Element enrichment and U-series isotopic characteristics of the hydrothermal sulfides at Jade site in the Okinawa Trough

The geochemical and U-series isotopic characteristics of hydrothermal sulfide samples from the Jade site (127A degrees 04.5'E, 27A degrees 15'N, water depth 1300-1450 m) at Jade site in the Okinawa Trough were analyzed. In the hydrothermal sulfide samples bearing sulfate (samples HOK1 and HOK2), the LREEs are relatively enriched. All the hydrothermal sulfide samples except HOK1 belong to Zn-rich hydrothermal sulfide. In comparison with Zn-rich hydrothermal sulfides from other fields, the contents of Zn, Pb, Ag, Cd, Au and Hg are higher, the contents of Fe, Al, Cr, Co, Ni, Sr, Te, Cs, Ti and U lower, and the Pb-210 radioactivity ratios and Pb-210/Pb ratios very low. In the hydrothermal sulfide mainly composed of sphalerite, the correlations between rare elements Hf and U, and Hf and Mn as well as that between dispersive elements Ga and Zn, are strongly positive; also the contents of Au and Ag are related to Fe-sulfide, because the low temperature promotes enrichment of Au and Ag. Meanwhile, the positive correlations between Fe and Bi and between Zn and Cd are not affected by the change of mineral assemblage. Based on the Pb-210/Pb ratios of hydrothermal sulfide samples (3.99x10(-5)-5.42x10(-5)), their U isotopic composition (U-238 content 1.15-2.53 ppm, U-238 activity 1.07-1.87 dpm/g, U-234 activity 1.15-2.09 dpm/g and U-234/U-238 ratio 1.07-1.14) and their Th-232 and Th-230 contents are at base level, and the chronological age of hydrothermal sulfide at Jade site in the Okinawa Trough is between 200 and 2000 yr.

Genesis of Th-230 excess in basalts from mid-ocean ridges and ocean islands: Constraints from the global U-series isotope database and major and rare earth element geochemistry

Based on Th-230-U-238 disequilibrium and major element data from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), this study calculates mantle melting parameters, and thereby investigates the origin of Th-230 excess. (Th-230/U-238) in global MORBs shows a positive correlation with Fe-8, P (o), Na-8, and F-melt (Fe-8 and Na-8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%, P (o)=pressure of initial melting and F (melt)=degree of melt), while Th-230 excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting. Furthermore, compared with the MORBs, higher (Th-230/U-238) in OIBs actually corresponds to a lower melting degree. This suggests that the Th-230 excess in MORBs is controlled by mantle melting conditions, while the Th-230 excess in OIBs is more likely related to the deep garnet control. The vast majority of calculated initial melting pressures of MORBs with excess Th-230 are between 1.0 and 2.5 GPa, which is consistent with the conclusion from experiments in recent years that D (U)> D (Th) for Al-clinopyroxene at pressures of > 1.0 GPa. The initial melting pressure of OIBs is 2.2-3.5 GPa (around the spinel-garnet transition zone), with their low excess Ra-226 compared to MORBs also suggesting a deeper mantle source. Accordingly, excess Th-230 in MORBs and OIBs may be formed respectively in the spinel and garnet stability field. In addition, there is no obvious correlation of K2O/TiO2 with (Th-230/U-238) and initial melting pressure (P (o)) of MORBs, so it is proposed that the melting depth producing excess Th-230 does not tap the spinel-garnet transition zone. OIBs and MORBs in both (Th-230/U-238) vs. K2O/TiO2 and (Th-230/U-238) vs. P (o) plots fall in two distinct areas, indicating that the mineral phases which dominate their excess Th-230 are different. Ce/Yb-Ce curves of fast and slow ridge MORBs are similar, while, in comparison, the Ce/Yb-Ce curve for OIBs shows more influence from garnet. The mechanisms generating excess Th-230 in MORBs and OIBs are significantly different, with formation of excess Th-230 in the garnet zone only being suitable for OIBs.

