V13C-01 13:40h
Chinese Continental Scientific Drilling (CCSD) Project: Progresses and Perspectives
By employing modern deep drilling techniques, CCSD project is going to drill a 5000 m borehole at the Sulu ultrahigh pressure metamorphic belt. The scientific objectives are to reveal the 3-D deep materials constitutions of convergent plate margins, to improve our understanding the physical and chemical processes that are responsible for the formation and evolution of UHP metamorphic terrains worldwide and finally to establish a deep laboratory for long-term observation on present-day crustal activities. The main hole was drilled to a depth of 4400m and 3 satellite holes have been completed. Many rock types including the UHP eclogite, gneiss, schist, quartzite, mylonite and garnet-peridotite, and large numbers of petrological boundaries have been revealed. Various logging profiles (e.g. petrology and geochemistry)up to 4000 meters long have been established. Some important scientific progresses from CCSD cores and field investigations have been obtained: (1) coesite and other UHP mineral inclusions are found widely in the gneisses, schists, amphibolites, marble, quartzite, and eclogites from the CCSD borehole, satellite holes and surface, which indicate that a large amount of material has been subducted to a great depth (> 100km); (2) 5 tectonic units and major shear boundaries are well-defined in the first 2000m long, detailed and orientated tectonic profile. Geochemical data and the early structures suggest that a rifting setting at a passive continental margin for the formation of the protoliths that have experienced HP-UHP metamorphism; (3) garnet peridotites from PP1 have olivine slip system (100)[001], which is similar to that of the Alps Arami ultramafic body, Central Alps, suggesting a LT/UHP condition. Omphacite slip system (100)[010] from eclogites of CCSD borehole also implies a LT/UHP condition; (4) the elasticity wave and thermal conductivity profiles have provided an important petrophysical constraints on the seismic reflection and geothermal structure. The VSP seismic profile reveals the deep structure of the tectonic slices, and indicates a close relationships between the strong seismic reflection layers and large ductile shear zones; (5) UHP peak metamorphic age (240-220Ma) and amphibolite facies retrograde ages (220-200Ma) are determined by the SHRIMP U-Pb dating of zircons from coesite-bearing paragneiss and orthogneiss. Ar/Ar ages for the southern Sulu HP metamorphic rocks indicate that exhumation started at 250-240 Ma, earlier than that of the north Sulu UHP rocks (220-200Ma). We propose a new model for diachronous subduction and exhumation of the Sulu HP-UHP Metamorphic Belt; (6) the seismic reflective and the tomographic profiles across the Sulu UH-UHP metamorphic belt reveal that the Sulu HP-UHP metamorphic belt occurs as an arched and eroded slab, translucent reflection body of low velocity and low density reflectors, and the Tanglu fault and Xiangshui fault probably occur as mantle shear zones; (7) many CH4, CO2 and He anomalies are found in the borehole, and these anomalies are more abundant with depth; (8) bacteria and archaea were discovered in eclogite, amphibolite, and gneiss at depth of 529, 730, 1080, 1179, 1930, 2026 m, respectively. Microbes living in extreme conditions are present in the drilling cores identified by DNA analysis; and (9) heat producing element (HPE) analyses show that the first 2000 m borehole has a stepwise or sandwich-like HPE distribution resulted from the juxtaposition of low HPE mafic to ultramafic rocks with high HPE felsic rocks.
V13C-02 14:00h
UHP Metamorphism and Continent Subduction/Collision - An Overview
Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural modification of predominantly continental and minor oceanic crustal protoliths and associated mafic-ultramafic rocks at P greater than $\sim$ 2.7 GPa ($>$ 80-90 km). Typical products include eclogite, garnet peridotite and associated metapelite, quartzite, marble, and gneisses. For appropriate bulk compositions, subduction-zone metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals. Thus far, at least 15 coesite-bearing eclogitic and 7 diamond-bearing UHP belts have been documented in the world; most lie within major continental collision zone in Eurasia. UHP metamorphic assemblages occur in long, discontinuous belts which may extend several hundred km or more and 10 to 50 km wide, and are associated with HP blueschist belts. Recent petrological data suggest that contrary to the long-held orthodoxy, low temperatures at very high pressures (the so-called 'forbidden zone') are realized in the Earth. A considerable number of collisional orogens also contain relics of coesite-grade metamorphism. They may have been overlooked due pervasive late-stage overprinting and Barrovian-type recrystallization during uplift. In such cases inclusions in zircon offer the best means to unravel the early UHP history. Majoritic garnet and HP clinoenstatite occur in garnet peridotites of some UHP terranes; supracrustal rocks together with the enclosing mafic-ultramafic rocks have experienced subduction zone UHP metamorphism. The discovery of these ultramafic denizens of the forbidden zone confirms cold subduction-zone models. These cold subduction zones are the sites of major recycling of water into the mantle, for HP experiments reveal that numerous, important hydrous phases are stable in the P-T forbidden zone. Research in the past 20 years on UHP rocks containing coesite and diamond has produced a dramatic restructuring of our understanding of plate tectonics. The discovery of garnet peridotites from the forbidden zone provides a revolutionary new window into the subduction of continental margins, the thermal structure of subduction zones, and the recycling of volatiles into the mantle.
