Garam Chashma granitic rocks intrude discordantly into the Hindukush metamorphic complex on the southern margin of the Asian plate, Trans-Himalayas, northwestern Pakistan. The rocks are fine to coarse-grained, two-mica leucogranite. K-Ar biotite analyses of four samples from the Garam Chashma pluton yield 18-20 Ma ages. These ages are younger than those of the other post-collisional granitic rocks in Trans-Himalayas of Pakistan and resemble those of the Higher Himalayan granitic rocks.

View full abstract

Sodic-calcic amphibole closely associated with aegirine-augite was newly found from a sandy hornfels at Mt. Hiei contact aureole, Kyoto, Japan. The amphibole shows a pleochroism of X=colorless, Y=blue and Z=light blue and is winchite with significant amounts of TiO₂ (1.5-2.5 wt.%) and of Mg-Fe-Mn-Li amphibole component at B site (ca. 0.1-0.4 cations for O=23 basis). The chemical composition and the coexisting phases of the winchite in this study are similar to those in the alkaline igneous complex.

View full abstract

The join diopside (Di)-nepheline (Ne) was studied to determine the upper stability limit of melilite (Mel) in nephelinitic magmas under both dry and wet conditions. Experiments at 28 kbar under dry condition show that the join Di-Ne has a pseudoeutectic at 1420°C and Ne₇₀Di₃₀. The solidus is attained at 1220±10°C in the compositional range of Di₁₀₀-Di₄₀Ne₆₀, but beyond the composition Di₄₀Ne₆₀ toward the nepheline-rich side, the solidus is reached at 1200±10°C. Melilite is absent in the 28 kbar polythermal join. In order to explore the upper stability of melilite in the join Di-Ne under dry condition, subsolidus runs were performed between 15 and 25 kbar at 1000°C. In the run at 15 kbar, melilite coexists with nepheline and diopside but this phase disappears in the runs above 20 kbar. Investigation at 10 kbar in the presence H₂O [P(H₂O)=P(Total)] shows that the join Di-Ne cuts through the primary phase volumes of diopside_ss, nepheline and amphibole_ss (Amph) with a pseudoeutectic at Di₅Ne₉₅ and 680°C. Melilite is not stable in the 10 kbar isobaric-polythermal H₂O-bearing join. Variation of phase assemblages between 1 to 10 kbars and under the presence of excess water for an arbitrarily chosen composition Di₄₀Ne₆₀ was also described. The study shows that rocks having melilite nephelinite compositions are represented at pressures above about 18 kbar (under dry condition) by nephelinite, but in the presence of excess water by an amphibole-bearing nephelinites. The study also demonstrates that an olivine-melilite nephelinite and a melilite nephelinite crystallize from an aqueous magma only under volcanic to subvolcanic conditions.

View full abstract

CT values (numerical expression for contrast of CT images) of 66 minerals, were measured by using a medical X-ray CT (computed tomography) scanner. These values were compared with the calculated X-ray linear attenuation coefficients of the minerals. The comparison permitted to evaluate the attenuation contrast resolution by polychromatic X-ray beam of the scanner and to estimate the practical restrictions to use the method for mineralogical purposes. Plots of the CT values vs. monochromatic linear attenuation coefficients calculated for different photon energies revealed the existence of the most efficient photon energy giving the best linear correlation. The energies are about 70 and 80 keV for accelerating voltages of 120 and 130 kV, respectively. These values roughly coincide with the peaks of the photon energy spectra of the incoming X-ray beams. The existence of the most efficient energies (hitherto called “effective energies”) shows that CT values can be evaluated quantitatively for minerals in a medical CT scanner with polychromatic X-ray beams. Resolution of the attenuation contrast is about 0.05 cm⁻¹ for these two effective energies in the scanner used. It is expected from the contrast resolution that the degree of Mg-Fe substitution in ferromagnesian silicate minerals is easily estimated if the Fe contents are sufficiently different. However, in the Mg-rich compositional range of ultramafic rocks (X_Mg≈0.9), olivine and Ca pyroxene may not be discriminated due to the overlap of the CT values. The CaAl-NaSi substitution in plagioclase solid solution is also detectable whereas the CT values of quartz and alkali feldspar are within the plagioclase range.

View full abstract