SHRIMP U-Pb analysis of zircon from charnockites and granite of the Archaean Ntem Complex, southern Cameroon was conducted to clarify the magmatic evolution of the northern part of Congo craton. Zircon morphologies and bulk chemistry are consistent with a magmatic origin for the charnockites. Zircons collected from two charnockites in the southern part of the complex yield differing magmatic ages of 3266 ± 5 Ma and 2883 ± 11 Ma. A xenocrystic zircon in a granite collected from the northern part of the complex gives a single grain age of 3477 ± 16 Ma, the oldest age ever reported from the Congo craton, while majority of zircons in the same rock yield magmatic age of 2853 ± 12 Ma. In addition, an inherited magmatic age of 3399 ± 6 Ma was obtained from the ~ 2883 Ma charnockite. The present data provide evidence of early Paleoarchaean magmatic activity, including charnockitic plutonism in the Congo craton.

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Radiometric ages of detrital zircons in three samples of psammitic schists from the Sanbagawa Belt, Kanto Mountains, were obtained from the ²³⁸U/²⁰⁶Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe (SHRIMP II). Most of the zircon ages cluster around Cretaceous, with a few ages corresponding to older zircons. The origins of the detrital zircons are mainly Cretaceous igneous rocks. The ages of the youngest zircons in samples AM48p, SnbE, and AM29p indicate Late Cretaceous time, and they are 78.8 ± 1.3 Ma, 91.4 ± 1.4 Ma, and 95.3 ±1.5 Ma, respectively. The samples AM48p and AM29p have K-Ar ages of 65.9 ± 1.4 Ma and 82.1 ± 1.8 Ma, respectively. The age difference between the youngest detrital zircon age and white mica K-Ar age is 13 Myr. The Sanbagawa Belt is believed to be a metamorphosed phase of the Chichibu Belt, which is a Middle Jurassic to earliest Cretaceous accretionary complex; however, the results of this study suggest that the protolith of the Sanbagawa belt was accreted in Late Cretaceous, similar to the Shimanto belt that runs parallel to the Sanbagawa and Chichibu belts.

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Using cold-seal hydrothermal bomb and piston-cylinder apparatus, we have carried out both forward and reversal experiments to investigate the phase boundary between nahcolite (NaHCO₃) and trona (NaHCO₃·Na₂CO₃·2H₂O). We found that the temperature of this phase boundary remains low at least up to 10 kbar, so that this phase transformation maintains its univariant nature in our investigated P-T space. The locus of this phase boundary in a log(pCO₂)-T space is defined as log(pCO₂) = 0.0240(±0.0001)T − 9.80(±0.06) (with pCO₂ in bar and T in K), in excellent agreement with earlier 1 atm experiments at different partial pressures of CO₂ (pCO₂) and theoretical calculation. Using this equation and literature thermodynamic data, the entropy of trona at 298.15 K is constrained to be 303.8 J mol⁻¹ K⁻¹, essentially identical to earlier estimates from different methods. Our experimental results have also been used to constrain the genesis of nahcolite in some fluid inclusions of diverse origins, and it is suggested that nahcolite in these occurrences is most likely a daughter mineral which crystallized from the fluids as temperature decreased, rather than an accidentally trapped phase.

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El Atshan area, situated in the central Eastern Desert of Egypt, is a good location for studying the influence of high- and low-T alterations on the formation of Zr- and U-bearing minerals within a trachyte sill. These minerals are mainly represented by an unidentified secondary Zr-rich silicate mineral, betafite, and liandratite. The unidentified Zr-rich silicate mineral is considered to be an alteration product of the precursor zirconolite during high-T alteration. This is indicated by the changes in the composition marked by an increase in the amount of hydration (H₂O), Si, and U and a decrease in the amount of Ca, Ti, Nb, and Fe. The morphology of this unidentified mineral is similar to that of zirconolite. For seven oxygen atoms, the calculated formula of the unidentified Zr-rich silicate mineral is (Si_1.45U_0.18Ca_0.32Pb_0.01Nb_0.06Zr_1.18Hf_0.01Fe_0.06Al_0.08Ti_0.11P_0.09Y_0.05REE_0.09)_Σ3.7O₇. With more extensive alteration, it was found that the unidentified secondary Zr-rich silicate mineral was unstable and it underwent re-equilibration with U-rich fluid, which led to alterations in betafite. The two possible mechanisms responsible for the alteration of the unidentified Zr-rich silicate mineral to betafite are as follows:(1) the dissolution of the unidentified Zr-rich silicate mineral and precipitation of betafite and (2) the ion-exchange between partially to fully amorphized zones and the U-rich fluid. Such alteration is indicated by a marked increase in the amount of U, Ti, and Nb and a decrease in the amount of Zr, Si, Ca, P, Y, and ΣREE. The single substitution of U⁴⁺ ↔ Zr⁴⁺ and the coupled substitution of (U^4+< + Ti⁴⁺ + Nb⁵⁺) ↔ (Zr⁴⁺ + Si⁴⁺ + Ca²⁺ + P5⁺ + Y3⁺ + ΣREE³⁺) appear to be the main causes for the formation of betafite. For 2.00 B-site cations, the calculated formula of the betafite is ^A(U_0.44Ca_0.25REE_0.05Y_0.03Pb_0.02)_Σ0.79^B(Si_0.79Zr_0.69Ti_0.23Nb_0.12Al_0.06Fe_0.05P_0.03V_0.02Hf_0.01)_Σ2.0O₇. Betafite was altered to liandratite in the late low-T alteration stage. For 2.00 Nb-site cations, the calculated formula of the liandratite is ^U(U_1.35Ca_0.41Pb_0.04REE_0.04Y_0.01)_Σ1.86^Nb(Ti_0.67Si_0.46Nb_0.40Zr_0.26e_0.09V_0.08Al_0.03Ta_0.01)_Σ2.0O₈. Among different uranium complexes, the influence of fluoride on the solubility and mobility of uranium is confirmed.

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The following is erratum for the letter entitled “Age determination of thorianite in phlogopite-bearing spinel-garnet peridotite in the Gföhl Unit, Moldanubian Zone of the Bohemian Massif” by Kosuke NAEMURA, Katsumi YOKOYAMA, Takao HIRAJIMA and Martin SVOJTKA (Vol. 103, no. 4, 285-290, 2008).

The second author's correct name is Kazumi YOKOYAMA.

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