Generation and evolution of magma beneath the East Pacific Rise: Constraints from U-series disequilibrium and plagioclase-hosted melt inclusions

Mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR 12 degrees 50'N) were analyzed for U-series isotopes and compositions of plagioclase-hosted melt inclusions. The Ra-226 and Th-230 excesses are negatively correlated; the Ra-226 excess is positively correlated with Mg# and Sm/Nd, and is negatively correlated with La/Sm and Fe-8; the Th-230 excess is positively correlated with Fe-8 and La/Sm and is negatively correlated with Mg# and Sm/Nd. Interpretation of these correlations is critical for understanding the magmatic process. There are two models (the dynamic model and the "two-porosity" model) for interpreting these correlations, however, some crucial parameters used in these models are not ascertained. We propose instead a model to explain the U-series isotopic compositions based on the control of melt density variation. For melting either peridotite or the "marble-cake" mantle, the FeOt content, Th-230 excess and La/Sm ratio increases and Sm/Nd decreases with increasing pressure. A deep melt will evolve to a higher density and lower Mg# than a shallow melt, the former corresponds to a long residence time, which lowers the Ra-226 excess significantly. This model is supported by the existence of low Ra-226 excesses and high Th-230 excesses in MORBs having a high Fe-8 content and high density. The positive correlation of Ra-226 excess and magma liquidus temperature implies that the shallow melt is cooled less than the deep melt due to its low density and short residence time. The correlations among Fe-8, Ti-8 and Ca-8/Al-8 in plagioclase-hosted melt inclusions further prove that MORBs are formed from melts having a negative correlation in melting depths and degrees. The negative correlation of Ra-226 excess vs. chemical diversity index (standard deviation of Fe-8, Ti-8 and Ca-8/Al-8) of the melt inclusions is in accordance with the influence of a density-controlled magma residence time. We conclude that the magma density variation exerts significant control on residence time and U-series isotopic compositions. (c) 2010 Elsevier B.V. All rights reserved.

石笋微层序列的断代和信号研究

Based on chronological and climatologic analysis of many young stalagmites in Shihua Cave at Beijing and Water Cave in Benxi, Liaoning Province, this dissertation aims to discuss some key points on quantitative reconstruction of high resolution climate change from micro bands time series in young stalagmites. On chronological aspect of micro bands time series in young stalagmite, the uppermost micro bands were testified formed in the sampling year or the year before sampling from long term and in situ experiment in Shihua Cave at Beijing. Have been observed detailed under microscope, the micro bands were counted and compared with high resolution TIMS-230Th or ICP-MS U series dating, the results show that the micro bands are annual bands. The microscopic characteristics of annual bands in North China is as follows after studying lots of young stalagmites: the interface appears to be carved, and there is a light line adjacent to it, which moves back and forth slightly when adjusting the microscope slide. The cross dating of annual bands from different stalagmites is experimented in this thesis. On climatologically aspect of micro bands times series in young stalagmites, one significant problem is that the deposition trend resulting from the variation of the stalagmite shape. Various deposition trends have been discussed and corrected before climatologically analysis of micro bands series. And the whole year in situ observation and analysis of dripping water in Shihua Cave provide us new evidence for climate information reserved in stalagmites micro bands. The stalagmites in the same cave may contain different climate information due to the micro hydrological condition. The annual bands of young stalagmites collected from North China can be used to reconstruct past climate changes quantitatively. The warm season temperature (May, June, July, August) is reconstructed using two annual bands time series from Shihua Cave, Beijing, which corresponds to the warm/cool periods very well in Chinese history documents. The precipitation record of past 500 years is reconstructed using two stalagmite annual bands series from Shihua Cave, which is consistent to the dryness/wetness records derived from historical documents too.

ISOTOPIC CHARACTERISTICS AND AGE DATING OF THE PUMICE IN OKINAWA TROUGH

The data on the isotope compositions of rubidium, strontium and oxygen in the pumice of Okinawa Trough are reported for the first time. The ages of the pumice were successfully dated with the method of U-series disequilibrium. Then, the material source, crystallization evolution of magma and activity cycles of volcanos are explored. Isotopic data show that pumice magma was originally from the mantle, but had undergone a full crystallization differentiation and had been contaminated to a fair extent by crust-derived materials before the magma was erupted out of the sea floor. According to the dating results available so far, the earliest volcanic eruption in Okinawa Trough occurred about 70,000 a ago and the latest eruption was about 10,000 a B.P. During this period, there were three volcanic eruption cycles which were respectively corresponding to the middle Late Pleistocene, the late Late Pleistocene and the Early Holocene.

