The Hangay-Hentey belt in Central Mongolia constitutes a Pacific-type accretionary orogen that formed through the evolution and closure of the Hangay-Hentey paleo-ocean during the Early Paleozoic to Early Mesozoic. From this belt, new geochemical and petrological results are presented for greenstones from the Erdenetsogt Formation hosted by the Tsetserleg accretionary terrane in the Hangay region, with particular emphasis on newly found picritic and andesitic rocks. These rocks occur mostly in the lower portion of the Erdenetsogt Formation as massive lavas, sills, and dikes closely associated with varicolored bedded ribbon cherts and siltstones. The protoliths of the studied greenstones comprise (1) enriched, plume-derived tholeiitic greenstones including picrites and ferrobasalts with oceanic plateau basalt affinity, (2) non-enriched, plume-derived tholeiitic basalts with E-MORB affinity, and (3) arc-derived high-Mg andesites (HMAs). The plume-derived rocks are characterized by chemical signatures such as slight LREE enrichment similar to that of tholeiitic OIB and the existence of ferropicrite with high FeO* (>14 wt%) and MgO (12-22 wt%), which is characteristic of large igneous provinces (LIPs), including oceanic plateaus. Their tholeiitic composition and high-Fe and -Ti contents require melting of the source mantle peridotite with addition of some recycled Fe- and Ti-rich basaltic material. The non-enriched basalts may have been generated by a higher degree of melting of the same source mantle. The HMAs are characterized by glassy texture, high MgO content (up to 7 wt%), and significant LREE enrichment with depletion in Nb and resemble sanukite of the Setouchi volcanic belt, SW Japan. We infer that the Hangay tholeiitic greenstones probably represent an accreted upper section of an oceanic plateau that developed in the deep-water region of the Hangay-Hentey paleo-ocean in the Devonian. The Hangay HMAs may have been produced by subduction of young oceanic plate after an oceanward back-stepping of the subduction zone that was a result of the collision during the Carboniferous of the oceanic plateau and the active continental margin of the Central Mongolian Massif.
The crystal structure of hydroxylbastnäsite-(Ce), (Ce0.49La0.21Nd0.16Pr0.06Y0.03Sm0.02Gd0.01Th0.01Eu0.01)(CO3)(OH0.85F0.15), was solved and refined R1 = 0.0363, wR2 = 0.0871 using single crystal CCD-XRD data. The hexagonal unit cell of space group P6 is identical to those of synthetic RE(CO3)OH, but is different from that of bastnäsite-(Ce), P62c. The refined dimensions are: a = 12.4730(9), c = 9.9607(14) Å, V = 1342.0(2) Å3, and Z = 18 for Ce(CO3)(OH). Each of the three crystallographically independent 9-hold RE cations is coordinated by three [(OH),F]-, four monodentate (CO3)2-, and one bidentate (CO3)2- ions in hydroxylbastnäsite-(Ce), in contrast to the 9-hold RE cation coordinated to three F- and six monodentate (CO3)2- ions in bastnäsite-(Ce). The replacement of OH for F modifies the coordination of RE polyhedra, thereby lowering the symmetry and enlarging the unit cell in the crystal structure of hydroxylbastnäsite-(Ce).
