metamorphic ophiolitic belt in Zermatt-Saas, western Alps, especially the petrologic characteristics and metamorphic evolution, is introduced in this paper. Based on the comparative study of geology, petrology and geochronology between two typical UHP ophiolitic belts in the Zermatt-Saas zone, western and the Chinese Tianshan belt, the general characteristics of UHP ophiolitic belt such as lower geothermal gradient and preservation of pillow lava structure of oceanic basalt are discussed in this paper.

H2O Behaviour in Subsolidus Metamorphic Processes

WEI Chun-jing, ZHANG Jing-sen

2007, 13(3): 507-514.

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Contours of H2O–content saturated to a mineral assemblage in the calculated p-T psaeudosections can provide much useful information on plausible evolution for the mineral assemblage. Guiraud et al. proposed that metamorphism in a closed system can only evolve via progressive dehydration, crossing contours of decreasing H2O–content, the fluid being lost from the rocks. If the metamorphic process crosses contours of increasing H2O–content, the mineral assemblage starts to become a fluid–absent state, which does not favor the further evelution of the mineral assemblage. Thus, a metamorphic peak should represent a point at which the metamorphic p-T path becomes tangential to a H2O–content contour or at which dehydration reactions complete and just start to rehydrate. It may not correspond to the maximum temperature and pressure reached along a p-T path. Using p-T pseudosections with contours of H2O–contents, the medium–low pressure metamorphism for pelite from the Altai orogen was documented. We proposed that the andalusite–type metamorphic zones in Altai were related to the uplifting of kyanite–type zones and are characteristic of isomorphic transformation between aluminosilicates. Generally, it is difficult for the low–pressure metamorphic assemblages to reach an equilibrium state. Distinctive from the low–medium pressure metamorphism of pelite, progressive dehydrations proceed even in the decompression process post metamorphic peak of UHP eclogites.

Two Different Metamorphic Paths of Eclogites and Differential Exhumation of Subducted Continental Crust: A Case Study of the Dulan UHP Terrane in the North Qaidam UHP Belt

SONG Shu-guang1, NIU Yao-ling2, ZHANG Li-fei1

2007, 13(3): 515-525.

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The Dulan eclogite-bearing terrane is the only coesite-bearing ultrahigh-pressure (UHP) metamorphic terrane so far recognized in the 700 km-long North Qaidam-Altun eclogite belt. The key feature of the Dulan terrane is that it consists of two sub-belts, the north Dulan belt (NDB) and the south Dulan belt (SDB) with different exhumation processes and histories. Coesite pseudomorphs in garnet and omphacite, together with p-T calculation, suggest that the peak metamorphic stage of eclogite in both belts is well within the stability field of coesite (p =2.8-3.3 GPa). The retrograde p-T paths are much different between the north and the south belts. The p-T path of the NDB eclogites suggests that the NDB eclogites underwent two stages of exhumation, an earlier rapid exhumation from a mantle depth to a middle crust level with a near-isothermal decompression, and a later uplift to the Earth’s surface. The SDB eclogites, on the other hand, were strongly overprinted by a high-pressure granulite-facies metamorphism under conditions of p =1.9~2.0 GPa and T =873~948℃, followed by temperature and pressure decrease to the amphibolitefacies. The process of temperature increase with decreasing pressure from UHP eclogite to high-pressure granulite suggests that the SDB eclogites exhumed at low speed and underwent strong thermal relaxation at a level of crustmantle transition or thickened lower crust. This thermal relaxation may occur in most UHP metamorphic terranes and may be the major mechanism for eclogite anatexis. The different metamorphic evolutional paths of the two sub-belts signify the differential exhumation histories of the subducted continental crust.

Progress, Controversies and Challenge of Studies on South Altyn Tagh-North Qaidam HP/UHP Metamorphic Belt

ZHANG Jian-xin1, MENG Fan-cong1, Mattinson C G2

2007, 13(3): 526-545.

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The South Altyn Tagh (SAT)-North Qaidam (NQD) HP/UHP metamorphic belt extends approximately 1000 km along the northern margin of the Tibetan Plateau，and is truncated by the Altyn Tagh sinistral strikeslip fault and splitted into two parts: the South Altyn Tagh region and the North Qaidam region. Based on rock associations, petrologic criteria and the field relationships, six HP/UHP metamorphic units can be distinguished in the South Altyn Tagh（SAT）-North Qaidam (NQD) region (Fig. 1A). From west to east, they are: the Jianggalesayi eclogite-gneiss unit (JSU), the Bashiwake garnet peridotite-high pressure granulite unit (BWU), the Yuka eclogitegneiss (schist) unit (YLU), the Luliangshan garnet peridotite-gneiss unit (LLU), the Xitieshan eclogite-gneiss unit (XTU) and the Dulan eclogite-gneiss unit (DLU). The geological relationship, petrology and geochronology indicate that these six HP/UHP metamorphic units have different rock associations, and underwent different p -T conditions and metamorphic histories. Geochronologyical dating for these eclogite-facies metamorphics yields ages between 500 and 420 Ma. This implies that a possible multiple or disachronous subduction took place along the SAT-NQD HP/UHP metamorphic belt.

