Masaki Yuhara

Sm-Nd data for granitoids across the Namaqua sector of the Namaqua-Natal Province, South Africa

Abstract Sm–Nd data for rocks of granitic composition in an east–west traverse across the Namaqua Sector of the Mesoproterozoic c. 1.2 Ga Namaqua–Natal Province of southern Africa provide new evidence about the timing of crustal extraction from the mantle. Recent ion probe zircon dating has shown that, contrary to previous indications that pre-Namaqua basement had been preserved in parts of the Namaqua Sector, the majority of the magmatic rocks were emplaced during the 1.4–1.0 Ga Namaqua tectogenesis and very little U–Pb evidence of older precursors remains. Sm–Nd model ages show that Mesoproterozoic crustal extraction did occur, but was also strong in the Palaeoproterozoic, with local evidence for the existence of Archaean crust. The distribution of model ages generally corresponds with the established terranes, subdivided using lithostratigraphic and structural criteria. The northern part of the Bushmanland Terrane shows early Palaeoproterozoic to Archaean model ages and is clearly different from the southern part, which has Mesoproterozoic model ages. This supports the previously published results of a north–south traverse and justifies the separation of the Garies Terrane from the Bushmanland Terrane, though further subdivisions are not supported. The continuation of the Richtersveld Province east of Pofadder is supported by one sample, and the terrane boundary between the Bushmanland and Kakamas terranes is marked by an abrupt change from Palaeoproterozoic to Mesoproterozoic model ages. Model ages for the Kakamas and Areachap terranes do not distinguish them well. They suggest a Mesoproterozoic to late Palaeoproterozoic origin for both terranes, neither of which has a purely juvenile character. The influence of Palaeoproterozoic crust-forming events is clear in the Kaaien Terrane to the east, possibly reflecting reworking of the Kaapvaal Craton. The Namaqua Sector of the Province thus has a history of crustal extraction and evolution which reaches back locally to the Archaean, with major Palaeoproterozoic and Mesoproterozoic crust-forming events. This differs from the Natal Sector, which has a largely juvenile character related to a Mesoproterozoic Wilson cycle. Attempts to reconstruct the Mesoproterozoic Supercontinent Rodinia will have to take into account the extensive Nd-isotopic evidence for older crustal events in the Namaqua sector.

Magnesiohögbomite-2N4S: A new polysome from the central Sør Rondane Mountains, East Antarctica

Abstract Högbomite-group minerals are complex Fe-Mg-Zn-Al-Ti oxides related to the spinel group. Their polysomatic structure is composed of spinel (S) and nolanite (N) modules. The new polysome magnesiohögbomite-2N4S (IMA 2010-084) was found in the Sør Rondane Mountains, East Antarctica. It occurs in Mg-Al-rich, Si-poor skarns, characterized by a corundum-spinel-phlogopite-clinochlore assemblage. The new magnesiohögbomite polysome formed during the retrograde metamorphic stage. Magnesiohögbomite-2N4S appears macroscopically orange red, the streak is light orange colored. Euhedral crystals are hexagonal plates or prisms with cleavage planes on {001}. The mineral is optically uniaxial (-) and pleochroic with O = reddish brown and E = pale brown. The mean refractive index calculated from reflectance data in air at 589 nm is 1.85(3). The calculated density is 3.702(2) g/cm3. The Mohs hardness is 6.5-7, and VHN300 = 1020-1051, mean 1032 kg/mm2. The crystal structure of the new polysome magnesiohögbomite-2N4S has been solved and refined (R1 = 2.74%) from single-crystal XRD data. The crystal chemical formula is T10M24O46(OH)2 where T and M represent tetrahedral and octahedral sites. The mineral is hexagonal, space group P63mc (no. 186), a = 5.71050(10), c = 27.6760(4) Å, Z = 1, V = 781.60(2) Å3. The strongest lines in the powder XRD pattern [d (Å), I (%), hkl] are: 2.8561(4), 37, 110; 2.6120(3), 39, 109; 2.42818(16), 100, 116; 2.4160(4), 39, 1010; 2.01181(13), 50, 208; 1.54892(16), 35, 2110; 1.42785(6), 57, 220. Strongest peaks in Raman spectra are at 302, 419, 479, 498, 709, 780, and 872 cm-1, with a broad OH-characteristic absorption around 3400 cm-1. The mean chemical composition (wt%) is SiO2 0.05, TiO2 7.08, SnO2 0.15, Al2O3 66.03, Cr2O3 0.02, Fe2O3 0.50, FeO 4.87, MnO 0.06, MgO 18.71, CaO 0.01, ZnO 0.96, NiO 0.01, CoO 0.02, F 0.06, Cl 0.01, H2O 1.00, sum 99.51. The simplified formula is (Mg8.2Fe1.2Zn0.2)2+(Al22.7Fe0.1)3+ Ti4+1.6O46(OH)2 and ideal formula is Mg10Al22Ti2O46(OH)2. This mineral is a solid solution between the two ideal end-members, (Mg,Fe,Zn)102+(Al,Fe)223+Ti24+O46(OH)2 and (Mg,Fe,Zn)82+(Al,Fe)263+O46(OH)2.

Rb-Sr and Sm-Nd Mineral Isochron Ages of the Metamorphic Rocks in the Namaqualand Metamorphic Complex, South Africa

Abstract Rb-Sr and Sm-Nd isotopic studies were carried out for metamorphic rocks in the Namaqualand Metamorphic Complex, South Africa. The metamorphic rocks give the Rb-Sr mineral isochron ages (whole-rock - biotite - felsic fractions) of 844±85 Ma and 811.6±6.6 Ma for the lower granulite zone and of 776.5±5.4 Ma for the upper granulite zone. The rocks yield the Sm-Nd mineral isochron ages of 1071±18 Ma (whole-rock - garnet - felsic fractions) and 1067±158 Ma (whole-rock - hornblende - biotite rich fraction - felsic fractions) for the lower granulite zone and of 1052.0±3.6 Ma and 1002.5±1.4 Ma (whole-rock - garnet - felsic fractions) for the upper granulite zone. These age data suggest that the granulite facies metamorphism took place at 1060-1000 Ma, and that the rocks cooled down at 850-780 Ma. The Sr and Nd isotopic compositions of metamorphic rocks are different between the lower and upper granulite zones.