Japanese Astrometry Satellite Mission for Infrared Exploration

JASMINE is the name of a Japanese infrared (K-band) scanning astrometric satellite. JASMINE (I and/or II-project) is planned to be launched between 2013 and 2017 and will measure parallaxes and proper motions with the precision of 10μas at K≃ 12 - 15 mag. JASMINE will observe a few hundred million stars belonging to the disk and the bulge components of our Galaxy, which are hidden by the interstellar dust extinction in optical bands. Furthermore, JASMINE will also obtain photometry of stars in K, J and H-bands. The main objective of JASMINE is to study the most fundamental structure and evolution of the disk and the bulge components of the Milky Way Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Existence of multiple units with different accretionary and metamorphic ages in the Sanbagawa Belt,Sakuma–Tenryu area,central Japan

U–Pb ages of detrital zircons and white mica K–Ar ages are obtained from two psammitic schists from the western and eastern units of the Sanbagawa Metamorphic Belt located in the Sakuma–Tenryu area. The detrital zircons in the sample from the western unit (T1) show an age cluster around 95 Ma, and the youngest age in the detrital zircons is 94.0 ± 0.6 Ma. The detrital zircons in the sample from the eastern unit (T5) show a main age cluster in the Late Cretaceous with some older ages, and the youngest age in the detrital zircons is 72.8 ± 0.9 Ma. The youngest zircon ages restrict the older limit of the depositional ages of each sample. White mica K–Ar ages of T1 and T5 are 69.8 ± 1.5 Ma and 56.1 ± 1.2 Ma, respectively, which indicate the age of exhumation and restrict the younger limit on the depositional age of each sample. The results show that the western and eastern units were different in their depositional and exhumation ages, suggesting the episodic subduction and exhumation of the Sanbagawa Belt in the Sakuma–Tenryu area. These results also suggest simultaneous existence of subduction and exhumation paths of metamorphic rocks in the high‐P/T Sanbagawa Metamorphic Belt.     

Detrital zircon geochronology of the Lutzow-Holm Complex,East Antarctica:Implications for Antarctica-Sri Lanka correlation

The Lützow-Holm Complex(LHC) of East Antarctica has been regarded as a collage of Neoarchean(ca.2.5 Ga), Paleoproterozoic(ca. 1.8 Ga), and Neoproterozoic(ca. 1.0 Ga) magmatic arcs which were amalgamated through the latest Neoproterozoic collisional events during the assembly of Gondwana supercontinent. Here, we report new geochronological data on detrital zircons in metasediments associated with the magmatic rocks from the LHC, and compare the age spectra with those in the adjacent terranes for evaluating the tectonic correlation of East Antarctica and Sri Lanka. Cores of detrital zircon grains with high Th/U ratio in eight metasediment samples can be subdivided into two dominant groups:(1) late Meso-to Neoproterozoic(1.1-0.63 Ga) zircons from the northeastern part of the LHC in Prince Olav Coast and northern Soya Coast areas, and(2) dominantly Neoarchean to Paleoproterozoic(2.8-2.4 Ga) zircons from the southwestern part of the LHC in southern Lutzow-Holm Bay area. The ca.1.0 Ga and ca. 2.5 Ga magmatic suites in the LHC could be proximal provenances of the detrital zircons in the northeastern and southwestern LHC, respectively. Subordinate middle to late Mesoproterozoic(1.3-1.2 Ga) detrital zircons obtained from Akarui Point and Langhovde could have been derived from adjacent Gondwana fragments(e.g., Rayner Complex, Eastern Ghats Belt). Meso-to Neoproterozoic domains such as Vijayan and Wanni Complexes of Sri Lanka, the southern Madurai Block of southern India, and the central-western Madagascar could be alternative distal sources of the late Meso-to Neoproterozoic zircons. Paleo-to Mesoarchean domains in India, Africa, and Antarctica might also be distal sources for the minor ~2.8 Ga detrital zircons from Skallevikshalsen. The detrital zircons from the Highland Complex of Sri Lanka show similar Neoarchean to Paleoproterozoic(ca. 2.5 Ga) and Neoproterozoic(ca. 1.0 Ga) ages, which are comparable with those of the LHC, suggesting that the two complexes might have formed under similar tectonic regimes. We consider that the Highland Complex and metasedimentary unit of the LHC formed a unified latest Neoproterozoic suture zone with a large block of northern LH-Vijayan Complex caught up as remnant of the ca. 1.0 Ga magmatic arc.     

