V14A-01 16:00h
The Study of Microstructures in UHP Rocks
Minerals with stability fields restricted to very high pressures (e.g. coesite and diamond) are now known from many continental collision zones of many ages. In addition, mineral associations and microstructures suggestive of extremely high pressures, perhaps representing the mantle transition zone or lower mantle, have now been documented from a wide variety of Ultra-High-Pressure metamorphic rocks, both from xenoliths in kimberlites and related volcanic rocks, and from continental collision zones. In this talk, I will discuss many of these microstructures in terms of how reliable may be the inferences of very great depth and some alternative interpretations that might be placed upon them. As additional new and perplexing observations emerge from UHPM terranes, the need will grow for experimental investigations of a wide variety of bulk chemistries representing continental rocks and sediments. We have begun such experiments on decompression of peridotite from conditions of the mantle transition zone; our successes and failures to date in reproducing natural UHP microstructures can be instrumental in designing similar experiments in other systems.
V14A-02 16:20h
Exhumation of UHP rocks: Elastic Deformation or Shearing Creep?
Since 1967, Chinese geophysicists have carried out deep seismic reflection, magnetotellulic, geothermal and potential-field observations in Sulu area where the Chinese continental scientific drilling site is located. In order to perfect interpretation of these data, we have measured physical properties of main rocks in the area at room pressure/temperature laboratories. Geophysical data reveal both external and internal structures along the UHP belt, including the Triassic collisional and subduction structures, occurrence of UHP and HP rocks in the upper crust, eclogite occurrence and magnetization history, multi-cycle ductile shear zones, etc. These features are useful to understand processes of the UHP metamorphic dynamics. The crustal structures in the area are characterized by following facts. (a) Some seismic signatures of Triassic collisional and subduction structures are remained in the current crust, but most seismic signatures illustrate extensional structures formed after the Triassic event. (b) The UHP rock slices only appear within the upper crust and are surrounded by non-UHP rocks, the seismic patterns of the middle and lower crust are typically extensional. (c) Multiple superimposed ductile shear zones densely develop in the crust and coexist with UHP rocks. (d) In the upper crust, UHP and HP rock slumps are coexist and superimposed each other with contact zones mostly the shearing faults. (e) The strike-dip of eclogite bodies is different from the shearing faults. (f) There are two groups of eclogites that have different permanent magnetization directions and magnetization history, possibly implying rotation of the UHP rock slumps. The exhumation dynamics of the UHP rocks can hardly explained by theories based on elastic deformation, which could hardly result the crustal structures mentioned above. On the contrary, the exhumation of the UHP rocks in the Sulu area might strongly relate to rheological creeps, especially complex shearing creep motions occurred in the lithosphere and upper mantle in the late Triassic. As a matter of fact, evidences of viscous flow can be seen not only in the UHP rock samples and cores, but also in the lithospheric structures, such as the ductile shearing faults. Though we have not measured the shear creep compliance of the UHP rocks, we know that these rocks have high yield strength and high fracture energy. However, their enclosing rocks, such as gneisses or most of lower-crust rocks, could act in shearing creep manners with a considerable rate under high temperature and pressure, pushing the UHP rock slices exhumed onto the upper crust.
V14A-03 16:32h
The Origin of Seismic Reflectivity in Ultra High-Pressure Metamorphic Terrain: the Case From Chinese Continental Scientific Drilling
The elastic wave velocities of 700 samples from main hole of Chinese Continental Scientific Drilling (CCSD) have been performed in the conditions at room temperature and pressure. The continuous profile of seismic velocities (Vp and Vs) from 100m to 2000m was constructed, which provides an important petrophysical data for interpretation of geophysical model. Fresh eclogite yields the biggest compressional wave velocity (7.86km/s), and gneiss has smaller velocity (5.53-5.71km/s). Seismic velocity of eclogite significantly decreases with increase of retrogressive metamorphism. Overall average Vp velocity of main hole of 2000m rocks is 6.2km/s, which is correspondent to conclusion with high velocity zone (6.2-6.3km/s) of the upper crust in Dabie-Sulu orogenic belt inferred from geophysical methods. Most rocks at the main hole have pronounced seismic anisotropy. The velocity of Vp and Vs with water-saturated rock increases by a factor of 19% and 10%, respectively. Conversely, anisotropy of Vp and Vs with water-saturated rocks reduces 3%-4%. Reflective coefficient (Rc) of different lithological boundaries is the main factor producing seismic reflection. The lithological boundaries between rutile-bearing eclogite and gneiss have the biggest reflective coefficient (0.24-0.31). The boundaries between fresh eclogite and retrogressive eclogite is characterized by strong reflection coefficient (Rc=0.27). Reflection coefficient of paragneiss contacting with fresh eclogite is 0.24. The margin between paragneiss and orthgneiss has the lowest seismic coefficient (Rc=0.02) if the rocks did not experience strong ductile shearing. The mylonitized gneiss in ductile shear zone is a good seismic reflector. P-wave anisotropy of gneiss increase with strengthening mylonitization. The measurements of elastic wave velocity in this study provide an important constraint on the origin of seismic reflection of Ultrahigh-pressure metamorphic terrain in Chinese Continental Scientific Drilling.
