V11A-1399 0800h
Multiple Geophysical Observations by a newly developed multi-component borehole instrument at the Continental Deep Drilling Site of the CCSD, Donghai, China
Multiple Geophysical Observations by a newly developed multi-component borehole instrument at the Continental Deep Drilling Site of the CCSD, Donghai, China Jiren Xu1 (+86-10-68992879; xujiren@ccsd.org.cn) Zhixin Zhao1 (+86-10-68999734; zhaozhixin@ccsd.org.cn) Hiroshi Ishii2 (+81-0572-67-3105; ishii@tries.gr.jp Tsuneo Yamauchi3 (+81-052-789-3045; yamauchi@seis.nagoya-u.ac.jp) 1 Institute of Geology, Chinese Academy of Geological Sciences, China 2 Tono Research Institute of Earthquake Science (TRIES), Japan 3 Graduate School of Environmental Studies, Nagoya University, Japan The Chinese Continental Scientific Drilling (CCSD) site is located in the Donghai area of the Dabie-Sulu belt, which is the largest UHPM belt in the world. The drilling of the main borehole with 5000m will finish in next year. Three satellite boreholes, PP1, PP2 and PP3 were drilled and various surveys have been performed in the Donghai area about 6 years ago. We are going to install a newly developed Multi-component Instrument for borehole observations in main hole near the large Tanlu fault, and establish a long-term underground observation laboratory, which is the first noiseless one in China. The seismic activity and various geophysical fields, viz. strain, geomagnetism, geothermy, tilt, pore pressure etc. will be investigated. Data from the underground laboratory will be open to scientific, engineering and public services. We will measure the initial stress in various depths of the borehole by overcoring method using a new developed wireless intelligent type strainmeter of in-situ stress. Establishing a long-term noiseless underground observation laboratory at deep borehole and investigating crustal movement in East China are important for observing the physical conditions of the earth's interior and solving many social problems, such as resources, disasters and environment. Multiple geophysical observations and the study in deep borehole will speed up and develop the study on tectonics and geodynamics. The accomplishment of this project may enrich the knowledge on the geophysics and geology.
V11A-1400 0800h
3-D Wave Velocity Structure for the Ultrahigh-Pressure Metamorphosed Terrace in the Dabie- Sulu region,Chind
3-D Wave Velocity Structure for the Ultrahigh-Pressure Metamorphosed Terrace in the Dabie- Sulu region,Chind Zhixin ZHAO 1 (+86-010-68999734; zhaozhixin@ccsd.org.cn) Jiren XU 1 (+86-10-68992879; xujiren@ccsd.org.cn) Wencai YANG 1 (yangwencai@ccsd.org.cn) Zhiqing XU 1 (xuzhiqing@ccsd.org.cn) 1 Institute of Geology, CAGS Baiwanzhuang Road 26, Beijing 100037, China 3-D wave velocity structure image in the crust and upper mantle in the east continent of China were analyzed in the present study. The velocity beneath the North China block is generally higher than that beneath the Yangtze block at 150 km depth. A velocity variation boundary appears beneath the Dabie-Sulu orogeny belt. Velocity values beneath Sulu region and the Shandong peninsula are in the identical contour line with that beneath the Dabie orogeny belt. In velocity profile crossing the Dabie mountain along near N-S direction, at the region shallower than 100 km depth, velocities beneath the North China and Yangtze blocks are lightly less than the average value. Velocity beneath the Yangtze block at the region deeper than 100 km however, is slower than that beneath North China block. A anomalous velocity zone lightly less than zero at the south side of velocity variation boundary beneath Dabie mountain thrusts downward greater than 300 km depth beneath the North China block from south to north, at the dip angle of about 30 degrees. A near zero velocity zone north of the velocity boundary seems to thrust upward the surface of the Yangtze block from 100 km depth along the N-S direction beneath the North China block. Such geometries of the velocity structure images beneath the region crossing the Dabie orogeny belt imply that the zones thrusting down and upward might be related to the evidences of the traces for the subduction and exhumation process of ultrahigh-pressure metamorphosed terrace beneath the Dabie mountain.
V11A-1401 0800h
Well Logging and Logging Analysis of UHP metamorphic Rocks in CCSD Main Hole (0-2000m)
CCSD logging engineering gather many modern high technologies and employs various advanced logging tools to survey the sidewall continuously. This can obtain various physical, chemical, geometrical, etc in-situ information of the borehole's profile. So well logging is one of the most important parts and pivotal technologies in the project of CCSD. The main logging methods in CCSD-MH(0-2000m) are laterolog (Rd,Rs), gamma ray(GR), nature gamma spectrometry(U, TH, K), density(DEN), photo electric section exponent (Pe), compensated neutron(CNL), multipole array acoustic (Vp, Vs, Vst), Simultaneous Acoustic-Resistivity-image(Star-II), temperature(T),magnetic susceptibility(MS), three component borehole magnetic and redox potential log,etc. The various metamorphic rocks can be classified by logging curves,and their physical parameters can be acquired by analyzing the response characters of various metamorphic rocks and by statistics. According to the logging cross plot, We can research the clustering of metamorphite's physical property. Five lithologic segments can be obtainend by logging curves. The GR, Th, U, K logging values of segment 1 is lower than the third, fourth and fiveth segment, higher than segment 2; The DEN, Pe values of segment 1 higher than the third, fourth and fiveth segments. The main rocks in segment 1,2,3,4,5 are eclogites, serpentinites, paragneiss, orthogneiss, and eclogites(containing silicon and muscovite ) respectively. Generally, eclogite contain rutile, silicon, muscovite, etc. minerals. These minerals have response obviously on log curves.There are rutile,ilmenite, pyrite mineralized, etc. Making use of DEN, Pe, susceptibility log values, these mineralized layers can be goodly demarcation. For example, on the rutile mineralzed layer, the logging curve response characters are of high density and Pe obviously. The key data of the synthetical seismic record is wave impedance. In this paper, Utilize the data of AC, DEN curves to calculate the wave impedance and compare with the VSP profile, finally analyze the reflectors of the CCSD-MH. Imaging log has a positioning system and very good vertical resolution, and can describe the geological features in detail. Various structure parameters (the size and occurrences of foliation, fracture, fault & vein) have been given by image logging. The main work of logging interpretation of CCSD have done as follows: 1) Character analysis of logging response and restoring the lithologic profile;2) Depth correction and Restoring orientation of cores; 3) Interpretation of imaging geological feature; 4) Research of rock's mechanics character, sonic anisotropy and formation stress; 5) Comparison wave impedance with the VSP profile,analyzing the reflectors; 6)The interpretation of magnetic susceptibility and temperature log. The logging analysis results of UHP metamorphic rocks in CCSD-MH(0-2000m) show that responses of logs curve are abundant, the physical properties of various metamorphic rocks are visibly different;image logging has a positioning system and very good vertical resolution, and can describe the geological features in detail;lithologic segments and mineralized layers can be goodly demarcation by log curves;the log curves can be used for standardizing the geophysical survey, for example, mark out the reflected interface of seismic wave;comparing the results of log curves value with core laboratory analysis,they have good consistency;and so on.
