Hironobu Hyodo

A new Late Miocene great ape from Kenya and its implications for the origins of African great apes and humans

Extant African great apes and humans are thought to have diverged from each other in the Late Miocene. However, few hominoid fossils are known from Africa during this period. Here we describe a new genus of great ape (Nakalipithecus nakayamai gen. et sp. nov.) recently discovered from the early Late Miocene of Nakali, Kenya. The new genus resembles Ouranopithecus macedoniensis (9.6–8.7 Ma, Greece) in size and some features but retains less specialized characters, such as less inflated cusps and better-developed cingula on cheek teeth, and it was recovered from a slightly older age (9.9–9.8 Ma). Although the affinity of Ouranopithecus to the extant African apes and humans has often been inferred, the former is known only from southeastern Europe. The discovery of N. nakayamai in East Africa, therefore, provides new evidence on the origins of African great apes and humans. N. nakayamai could be close to the last common ancestor of the extant African apes and humans. In addition, the associated primate fauna from Nakali shows that hominoids and other non-cercopithecoid catarrhines retained higher diversity into the early Late Miocene in East Africa than previously recognized.

Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet

New zircon U-Pb and mica 40Ar/39Ar dating combined with structural studies in the Longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of Tibet is Mesozoic. However, at lower structural levels, apatite U-Pb and monazite electron microprobe dating reveals a previously unknown domain of Cenozoic (ca. 65 Ma) Barrovian-type metamorphism and deformation. This discovery shows that the crust in the eastern margin of Tibet was already a substantial thickness around the time of the India-Asia collision. Associated deformation has a N-S-oriented stretching lineation, implying that deformation was not driven by topographic gradients in the Tibetan Plateau. The observed moderate amounts of distributed postmetamorphic E-W shortening can probably explain the present thickness of the continental crust in the area. These results do not support models of crustal thickening caused by solid-state lateral flow of midcrustal metamorphic rocks.

Effect of anisotropy on the paleomagnetic contact test for a Grenville Dike

We report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the Precambrian Grenville Province of Canada. Directions of natural remanent magnetization (NRM) in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric, using the measured anisotropy of laboratory-induced thermoremanent magnetization (TRM). The TRM ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility (AMS) ellipsoids, but the anisotropy of remanence is more pronounced than the AMS. Anisotropy correction moves NRM directions in the unbaked zone almost 20°, bringing them close to typical Grenvillian paleofield directions. The anisotropy seems to be controlled by multidomain rather than single-domain magnetic minerals. NRM directions from the baked gneiss near the dike contact after anisotropy correction agree well with the NRM directions from the chilled margin of the early Cambrian (570 ± 3 Ma) dike. This positive baked contact test is augmented by the presence of a “hybrid zone”, in which country rock NRMs are increasingly overprinted by the dike NRM as one moves from the unbaked toward the baked zone. The dike NRM is therefore primary. However, the virtual geomagnetic pole (VGP) for the dike NRM: 51°N, 131°W (dp=10°, dm=12°, N=24 samples) or 41°N, 153°W (dp=13°, dm=18°, N=4 chilled margin samples) does not fall near most established Cambrian paleopoles. Post-emplacement tectonic movement of the dike is unlikely; it may be that the dike cooled and acquired TRM during a geomagnetic field excursion because of its geologically short cooling (blocking) period of a few tens of years.

Observations of Short-Range Order by Optical Birefringence in One-Dimensional Antiferromagnets CsNiCl3, RbNiCl3 and CsCoCl3

Magnetic short-range order was observed in one-dimensional S =1 Heisenberg (CsNiCl 3 , RbNiCl 3 ) and \(S{=}\frac{1}{2}\) Ising antiferromagnet (CsCoCl 3 ) up to k T ∼10| J | and 3| J |, respectively, by measuring optical birefringences. An isomorphic diamagnet CsMgCl 3 was used, with corresponding states argument, to extract the magnetic contribution Δ n m from the resultant birefringence Δ n in these magnetic systems. The temperature T max , at which the temperature derivative of the magnetic part Δ n m displays a broad maximutm, is in agreement with that predicted by the rigorous theory for the specific heat in each linear chain system.

Geochemistry and Chronology of Tectonic Blocks in Serpentinite Mélange of the Southern New England Fold Belt, NSW, Australia

Abstract Tectonic blocks are widely distributed throughout the serpentinite melange of the Peel-Manning Fault System, southern New England Fold Belt, Australia. Two meta-diorite blocks from the Pigna Barney area have distinctly differing chemical signatures. One resembles boninite, containing high Ni and Cr, low rare earth element (REEs), and high field strength element (HFSEs) contents. Further, it exhibits an enriched chondrite normalized LREE pattern and an atypical eNd536Ma value of –4. Well-defined isochrons for this sample gave Sm–Nd and Rb–Sr ages of 536±177;38 Ma and 426±177;38 Ma respectively. The former represents the emplacement age and the latter resetting of the Rb–Sr system during a later metamorphic event. The initial Nd isotopic composition suggests an enriched source. The other meta-diorite block has a flat REE pattern, a Ti/V ratio >10 and Y, La and Nb values typical of tholeiitic magmas erupted in an island arc setting. By contrast, blueschists from this locality and from the Glenrock Station area show depleted REE patterns, Ti/V = 20–50, and eNd480Ma = +8, features characteristic of normal mid-ocean ridge basalts (N-MORB). Meta-diorites from the Glenrock Station area have moderately depleted signatures (eNd400Ma = +4.5) and Ti/V ratios and REE patterns typical of calc-alkaline and tholeiitic island arc magmas. These contrast with hornblende cumulate rocks from this location, which have very high Cr and Ni, low Zr and Y, and show slight LREE-enriched chondrite normalized patterns. Chromites (100 Cr/Cr+Al = 85) and clinopyroxenes (Wo93En42.3Fs7.7 – Wo48En43.7Fs8.3) in these rocks reveal an island arc affinity. The eNd420Ma values vary from +2 to +4 and Rb–Sr ages from 425±177;44 Ma to 411±177;15 Ma. The core of a single grain of cumulus hornblende gave a poorly defined 40Ar/39Ar age of 394 Ma in contrast to younger ages (235–263 Ma) obtained from other grains. The younger ages are attributed to resetting of the K-Ar system during a metamorphic event. This study has provided further evidence for an Early Paleozoic arc-trench system along the eastern Gondwana margin.