Depositional age of the Himenoura Group on the Amakusa‐Kamishima area,Kyushu, southwest Japan: Using zircon U–Pb dating of the acidic tuffs

The Upper Cretaceous Himenoura Group in the Amakusa‐Kamishima Island area, southwest Japan is subdivided into the Hinoshima and Amura Formations. In order to determine the numerical depositional age of the formations, zircon U–Pb ages were investigated using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) for acidic tuff samples from the lower part of the Hinoshima Formation and the upper part of the Amura Formation. Although the two samples contain some accidental zircons, the samples have a definite youngest age cluster and their weighted mean ages are 85.4 ± 1.3 and 81.5 ± 1.1 Ma, respectively (errors are 95 % confidence interval). These age data indicate that the Himenoura Group in the Amakusa‐Kamishima Island area was deposited mainly in the early Santonian to early Campanian which is consistent with biostratigraphic ages. Additionally, zircon age distributions of the two tuff samples from the upper part of the Hinoshima Formation do not show a distinct youngest peak of eruption age but characteristics of detrital zircons suggestive of maximum depositional age of the host sediments. These results demonstrate that the mean age of the youngest zircon age cluster of a tuff sample does not always indicate depositional age of the tuff, and statistical evaluation of age data is effective to determine depositional age of a tuff bed using zircon U–Pb ages.     

A major change in magma sources in late Mesozoic active margin of the circum-Sea of Japan domain: Geochemical constraints from late Paleozoic to Paleogene mafic dykes in the Sergeevka belt,southern Primorye,Russia

More than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major- and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow-dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high-Mg and high-Al gabbro-dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)-type mantle, whereas the younger suite from an enriched mantle II (EM2)-type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic-earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long-continuing subduction-related magmatism. The plume influence reached the maximum when the unique meimechite-picrite complex formed in the region.     

Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.     

Ion microprobe U–Pb dating of zircon with a 15 micrometer spatial resolution using NanoSIMS

We developed a ²³⁸U–²⁰⁶Pb and ²⁰⁷Pb–²⁰⁶Pb zircon dating method using a Cameca NanoSIMS NS50 ion microprobe. A 7-to 9-nA O⁻ primary beam was used to sputter a 15-μm crater, and secondary positive ions were extracted for mass analysis using the Mattauch–Herzog geometry. The multicollector system was modified to detect ⁹⁰Zr⁺, ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²³⁸U¹⁶O⁺, and ²³⁸U¹⁶O₂⁺ ions simultaneously. A mass resolution of about 4000 at 10% peak height and with a flat peak top was attained, and the sensitivity of Pb was about 4 cps·nA^− 1·ppm^− 1. A multicrystal zircon standard (QGNG) from South Australia with a U–Pb age of 1842 Ma was used as a reference for Pb⁺/UO⁺–UO₂⁺/UO⁺ calibration, and on the basis of the positive correlation between these ratios, we determined the sample ²⁰⁶Pb/²³⁸U ratios. ²⁰⁷Pb/²⁰⁶Pb ratios were measured by magnetic scanning in single-collector mode. The standard zircons 91500, from Canada, and SL13, from Sri Lanka, were analyzed against QGNG. Observed ²³⁸U–²⁰⁶Pb and ²⁰⁷Pb–²⁰⁶Pb ages agreed well with published ages within experimental error. Then, 16 zircon grains in a metamorphic rock from Nagasaki, Japan, were analyzed. Observed ages were compatible with SHRIMP ages, suggesting that the NanoSIMS with a 15-μm probe diameter is suitable for ion microprobe U–Pb zircon dating.     