Geochemical character and material source of sediments in the eastern Philippine Sea

Based upon analyses of grain-size, rare earth element (REE) compositions, elemental occurrence phases of REE, and U-series isotopic dating, the sediment characteristics and material sources of the study area were examined for the recently formed deep-sea clays in the eastern Philippine Sea. The analytical results are summarized as follows. (1) Low accumulation rate, poor sorting and roundness, and high contents of grains coarser than fine silt indicate relatively low sediment input, with localized material source without long distance transport. (2) The REE Contents are relatively high. Shale-normalized patterns of REE indicate weak enrichment in heavy REE (HREE), Ce-passive anomaly, and Eu-positive anomaly. (3) Elemental occurrence phases of REE between the sediments with and without crust are similar. REE mainly concentrate in residual phase and then in ferromanganese oxide phase. The light REE (LREE) enrichment, Ce-positive anomaly, and Eu-positive anomaly occur in residual phase. Ferromanganese oxide phase shows the characteristics of relatively high HREE content and Ce-passive anomaly. (4) There are differences in each above mentioned aspect between the sediments with and without ferromanganese crust. (5) Synthesizing the above characteristics and source discriminant analysis, the study sediments are deduced to mainly result from the alteration of local and nearby volcanic materials. Continental materials transported by wind and/or river (ocean) flows also have minor contributions.

Isotopic feature and uranium dating of the volcanic rocks in the Okinawa Trough

Volcanic rocks from the northern and middle Okinawa Trough were dated by uranium-series dating method. Differential fractions using magnetic procedure were designed to separate samples. New report on the ages and isotopic data of rocks in the northern trough (especially black pumice) was discussed. Based on the uranium dates and Sr-Nd isotopic ratio, magmatic evolution process of the Okinawa Trough was noted. Firstly, there have been wide silicic volcanic activities in the Okinawa Trough from late Pleistocene to present, and the volcanic rocks can be divided into three subgroups. Secondly, magma generally came from PREMA source area under the Okinawa Trough. Magmatic evolution in the northern trough was similar to the middle, but different to the south. Finally, volcanic activities indicated that opening of the southern Okinawa Trough did not happen due to the collision between Luson Arc and Eurasian Plate until the early Pleistocene.

天山南北山前盆地中-新生代沉积记录及其动力学分析

It has been long known that intense multiple Mesozoic-Cenozoic intracontinental deformations have controlled the grand scale basin-range structural evolution of the Tianshan and its adjacent basins. So it is important to study the sedimentary records of the piedmont basins along the two sides of the Tianshan synthetically for the continental geodynamic research.We carried out a magnetostratigraphy study on Cretaceous- Tertiary succession and U-Pb dating analysis of detrital zircons from the representative sandstone samples of the Mesozoic-Cenozoic deposits in Kuqa Subbasin, northern Tarim Basin, combining our previous results of multiple depositional records from different profiles including paleocurrent data, conglomerate clast, sandstone framswork grains, detrital heavy minerals and geochemistry analysis, so the multiple intracontinental tectonic processes of Tianshan and their depositional response in the Kuqa Subbasin can be revealed. The results show that the tectonic evolution of the Tianshan Orogen and the sedimentary processes of the Kuqa Subbasin can be divided into four periods: early Triassic(active period), from middle Triassic to late Jurassic(placid period), from early Cretaceous to Tertiary Paleocene(active period) and from Neogene to present (intensely active period). Simultaneously，the depositional records reveal the provenance types and tectonic attributes in different periods. As follows, the lower Triassic with a dominant age ranging from 250 to 290Ma of the Zircons, which were principally derived from alkali feldspar granites and alkaline intrusion obviously, relative to the magma activity in Permian. In middle Triassic-late Jurassic, the two samples collected from the Taliqike formation and the Qiakemake formation respectively show the age peak at 350～450Ma, which was relative to the subduction of the Tarim Block to Yili-Central Tianshan Plate. In this period the provenance of the Kuqa deposits was the Central Tianshan arc orogenic belts distantly with little height predominance.During early Cretaceous-Paleogene, two major zircons age spectra at 240~330Ma and 370~480Ma have been acquired, with some other not dominant age ranges, indicating complicated provenance types. In Neogene, the detrital zircons age dating ranges from 460 to 390 Ma primarily. What’s more, the newer chronology of the stratigraphy and the older source age, indicating that Tianshan was uplifted and exhumated further strongly. Further study on the heavy mineral and the detrital zircons age dating of the Mesozoic-Paleogene representative profiles in southern Junggar Basin, combined with the published results of the sandstone framework grains, we consider that it occurred obvious sedimentary and tectonic changes occurred in the inside of Jurassic, from late Jurassic to early Cretaceous and form early Cretaceous to late Cretaceous. On this faces, there are remarkable changes of the steady minerals and unstable minerals, the sandstone maturity and the age spectra of the detrital zircons. Compared the sedimentary records from the two sides of the Tianshan, We find that they are different obviously since Middle Jurassic. It can be concluded that Tianshan have uplifted highly enough to influence the paleo-climatic. According to the current strata division, the structural activity apparently showed a migration from north to south. That is to say, the South Tianshan uplift later than the north, especially from late Jurassic to early Cretaceous , but it was uplifted and exhumated more strongly. Furthermore, correlating the depositional records and tectonic styles in the Kuqa-South Tianshan basin-range conjugation site in the east with the west, the obvious differentiation between the west and the east from the Cretaceous especially in Tertiary along the Tianshan-Kuqa belt was revealed, probably showing earlier uplifting in the east while greater exhumation depth and sediment rates in the west. In addition, the contacting style of Kuqa subbasin to the Tianshan Orogenic belts and the basement structure are also inconsistent at different basin-range conjugation sites. It is probably controlled by a series of N-S strike adjusting belts within the Kuqa subbasin, or probably correlated with the material difference at the complicated basin-range boundary. The research on the Mesozoic-Cenozoic tectonic-depositional response in the piedmont basins along the two sides of the Tianshan shows that the basin-filling process was controlled by the intracontinental multicyclic basin-range interactions, especially affected by the intense tectonic differentiations of basin-range system, which can’t be illuminated using a single evolutionary model.