Late Neoproterozoic-Early Paleozoic Tectonothermal Events in East Antarctica: Implications for Amalgamation of the Gondwana Supercontinent

LIU Xiao-chun1, ZHAO Yue1, LIU Xiao-han2, HU Jian-min1

2007, 13(3): 546-560.

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Three late Neoproterozoic-early Paleozoic orogenic belts, East African Orogen, Prydz Belt and Ross Orogen, occur in East Antarctica. The East African Orogen extends southward to the Lützow-Holm Bay-Dronning Maud Land-Shackleton Range area. The area is interpreted as the southern part of the suture between East and West Gondwana blocks due to the appearance of ophiolites, eclogite-facies ultramafic rocks and thrust-nappe structures.The Ross Orogen is exposed in the Transantarctic Mountains and is considered as the active margin of the Gondwana supercontinent. The geological records of continental break-up, oceanic crustal subduction and terrane accretion are preserved in the orogen. The Prydz Belt occurs in the Prydz Bay and Denman Glacier, and is located in the interior of a previously proclaimed unified East Gondwana. The absence of subduction-related rocks, such as ophiolite suite, arc accretionary complex and high-pressure blueschist and eclogite, resulted in the argument that the Prydz Belt represents whether a collisional zone or an intraplate reworking belt. The determination of the nature of the Prydz Belt would constrain not only the amalgamation scenario of the Gondwana supercontinent, but also the reconstruction pattern of the Rodinia supercontinent. Therefore, the Prydz Belt is the key to understand the global tectonic evolution during the Neoproterozoic to early Paleozoic.

Determination of Constants C 1-C 14 in 14 Equations of the Relationships between the K¨ubler，Weaver and Weber Indices and Their Applications

WANG He-jin1, TAO Xiao-feng2, RAHN Meinert3

2007, 13(3): 561-565.

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Constants C 1-C 14 in 14 equations of the relationships between the K¨ubler，Weaver and Weber indices were deduced and calculated. Four formulae of the relationships between exponent u of the Pearson VII function and C 5, C 6, C 11, C 12 were deduced also. Therefore, those 14 equations can be directly used for actual converting analysis. According to the shape parameter Sc＝0.731 for the XRD peak of illite from Alpine flysch and using these equations the boundaries of anchizone of Weaver and Weber indices were calculated as 5.8-17.6 and 300-179, respectively.Application in distinguishing illite/smectite mixed-layer phase in air-dried condition was discussed on the basis of the added principle of XRD peaks.

Permian CHIME Ages of Monazites for the Kyanite-Sillimanite Type Metamorphic Belt in Chonghuer Area, Altai, Xinjiang and Their Geological Implications

ZHENG Chang-qing 1，XU Xue-chun1，KATO Takenori2，ENAMI Masaki2

2007, 13(3): 566-573.

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The kyanite-sillimanite type progressive metamorphic belt developed in Chonghuer area, Altai, Xinjiang,is composed of chlorite-biotite, biotite-garnet, garnet-staurolite, staurolite-kyanite and sillimanite zones. The metamorphic peak temperature and pressure of staurolite-kyanite and sillimanite zones are 560~680 ℃ and 4.4~5.8 kbar, respectively. There are abundant accessory monazites of metamorphic origin with high Th-U-Pb contents in rocks suitable for CHIME dating. The dating results show that the metamorphic age of these rocks is 268±10 Ma~ 261±20 Ma, consistent with the age of the andalusite-sillimanite type (low pressure type) metamorphism in Altai area, indicating that the kyanite-sillimanite and andalusite-sillimanite type progressive metamorphic belts were formed in the same regional tectonothermal event at Middle Permian. These ages provided the direct chronological evidence for the closure of the Paleo-Asian Ocean.

LA-ICPMS U-Pb Dating of Zircons from Two Types of Leucosomes in North Dabie Unit: Evidences for Paleoproterozoic Anatexis and Triassic Subduction?

GONG Song-lin1, CHEN Neng-song1, LI Xiao-yan1, LIU Xiao-ming2

2007, 13(3): 574-580.