V14A-04 16:44h
Re-interpretation of Progressive Metamorphism, Facies Series, P-T-t Path, and Exhumation Model for Global Collisional Orogenic Belts
Recent discovery of ultrahigh (UHP) minerals, and descriptions of geology, petrology, geochronology and geochemistry of the global collisional orogens leads us to re-interpret some basic concepts of orogeny. (1) A collisional belt consists of a thin tectonic slice with P-T maximum at structural intermediate. The UHP-HP unit is a thin slab up to a few km thick and bounded on the top and bottom by normal and reverse faults, respectively, and separated from the low-grade or low-P metamorphic rocks. (2) The underlying unit is thermally metamorphosed to form andalusite-sillimanite metamorphic rocks in some terranes by the tectonic intrusion of UHP-HP rocks. (3) Metamorphic facies series ranges from the greenschist/blueschist transition or the blueschist, through epidote-amphibolite, quartz- and zoisite-eclogite, to dry eclogite facies with a sharp kink point, indicating anti-clockwise P-T path. This facies series is consistent with the calculated P-T path by inclusion mineralogy in garnet combined with compositions of zoned garnet and paragenesis of mineral inclusions in zircon. (5) The extensive hydration at mid-crustal level obliterated the pre-existing UHP-HP mineralogy, except for mineral inclusions in garnet, zircon and omphacite. (6) Zoned zircons with UHP-HP minerals mantled by late-stage hydration stage with Barrovian minerals from the Kokchetav Massif are dated as ca. 30 m.y. older from that of hydration stage at rim, indicating a slow tectonic exhumation. (7) Combining above constraints, a tectonic extrusion model is most probable (Maruyama, 1990; Maruyama & Liou, 1994; Maruyama et al., 1996). (8) Previous interpretation (e.g., England and Thompson, 1984) of progressive metamorphism, facies series, P-T-t path are all quite different from those we summarized. Most of the observed assemblages exhibit the late-stage crustal metamorphism due to extensive hydration and retrograde recrystallization. The role of water must be re-evaluated to mask the progressive nature of metamorphism during subduction.
V14A-05 16:56h
Tracing The Boundary Between UHP And HP Metamorphic Belts In The Southwestern Sulu Terrane, Eastern China: Evidence From Mineral Inclusions In Zircons Of Metamorphic Rocks
The southwestern Sulu terrane, eastern China is divided into four fault-bounded lithological units. From northwest to southeast, they are orthogneiss unit, supracrustal rock unit, kyanite-bearing quartzite-marble unit and paragneiss-schist unit. Inclusions of index minerals in zircon separates from more than 90 samples from these units were identified using laser Raman spectroscopy and electron microprobe analysis. Coesite and coesite-bearing ultrahigh-pressure mineral assemblages occur in zircon separates from various amphibolite-facies metamorphic rocks in the orthogneiss and supracrustal rock units. Most coesite-bearing zircons contain the following UHP mineral assemblages: coesite + garnet + omphacite + phengite and coesite + garnet + jadeite + rutile + apatite in the paragneiss, coesite + kyanite + rutile + apatite in the kyanite-bearing quartzite, coesite + aragonite in the marble and coesite + phengite in the orthogneiss. Whereas aragonite- and phengite-bearing high-pressure mineral assemblages are preserved in zircons from many amphibolite- to greenschist-facies metamorphic rocks in kyanite-bearing quartzite-marble and paragneiss-schist units. The index HP mineral inclusion assemblages are characterized by aragonite + garnet + phengite + quartz and garnet + phengite + albite + quartz in the paragneiss, kyanite + phengite + quartz in the kyanite-bearing quartzite, aragonite + quartz in the marble, and phengite + quartz + apatite in the orthogneiss. The spatial distribution of the pressure and temperature estimates and recent surface mapping result constrain an exact boundary between UHP and HP belts in the southern Sulu terrane along a ductile shear zones in the Donghai region.