内蒙古中部道郎和都格铜多金属矿的地质特征矿床类型及找矿模式研究

Daolangheduge copper polymetallic deposit is located on east edge of Ondor Sum-Bainaimiao metallogenic belt, which is a prospective area of porphyry copper deposit, in Xianghuangqi of central Inner Mongolia. Geotectonically, it occurred in the continental margin accretion belt along the north margin of North China Plate, south of the suture zone between North China Plate and Siberian Plate. The intrusive rocks in this area mainly consist of intermediate-acid magmatic rocks, and the quartz veins, tourmaline veins and the transitional phase are comparatively developed. According to our research, the ore-bearing rock body is mainly quartz diorite while the surrounding rock is mainly biotite granite. Besides, the wall rock alteration are mainly propylitization, pyritization and silicification, which consist of epidotization, actinolitization, chloritzation and so on. The metallic minerals are mainly chalcopyrite and pyrite. In addition, the primary ore is mainly of quartz-chalcopyrite-pyrite type. Above all, Daolangheduge copper polymetallic deposit is suggested to be categorized in the porphyry copper type. With isotopic dating and geochemical research on quartz diorite of ore-bearing rock body, the zircon LA-ICP-MS U-Pb dating of two samples yields an age of 266±2 Ma, falling into the range of late Permian Epoch. It is the first accurate age data in Xianghuangqi area, so it should play a key role in the research of deposit and magmatic rocks in this area. With the major elements and trace elements analysis of 14 samples, the quartz diorite should be among the calc-alkaline series, the geochemical characteristics show higher large-ion lithophile elements of Rb, Sr and LREE, low high-field strength elements of Nb, Ta and high transition elements of Cu, Cr . Also, the REE patterns have negative Eu anomalies. With the same analysis of 4 sample for the biotite granite, the geochemical characteristics show higher Rb, Th,, Zr, Hf and LREE, low Nb, Sm and HREE and Eu has no anomaly. It should be among the calc-alkaline series, over aluminum quality and has characteristics of Adakites. According to isotopic dating and geochemical characteristics of ore-bearing rock body, it is suggested that its materials mainly derived from upper mantle that had fractional crystallization and its magma source region may be affected by fluid metasomatism of paleo-asian ocean. It should be an extensional process of post-orogeny according to regional tectonic evolution. Consequently, because of the decrease of temperature and pressure, the ore forming fluid was raised to surface and mineralized accompanied by magmatic activity which might occur in south of the suture zone. By geological survey, further geophysical and geochemical work is needed. In this area, we have accomplished high precision magnetic prospecting, high density electrical survey, gravity prospecting, soil geochemical prospecting, X-ray fluorescence analyzer prospecting and so on. According to geophysical and geochemical abnormal and surface occurrence, 11 drills are arranged to verification. The type of ores are mainly quartz-chalcopyrite-pyrite ores within 3 drills by drill core logging. Although the grade as well as the scale of already-found Cu deposits are insufficient for industrial exploitation, the mineralization prospect in this region is supposed to be great and the potential in mineral exploration at depth is excellent.