V13C-03 14:12h
The Chinese Continental Drilling Program: Geochemical and Isotopic (O, Sr, Nd, Pb), Investigations on a Vertical Sequence of UHP metamorphism
Various UHP metamorphic rocks - orthogneiss, paragneiss, eclogite, amphibolite, ultra-mafic rocks, and rare schist and quartzite - have been drilled within the main hole of the Chinese Continental Drilling Program (CCSD) in Donghai, Sulu. Petrographic studies indicate that all the rocks have been subjected to UHP metamorphism. R-mode factor analysis vs. depth, combined with rock associations and lithologies, indicate that the 2050-m-thick slab can be divided into 6 units in order of increasing depth. Geochemical characteristics of eclogites from different units suggest that their protoliths may be of diverse origins: eclogites from unit 1(0-530m) are probably of cumulate origin being similar to Bixiling and Maowu in Dabie Shan; the protolith of the eclogite from unit 2 (530-600m) is a Fe-Ti-rich gabbroic rock-body, whereas that of the eclogite within ultramafic rocks from unit 3 (600-690m) is of mantle origin; eclogites fom unit 4 (690-1160m) and unit 6 (1600-2050m) that interlayered with paragneiss are metamorphic supracrustal rocks. Geochemical characteristics of orthogneiss (unit 5, 1160-1600m) and paragneiss suggest that their protoliths are probably of granitic and of supracrustal sedimentary origins, respectively. Oxygen isotope data indicate that eclogites and gneiss from depths above 1600m of the main hole have depleted bulk d18O values of -5.3 to 4.5%, indicating significant meteoric water/rock interaction; by contrast, samples from depths between 1600-2050m show bulk d18O values greater than 5.8%, without any indication of a meteoric water/rock interaction. This suggests an "in situ" origin and structural coherence throughout subduction and exhumation. The lowest d18O values occur at depths of 1100-1200m, which corresponds to the boundary between eclogites and the about 400m-thick ortho- (granitic) gneiss layer (boundary between unit 4 and unit 5; this implies that the intrusion of the granitic body could be a heat source for initiating meteoric water/rock interaction. Correlations in Sr-Nd-Pb isotopic ratios among the UHP drill cores, Mesozoic and Cenozoic volcanics from the North China craton, and depleted MORB mantle under eastern China suggest interactions between the UHP rocks (formerly subducted continental crust) and the eastern China mantle. This conclusion from radiogenic isotopes contrasts sharply with the oxygen isotope data, which suggest very limited crust/mantle interaction.
V13C-04 14:24h
Mineral and Fluid Inclusions in Zircon of UHP Metamorphic Rocks from the CCSD-Main Hole: a Record of Metamorphic Evolution and Fluid Activity
The recovered cores from depths of 0-3000 m of the Chinese Continental Scientific Drilling (CCSD) Main Hole in Donghai, southern Sulu, comprise of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Zircon grains ranging from 50 to 300 micrometers in size from all rock types except ultramafic rocks were investigated by Raman, cathodoluminescence, and microprobe analysis. Based on their growth structure and mineral inclusion assemblages, zircons were divided into three types. Type I has an oscillatory zoned magmatic core with low-P mineral inclusions of Qtz, Pl and Ap, and a metamorphic rim with uniform luminescence and UHP inclusions of Grt, Omp, Phn, Coe and Rt. This type occurs in most eclogites and orthogneisses, and minor schists, suggesting that the host rocks have been derived from intrusive rocks, which have been subjected to UHP metamorphism. Type II is generally unzoned, and occurs in a few eclogites, paragneisses and schists. The cores of the zircons contain UHP inclusions of Grt, Omp, Phn, Coe and Rt, whereas the rims have HP inclusions of Grt, Omp and Qtz, suggesting that they are of metamorphic origin. The third type of zircons occurs only in some orthogneisses, and show euhedral, oscillatory zonations without a core-rim structure, typical for a magmatic origin; in these zircons only low-P mineral inclusions were found, implying that the host rock did not experience UHP metamorphism. In a few cases, magmatic cores of type I zircons contain coesite inclusions, which implies that former magmatic quartz has been transformed to coesite during UHP metamorphism. In addition, polycrystalline quartz pseudomorph inclusions after coesite and symplectite after omphacite were observed in the metamorphic rim, suggesting that UHP mineral inclusions have been replaced by low-P minerals during retrogression. Abundant H2O-CO2, H2O- or CO2-dominanted fluid inclusions occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These primary fluid inclusions should have been trapped during magmatic crystallization. Rarely fluid inclusions occur together with UHP mineral inclusions in zircons of metamorphic origin. They have similar compositions to those within the magmatic cores of zircons. These data suggest that the UHP metamorphism occurred under relatively dry conditions and as a closed fluid system. Fluid components during UHP metamorphism were obviously inherited from their protoliths. This work is sponsored by 973-project 2003CB716501 and NSFC-40399142.
