Jingsui Yang

Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet

Diamonds and other ultrahigh pressure (UHP) minerals have been reported previously from the Luobusa ophiolite of Tibet, but these minerals have thus far been found only as individual grains. Here we report the occurrence of diamond as an inclusion in OsIr alloy and coesite as part of a silicate assemblage rimming a grain of FeTi alloy, both of which were recovered from chromitite. These occurrences confirm the presence of UHP minerals in the Luobusa chromitite requiring minimum pressures of ∼2.8–4 GPa. Individual coesite “crystals” have external form similar to that of stishovite and are polycrystalline, suggesting pseudo morphic replacement and implying a pressure >9 GPa. We propose that the UHP minerals were incorporated into the chromitites in the deep upper mantle or that they have an impact origin; the preponderance of evidence favors the former.

Two Ultrahigh-Pressure Metamorphic Events Recognized in the Central Orogenic Belt of China: Evidence from the U-Pb Dating of Coesite-Bearing Zircons

A ~4000 km long ultrahigh-pressure metamorphic (UHPM) belt in northern China has been documented on the basis of the discovery of coesite-bearing rocks in the Altun-North Qaidam terrane in the western Central Orogenic Belt (COB), and diamond-bearing rocks in Qinling in the central and Dabie-Sulu terrane in the east. New SIMS and SHRIMP U-Pb dates of zircons from coesitebearing UHPM rocks indicate two UHPM events: one in the early Paleozoic and the other in the Triassic. Coesite-bearing zircons from a North Qaidam gneiss yielded UHP metamorphic ages of 452 ± 13.8 Ma and retrograde ages of 419 ± 6.7 Ma. A diamond-bearing gneiss from Qinling gave a lower intercept age of 502 ± 45 Ma, and an upper intercept age of 1545 ± 100 Ma, whereas a Qinling eclogite sample gave a lower intercept age of 493 ± 170 Ma and an upper intercept age of 1381 ± 82 Ma. The lower and upper intercept ages of the Qinling samples are interpreted as UHPM and protolith ages of the rocks, respectively. Coesite-bearing zircons from a Qinglongshan eclogite in the south Sulu belt yielded early Paleozoic UHPM ages of 441 ± 9 Ma, 449 ± 9 Ma, and 442 ± 9 Ma, whereas the core of a zircon containing plagioclase and apatite inclusions gave a protolith age of 761 ± 13 Ma. These age data suggest that the early Paleozoic UHPM rocks extend from west to east for about 4000 km across the COB, whereas the Triassic UHPM belt extends across the Dabie-Sulu region for about 1000 km. Based on available geochronological and geochemical data, we suggest the following tectonic model for evolution of the COB. At about 1000 Ma, the area was amalgamated to form the Rodinian continent, which contained ophiolitic fragments of oceanic affinity. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and intruded by granitic plutons. Closure of this ocean basin produced Neoproterozoic ophiolites and granitic gneisses. The UHPM rocks, along with subduction-related island-arc volcanics and granites of early Paleozoic age suggest a second cycle of rifting and subduction along the COB, whereas the Triassic UHPM rocks record a final subduction and collision event between the North China and South China blocks.

Petrology and geochronology of eclogites from the western segment of the Altyn Tagh, northwestern China

Abstract The eclogites from the western segment of the Altyn Tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses. The petrographic features and reaction textures testify to three main metamorphic stages: (1) the peak eclogite facies stage (P>15 kbar, T=665–880°C), (2) the granulitic facies stage (P=11–14 kbar, T=670–800°C) and (3) the amphibolite facies stage (P=6.3–9.5 kbar, T=619–738°C) during decompression. The trace element, rare earth element and Sm–Nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type (T-type) mid-ocean ridge basalt (MORB). The well preserved eclogite was selected for Sm–Nd and U–Pb isotopic dating. The Sm–Nd isotopic data yield a whole rock–garnet–omphacite isochron of 500±10 Ma age. The U–Pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia, giving a weighted mean age of 503.9±5.3 Ma. The two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites, and reflects the existence of a mountain root related to continental subduction during Early Paleozoic. Moreover, based on the comparison with similar rocks in the northern margin of the Qaidam basin, a displacement of 350–400 km for the Altyn Tagh fault have been estimated.

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

The deepest rocks known from within Earth are fragments of normal mantle (≈400 km) and metamorphosed sediments (≈350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO2, native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe2+ at very high pressure and consequent low oxygen fugacity (fO2) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO2 environment at the very high temperatures of oceanic spreading centers.