V14A-06 INVITED 17:08h
Garnet peridotites from the Sulu UHP terrane, China: A window insights into mantle and subduction processes
Garnet peridotites from the Sulu UHP terrane of China occur as block and lens within granulite-amphibolite facies gneiss and contain lenses of clinopyroxenite and coesite-bearing eclogite. In addition to the assemblage Ol + Opx + Cpx + Grt, some peridotites contain additional hydrous phases (e.g., phlogopite, Ti-clinohumite and pargasite) and magnesite. The abundances of "fertile elements" (TiO$_{2}$, Al$_{2}$O$_{3}$, CaO and FeO) of the Sulu peridotites are lower than those of the primitive mantle and show negative correlation with MgO content. The garnet peridotites and pyroxenites preserved mantle $\delta$$^{18}$O values: +4.8 to +5.7 for Grt, +4.7 to +5.4 for Ol, +4.5 to 5.6 for Cpx and +5.7 to +5.8 $\permil$ for Opx. These characteristics suggest that the Sulu peridotites represent pieces of a depleted, residual mantle. Most peridotites with LREE-enrichment patterns, high $^{87}$Sr/$^{86}$Sr (0.7071-0.7100) and low $^{143}$Nd/$^{144}$Nd ratios (0.5122-0.5128) have been subjected to the effects of mantle metasomatism and crustal contamination. In Donghai, the site area of the Chinese Continental Scientific Drelling (CCSD), peridotites with or without magnesite are enriched in Th, U, Sr, Ba and LREE, and show Hf and Ti negative anomalies; these may be attributed to carbonatitic metasomatism. Zircon separates from peridotitic cores of the PP1 drill hole show concentric zoning without inherited cores indicating a metamorphic origin; they yield SHRIMP U-Pb age of 221 $\pm$ 3 Ma (weighted mean), consistent with the Dabie-Sulu regional UHP metamorphism. Polymorph transformation (OREN to CLEN) of orthopyroxene and exsolution microstructures in peridotitic minerals are common; these include Ilm in olivine, Qtz, Ilm, Phl and Grt + Ilm $\pm$ Amp in diopside, and Rt $\pm$ Cpx + Ap in garnet. These petrochemical and age data, microstructures and P-T estimates conclude the mantle-derived Sulu garnet peridotites were inserted into a Triassic continental subduction zone and experienced in situ UHP metamorphism together with subducted slab at 750-1000$\deg$C and 4-7 GPa with an extremely low thermal gradient of $<$ $5\deg$C/km.
V14A-07 17:20h
Unusual garnet-clinopyroxenite core from the main hole of the Chinese Continental Scientific Drilling Project in Donghai
The Chinese Continental Scientific Drilling (CCSD) in Donghai has reached 4360m depth in September, 2004 and recovered more than 80% core samples for scientific investigation. These core samples include both para- and ortho-gneisses with less than 25% eclogitic and garnet peridotitic rocks. Inclusions of coesite and coesite pseudomorph occur in garnet and omphacite of some eclogites and in zircon separates from many gneisses, eclogites and amphibolites; their occurrences suggest in-situ UHP metamorphism. One moderately-oriented garnet-clinopyroxenite (558.65m) occurs as thin layer within rutile-rich eclogites (90m thick) and contains abundant ilmenite (15vol%) together with granoblastic garnet (Pyr$_{29-32}$Alm$_{42-47}$Grs$_{16-21}$Spr$_{1-2}$Adr$_{2-7}$; X$_{Mg}$= 0.37-0.42), clinopyroxene (Jd$_{6-13}$Ac$_{13-24}$Aug$_{67-73}$CaTs$_{<4}$Opx$_{<3}$; X$_{Mg}$= 0.88-0.97) and minor amount of iron sulfide. Ilmenite occurs as either coarse-grained granoblast ($\sim$3mm) in matrix or inclusions in garnet and clinopyroxene; matrix ilmenite contains inclusions of garnet and clinopyroxene. Both matrix and inclusion ilmenite exhibit coarse exsolution lamellae (20-80$\mu$m) of hemoilmenite (Ilm$_{71-80}$) and ilmenohematite (Ilm$_{14-18}$); the hemoilmenite and ilmenohematite themselves carry further fine-scale exsolution lamellae of hematite and ilmenite ($<$2$\m$um), respectively. These textural evidences indicate that pre-exsolved ilmenite-hematite solid solution was in equilibrium with garnet and clinopyroxene. Considering those petrologic features and bulk compositions of surrounding eclogite cores, the garnet-clinopyroxenite may initially have been a titanomagnetite-rich ultramafic layer in a coherent Fe-Ti gabbro sequence. Original titanomagnetite may have been oxidized and recrystallized to form Fe$^{3+}$-rich ilmenite at an UHP condition, and subsequent cooling during exhumation exsolved hematite from ilmenite. The local ultramafic composition may have allowed the occurrence of ilmenite instead of rutile, and oxidation may play an important role to accommodate significant amounts of acmite (Ac$>$Jd) in clinopyroxene and andradite ($\sim$37mol%) in garnet.