Discovery of Late Cretaceous garnet two-pyroxene granulite in the southern Lhasa terrane, Tibet and its tectonic significances

The Lhasa terrane, located between the Bangonghu-Nujiang suture zone and the Indus-Yalung Tsangpo suture zone in the southern Tibetan Plateau, was considered previously as a Precambrian continental block. Mesozoic and Cenozoic tectonic evolution of the Lhasa terrane is closely related to the subduction of the Tethys ocean and the collision between the Indian and European continents; so it is one of the keys to reveal the formation and evolution of the Tibetan plateau. The garnet two-pyroxene granulite which was found at the Nyingtri rock group of the southeastern Lhasa terrene consists of garnet, clinopyroxene, orthopyroxene, labradorite, Ti-rich amphibolite and biotite, with a chemical composition of mafic rock. The metamorphic conditions were estimated to be at T = 747 similar to 834 degrees C and P = 0.90 similar to 1.35GPa, suggesting a formation depth of 45km. The zircon U-Pb dating for the garnet amphibolite and marble associated with the granulite give a metamorphic age of 85 similar to 90Ma. This granulite-facies metamorphic event together with a contemporaneous magmatism demonstrated that the southern Lhasa terrane has undergone an Andean-type orogeny at Late Mesozoic time.

Paleoenvironment evolution of the East Philippine Sea recorded in the new-type ferromanganese crust since the terminal Late Miocene

From systemic research of microstructure, geochemistry, uranium-series and Be-10 isotope dating on a new-type deepwater ferromanganese crust from the East Philippine Sea, the paleoenvironment evolution of the target area since the terminal Late Miocene was recovered. The vertical section changes of microstructure and chemical composition are consistent in the studied crust, which indicate three major accretion periods and corresponding paleoenvironment evolution of the crust. The bottom crust zone was formed in the terminal Late Miocene (5.6 Ma) with loose microstructure, high detritus content and high growth rate. Reductions of mineral element content, accretion rate and positive Ce-anomaly degree at 4.6 Ma indicate temporal warming, which went against the crust accretion and finally formed an accretion gap in the terminal Middle Pliocene (2.8-2.7 Ma). The more active Antarctic bottom seawaters in the Late Pliocene (2.7 Ma) facilitated the fast transfer to the top pure crust zone. Hereafter, with the further apart of volcanic source and the keeping increase of eolian material (1.0 Ma), although surrounding conditions were still favorable, mineral element content still shows an obvious reducing trend. It thereby offers new carrier and data for the unclear paleoceanographic research of the target area since the terminal Late Miocene.