V11A-1402 0800h
Tectonic stress field of brittle deformation within 2000 m of the main borehole of Chinese Continental Scientific Drilling
Four phases of tectonic stress fields: There exist a complex brittle fracture system and a tectonic stress field within 2000 m of the main hole. According to the features of its fillings, three different types of brittle strain phenomena may distinguished; they are microfractures filled by quartz, calcite, chlorite and other minerals, microfractures with films of minerals such as quartz and calcite or with striation lineation, and microfractures with neither mineral filling nor mineral films. They represent the early, middle and late phases and deep, middle and shallow tectonic levels of brittle deformation, respectively. Preliminary analysis indicates that four phases of tectonic stress fields, i.e. dominantly ESE and WNW compression, NE-SW regional compression, N-S compression and vertical extension, occur in the stage of brittle deformation. The modern tectonic stress field on the eastern side of the Tanlu fault is regionally very stable. In the brittle and brittle-ductile transition belts, the dominant mode of stress action constraining the emplacement of the Sulu high-pressure and ultrahigh-pressure metamorphic belt is ESE-WNW compression, which shows certain stability in time and space. The determination of in-situ stress from wellbore breakouts: The breakouts began to occur under the depth of 1200m in the main borehole of Chinese Continental Scientific Drilling (CCSD), 82 breakout images are collected from acoustic borehole televiewer data between 1200m and 2010 m. The averaged orientation of breakouts is 324.8-a-A3.3-a. The averaged orientation of maximum horizontal stress is 54.8-a-A3.3-a. Using parameters measured from breakouts (depth and width of breakout) and rock cohesive strength determined from triaxial rock compression and deformation test, the magnitudes of principal stress are calculated at 16 different depths of 1269m, 1500m, 2000m and so on. Overburden stress is calculated using the density logging data. According to the orientations and magnitudes of three principal stresses, the least principal stress is horizontal, and stress state (SH>Sv>Sh) determined from the analysis of wellbore breakouts of CCSD indicates the strike-slip stress regime and is coincident with the strike-slip stress field observed from earthquake focal plane mechanisms.
V11A-1403 0800h
Deformation Induced Anisotropy of Thermal Conductivity and Seismic Velocity in UHP Rocks From CCSD Main Hole
Both thermal conductivity and seismic velocity of rocks are of great interests to geologists because they provide important constrains on interpretation of rock dynamics at depths. Numerous investigations have shown that there is a strong correlation between seismic anisotropy and deformation in rocks. In principle deformation should have similar impact on thermal conductivity anisotropy. We present here preliminary results from thermal conductivity and seismic velocity measurements at atmospheric pressure and room temperature on drill cores from the Chinese Continental Scientific Drilling (CCSD) main hole located at Donghai County, eastern China. We measured thermal conductivity and seismic velocity of about 300 mini core samples including eclogites, gneisses and serpentinized garnet peridotites. A half-space line needle probe with transient heat flow method was applied to determine the anisotropy of thermal conductivity. The investigation gives information that thermal conductivity varies with lithology, mineral assemblage, even the degree of retrogression, but the most important influence factor is directional dependence, that is, to the same rock sample, both the thermal conductivity and the seismic velocity vary significantly with the azimuth of the measurement relative to the foliation if it developed in UHP rocks. The thermal conductivity parallel to the foliation can be up to 24% larger than that normal to the foliation while the P-wave velocity parallel to the foliation is 14% faster in average than that perpendicular to the foliation. Though there is no obvious correlation between the thermal conductivity and the seismic velocity in these rocks, the thermal conductivity anisotropy increases approximate linearly with the seismic anisotropy with a correlation coefficient of 0.91. Our results suggest that deformation controls not only the seismic anisotropy but also the thermal conductivity anisotropy in UHP rocks. It is necessary to investigate into the corresponding mineral fabrics for a better understanding of rock anisotropy.