Microstructurally controlled monazite chronology of ultrahigh-temperature granulites from southern India: Implications for the timing of Gondwana assembly

Ultrahigh‐temperature (UHT) granulite facies rocks from the Achankovil Shear Zone area and the southern domain of the Madurai Granulite Block in South India contain monazite useful for in situ microprobe U–Pb dating. The UHT rocks examined consist of garnet + cordierite (retrograde) + quartz + mesoperthite + biotite + plagioclase + Fe‐Ti oxides ± orthopyroxene ± sillimanite and accessory zircon and monazite. Sillimanite occurs only as inclusions in garnet. Microstructural observations suggest garnet, orthopyroxene, spinel and mesoperthite are products of peak metamorphism. Post‐peak formation of cordierite ± orthopyroxene ± quartz and cordierite + spinel + Fe‐Ti oxides assemblages is also observed. Geothermobarometry on orthopyroxene and garnet‐orthopyroxene bearing assemblages suggest peak UHT conditions of T = 940–1040°C and P = 8.5–9.5 kbar. This was followed by a retrograde stage of 3.5–4.5 kbar and 720 ± 60°C, estimated from garnet‐cordierite assemblages. A small population of rounded, probably detrital, monazites in these rocks yield ages from Meso‐ to Neoproterozoic indicating a heterogeneous source. The youngest associated spot ages are 660–600 Ma suggesting protolith deposition up to ca 600 Ma. In contrast, the vast majority of monazites that crystallized during the latest metamorphic event show late Neoproterozoic to Cambrian ages. Probability‐density plots of monazite age data show a ‘peak’ between 533 and 565 Ma, but this peak need not reflect a particular thermal event. Collating ages from homogenous metamorphic monazites associated with minerals stable at peak P‐T conditions suggests peak metamorphism in these rocks occurred at 580–600 Ma. Together with a re‐evaluation of available data from adjacent granulite blocks in southern India, these data suggest the main metamorphic event coinciding with the suturing of India with the Gondwana amalgam probably occurred 580–600 Ma. The 500–550 Ma ages commonly reported in previous studies might represent post‐peak thermal events.     

Metamorphic phase equilibria modelling and zircon U–Pb geochronology of ultrahigh-temperature cordierite granulites from the Madurai Block,India: implications for hot Gondwana crust

The Madurai Block (MB) is the largest Precambrian crustal block in the Southern Granulite Terrane (SGT) of India and hosts rare cordierite- and orthopyroxene-bearing granulites. Investigations based on field study, petrology, metamorphic P–T estimation, and detrital zircon geochronology of these granulites are crucial for understanding the ultrahigh-temperature (UHT) metamorphism and crustal evolution in this block. Here we investigate the petrology and zircon U–Pb geochronology of two new localities of cordierite granulites at Kottayam (southern MB; SMB) and Munnar (central MB; CMB). Petrographic observations and phase equilibria modelling results indicate that these rocks experienced UHT metamorphism with the peak temperature exceeding 950℃ and involving clockwise P–T paths. The prograde mineral assemblages define the P–T conditions of 6.8–8.7 kbar and 750–875℃. The peak conditions are estimated using pseudosection modelling and geothermometry, which yield P–T estimates of 7.1–9.1 kbar and 955–985℃. The retrograde cooling and decompression are inferred at 860–790℃ and <6.5 kbar, respectively. Partial melting played an important role during metamorphism and contributed to the overgrowth around detrital zircons. The melt production process was probably related to biotite dehydration melting, and was mainly triggered by heating, with or without the effect of decompression. Detrital zircons in cordierite granulite samples from the two localities show similar age distributions and have dominantly Neoproterozoic ages (1024–760 Ma). The zircon cores show oscillatory zoning with a wide range of Th/U ratios (0.01–0.96), implying complex protoliths from multiple Neoproterozoic provenances from both southern and central domains of the MBs. Zircon rims and homogeneous bright zircons yield mean ages of 549 ± 5 Ma, 536 ± 6 Ma, and 544 ± 6 Ma, which are interpreted to represent zircon overgrowths during the post-peak cooling and decompression process. The timing of peak UHT metamorphism is constrained as 549–599 Ma, which coincides with the assembly of the Gondwana supercontinent.