V13C-05 14:36h
The metamorphic evolution of the eclogitic rocks from the 100-2000m core of Chinese Continental Scientific Drilling, China
On the basis of petrographic study of the metamorphic rocks from 100-2000m in the main hole of the Chinese Continental Scientific Drilling (CCSD) it is revealed that the major lithological types are: (1)Eclogites and garnet clinopyroxenites; (2)Eclogitic gneisses; (3)Garnet peridotites; (4)Biotite (hornblende) two-feldspar gneiss (granitic gneiss) and (5) Fault breccia and mylonites. Most of the eclogites and eclogitic gneisses had a long residence history in the crust before the deep subduction of the continental slab. However the garnet clinopyroxenites are closely related with garnet peridotites which occur within 607-783m of the drill hole. Based on their mineralogical and petrochemical characteristics they were ultramafic-mafic complexes incorporated into the subducted slab from the overlying mantle wedge during the subduction of the continental slab and had undergone ultrahigh"Cpressure metamorphism (UHPM). The eclogitic gneisses, equivalent partly to paragneiss as called by some geologists, are UHPM rocks of intermediate acidic protolith associated with crustal eclogites. The granitic gneisses are of diverse origin, most of them are products of decompressive melting of the retrogressive amphibolite and biotite gneiss derived from eclogitic rocks. The metamorphic evolution of the eclogitic rocks can be divided into 3 major stages. The first (M1) is the ultrahigh pressure metamorphism (UHPM) as evidenced by coesite inclusions within garnet and omphacite grains in eclogites. The second (M2) is a retrogressive stage sequence during which most of the UHPM rocks were turned to rocks of high-pressure eclogite facies, amphibolite facies and then epidote amphibolite facies. At this stage the eclogites are characterized by the growth of symplectites and coronas. The eclogitic gneiss is retrograded to be biotite (hornblende) plagioclase gneiss and epidote biotite (amphibole) gneiss. The granitic rocks are turned to biotite hornblende two feldspar gneiss (so-call orthogneiss) due to partial melting or K-metasomatism of the HP supercritical fluid. Some of them are allanite (Ce)"Cbearing, the La/Ce=0.42ͦ0.72. The allanite are zonal and are rimmed by epidote, representing the later overprint of epidote amphibolite facies retrometamorphism. The third stage (M3) is characterized by the formation of tectonic breccia and mylonites in response to the brittle and ductile-brittle deformation related with uplift. The matrix of these tectonites containing chlorite, actinolite and calcite indicates the greenschist facies of metamorphism (M3). All of the above-mentioned 3 stages of metamorphism are records of the tectonic processes surpassed by these eclogitic rocks. They delineate a clock-wise metamorphic PTt D path of the Donghai UHP terrane which is comparable in pattern with those of Dabieshan. The metamorphic evolution and the successive deformational events found in the 100ͦ2000m core from the CCSD main hole confirms that during the collision of the two (Yangtze and North China) plates voluminous crustal materials including granitic rocks can deeply be subducted to mantle depth and then rapidly returned back to the earth surface. Acknowledgement: This work is sponsored by the Major State Key Project to CCSD: 2003CB716501 and major project from NSFC: 40399142.