V11A-1404 0800h
Tomography in CCSD area of Jiangsu Donghai
Sulu-Dabie Orogeny is typical UHP zone containing wide-spreading eclogite. CCSD choose largely covered eclogite areaͦJiangsu Donghai as first site to commence deep drilling because this will disclose the geodynamic processes of orogeny and deformation and diapiring afterwards. Less of geostatistic data and documentation for deep geology of this area makes tomography one of the most important methods for velocity perturbation analysis of deep material. And this can be the preceding procedure for deep drilling and deep survey, as we deployed a crossing profile centered at the drilling address with NS azimuth of 340. Approximate 5000 events were acquired between May and October of 2000 from IRIS, among which P wave coverage events has 3500 pieces and PKP 870 pieces. After carefully arrival time picking , 355 utilizable P events were collected in which 27 events has magnitude bigger than 6. Using 41 stations tomography inversed 917 model blockstransversed by 1185 rays. Then traval time residue less than 1s were selected to revise the model, through which data residue reduced from 0.64 to 0.3 and the diagonal elements sum of resolution matrix reachs 2000. Observed from time travel time residue data, southern stations have bigger residue due to southward events and get smallest at the center part of model,then become bigger again to the north. But this rule is not effect for the E-W profile. This kind of distribution illustrate the tectonic strike may go through EW direction and the center part has the highest velocity and southern part have more low velocity material than the north part. Applying tomography inversion, we get the image of velocity at different depth. The 3rd layer is consistent with Bouger gravity anormality , where high velocity perturbation and high gravity both appear at the center, and Tanlu, Jiashan-Xiangshui Fault both serve as boundary of high and low velocity and gravity. NS section perpendicular to tectonic strike can be divided into three parts which represent Yangtze, Sulu and North China respectively. The fault normal in south part is dipper with a dipping high velocity body of indistinct discontinuity, but faults in the north part have coherently northward inclination controlled by Tanlu Fault. Upper the depth of 200km, there are three horizontal changing zone at 30km,80km and 130km, which is about the depth of Moho, lithosphere and asthenosphere. This scheme illustrates the feature of lithosphere activity of Sulu area controlled by deep faults.
V11A-1405 0800h
An unusual clinopyroxenite nodule from the northeastern Sulu ultrahigh-pressure metamorphic (UHPM) terrane, East China: Its P-T path and implications for subduction erosion
Pyroxenite consisting only of clinopyroxenes is rare. We present bulk composition and microtexture observation data on a garnet-pyroxenite nodule enclosed within a granitic gneiss in Rongcheng, northeastern Sulu UHPM terrane to (1) reconstruct its protolith; (2) estimate its P-T path; and (3) its implications for the continent-continent subduction and collision dynamics. Bulk composition analyses and micro-textural observations demonstrated that the Rongcheng garnet-pyroxenite originally was a mono-mineralic pyroxenite consisting only of clinopyroxene. Prograde and/or retrograde metamorphic reactions have led to the formation of pyroxenes that are surrounded by an interconnected network of garnets. Within the pyroxenes that consist of clinopyroxene and solid exsolutions, four sets of garnet lamellae occur strictly along four crystallographic planes. Hydrous minerals such as amphibole and epidote were formed at later stages of retrograde metamorphism. The P-T conditions determined for the pyroxenite indicate it has undergone a P-increasing and/or T-decreasing earlier stage and an isothermal decompression later stage metamorphism, followed by the latest stage metamorphism of decreasing both in P and T. The last two stages experienced by the pyroxenite are similar to those of the nearby eclogites from the northeastern Sulu UHPM terrane. The Rongcheng pyroxenite and granitic gneiss host may belong to the hanging wall of the Sulu subduction zone. This observation also suggests that parts of the Northern China Block (NCB) might have been dragged into deeper mantle due to subduction erosion during the continent subduction and collision between the NCB and the Southern China Block (SCB), and then returned to the surface together. This interpretation is significant to resolve some differences in geologic features between the northeastern and southwestern Sulu UHPM terrane.
V11A-1406 0800h
The Extremely Depleted Peridotites in Sulu UHP Belt: Geochemistry of PP3 Ultramafic Rocks
PP3 ultramafic massif is located in the east of Sulu UHP belt, as a third pre-pilot hole of Chinese Continental Scientific Drilling (CCSD) Program; PP3 had drilled 705.45 m depth. Ultramafic rocks about 480 metres sandwiching eclogites and gneisses are revealed. Coesite occurred as an inclusion in omphacite, which means the eclogite ever metamorphosed in the UHP condition. Ultramafic rocks include garnet peridotite, dunite and serpentinite with few hargzburgite; contain olivine, chromium spinel, clinopyroxene, or/and garnet, and few orthopyroxene. About 55 ultramafic rock samples are analyzed systematically, all the samples in PP3 peridotites concentrate in hargzburgite area in the modal mineralogy compositions. SiO$_{2}$ content of PP3 peridotites varies from 37% to 43.16%, Al2O3 from 0.24% to 1.03%, MgO from 38.7% to 45.4%, Cr$_{2}$O$_{3}$ from 0.27% to 0.41% NiO from 0.26% to 0.39%. Most of CaO and Na$_{2}$O values from under detection to 0.29% and 0.32% respectively. These samples characterize relatively lower contents of CaO, Al$_{2}$O$_{3}$, TiO$_{2}$, Na$_{2}$O, K$_{2}$O and FeO and higher contents of MgO, compared to the depletion trend. Mg\# (Molar Mg/(Mg+Fe)-A100) is generally high (92 to 93) and Cr\# (Molar Cr/(Cr+Fe) -A100) is also high (30 to 45), and variable in a small scale. Garnet-bearing peridotites have the same Mg\# and Mg/Si ( $\sym$ 1.6) as garnet-free peridotites, indicating that they are residues of high degrees of partial melting (between 35-50%), which demonstrating depleted mantle. PP3 peridotites were extremely depleted, not only major and trace elements, but minerals as well. Mg\# of olivine is about 92.8, Cr\# of chromium spinels is from 60 to 90.Compared to previously published compilations of the average composition of either spinel lherzolites, garnet lherzolites or spinel harzburgites, The compositions of PP3 peridotites is significantly lower CaO and Al$_{2}$O$_{3}$, and higher Mg\#, suggesting that they are the extremely depleted ultramafic rocks in east part of China. PP3 peridotites show huge variations in rare earth element (REE) abundances (from $\sym$ 0.07 to $\sym$ 0.53 times chondritic valuesthe average value is 0.18, and exhibits depleted, but smooth chondrite-normalized REE patterns, which suggests that REE compositions of PP3 garnet peridotites and dunites are severly depleted. (La/Yb)$_{N}$ ratio is from 6.9 to 51.2the average is 16.1, which shows slightly rich in LREE. PGE in PP3 peridotites is higher than mantle nodule and orogenic peridotites, but less than one-two order of magnitute comparing with chondrite. (Pd/Ir)$_{N}$ varies from 0.11 to 0.92, the average is 0.52, (Rh/Ir)$_{N}$ from 0.32 to 0.91the average is 0.61 and (Pt/Ru)$_{N}$ varies from 0.02 to 0.54, the average is 0.09. Their PGE patterns are similar to chromite PGE patterns in the world. The relatively incompatible elements Pd and Pt are more depleted than that of pyrolite, represents the most depleted mantle and is apparently different from other ultramafic rocks in the east part of China.