塔里木盆地塔中—巴楚地区早二叠世岩浆岩地球化学特征及其成因

Tazhong-Bachu region is located in the Western Tarim basin.The early Permian magmatic rocks occur in the earth surface of Tazhong-Bachu region are mainly distributed in Kepintag,Mazhartag and Wajilitag region. There are a lot of wells, in which researchers found the early Permian magmatic rocks,in desert cover area.Most magmatic rocks are basic rocks, a few of which are ultrabasic rocks and intermediate-acid magmatic rocks.The ultrabasic rocks are are mainly occur in the Cryptoexplosive Breccia Pipes ,which is located in the volcanic complex body of Wajilitag region.The basic rocks can be divided into three rock types：The first type of the magmatic rocks in Tazhong-Bachu region is volcanic rock ,which occurs in the Lower Permian Kupukuziman Formation and Kaipaizileike Formation. Most Volcanic rocks are basalts,a few of which are volcanic breccias and pyroclastic rocks.The basalts are distributed in stratiform occurrences and interbeded the clastic rocks in Kepintag region.The attitudes of the basalts are nearly horizontal.Columnar Joints, gas pore textures and amygdaloidal structure are to develop in basalts.The second type of the magmatic rocks in Tazhong-Bachu region is diabase,which occurs in Mazhartag region.Diabase dike swarms occur in the stratums of Silurian, Devonian, Carboniferous and Lower Permian.They make from NNW direction to SSE direction, the obliquity of stratum is greater than 60°, and the dike thickness is form several cm to several meters. Diabasic texture is found in the rocks .The first type of the magmatic rocks in Tazhong-Bachu region are gabbro- pyroxenite rocks ,which occur in the Wajilitag igneous complex body. The intermediate-acid magmatic rocks, which are mainly syenites, are located in Mazhartag and Wajiltag region. But they are small in the whole Tazhong-Bachu region.There are intermediate-acid magmatic rocks,which are mainly dacite,in the northeast part of the wells in Tazhong-Bachu region.But ,it is not found in earth surface.Through systematical geochemical research of early Permian magmatic rocks,which are distributed in Kepintag,Mazhartag, Wajilitag region and the wells such as F1 well、Z1 well、Z13 well、TZ18 well、H3 well、H4 well et al., the focus on the geochronologic characteristics, the main element,trace element and REE geochemistry, the mineralogic characteristics, the Sr-Nd and Pb isotopic characteristics are put forward. The main points are: 1、A combined study of CL imaging and LA-ICP-MS U-Pb dating were carried out for zircon grains of the magmatic rocks in the Tazhong-Bachu region from the Tarim basin.The results of the systematic zircon LA-ICP-MS U-Pb dating reveal 272±6Ma to 291±10Ma for the magamatic rocks. It indicated that Early Permian is an important period of magmatic acvivity in the Tazhong-Bachu region. 2、There are a big hunch in the curves of primitive mantle-normalized trace element concentrations in the early Permian magmatic rocks from Kepintag, Mazhartag, Wajilitag region and the 14 wells. Light rare earth elements are comparatively rich and heavy rare earth elements are comparatively poor. The slope rates are same between light rare erath elements and heavy rare earth elements. It is not like the curves of the basalts in the convergent margin of plate , in which the slope rates of light rare erath elements is bigger than the alope rates of heavy rare erath elements, and the curves of heavy rare earth elements are comparatively flat. The magmatic rocks of Tazhong-Bachu region rarely have the characteristics of the basalts in the convergent margin of plate, which is that Tantalum, Niobium and Titanium are much poor, and Zirconium, Hafnium and Phosphorus are moderately poor. The magamatic rocks are mostly alkaline, which is indicated by the dots of the (Na2O+K2O)-SiO2 identification diagram. All of these indicate that the early Permian magmatic rocks were formed in an extension environment of intraplate. 3、The Thorium abundance is high and Tantalum abundance is low in most magmatic rocks from Tazhong-Bachu reguion, which is formed for crustal contamination.In the Th/Yb-Ta/Yb identification diagram，most dots are in the region, which means active continental margin, but a few dots are in the region, which means mantle source. It indicated the feeding of continental crust materials. 4、The magnesium content of the olvines from Wagilitag region is richest, and the olvines from Kepintag region is poorest in the tree region. 5、Through the the Sr-Nd and Pb isotopic study of the basalts and diabases from the F1 well core, Z1 well core, Z13 well core,TZ18 well core, and the basalts，gabbros, diabases(diabase-prophyrites) and pl-peridotites from Kepintag,Mazhartag, Wajilitag region , it indicated that all isotopic data is similar and close to enriched mantle.