Discovery of eclogite at northern margin of Qaidam Basin, NW China

Eclogite was first discovered at the northern margin of the Qaidam Basin in this study. It occurs as pods in the gneiss sequence of Middle to Upper Proterozoic age and is mainly composed of garnet, omphacite, phengite and rutile. The garnets contain 44%–62% of almandine, 15%–33% of grossular and 12%–30% of pyrope molecules, and the omphacites contain 40%–46% of jadeite. Applying garnet-clinopyroxene thermometry and jadeite geobarometry, the peak conditions of eclogite facies metamorphism occurred at about (722±123)°C and at the pressure of up to c. 22 × 108 Pa.

The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt——Evidence for existence of Caledonian orogenic root

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses characterized by amphibolite-facies mineral parageneses. The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of (500 ± 10) Ma. The U-Pb isotopic measurements of zircons show that the four grain populations are nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 ± 5.3) Ma. Two kinds of methods obtain a similar age which reveals the peak metamorphic age of eclogites, and reflect the existence of orogenic root related to Caledonian subduction and continent-continent collision.

A New HP/LT Metamorphic Terrane in the Northern Altyn Tagh, Western China

Field observations and petrological data suggest a new high-pressure/low temperature (HP/LT) metamorphic terrane in the northern Altyn Tagh. Newly recognized eclogites, blueschists, and metapelites constitute part of a coherent HP/LT metamorphic terrane that occurs as a tectonic slab in an ophiolitic mélange. Geobarometry and geothemometry show that peak metamorphic conditions of the eclogites were T = 430-540°C and P = 20-23 kbar. Petrographic data, P-T estimates, and amphibole compositions indicate distinctly different metamorphic histories for two outcrops about 1.2 km apart. For blueschists from outcrop I, the retrograde P-T path is a relatively straight line directed towards the greenschist facies, suggesting simultaneous cooling and decompression. In contrast, interlayered blueschist and eclogite in outcrop II underwent isothermal decompression from eclogite/blueschist facies to epidote-amphibole facies conditions during exhumation. This implies that rocks from these two outcrops experienced different tectono-thermal histories. Preliminary data regarding geological setting, lithologies, and P-T estimates reveal significant similarities between the HP/LT metamorphic terranes in the north Altyn Tagh and the north Qilian Mountains, and suggest that they may be parts of the same early Paleozoic HP/LT belt, subsequently displaced by the Altyn Tagh fault.

Hematite and magnetite precipitates in olivine from the Sulu peridotite: A result of dehydrogenation-oxidation reaction of mantle olivine?

Abstract Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3-6.6 GPa; 853-957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101]Mt//[001]Ol, [110]Mt//[01̄1]Ol, and [01̄1]Mt//[011]Ol; and [0001]Hm//[100]Ol and [101̄0]Hm//[001]Ol. These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe2+ to Fe3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6-11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

Discovery of khondalite series from the western segment of Altyn Tagh and their petrological and geochronological studies

The khondalite series, which are characterized by aluminum-rich gneisses (schists) consisting of sillimanite-garnet-biotite-monzonite gneiss, garnet-biotite-monzonite gneiss, graphite-sillimanite-biotite schist, and garnet-amphibole two-pyroxene granulites occurring as lenses and layers within gneisses (schists), were discovered in Tula area of western segment of Altyn Tagh. The petrology and geochemistry indicate that the protoliths of aluminum-rich gneisses (schists) are aluminum-rich pelitic and pelitic arenaceous sedimentary rocks, the protoliths of basic granulites are continental tholeiitic basalts. Therefore, the khondalite series may be produced at continental margin. They had suffered granulitic facies metamorphism with peak temperatures of 700–850°C and pressures of 0.8–1.2 GPa. The U-Pb and Pb-Pb isotopic dating of zircons provided the ages of 447–462 Ma representing the ages of peak granulitic metamorphism. The U-Pb dating of detrital zircons from aluminum-rich gneisses yielded older upper intercept ages which reflect the times of older materials derived from source rocks of the gneiss protoliths.

Qusongite (WC): A new mineral

Abstract An unusual group of mantle minerals including about 70-80 species has been recovered from podiform chromitites of the Luobusa ophiolite, Qusong County, Tibet, China. All of the minerals were hand picked from heavy mineral separates of the chromitite. The minerals include diamond, coesite, moissanite, wüstite, intermetallic compounds, Os-Ir alloys with diamond inclusions, Fe-silicides, and a new mineral, qusongite. Qusongite is associated with chromian chlorite, calcite, (W,Ti)C and (Ti,W) C alloys, and chromite. It occurs as angular grains generally 4-8 μm in diameter, but some are as large as 0.2 × 0.3 × 0.25 mm. The grains are opaque and steel-gray with a metallic luster and grayish-yellow reflection. The empirical formula (based on 2 atoms) is W1.006Cr0.02C0.992, and the simplified formula is WC. Qusongite has a hexagonal structure and belongs to space group P6m2, with a = 2.902(1) Å, c = 2.831(1) Å, c:a = 0.9775, V = 20.05 (1) Å3, Z = 1.