V11A-1407 0800h
Rodingite and its geochemistry at the Changawuzi Serpentinite complex, Western Tianshan, China
Tianshan rodingites included in the serpentinite complex were formed at the expense of eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites. The Southwestern Tianshan ophiolitic melange extends along the NE-SW axis of the tectonic ridges and faults of the Tianshan Mountain Range. Serpentinized ultramafic rocks occur together with interlayered south-dipping blueschists and greenschists in Changawuzi ophiolites suggesting to represent relics of a Silurian oceanic crust. They contain a mineral assemblage of prehnite, clinozoisite, hydrogrossular, diopside, vesuvianite and chlorite, while partial rodingitized rocks still preserve the relict omphacite and Fe-Mg-Al garnet. Complete rodingitized rocks have the simplest mineral assemblage of hydrogrossular, vesuvianite, chlorite and occasionally diopside. Our former study indicated that the rodingitization started at 370~410 -aC/6.5~8.5 kbar, while pervasive rodingitization took place under conditions of 200~350-aC/2~6 kbar. Major Elements of protolithic retrogressive eclogites and two partial rodingitized rocks show a good fit to the basalt compositions for most major components except for Na2O and K2O. The high TiO2 and also melt-like P2O5 contents as well as higher total iron content suggest basalt origin rather than gabbros and mafic dykes containing lots of cumulate minerals low in Ti and P. Rodingites suffered intensive metasomatism, however, mostly contains low TiO2 <0.1%, total iron <5% and P2O5<0.01, suggesting a removal process of these elements in intensive rodingitized rocks. Trace Element data of retrogressive eclogites show moderate LREE enrichment patterns. The mantle normalized spidergrams show a slight depletion of HFSE from Nb, Ta and Ti and enrichment of Th and LILE with Nb/La 0.32~0.48 and Nb/Th 3.52~4.36. These features are different from OIBs and MORBs. Trace element characteristic gives a hint of subduction zone magmatic origin. As the rodingitization progressed, many immobile elements, including Zr, Nb, Y, rare earth elements (REE), show continuous and significant depletion, indicationg fluids produced by serpentinization could leach out even most immobile elements from the protolith of rodingites. Therefore, the geochemical patterns of trace elements including REE from rodingites (at least completely rodingitized rocks) could neither represent the geochemical features of the igneous protoliths, nor be used to indicate tectonic environment where the igneous protoliths were formed. Key Words: rodingite eclogite geochemistry Western Tianshan Xinjiang
V11A-1408 0800h
Geochemical Characteristics of Ultramafic Rocks From Main Hole of CCSDP, Sulu UHPM Belt
In the main hole of Chinese Continental Scientific Drilling Project (CCSDP), the ultramafic rocks, hosted by rutile eclogite, occur at the depth between 603.2$\sim$.683.5 m. Ultramafic rocks are composed of mainly wehrlite and minor lherzolite. Most of these rocks contain abundant garnet and Ti-clinohumite, but minor rocks have no garnet and Ti-clinohumite. Serpentinization varies in different degrees ,through the depth and minor talc and prehnite, present mainly at the lowermost part of the profile. Besides, there is a 10 m thick layer and many thin lens of rutile phengite eclogite, as well as small blocks of eclogite and thin layers of phlogopite orthopyroxenite and megacrystic amphibolite in the ultramfic sequence. The contacts between the ultramafic rocks and hosting eclogite and lens in them are sharp. The uppermost ultramafic rock possibly has a tectonic contact, with the host eclogite, characterized by mixed agglomerates of them. Major element concentrations of wehrlite and its altered product serpentinite range between SiO$_{2}$ 40.60%$\sim$.43.42%, TiO$_{2}$$<$0.31%, Al$_{2}$O$_{3}$ 3.16%$\sim$.8.96%, total FeO 12.27%$\sim$.16.77%, MnO 0.18%$\sim$.0.27%, MgO 27.65%$\sim$.39.20%, CaO 1.02%$\sim$.5.32%, Na$_{2}$O 0.01%$\sim$.0.74%, K$_{2}$O 0.01%$\sim$.0.81% and P$_{2}$O$_{5}$ 0.01%$\sim$.0.12%. The average fusible compositions are fairly high(TiO$_{2}$ 0.21%͛Al$_{2}$O$_{3}$ 5.06%͛CaO 2.53% and Na$_{2}$O 0.25%) and closing to or exceeding the corresponding compositions of primary mantle .Representative refractory composition MgO is lower than that of primary mantle. Mg$^{\#}$ is lower and varies narrowly between 77.26$\sim$.85.07. When compared with A type Zhimafang peridotite(Zhang et al, 2000), the wehrlite in borehole has relatively lower SiO$_{2}$ and MgO and higher TiO$_{2}$, Al$_{2}$O$_{3}$, CaO and total FeO. In a word, wehrlite shows more fertile characteristics. Total REE concentrations range 4.38$\times$10$^{-6}$$\sim$.43.26$\times$10$^{-6}$, most of which are higher than that of primary mantle(PM, 6.86$\times$10$^{-6}$). Most of the samples show moderate LREE enriched characteristics with "La/Ybc$_{N}$ ratios of 1.02$\sim$21.03 and slight Eu positive anomaly in PM normalized partition pattern diargram. Trace element spider diagram shows no evident incline with "Rb/Ybc$_{N}$ ratios of 0.25$\sim$.2.98, but has U, La, Zr, Hf and Sr positive anomalies and Ba, Nb and Ta negative anomalies. Tatol content of PGE is very low ranging between 1.14$\times$10$^{-9}$$\sim$.2.72$\times$10$^{-9}$ which are fairly lower than that of primary mantle(23.5$\times$10$^{-9}$), that of mantle xenoliths in Cenozoic basalt in East China(16.5$\times$10$^{-9}$$\sim$.50.7$\times$10$^{-9}$) and that of Ronda peridotite. All the data above indicate that the protolith of wehrlite was possibly not a residual mantle block but a ultrmafic body in the crustal depth, i. e. , B type ultramafic body in UHP belt.