V13C-06 14:48h
HP/UHP Eclogites From the Hong'an Block, China: Isotope Disequilibrium and Geochronological Controversy
The Hong'an Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist-greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The Block appears to have preserved excellent archives of tectonic evolution of the Dabie Orogen. Geochronology plays a crucial role in our understanding of the processes but the published age data have shown much complexity as a result of the lack of isotopic equilibrium during the metamorphic recrystallization (ca. 700 to 500°C). In comparison with the Sulu and Dabie terranes, the metamorphic temperatures in the Hong'an Block are lower by 50 to 150°C. New trace element and Rb-Sr, Sm-Nd and O isotopic analyses on minerals provide much insight into the geochronological complexity. The present results indicate that: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g. epidote) in garnet and omphacite has further contributed to the isotope disequilibrium. (2) Sm-Nd isotope analyses yielded no isochron ages for the Hong'an eclogites. (3) Rb-Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established. However, the pattern of Rb-Sr isotope disequilibrium is random and independent of the petrological and O-isotope temperatures. The established isochrons yield ages from 210 to 225 Ma. (4) In contrast to the disequilibrated Sm-Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature from the mass balance consideration. (5) Whole-rock d18O values show a large variation from +10% to -8%, suggesting that their protoliths have undergone very different processes of water-rock interaction. (6) In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hong'an Block took place in the Triassic at about 220-230 Ma. This is identical to that observed in the Dabie and Sulu terranes. A variety of Paleozoic dates (450 to 300 Ma) reported for the Xiongdian eclogite are still difficult to understand. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are beyond our comprehension.
V13C-07 15:00h
A study of oxygen isotopes and hydroxyl content in minerals of UHP metamorphic rocks from CCSD core samples (734 to 933 m)
Two continuous core segments between eclogite and gneiss from the main hole of CCSD project, at the depths of 734.21 to 737.16m (02CCSD-I) and 929.67 to 932.86m (02CCSD-II), respectively, are systematically analyzed for oxygen isotopes by the laser fluorination technique and for hydroxyl (OH) content in nominally anhydrous minerals by the Fourier transform infrared spectroscopy (FTIR). Oxygen isotope analysis shows that garnet from eclogites in the two core segments has d18O values of +1.0 to +3.8͉ and -4.1 to -3.3͉, respectively, and that garnet from gneisses in the two core segments has d18O values as low as -4.0͉. These suggest that protoliths of both eclogite and gneiss experienced meteoric-hydrothermal alteration before continental subduction. An oxygen isotope front is observed in the contact zone between eclogite and gneiss in the core segment 02CCSD-I. This is interpreted as a result of fluid-rock interaction between eclogite near the contact zone and fluid mainly from gneiss nearby, which is controlled by fluid transport distance. The FTIR analysis shows concentrations of hydroxyl in 50 to 776 ppm for garnet and in 321 to 1636 ppm for omphacite. For quartz, we have only found the characteristic band of hydroxyl at 3596cm-1 but its hydroxyl content is very low. A profile-comparison of oxygen isotope ratio and hydroxyl content shows a consistent change with depth between d18O value and OH content in garnet, omphacite and other minerals. This suggests that retrograde alteration and associated local fluid flow may be the main cause for the observations that the ultrahigh- and high-pressure minerals deviate from oxygen isotope equilibrium fractionations and the degree of disequilibrium is controlled by local water/rock ratios. Therefore, the significant amounts of hydroxyl in garnet and omphacite, together with other nominally anhydrous minerals, may constitute a major source of retrograde fluid by releasing water during decompression exhumation.
V13C-08 15:12h
Length-Scales of Oxygen, Hydrogen, and Argon Isotope Exchange During UHP Metamorphism: an Example From the Qinglongshan UHP Eclogites (Sulu Terrain, China)
New and previously published stable isotope (oxygen and hydrogen) and $^{40}$Ar/$^{39}$Ar data are compared from selected outcrops of the Sulu UHP terrain near Qinglongshan (Jiangsu Province, China). These rocks retain unusually low and heterogeneous $\delta$$^{18}$O and $\delta$D values acquired in a Neoproterozoic geothermal system despite undergoing Triassic (220-240 Ma) UHP metamorphism. Incremental heating $^{40}$Ar/$^{39}$Ar analyses of muscovite, biotite, and K-feldspar from metagranite, quartzite, and gneiss (all metamorphosed to the coesite-eclogite facies) yield cooling ages between 190 and 204 Ma. In contrast, phengite $^{40}$Ar/$^{39}$Ar data from eclogite, quartzite, and gneiss contain variable amounts of extraneous argon, consistent with inheritance from a Neoproterozoic protolith. Plots comparing phengite $\delta$$^{18}$O, $\delta$D, and $^{40}$Ar/$^{39}$Ar total gas ages from from different lithologies within individual Qinglongshan outcrops only meters apart highlight significant