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Four types of leucosomes are recognized for the North Dabie complex in Huangtuling area, of which the second type leucosome is adamellitic in the biotite gneisses, and the third type of alkali-feldspar granitic composition developes synchronously with the hypersthene-bearing corona around garnet porphyroblasts in the felsic granulite. LA-ICPMS U-Pb dating yielded upper and lower intercepted ages of 2 040 ± 14 Ma and 242 ±27 Ma for melt-crystallized zircons from the second type leucosome, and 2 056 +35/-26 Ma and 244 +180/-210 Ma for metamorphic zircons from the third type. These results indicate that both the second type leucosome in the biotite gneisses and the first type leucosome were produced synchronously in the Paleoproterozoic granulite-facies metamorphism, and that both the melt-derived and the metamorphic Paleoproterozoic zircons suffered significant lead-loss in the Triaasic, suggesting that both felsic granulite and biotite gneisses be responses to the Triassic deep subduction event.

Discovery of Coesite in Sanqinge Eclogite, Sulu UHP Terrane,E China, and Its Geological Significance

ZHU Meng-fan, ZHU Yong-feng

2007, 13(3): 581-589.

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Here we report the discovery of coesite in phengite eclogite at Sanqinge, Ganyu county, Sulu UHP terrane, east China. These eclogites coexist with the UHP magnesite-bearing marble. The coesite relics and polycrystalline quartz after coesite are observed within garnet grains with radial fractures. This finding indicates that the Sanqinge phengite eclogites underwent metamorphism at pressure of >2.8 GPa. Metamorphic pressure for the mineral assemblages of cores correspond to 3.1~3.5 GPa ( T＝650~689℃ ) and their rim assemblages correspond to 2.9~3.3 GPa ( T＝652~671℃ ) by using garnet-omphacite Fe2+-Mg geothermometer and phengite baromemter.Quartz exsolution and sulfide exsolution textures occur in omphacites and apatites, respectively. Apatite contains abundant volatiles such as OH, F, Cl, and S, and its subduction to deep mantle is of great significance for the cycle of these volatiles. Based on our approximate estimation, the mass of OH, F and Cl subducted into deep mantle via Sulu-Dabie subduction zone was in the order of 80 000 Mt.

Geology of the Baobei Gold Deposit in Western Juggar and Zircon SHRIMP Age of Its Wall-Rocks, Western Junggar (Xinjiang, NW China)

WANG Rui, ZHU Yong-feng

2007, 13(3): 590-602.

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The Baobei gold deposit is located in the Early Carboniferous volcanic–sedimentary formation named as Tailegula group. The Baobei gold deposit is mainly composed of gold–bearing quartz veins. The major gold mineral is electrum, which mainly occurs as inclusions in arsenopyrite. Pyrite with electrum inclusions contains relatively higher As. The mineral paragenesis analysis suggests four stages of ore–formation including albite–quartz stage (I), electrum–pyrite–arsenopyrite–quartz stage (II), sulphide stage (III), and carbonatization stage (IV). The stages II and III correspond to the major periods of gold mineralization. The zircon SHRIMP dating for the zircons separated from the altered felsic tuff (the wall–rock of the gold–bearing quartz vein) gives a concordant U–Pb age of 328.1±1.8 Ma (MSWD=1.6, n =13).

Geochemical Characteristics and Origin of the Tiaojishan and Donglingtai Formations from Western Hills, Beijing

WANG Rui1, CHEN Bin1, LIU Xiao-ming2

2007, 13(3): 603-612.

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Voluminous volcanic rocks of the Tiaojishan Formation outcrop in the Western Hills of western Beijing, which are unconformably overlain by the volcanics of the Donglingtai Formation. These volcanic rocks vary significantly in chemical compositions, with SiO2=51.94~77.30%，Al2O3=12.85~19.17%，Na2O=1.65 ~5.82%，K2O=0.83~4.52%，Fe2O3=0.95~9.30%，and CaO=0.13~7.08%. Silica contents show approximately linear covariations with other oxides. These volcanic rocks are characterized by highly enriched LREE and depleted HREE, and minor Eu anomalies in the chondride-normalized REE patterns. Meanwhile, they show high abundances of Sr and Ba and high Sr/Y and La/Yb. The volcanic rocks have varied and enriched Nd isotopic compositions (εNd = -11~-17), which plot intermediately between the field of the ancient continental crust and the field of the enriched subcontinental lithospheric mantle. These geochemical signatures indicate that the volcanic rocks of the Tiaojishan and Donglingtai Formations were originated from a process of hybridization between the enriched mantle-derived mafic magma and the lower continental crust.

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