V11A-1409 0800h
SHRIMP U-Pb Dating of Zircon From the Xugou UHP Eclogite, Sulu Terrane, Eastern China
Eclogites, together with garnet clinopyroxenites, occur as lenses within the Xugou garnet peridotite body from the southern Sulu ultrahigh-pressure (UHP) terrane. Combined cathodoluminescence (CL) imaging and SHRIMP U-Pb dating of zircon from two Xugou eclogites provide added constraints on the timing of UHP metamorphism in this area. Zircon grains from both mafic eclogite samples show subrounded to rounded shapes, ranging from 50 to 250 |?m in diameter. CL images reveal that zircons from both samples are not oscillatory zoned, and have no obvious cores of magmatic or xenocrystic origin. Most zircons show a homogeneous CL image, in a few cases with a weak cloudy or patchy pattern. Th-U chemistry of studied zircons is characterized by variable but low U and Th contents (U: 4-423 ppm; Th: 0.2-67 ppm) and variable Th/U ratios, ranging from 0.01 to 1.78 for sample XG-3C and 0.06 to 0.50 for sample XJ-1E. There is also a weak linear relation between Th/U ratios and 206Pb/238U age, as well as U and Th, indicating that these eclogites might have undergone systematic variation during UHP metamorphism. All these features suggest that zircons from our samples are metamorphic. The analyses of both samples fall into one population on a Tera-Wasserburg (TW) diagram and a histogram. U-Pb data of both samples yield apparent 206Pb/238U ages ranging from 214 to 280 Ma with a weighted mean age of 237+/-8 Ma (n=30). This age is interpreted as the time of peak UHP metamorphism of the Xugou eclogites and is consistent with previous reported UHP metamorphic ages from eclogite pods and country rock gneisses in the Sulu terrane. The Xugou mafic lenses may have formed by partial melting of the enclosing peridotites in the mantle before subduction (Zhang et al., 2003); then these eclogites, together with host peridotites, were emplaced into the subduction zone and subjected to UHP metamorphism at 237+/-8 Ma.
V11A-1410 0800h
Allanite and apatite from the Sulu UHP eclogites (CCSD): light rare earth elements and thorium carriers in subducted crust
Light rare earth elements (LREE) and Th are ones of important elements for the characterization of the components in subduction zones. Therefore, a profound understanding of the behaviour of these elements in subduction zone processes is only possible if we know in which phases LREE and Th are incorporated, how stable these phases are and how they interact with subduction zone liquids. Allanite and apatite are considered two accessory minerals incorporating important LREE and Th in subduction-zone rocks. Chinese Continental Scientific Drilling (CCSD) provides us an opportunity to make a systematic study on distribution of REE-bearing accessory minerals in the Sulu UHP eclogites. The rocks contain garnet, omphacite, quartz (probably pseudomorph after coesite), phengite and rutile. Accessory allanite and apatite often form aggregates. The following features can be taken for preliminarily characterizing the aggregates. (1) allanite grains are cored by apatite, and mantled by epidote; (2) micro-inclusions of thorite are found within the apatite core or allanite, or at the contact between them; (3) allanite transforms progressively to epidote; (4) epidote reveals zoned texture (e.g., lower right part in Fig. 1a), which is represented by variations in Fe and Al contents. Electron-microprobe analyses reveal that the apatite contains up to 1 wt%(LREE)2O3 and 0.5 wt% ThO2, particularly allanite incorporates not only LREE, but also as high as 2.1 wt% ThO2. The above observations indicate that there is a continuous transition from epidote to allanite, which is stabilized due to the presence of LREE, deriving possibly from the breakdown of primary LREE- and Th-bearing apatite in the course of UHP metamorphism. This work was financially supported by Chinese Ministry of Science and Technology (2003CB716507).
