The peculiar Co, Zn and Mn-rich chromian spinels are hosted by magnetite veins, serpentinites and chromitites of the mantle section of the Proterozoic Bou-Azzer ophiolite, Morocco. The spinel is complexly zoned either optically or chemically, and exhibits anomalously high MnO, ZnO and CoO contents (up to 22, 7.5 and 2 wt%, respectively). It has four distinct optical zones particularly in the magnetite veins and less typically in serpentinites. The highest level of these elements, probably divalent, is recorded within the ferritchromite zone and/or within the core zone if the ferritchromite zone is absent. These elements as well as Fe exhibit enrichment along grain boundaries and fractures of the altered spinels. Fe, Mn, Zn and Co were most probably supplied from olivine upon severe serpentinization during and after obduction of the ophiolite.
     The enrichment of the Bou-Azzer chromian spinel in Mn, Zn and Co was governed mainly by the fluid/mineral (spinel) ratio and partition coefficient between spinel and the relevant fluid among many factors. The Co-, Zn- and Mn-rich chromian spinels can be used as an exploration guide for Co-Ni-Zn-Cu sulfide mineralization associated with serpentinized peridotites.

View full abstract

The Samnua Depresion Zone (SNDZ) extends NW-SE for more than 400 km from Northern Vietnam to Southwestern China. It represents the northern edge of the Indochina block and is separated from the South China block by the Songma fault. The zone is occupied mainly by Paleozoic sedimentary rocks and Mesozoic volcano-sedimentary successions that are intruded by Mesozoic intrusives and extrusives and by Cenozoic extrusives. Representative igneous rocks of the SNDZ were geochronologically investigated by the Rb-Sr whole rock and K-Ar biotite methods. The Rb-Sr ages obtained are 222 Ma for the Nuichua gabbros, 213 Ma for the Songma granitoid (SMG), 203 Ma for the Piabioc granitoid (PBG), 138 Ma for the Banmuong subvolcanic granite, 27 Ma for the Banchieng granitoid (BCG), 218 Ma for the Dongtrau felsic volcanics, 186 Ma for the Muonghinh felsic volcanics and 5.8 Ma for the Cenozoic basaltic rocks (CBR). The K-Ar methods give 252 Ma for the SMG, 237 Ma for the PBG and 24 Ma for the BCG. The groundmass fraction of the CBR yields an age of 1.9 Ma. Geochronological comparison and using fossil records suggest that the Rb-Sr ages of Mesozoic igneous rocks provide apparent young age, a pseudo isochron age. The major igneous activity of the SNDZ took place during Late Permian-Triassic time to form granitic and felsic volcanic rocks, which were related to the continental volcanic arc magmatism. Minor intraplate igneous activity occurred in the Paleogene. This suggests that the tectonic environment of the SNDZ changed from a convergent plate margin to a matured continental margin in the time span between the Triassic and Paleogene.

View full abstract

The upper part (Stage IV) of the Fongen-Hyllingen intrusion, southeast of Trondheim, Norway, comprises a ∼ 1800 m thick sequence of cumulates showing a strong normal fractionation trend from olivine gabbros at the bottom through diorites to quartz-bearing syenites at the roof. Mineral compositions show extreme ranges: olivine Fo_74-0; Ca-rich pyroxene Mg#79-0; plagioclase An_60-0. The cumulate sequence has been divided into five intervals based on cumulus assemblages. The summation method has been used to calculate successive magma compositions for these intervals based on whole-rock major element chemical analyses of 49 samples. The most primitive calculated magma composition has 51.7% SiO₂, 5.0% MgO, 5.3% total alkalis and a Mg-number of 47.5; it lies near the base of the basaltic trachyandesite field in the Total Alkali Silica (TAS) diagram, very close to the dividing line between tholeiitic and alkaline suites. This magma evolved through the trachyandesite field into the trachyte field. The most evolved magma composition has ∼ 11.4% alkalies; alkali enrichment is not due to assimilation of metapelites. There was very little iron-enrichment during fractionation and magma density decreased continuously, enabling repeated influxes of new magma to pond and to elevate the resident melt, facilitating magma stratification. The parental magma may have resided in a lower crustal chamber where it differentiated before repeatedly supplying the overlying Fongen-Hyllingen chamber.

View full abstract

A least squares refinement with a generalized structure factor expression in the Gram-Charlier expansion was applied to the structure of elbaite, XY₃Z₆(BO₃)₃T₆O₁₈A₃⁽¹⁾A⁽²⁾ (X = Na_0.640Ca_0.090K_0.006[]_0.264, Y = Al_0.415Fe²⁺_0.084Mn²⁺_0.034Mg_0.005Ti⁴⁺_0.003Li_0.458, Z = Al, T = Si, A⁽¹⁾ = (OH)_0.750, and A⁽²⁾ = F_0.179(O,OH)_0.071). Elbaite has space group R3m, and hexagonal cell dimensions of a = 15.874(3) and c = 7.116(3) Å. The least squares refinement of the X-ray single-crystal data measured at room temperature converged at R = 0.0221 for 179 variables, including terms up to the fourth order coefficients. The O1 and O2 sites showed multiple split probability density functions (pdfs), and the X site showed a circular ring-like pdf, providing evidence of positional disorder at these sites. The disorder at the O1 and O2 sites was interpreted to be due to substitutional disorder of the Y site cations. The remaining atom sites all showed unimodal pdfs with a more or less asymmetric tendency along the hexagonal c-axis. The asymmetry is related to the primary effect of pyroelectricity.