内蒙古白云鄂博地区元古宙构造-岩浆演化史与超大型REE-Nb-Fe矿床成因

Bayan Obo giant REE-Nb-Fe deposit in the northen margin of the North China Craton (NCC) is well known in the world for its abundant rare earth element resources. There is nearly one hundred year of studying history in substance component, chronology and geochemistry of the ore deposit, since the main ore body was found in 1927. However, there still exist remarkable divergences in genesis, mineralized age and material origin. Especially the REE enrichment mechanism leaves us a secret. Recent research shows that the Bayan Obo ore deposit likely resulted from the carbonatite magma activity, which is a favorable factor for REE accumulation. Based on the analysis of tectonic evolution history of north margin of NCC this thesis mainly discussed the formation background of cratonic margined rifts in Bayan Obo, and presented the analytical results of formation environment, intrusion age and deep origin of Proterozoic carbonatite magma. These research results can provide evidence for ore genesis. LA ICP-MS U-Pb dating on zircon shows that the Neoarchean basement was mainly composed of calc-alkaline TTG gneisses (2588±16Ma). The collision orogeny movement of the northen margin of the NCC between 2.0 Ga to 1.9 Ga brought the swarm of diorite-granodiotite magma (2023±16Ma) and intense regional metamorphism event (1906.3±7.7 Ma to 1892.7±6.7 Ma). In the sequent super continent break up background, intense metamorphic and deformed basement complex was uplifted to the surface suffered denudation, forming Mesoproterozoic Bayan Obo group in the contemporary continental margin rifts. The uplift of basement complex and formation of continental rifts were likely related with mantle plume activity. Evidence from petrological and geochemical data suggests that abundant alkaline-basic magma resulted from enhancement of continental breakup activity, that separated into carbonatite veins and mafic dykes by melt immiscibility mechanism, intruded in Bayan Obo margin rifts at the late stage of extension movement. Carbonatite veins can be divided into three main types by mineral composition: dolomite carbonatite, dolomite-calcite coexistent carbonatite and calcite carbonatite. Intrusion relationship between different types of carbonatite veins show that the calcite carbonatite veins were formed latter than the dolomite type as well as the coexistent type. Moreover, geochemical data also reveals successive and evolutive character between them. The content of REE increases together with the calcite minerals component. That is to say that REE gradually accumulated as the evolution of carbonatite magma. High precision Sm-Nd isochron data shows that the intrusion age of carbonatite veins was at 1319±48Ma. Moreover, the REE mineralization age in calcite carbonatite veins was around 1275±87Ma that is consistent with the intrusion age in error range. According to these data the abundant REE already existed in the carbonatite magma before intrusion and result in the earlier ore mineralization. The average age of mineralized dolomite was at 1353±100Ma, and the mineralization age of apatite in coarse grain dolomite was around 1329±150Ma. These data is consistent with carbonatite. Considering the coincident rare, trace element and isochron composition between them, it is presumed that mineralized dolomite was also the carbonatite intrusion and was the mainly factor for huge REE enrichment.

内蒙古克什克腾旗小东沟斑岩型钼矿床的成矿作用

In this paper, the Xiaodonggou porphyry molybdenum deposit located in the Xarmoron molybdenum metallogenic belt is chose as the research area. We have analyzed the petrology of the Xiaodonggou pluton in detail and made chemical analysis of the major and trace elements, Rb-Sr and Sm-Nd isotope, common lead isotope and SHRIMP zircon U-Pb dating et al; in the other hand, we use the molybdenite to make common lead analysis and Re-Os isotopic dating. The Xiaodonggou pluton is rich in silicon, potass, zirconium, and low in REE. In addition, it has no minus Eu abnormity and show a isotopic composition high in εNd(t) and low in Sri, indicating its magma origining from the melting of juvenile thicken lower crust. In the meanwhile, it contained the features of high temperature, quick melting, quick segregation and quick emplacement. The common lead analysis of the pluton orthoclase and molybdenite show that the former transfer from orogen to mantle and the latter come from mantle, which is consistent to the molybdenite sulfur isotopic and quartz oxygen isotopic composition, demonstrating that the rock and ore-forming materials of deposit having different sources, magma from the lower crust mixing with mantle fluid. In plus, we use the physical experiments results of the water-magma reaction to explain the interaction of magma and mantle fluid. In the deep crust, these two systems uplifted in a immiscible state; when they reached low depth, the stream film between fluid-magma collapsed, and the magma was broken into small agglomerates by the fluid, then they mixed thoroughly. The SHRIMP zircon U-Pb dating gave a result of 142±2Ma and the molybdenite Re-Os dating result is 138.1±2.8Ma, corresponding to the big tectonic transition period of 140Ma, when the major stress field changing from south and north to west and east. At this time, the Da Hinggan ling ranges area was under an extensive background, underplating proceeded and mantle materials could add into the magmas forming in the lower crust. So, from the above analysis, we propose the following model for the Xiaodonggou porphyry molybdenum deposit: in the early Cretaceous period, the Da Hinggan ling ranges area was under a extensive background, the adding of mantle fluid containing ore materials into heated lower crust made it melting to produce magmas. Following more mantle fluid got into the magma room and urged the magma to segregate from the source quickly. The fluid and magma uplifted together, when they arrived at shallow depth, the fluid-magma became unstable and the latter was broken into many small agglomerates with fluid connecting them in the interspaces. Because of the H+, K+ and various elements existing in the fluid, it would reacted with the magma and the rock through alteration and ore minerals crystallized out, forming the Xiaodonggou porphyry deposit with disseminated mineralization phenomenon.