V11A-1411 0800h
Retrograde Processes of Eclogite from 100-2000m in the Main hole of Chinese Continental Scientific Drilling Project,Eastern China
The Chinese Continental Scientific Drilling (CCSD) Project is located in the Maobei Village, Donghai County, Jiangsu Province, Eastern China. Based on the detailed study on mineralogy, petrology, and geochemistry of eclogite from 100 to 2000m of the main borehole, the major achievements are as following: 1. Four retrograde processes have been identified. That is: weakly retrograde stage, partially retrograde stage, retrograde stage and strongly retrograde stage. During the weakly retrograde stage, only a few symplectitic coronas of amphiboles and plagioclases distributed along the boundry of garnet and omphacite. Rutile is stable. During the partially retrograde stage, the rate of omphacite disappear is rapid, omphacite exist only as a relict. Aegirine rim exist along the boundry of quartz and omphacite. Rutile retrograde into ilmenite. During the retrograde stage, garnets were gradually replaced by pargasite or epidote+biotite. omphacite disappear totally, and It is replaced by symplectitic coronas of amphiboles and plagioclases, but the pseudomorph of omphacites preserved. Aegirine rim become wider and sometimes exist as a lump. During the strongly retrograde stage, Aegirine almost disappear. Amphibolite replaced Aegirine, and It distributes along the boundry of quartz. 2. From fresh eclogites to strongly retrograde eclogites, MgO, CaO and FeO of the whole rock gradually decrease, SiO2, K2O and Na2O gradually increase, and Al2O3 almost keep constant. The Pb value increase obviously from weakly retrograde stage to strong retrograde stage. 3. The peak P-T condition of eclogites: 697-831͑ and 3.0Gpa-A; partially retrograde phase: 629-776͑ and 1.2-1.6Gpa; retrograde phase: 550-650͑ and 0.5-0.7Gpa (Xu, Z Q et al., 2004); strongly retrograde phase: 300-400͑ and 0.30-0.35Gpa (Xu, Z Q et al., 2004). 4. The UHP metamorphic rocks had undergone two-phase exhumation: first, nearly isothermal and decompression, suggested that fast exhumation occurred before the completely retrograde of the omphacite (during this phase, eclogites had undergone weakly and partially retrograde); second, decreasing temperature and decreasing pressure by slow uplifting after omphacites being completely replaced by symplectite (during this phase, eclogites have undergone retrograde phase and strongly retrograde phase). Acknowledgement: This work is sponsored by the Major State Key Project to CCSD: 2003CB716501 and major project from NSFC: 40399142
V11A-1412 0800h
Textural Characterization and its Formation of Nanometer Ultra-fine Rutile from Eclogite of Sulu UHP terrane, China
Detailed TEM investigations present the ultra-fine microscopic textures of rutile in rutile eclogites of Maobei, Su-Lu UHP terrane. The mineral assemblage contains garnet, omphacite, rutile, quartz/coesite and minor apatite. Two types of rutiles occur in investigated samples. Rutile (A) associates with garnet and omphacitewith the grain-size of 0.2-2 mm. The dense defects and modulated structures regularly oriented nanometer-size kinks are discovered in the rutile crystal, which implicate a strong stress and strain effect on the rutile crystals. The kinks in the rutile are observed with the sliding planes {110} and sliding direction [100]. That is intimately related to continental collision. Rutile (B) is included in the garnets and omphacites with the grain size of 1-40 Im. The crystal of later type is well preserved in the host minerals, and has not undergone crystal distortion during exhumation of subducted slab. The rutile eclogite under study occurs in the Daibei-sulu UHPM belt, consisting of garnet (~43 vol. %), omphacite (~45 vol. %), rutile (~7 vol. %), Quartz/Coesite (~3 vol. %) and minor amount of apatite and spinel. There are two different types of rutiles in the samples, in which rutile (A) associates with garnet and omphacite with the grain-size of 0.2-2 mm.; and rutile B included in the garnets and omphacites has grain size of 1-40 µm. The ultra-fine microscopic textures of rutile are discovered in type A rutile, having been investigated in detail by means of TEM technique. Two type rutiles in the eclogites of Maobei were investigated in detail by TEM. They exhibit completely different micro textures, indicating these two types of rutiles form from different tectonic environment and have experienced different geological events. Rutile (A) associated with garnet and omphacite contains a lot of dense defects, modulated texture and nanometer-size kinks in the crystal. The deformed kink twins in the rutile are observed regularly oriented with the sliding planes {110} and sliding direction [100], indicating a strong strain effect on the rutile crystals, and may intimately relate to the subduction, exhumation and subsequent continental collision. The rutile (B) is included in the garnets and omphacites with the grain size of 1-40 Im . Rutile (B) shows a perfect configuration due to the well preservation from host minerals (garnets or omphacites), and no defects has been found in the crystal, which suggests rutile (B) has not undergone crystal distortion during exhumation of subducted slab. Discovery of many oriented exsolution rods of quartz indicates the host primary clinopyroxenes could be supersilicic. Former researches conclude that pyroxenes from deep mantle or deeply subducted UHP terrane could be of supersilicic characteristics. In addition, several coesites have been found inside the rutile and among the grain boundaries of rutiles under TEM in this study, also suggesting the rock has experienced a UHP metamorphism. The formation of rutile (A), contained a lot of dense defects, modulated texture and nanometer-size kinks in the crystal, may intimately relate to the exhumation process of subducted terrane. Rutile (B) crystal is well preserved in the host minerals, and has not undergone crystal distortion during exhumation of subducted slab.