Takanawaite-(Y), a new mineral of the M-type polymorph with an generalized formula of Y(Ta,Nb)O4, is identified from Takanawa Mountain, Ehime Prefecture, Japan. It is dark brown in color and occurs as a single crystal with a tabular habit (up to 5 mm) or a radial aggregate of fine crystals. The hardness is 5 1/2 on Mohs scale. The empirical formula of takanawaite-(Y) is (Y0.75Dy0.08Yb0.05Gd0.02U0.03Ti0.02Fe0.02)Σ0.97(Ta0.57Nb0.45)Σ1.02O4, leading to a simplified formula of Y(Ta,Nb)O4. The Ta/(Ta + Nb) ratio varies between 0.55 and 0.57. Takanawaite-(Y) naturally occurs in a submetamict state, and a heating treatment is required to determine the crystal structure. The heating experiment revealed the consistency of takanawaite-(Y) and its M-type polymorph. The mineral crystallizes in a monoclinic M-type structure with a space group I2/a, unit cell parameters a = 5.3182(8), b = 10.9583 (13), c = 5.0595 (7) Å, β = 94.993(14)°, and V = 293.74(7) Å3, and Z = 4, with a calculated density 6.97 g/cm3. The eight strongest lines in the powder XRD pattern [d (Å), (I/I0), hkl] are 3.1334 (100) 121, 2.9525 (85) 121, 2.7391 (29) 040, 2.6494 (21) 200, 1.9115 (24) 202, 1.9050 (39) 240, 1.8546 (26) 042, and 1.5681 (19) 242. Takanawaite-(Y) is a distinct mineral compared to iwashiroite-(Y), yttrotantalite-(Y), and formanite-(Y).
We report new petrological and P-T data for spinel-sapphirine-bearing mafic granulite from Akarui Point in the upper amphibolite- to granulite-facies transitional zone of the Lützow-Holm Complex (LHC), East Antarctica, and provide unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during the Neoproterozoic. The reaction microstructures in the rock suggest the stability of the prograde pargasite + garnet + orthopyroxene + plagioclase + hematite assemblage followed by the formation of plagioclase + orthopyroxene + spinel + sapphirine symplectite after garnet. The application of mineral equilibrium modeling on the mafic granulite in the system NCFMASHTO yields a prograde condition of 11-12 kbar and ∼ 900 °C, followed by decompression to the peak stage (Pgs + Pl + Opx + Spl ± H2O) of 5-6 kbar and 900-920 °C, and subsequent cooling to less than 890 °C, possibly along a clockwise P-T path. The high-temperature and possibly ultrahigh-temperature condition observed at Akarui Point might reflect an effect of a local thermal event or the presence of an exotic crustal block within the LHC, which are essentially associated with the complex collisional evolution of this region during the assembly of Gondwana.
Smithsonite occasionally exhibits a characteristic blue emission, known as cathodoluminescence (CL), which can be assigned to a lattice defect center by CL spectral analysis. The intensity of this emission is reduced at higher temperatures, suggesting a temperature quenching phenomenon. The activation energy in the quenching process was evaluated by a least-square fit of the Arrhenius plots using the integrated intensity of the emission component, and was found to be ∼ 0.03 eV for the defect center. According to the Mott-Seitz model, the quenching process can be interpreted by an increase in non-radiative transition at higher temperatures. The value of the activation energy for a blue emission caused by the defect center corresponds to the vibration energy of the O-Zn-O bending mode in the lattice. It implies that the temperature quenching energy might be transferred as a phonon to the specific lattice vibration.
The transmittance of 500-800 nm light was shown to change significantly at the critical points of three pure fluids. Spectroscopic measurements of H2O, CO2, and C2H5OH were made in a visible-type autoclave at high temperature and pressure. Each fluid showed significantly decreased transmittance as its critical point was approached; transmittance reached a local minimum at the critical temperature and pressure. The experimentally determined critical temperature and pressure of H2O are 374 °C and 22.07-22.09 MPa, respectively, similar to reference values of 374.15 °C and 22.12 MPa; C2H5OH showed values of 243 °C and 6.30-6.31 MPa (versus reference values of 241 ± 7 and 243.4 °C and 6.3 ± 0.4 and 6.14 MPa); and CO2 showed values of 32 °C and 7.36-7.43 MPa (versus reference values of 31.04 ± 0.2 °C and 7.37 ± 0.04 MPa). The good agreement between the experimental results and the reference values indicates that the spectroscopic method used here could be applied to other fluids, including mixed geofluids.
Vol.31 (1944) No. 5 and No.6 in the predecessor journal ″The Journal of the Japanese Association of
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April 03, 2015
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