New geological and palaeontological age constraint for the gorilla–human lineage split

The palaeobiological record of 12 million to 7 million years ago (Ma) is crucial to the elucidation of African ape and human origins, but few fossil assemblages of this period have been reported from sub-Saharan Africa. Since the 1970s, the Chorora Formation, Ethiopia, has been widely considered to contain ~10.5 million year (Myr) old mammalian fossils. More recently, Chororapithecus abyssinicus, a probable primitive member of the gorilla clade, was discovered from the formation. Here we report new field observations and geochemical, magnetostratigraphic and radioisotopic results that securely place the Chorora Formation sediments to between ~9 and ~7 Ma. The C. abyssinicus fossils are ~8.0 Myr old, forming a revised age constraint of the human–gorilla split. Other Chorora fossils range in age from ~8.5 to 7 Ma and comprise the first sub-Saharan mammalian assemblage that spans this period. These fossils suggest indigenous African evolution of multiple mammalian lineages/groups between 10 and 7 Ma, including a possible ancestral-descendent relationship between the ~9.8 Myr old Nakalipithecus nakayamai and C. abyssinicus. The new chronology and fossils suggest that faunal provinciality between eastern Africa and Eurasia had intensified by ~9 Ma, with decreased faunal interchange thereafter. The Chorora evidence supports the hypothesis of in situ African evolution of the Gorilla–Pan–human clade, and is concordant with the deeper divergence estimates of humans and great apes based on lower mutation rates of ~0.5 × 10−9 per site per year (refs 13, 14, 15).

The discovery and significance of a fossilized radiogenic argon wave (Argonami) in the Earth's crust

We have discovered a curious “fossilized” asymmetric radiogenic argon wave trapped in single grains of biotite in the contact zones between a diabase dike (≈570 Ma) and the Grenville country rock (tonalitic gneiss ≈1000 Ma) near Mattawa, Ontario, Canada. The phenomenon is presumably the result of the interplay between the partial pressure of the ambient radiogenic argon and the opening and closing (blocking) of biotite to argon movement during the period of thermal (and fluid?) perturbation following the dike intrusion. The wavelength is roughly equal to one dike width (≈30 m). The only previously reported example of such a wave (which we call an “argonami”) was one discovered in 1970 by Wanless and his colleagues in a study of biotites from a traverse across the Grenville Front. The two argonamis are remarkably similar, even in their asymmetry, but the wavelength scales differ by roughly two order of magnitude, presumably reflecting the relative sizes of the Mattawa dike and the continental-scale Grenville Front. The occurrence of the argonami phenomenon suggests that in some instances, the analysis of excess argon may become a useful tool for the unraveling of complex tectono-thermal and fluid-migratory events taking place at plate boundaries and igneous contacts.

Study of Magnetic Energy in One-Dimensional S=1/2 Heisenberg Antiferromagnet KCuF3 by Optical Birefringence

Measurements of optical birefringence have been made on single crystals of the a-type KCuF 3 between 6 and 600 K. The observed temperature dependence of birefringence was found to be in good agreement over a wider temperature range above T N with Bonner-Fishers theoretical results for the magnetic energy of S =1/2 Heisenberg linear chain systems.

Optical Birefringence in CsCuCl_3 : A Quasi One-Dimensional S=1/2 Ferromagnetic Heisenberg System

Short range order effects on the optical birefringence were investigated in an antiferromagnetically coupled linear-chain S =1/2 Heisenberg ferromagnet CsCuCl 3 in a temperature range between 6 K and 450 K. A nearly isomorphic diamagnet CsMgCl 3 was used to extract the magnetic part of birefringence in CsCuCl 3 . The magnetic birefringence was found to have a broad maximum at about 32 K. The effect of the interchain interaction on the birefringence was calculated in terms of a perturbation approach. By comparing the results with the observed temperature dependence of the birefringence, the ratio of inter- to intrachain exchange integrals was obtained as -0.07.

Metamorphic Evolution of the Southwest Okcheon Metamorphic Belt in South Korea and Its Regional Tectonic Implications

The Hwasan area of the southwest Okcheon metamorphic belt in South Korea underwent intermediate-P/T M1 metamorphism in the Late Carboniferous to Early Permian. Mica K-Ar and 40Ar/39Ar ages of the pelitic and psammitic rocks are concentrated at ˜160 Ma. Granites in the northern and western parts of the Hwasan area have K-Ar mica ages of ˜156 Ma. Carbonaceous material throughout the Hwasan metasediments exhibits a narrow range in d002 of 3.353 to 3.359 Å, corresponding to that of fully ordered graphite, consistent with amphibolite-facies conditions of ˜500°C. Graphitic carbon is also distributed throughout lower-grade greenschist-facies rocks formed during the main M1 metamorphic episode. These results, along with the occurrence of a thermal metamorphic aureole (>500°C) in the metasediments close to the mid-Jurassic granites, suggest that the study area was overprinted by low-P/T M2 metamorphism due to the intrusions of the plutons. This latter event, however, failed to re-equilibrate mineral assemblages of the earlier metamorphism, except within a narrow aureole of 1-2 km around the Jurassic granites.