V11A-1413 0800h
Recent Fluids in Chinese Continental Scientific Drilling Project
The fluids and their origins in continental scientific drilling programs have widely been applied to the studies of crustal extension, fluid transportation paths and tectonization processes. The rare gases are good indicators of mantle fluids. The isotopes of carbon and hydrogen and the relationships between them can be used in revealing the fluid sources. And C/3He can provide more ambiguous distinguish between sources. The recent fluids in Chinese continental scientific drilling project (CCSD) have been analyzed and profiles were obtained. He, CO2, Ar, N2, O2, H2 and C1-C4 were determined by two on-line units, a mass spectrometer and a gas chromatograph. Cations and anions in mud samples were analyzed by an on-site high performance liquid chromatograph. Rare earth elements and other inorganic components were measured by ICP-AES and ICP-MS in our laboratory in Beijing. The isotopes of carbon, hydrogen, oxygen, and rare gases, especially helium, were analyzed by mass spectrometers in different laboratories. One key in studying the recent fluids in CCSD project is to identify whether the recent fluids were from the deep earth or not, even when their concentrations were higher than normal levels. Many disturbance components would usually be produced during drilling process. Such the disturbance includes many artifact gases from mud ferment, organic additive decomposition, bit erosion, etc. The analytical data of recent fluids could not be used in the investigation before removing the artifact components. It was found that the high contents of elements were related to the special rocks and minerals, such as sulfide and radiation ores. Carbon dioxide was related with carbonate. The high contents of gases were often found when the cracks or fissures occurred. The distribution of rare earth elements changed with the recent fluids. In some cases, a certain amount of helium gas was found with a high intensity of radiation detected. The high content of methane was once observed with a crystal hole in CCSD project. The samples for isotope analyses were collected in glass bottles and sent to several laboratories in China and Germany, separately. When helium and carbon isotopes in samples were found above the average values in CCSD samples, they would be measured again to confirm the safe conservation of these samples and there was no significant leak of the gases from the bottles. The isotope data show that the abnormal contents of gases found in the CCSD drilling well come from multiple sources and are related to the geological structure in the Sulu ultrahigh-pressure metamorphic belt of China.
V11A-1414 0800h
Heat-producing elements in the first 2000 meters CCSD drilling core: Implications for heat production distribution in the high to ultrahigh pressure metamorphic terrains
How radiogenic heat production elements (HPE) distribute as a function of depth in the Earth is critical to determine crustal thermal and rheological structure and to constrain geochemical, petrological, and tectonic models of crustal evolution. Our knowledge on HPE distributions within the crust have been derived predominantly from studies on large-scale granitic batholiths. The HPE distributions within high-grade metamorphic terrains are not well studied and thus poorly understood. The Chinese Continental Scientific Drilling (CCSD) project provides us an unparallel opportunity to characterize the vertical HPE distribution in a type example of high to ultrahigh pressure metamorphic terrains worldwide. U, Th, and K$_{2}$O contents have been measured on 734 core samples with a sampling interval of 1 to 5 meters and used to determine the distribution pattern of radiogenic heat production. Petrographic studies have demonstrated that the uppermost 2000 meters core can be subdivided into seven lithologic sections which consist of (I) quartz eclogite (100-248 m); (II) quartz eclogite alternate with rutile eclogite (248-530 m); (III) rutile eclogite (530-600 m); (IV) garnet peridotite with interlayers of eclogites (600-695 m); (V) paragneiss with intercalated eclogite layers (695-1160m); (VI) granitic gneiss with intercalated eclogite layers (1160-1600 m); and (VII) eclogite (1600-2050 m), respectively. Preliminary results show that: (1) the granitic gneisses have the highest heat production with an average of 1800-A10$^{-11}$W/kg; (2) the paragneisses have the intermediate heat production values; (3) the rutile eclogites and garnet peridotites have the lowest and similar values at about 4~5-A10$^{-11}$W/kg; (4) radiogenic HPE concentrations and heat production change stepwisly as lithology changes downward. This data reveals a sandwich-like structure for the HPE in the drilling core in which relatively high heat production sections are enclosed by relatively low heat production sections. Such a pattern is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. However, it may represent a typical structure in UHP metamorphic terrains as a result of deformation during or after the exhumation which resulted in the juxtaposition of mafic to ultramafic blocks with felsic blocks.
V11A-1415 0800h
An exotic terrane in the Sulu UHP region, China
The Haiyangsuo region of about 15 km2 along the coast in the NE part of the Triassic Sulu UHP terrane occurs three major rock types: amphibolitized metagabbro, gneiss and granitic dikes. Three different gneisses were observed in the field: A) Light color felsic gneiss is the dominant country rock and contains Qtz, Pl, Ms and Bi. B) Dark color plagioclase-amphibole gneiss occurs as thin layers within country rock; C) Granulite facies rock occurs as discontinuous lens. The amphibolitized metagabbros intrude into the gneisses as massive bodies (several m to hundreds of m in size) and thin dikes. Both metamorphic intrusives and gneisses are cross-cut by granitic dikes. The amphibolitized metagabbro was divided into three types: coronal metagabbro, transitional rock and garnet amphibolite: 1) Coronal metagabbro preserves gabbroic texture and primary assemblage of Opx+Cpx+Pl+Amp+Ilm. Most pyroxene grains are partially rimmed by thin corona of Amp+Ab+Qtz. Garnet occurs as fine-grained coronas at interface between plagioclase, pyroxene or ilmenite. 2) Transitional rocks contain similar assemblage and texture but most orthopyroxenes were partially or totally replaced by Amp+Qtz; garnet increases in content and size. Some gabbroic textures are preserved, but calcic plagioclase was replaced by zoisite, albite and muscovite. 3) Garnet amphibolite occurs at the margins of intrusive bodies and boudins where only minor relict clinopyroxenes preserve. Garnet coronal chains are not clear any more. Granitic dikes show pronounced deformation with mylonitic texture and contain 40-50% quartz porphyroclasts. Zircon separates from 2 metagabbros, 4 gneisses and 1 granitic rock were dated by using Stanford SHRIMP-RG. Metagabbroic zircons are angular and fractured shapes. The upper-intercept ages of gneisses rang from 1730 to about 2400 Ma, indicating variable protoith age. The 2 garnet amphibolites have upper-intercept ages 1734$\pm$5Ma and 1735$\pm21$Ma respectively. They are much older than the protolith ages (680-850 Ma) of the Sulu eclogite and country rock. At least two possible metamorphic events are indicated by the lower-intercept ages. One metagabbroic rock has a lower-intercept age of 842$\pm$37 Ma; this records a garnet amphibolite metamorphic event. One gneiss and one metagabbroic rock yield lower-intercept ages of about 340$\pm$25 Ma, suggesting a second metamorphic event. The granitic dike has upper-intercept age 749$\pm$43 Ma and lower-intercept age 150$\pm$17 Ma. All these petrological and geochronological results indicate that the Haiyangsuo region is not part of the Triassic Sulu UHP terrane.
