Yang Jingsui

Granitic Magmatism in the North Qaidam Early Paleozoic Ultrahigh-Pressure Metamorphic Belt, Northwest China

Three suites of granitic rocks are present in the early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt, which has eclogite ages of 466-495 Ma. The first suite consists of quartz monzodiorite + granodiorite + monzogranite, the second of monzogranite + two-mica granite + muscovite granite + syenogranite, and the third of granodiorite + monzogranite + biotite granite. The country rocks into which these bodies were intruded consist chiefly of medium- and high-grade Proterozoic gneisses and schists and Paleozoic sedimentary and volcanic rocks. Monzogranite of the first suite has a zircon SHRIMP age of 473 Ma, whereas syenogranite in the second suite has an age of 446 Ma, and granodiorite in the third suite has an age of 397 Ma. Rocks of the first and third suites have similar mineralogies consisting of plagioclase, hornblende, quartz, alkali feldspar, and biotite, whereas those of the second suite consist of potassium feldspar, quartz, muscovite, biotite, and plagioclase. Rocks of the first and third suites have I-type compositions with SiO2 = 61-69 wt%, Na2O/K2O >1, ANK <1, and Eu/Eu* = 0.7-1.0, whereas the second suite consists of S-type granites, with SiO2 = 70-76 wt%, Na2O/K2O <1, ANK >1, and Eu/Eu* = 0.1-0.3. The first suite is interpreted as island arc or active continental margin magmatism, the second as igneous activity related to collision between the Qaidam and Qilian blocks, and the third as post-collisional plutonic activity.

The Discovery of Diamonds in Chromitites of the Hegenshan Ophiolite, Inner Mongolia, China

Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr# [100Cr/(Cr+Al)] of about 47–53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.

Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane, eastern China

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz+Phe+Ap+impurities and Qtz+Phe+impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe+Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe+Ap and Coe+Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP meta-morphic stage and overprinted by the late retrogressive metamorphism related to the Sulu ter-rane exhumation. SHRIMP U-Pb dating reveals that zircons separated from the paragneisses recorded complicated geochronological traces. Some zircons retain inherited cores with the 206Pb/238U ages of 284–754 Ma, indicating the complexity of the detrital zircons in protoliths. Coesite-bearing zircon domains recorded 238–266 Ma (with the average age of 245±14 Ma) for the UHP metamorphic event. The retrogressive rims of zircons recorded 213–223 Ma (with the average age of 217±15 Ma) for the late-stage amphibolite facies metamorphic event, indicating that the rapid exhumation time for the Sulu UHP terrane is about 220 Ma.

Origin of Listwanite in the Luobusa Ophiolite,Tibet,Implications for Chromite Stability in Hydrothermal Systems

Listwanite from the Luobusa ophiolite, Tibet, forms a narrow, discontinuous band along the eastern part of the southern boundary fault. We undertook a detailed petrographic and geochemical study to understand the mineral transformation processes and the behaviour of major and trace elements during listwanite formation. Three alteration zones characterized by distinct mineral components and texture are recognized and, in order of increasing degree of alteration, these are: zoneIII is rich in serpentine minerals; zoneII is rich in talc and carbonates; and zoneI is mainly composed of carbonates and quartz. Geochemical data for the three alteration zones show significant modification of some major and trace elements in the protolith, although some oxides show linear correlations with MgO. Gold mineralization is recognized in the Luobusa listwanite and may signify an important target for future mineral exploration. Gold enrichment occurs in both zoneI and zoneII and is up to 0.91 g/t in one sample from zoneI. We show that CO2-rich hydrothermal fluids can modify both the occurrence and composition of chromite grains, indicating some degree of chromite mobility. Low-Cr anhedral grains are more easily altered than high-Cr varieties. The compositions of chromite and olivine grains in the listwanite suggest a dunite protolith.