inter-outcrop isotopic heterogeneities ($\Delta$D$_{phengite}$ = 26$\permil$; $\Delta$$^{18}O$_{phengite}$ = 8.8$\permil$; $\Delta$$^{40}$Ar/$^{39}$Ar$_{phengite}$ = 664 Ma), however maximum intra-outcrop isotopic variations between lithologies are limited ($\Delta$D$_{phengite}$ = 5$\permil$; $\Delta$$^{18}O$_{phengite}$ = 1.8$\permil$). The oxygen and hydrogen isotopic variations are interpreted to reflect primary isotopic heterogeneities acquired during Neoproterozoic hydrothermal fluid circulation with cold-climate meteoric waters. The limited intra-outcrop isotopic variations suggest extensive isotopic exchange occurred during UHP metamorphism within discrete outcrops, irrespective of lithology. Likewise, extraneous argon within the phengites reflects inheritance from a Neoproterozoic protolith, and the age variations between outcrops is probably due to differential argon loss during thermal and baric equilibration accompanying differential exhumation following UHP metamorphism. The retentivity of extraneous argon in phengite is partially controlled by the host rock lithology; for example the armoring effects of basaltic eclogite are greater than quartzite or gneiss. Collectively these data indicate that, while extensive, isotopic exchange during subduction and UHP metamorphism was limited to contiguous blocks of coherent subducted crust with each block preserving small isotopic heterogeneities acquired from their protoliths roughly 0.5 Ga previously. Such length-scales of isotopic exchange, defined by the size of a contiguous outcrop, underscore the closed system isotopic behavior that can occur during continental subduction, collision, and uplift from UHP conditions.
V13C-09 15:24h
Microbial Communities in Ultra-High Pressure Rocks and Fluids From Chinese Continental Scientific Drilling (CCSD): A Unique Opportunity to Study Microbial Adaptation and Survival
A major obstacle to understanding the subsurface biosphere has been our limited ability to access the deep subsurface, to acquire uncontaminated samples and to place our knowledge of isolated microorganisms into environmental context. We studied deep subsurface microbiology by taking an advantage of the Chinese continental scientific drilling (CCSD) project currently underway in China. The project is to drill a 5-km deep borehole in the Dabie-Sulu ultra high-pressure (UHP) metamorphic belt in China that is located at the convergent plate boundary between Sino-Korean and Yangtze Plate. The collision began at ~240 Ma ago followed by exhumation ~220 Ma ago. The products of such a plate convergence are the formation of unique UHP rocks and minerals. These rocks are typically separated by a series of structurally weak shear zones and faults. The macroscopic shear zones/faults are potential storage space for large pockets of fluids/gases and they may serve as a potential microbial habitat. Fluids/gas bubbles also exist inside minerals, and they are called fluid/gas inclusions. The inclusions are microscopic and they serve as another potential habitat for microbes. The 5-km contiguous drilling intercepts both habitats and spans a range of environmental gradients. Our cultivation and SSU rRNA gene analyses appear to indicate that unique microbial communities may exist in both habitats. A variety of methods were used to assess possible contamination, and contamination was minimal. Acridine orange direct count method was employed to determine the total number of cells in the rocks, and the results indicated that the biomass ranged from 5.2 x 103 to 2.4 x 104 cells/g (dry weight). Total counts indicated a much higher biomass in the drilling fluids, ranging from 3.5 x 108 to 4.2 x 109 cells/g (wet weight). The PLFA profiles for one rock and multiple drilling fluids indicated the presence of sulfate and metal reducers. Cultivation attempts have identified the presence of mesophilic Fe(III) reducers in the rocks, but thermophilic (37 to 68oC) and alkaliphilic metal reducers and fermenters in the drilling fluids. SSU rRNA gene analyses detected clone sequences in the rocks that have previously been isolated from cold, alkaline and saline environments (including mesophilic, facultative, heterotrophic, halotolerant or halophilic nitrate and Fe(III) reducers). These microbial growth habitats are inconsistent with the present day geochemical conditions (geothermal gradient, for example). We speculate that these microbes reside in mineral fluid/gas inclusions. Because the inclusions are isolated and heat conductivity is low, microenvironment inside the inclusions may be out of equilibrium with the bulk subsurface conditions. The microbial communities in the drilling fluids include anaerobic, alkaliphilic, chemoorganotrophic or chemolithoautotrophic, halotolerant or halophilic Fe(III) and sulfate reducers, fermenters, acetogens, and methanogens/methanotrophs. This microbial growth habitat suggests that the detected microbes in the drilling fluids may be of different origin, and they may be derived from macroscopic fluids/gases associated with structurally weak shear zones/faults. Because of possible connectivity to flow channels and shear zones, these fluids/gases may be in equilibrium with the in-situ subsurface conditions, and microbes from this habitat are expected to change in environmental gradients.