V14A-08 17:32h
Ultra-high Pressure Minerals in Ophiolites
Most ultra-high pressure (UHP) minerals are found in kimberlites, deeply subducted continental crustal rocks and meteorites or meteorite impact craters. Graphitized diamonds have been found in subcontinental peridotites of the Ronda and Beni Boursera massifs. Numerous UHP minerals have also been reported from ophiolites, particularly in Russia, Indonesia, Canada, the USA and China. Many of these reported discoveries are from placer deposits and the origin of the UHP minerals has not been confirmed. The best documented occurrence of an UHP mineral-bearing ophiolite is the Luobusa ophiolite, which lies in the Yarlung-Zangbo suture zone of southern Tibet. A wide variety of UHP minerals, including diamond, moissanite, coesite, Fe-silicides, wüstite, silicon rutile, silicon spinel, and CrC alloys, has been recovered from podiform chromitites in Luobusa. These minerals are associated with native elements, such as Si, Fe, Ti, and Cr, and with PGE alloys, some of which may also have an UHP origin. Diamonds have also been reported from the Donqiao ophiolite of the Nujiang-Bangong Lake suture zone in central Tibet but this occurrence as not been confirmed. The demonstrated occurrence of UHP minerals in Luobusa and the reported occurrences from other bodies suggest that ophiolites may be common repositories of UHP minerals.
V14A-09 17:44h
Ultra-high pressure and associated minerals separated from eclogite and garnet peridotite sampled in the CCSD Main Hole
The Chinese Continental Scientific Drilling project (CCSD) is currently drilling a projected 5000-m-deep hole on the Sulu UHP metamorphic slab, which has now reached a depth of about 4400 m. Cuttings of eclogite and garnet peridotite from the CCSD main hole were collected for mineral studies. Minerals were separated by a variety of standard techniques and handpicked under a binocular microscope. Mineral identification was by a combination of microprobe, Laser Raman, and X-ray techniques. Thus far, over 50 individual minerals have been identified, including silicates, oxides, sulfides, carbonates, native elements, and metallic alloys. Numerous ultra-high pressure (UHP) minerals have been identified, such as diamond in eclogite, coesite in eclogite, moissanite, and w"1stite. Other phases include native Fe, Cr, Ni, Au, Cu, and Al, taenite, kamacite, Ni-Fe-Cr alloy, Ni-Fe alloy, Fe-Cr alloy, Si-Fe alloy (Fe3Si7), Co-Ni alloy, chalcopyrite, sphalerite, galena, limonite, titanomagnetite and kyanite. Common silicate minerals, such as olivine, pyroxene, garnet, rutile, phlogopite, and some alteration minerals, are also present. Most of the identified minerals are attached to, or included in, other minerals suggesting that they are natural phases, not contaminants. Some of the minerals, e.g., badeleyite, native Fe, taenite, Fe-Ni alloy, and magnetite are spheroidal, suggesting that they formed as liquid droplets from high-temperature melts. These minerals may have formed from mixtures of mantle and core materials. The minerals separated from the eclogites and peridotites of the CCSD-MH are comparable to those collected from the Luobusa ophiolite, Tibet, as well as those from Alpine-type ophiolites in the South Urals. However, the eclogites and peridotites of the CCSD-MH lack PGE minerals, indicating that the mantle rocks in the various localities had different origins.