V11A-1416 0800h
High-pressure Metamorphism in the Evolutionary History of the Earth's Crust in the Urals
Eclogite ages from the Urals mainly fall within the interval 390 ± 240 Ma. Grounding on these dates, many researchers limit the time of formation of eclogite-bearing rock associations by the above-mentioned interval, beginning with crystallization of high-pressure parageneses in deep zones and finishing at eclogite exhumation during the collision and post-collision stages of fold-orogenic belt development. At the same time, there are more ancient dates of the objects to be studied. Hence, not denying a theoretical possibility of eclogite formation and exhumation during a single geodynamic cycle, we think that the models, which allow for a significant time interval between the eclogite formation and exhumation of the high-pressure associations out of the deep zones, are more realistic for the Uralian eclogites. On the basis of available geochronological data on the Uralian eclogites, one can single out three time levels of evolution of high-pressure metamorphic rocks: over 1560, 720-550, and 390-240 Ma. At present, most researchers think that conditions for metamorphic eclogite crystallization can be realized only in subduction zones. This could be accepted, however, repeated high-pressure metamorphic manifestations in the same complexes for more than 1000 million years seem unlikely. It appears that only the most ancient eclogite dates (over 1560 Ma) record the time of crystallization of these rocks in subduction zones. Neoproterozoic and Paleozoic ages indicate the time of collision and post-collision processes at later stages of geodynamic development. Movement of eclogite-bearing complexes to the upper layers of the Earth's crust is connected with the processes.
V11A-1417 0800h
An Early Paleozoic HP granulite-garnet peridotite unit in the south Altyn Tagh, North Tibet: Petrological and SHRIMP dating constraints
Petrological investigations and SHRIMP dating were undertaken on a newly recognized HP granulite unit in the Bashiwake area of the south Altyn Tagh, western China. The HP granulite unit consists of bearing-kyanite felsic granulites, mafic granulites, sapphirine-bearing garnet clinopyroxenite and associated garnet peridotites. The predominant felsic granulites are quartz + Ternary feldspar (present perthite)-rich rocks containing various amounts of garnet, kyanite, plagioclase, biotite, rutile, spinel and corundum, and minor zircon, apatite. The mafic granulites enclosed directly in felsic granulites contain a peak assemblage of Grt + CpxI + Ternary feldspar (now mesoperthite) + Qtz + Rt. The growth of new clinopyroxene (CpxII), Opx and Pl in the matrix was related to decompression. The bearing-sapphirine garnet clinopyroxenite occurs as mafic layers in garnet peridotite. Petrographical data suggests a peak assemblage of Grt + CpxI + Ky + Rt. The early kyanite is inferred by the symplectite of Spr + Cor + Pl ,b Spl formed during decompression. Garnet peridotite contains an assemblage of Grt + Ol + Opx + Cpx. The fibrous kelyphites composed of Opx + Cpx + Spl around garnet suggests decompression. The peak assemblage of garnet + kyanite + ternary feldspar + quartz in the felsic granulite give a P-T conditions of around 925-995,aC and 20.0-24.5kbar, consistent with the estimates of T = 950-1020,aC and P = 18.5-25.3 kbar obtained from the peak assemblage of mafic granulite, and T = 870-1050,aC and P = 18.5-27.3 kbar of garnet peridotite within error. A medium-high pressure granulite facies overprint (780-820,aC, 9.5-12 kbar) is well defined by the formation of secondary CpxII ,b Opx + Pl at expense of garnet and early Cpx in the mafic graunlites, as well as by growth of Spl and Pl at expense of garnet and ky in the felsic granulite. The separated zircons from felsic granulite, sapphirine-bearing metabasites and garnet peridotite were measured on SHRIMP II iron microprobe. Most zircons are spherical, multifaceted garins, and some of the zircons contain mineral inclusions which are consistent with peak assemblages observed in thin section. The metamorphic zircons from the felsic granulite yielded an average age, for individual grain spots, of 496 ,b 9 Ma (mean of 11). Metamorphic grains from sapphirine-bearing metabasite and garnet peridotite gave the same ages of 497 ,b 11Ma (mean of 11) and 501 ,b 16 Ma (mean of 12), respectively. Therefore, the age of 500 Ma is interpreted as dating the HP/UHT metamorphism. It is identical to ages determined for eclogites from the western segment of the south Altyn Tagh. This suggests that south Altyn Tagh region experienced the same crustal thicken event that resulted in the formation of eclogites and HP granulites in different areas, respectively. The compatible P-T paths of the HP granulites and associated garnet peridotites from the Bashiwake area of the south Altyn Tagh and their metamorphic ages around 500 Ma argues for a shared metamorphic history.
V11A-1418 0800h
Evidence for Cretaceous uplift of the Northern Tibetan plateau
The northern part of Tibet Plateau became a coherent block after closure of the Paleo-Tethys Ocean in the Triassic. During the closure, a series of large-scale strike slip faultssuch as the Altyn Tagh, Anemaqen and North Qaidam faults were formed as a result of the NE-directed oblique subduction of the Qiangtan block, which had previously separated from the Gondwanaland. The N-S Longmenshan fold belt was also formed at this time due to the buttressing effect of the South China Plate to the east. Uplift of the northern Tibet Plateau began in the Cretaceous as a result of closure of the Middle Tethys Ocean and subduction of the Gangdes Block beneath North Tibet along the Bankong Lake-Nujiang suture zone. Uplift of the plateau was aided by the buttressing effect of the Tarim-Alax platform to the north and by the westward subduction of the Pacific Plate beneath the China Continent.