View full abstract

Orbicular granite occurs at Minederayama in the Tsukuba Mountains, Ibaraki Prefecture, Japan. The orbs consist of a migmatitic core, originally pelitic hornfels, and a mantle that consists mainly of radial cordierite containing apatite inclusions and subordinate feldspar and micas. The orientation and size distribution of the orbs indicates that isolated orbs had floated within the host granitic magma. According to the geometrical selection theory of crystal growth, we propose that radial cordierite grew from the orb shells to the orb cores. Melting experiments demonstrate that the mineral assemblage of the orbicular granite reflects the incongruent melting of pelitic hornfels. Important factors in the formation of the orbicular granite are the unmixing between the incongruent melt of cordierite hornfels and host granite melt, and the infiltration of the incongruent melt into the host granite melt as the cordierite crystallized. The temperature of the host granite magma, as estimated from melting experiments and mineral assemblage, was above 730 °C at 0.3 GPa, which may represent superheated conditions for the host granitic magma. Cooling of the host granite magma placed the orbs under supercooled conditions as the radial cordierite grew from the shells of the orbs to the cores.

View full abstract

Ultramafic xenoliths in Pliocene-Quaternary basalts from Bir Ali, the Shabwa Governorate, southern Yemen, were studied to examine upper mantle processes related to continental rifting. They show a great variety of rock types: spinel lherzolites, harzburgites, dunites, clinopyroxene (= cpx)-rich lherzolites, wehrlites and pyroxenites. The cpx-rich lherzolites and wehrlites with high clinopyroxene/orthopyroxene ratios (> 1), which are characteristic of the Bir Ali xenolith suite, contain lower Fo contents of olivine than ordinary lherzolites at a given Cr# of spinel. Their clinopyroxenes are higher in TiO₂ and REE contents than those in the ordinary lherzolites, which are restite. These features strongly indicate metasomatic addition of clinopyroxene from alkali basalts at the expense of orthopyroxene to depleted peridotites for genesis of the cpx-rich lherzolites. The cpx-rich lherzolites are one of common constituents of lithospheric mantle at rifted continental margins.

View full abstract

We discovered a new kind of dyke in the mantle section of the Oman ophiolite composed mainly of diopside that we called diopsidite, although traces of forsterite, anorthite, titanite or andradite may be also observed. The texture and chemical composition, and the relationship between antigorite and diopside or rare forsterite, suggest that they formed by high-T metamorphism of serpentine (antigorite) and are the result of interaction between hydrothermal fluid and percolating magma in the shallow mantle. Chemical data, obtained by electron microprobe and in situ laser ablation coupled to ICP-MS, show large heterogeneities from one sample to another and between diopside crystals in each sample. The trace element patterns are characterised by strong positive anomalies in Sr and Eu and a negative anomaly in Ti. Small-scale heterogeneities reflect the disequilibrium conditions in the dyke formation and show that a relatively Ti-rich- and Al-Eu-Sr-LREE-poor phase may be included in the diopside. The exact nature of this phase is difficult to determine but the Eu negative anomaly, and the low Sr and LREE concentration suggest that it is probably some partially altered, relict protolith mineral.

View full abstract

Garnet and omphacite in eclogites from the Aktyuz area, northern Kyrgyz Tien-Shan, preserve evidence of two distinct metamorphic events. Relic polyphase inclusions of Fe-rich staurolite (X_Mg < 0.29), Mg-taramite, paragonite, and oligoclase (An < 16) contained in garnet cores represent a separate pre-eclogitic amphibolite or epidote-amphibolite facies metamorphism (T = 560-650 °C, P = 4-10 kbar). This relatively low- to medium-pressure metamorphic event probably resulted from the heat of the hot hanging wall at the inception of subduction. Inclusions in the garnet cores and rims and matrix omphacite (glaucophane, sodic-calcic and calcic amphiboles, paragonite, phengite, epidote, albite, rutile, hematite, and quartz) suggest that epidote-blueschist facies conditions (T = 330-570 °C, P = 8-16 kbar) were experienced during the prograde stage of the eclogitic metamorphism. This event occurred within a subduction zone. The estimated peak metamorphic conditions of the eclogites are T = 600-710 °C and P = 15-25 kbar. The subsequent retrograde path through epidote-amphibolite and greenschist facies suggests a clockwise P-T path, in contrast to the anticlockwise P-T path proposed by previous work.

View full abstract

The above PDF file shows errata in the paper entitled “Cation vacancy and possible hydrogen positions in hydrous forsterite, Mg_1.985Si_0.993H_0.06O4, synthesized at 13.5 GPa and 1300 °C” by Yasuhiro KUDOH, Takahiro KURIBAYASHI, Hiroyuki KAGI and Toru INOUE (Vol. 101, no. 5, 265-269, 2006) Wrong:See PDF attached
Right:See PDF attached

View full abstract