Age and early metamorphic history of the Sanbagawa belt: Lu–Hf and P–T constraints from the Western Iratsu eclogite

Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c.  120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c.  88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c.  116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap ( c.  27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.     

柴达木北缘超高压变质带中的岛弧火山岩

青藏高原北部柴北缘发育一套与超高压变质带并行的早古生代岛弧火山岩带,岛弧火山岩以玄武岩类为主,包括一些中酸性岩类,岩石以普遍遭受绿片岩相蚀变为特征,区别于该地区普遍遭受角闪岩相区域变质的元古代的基性火山岩。该早古生代的岛孤火山岩显示三组地球化学特征:①VTG-Ⅰ,岛弧拉斑玄武岩(IAT);②VTG-Ⅱ,高Al次钙碱性-碱性过渡型玄武岩;③VTG-Ⅲ,较N-MORB更亏损的拉斑玄武岩(异常MORB)。研究认为前两组火山岩是成熟岛弧两个发育阶段的特征性产物:洋壳俯冲到陆壳的初用,由俯冲洋壳和地幔楔的部分熔融形成岛弧拉斑玄武岩(IAT),随着俯冲板块的速度加快和岛弧周围地壳的加厚,则形成钙碱性玄武岩(CA)、高Al玄武岩。第三组火山岩形成于弧间盆地,由亏损的地幔楔高度部分熔融形成比N-MORB亏损的的火山岩(异常MORB)。岛弧火山岩的锆石LA-ICP-MS法U-Pb年龄为514.2±8.5 Ma,说明柴北缘在早古生代发生过洋壳向陆壳的俯冲作用。鉴于该地区代表陆-陆俯冲作用的柴北缘超高压变质岩石也是形成于早古生代(494Ma),认为陆-陆俯冲作用发生在洋-陆俯冲作用之后,二者时间和空间相伴随。     

Plate movements, ductile deformation and geochronology of the Sanbagawa belt, SW Japan: tectonic significance of 89–88 Ma Lu–Hf eclogite ages

Prograde P–T paths and thermal modelling suggest metamorphism in the Sanbagawa belt represents unusually warm conditions for subduction-type metamorphic belts, and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge. Radiometric age data and kinematic indicators of ductile deformation suggest the Sanbagawa belt formed in a Cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt, the East Asian margin. Plate reconstructions for the Cretaceous to Tertiary for this region show that the only plausible plate compatible with such motion at this time is the Izanagi plate. These reconstructions also show that progressively younger sections of the Izanagi plate were subducted beneath eastern Asia, i.e. a spreading ridge approached, until 85–83 Ma when the Izanagi Plate ceased to exist as an independent plate. The major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin. The ridge-approach model for the Sanbagawa metamorphism, therefore, predicts that peak metamorphism is a few million years older than this age range. New Lu–Hf dating of eclogite in the Sanbagawa belt gives ages of 89–88 Ma, in excellent agreement with the prediction. Combining this estimate for the peak age of metamorphism with published P–T-t results implies vertical exhumation rates of greater than 2.5 cm yr⁻¹. This high rate of exhumation can explain the lack of a significant thermal overprint in the Sanbagawa belt during subduction of the ridge.     

越南东北部Cat Ba岛泥盆系-石炭系过渡层的初探

The Pho Han Formation is exposed on southern Cat Ba Island, Hai Phong Province in northeastern Vietnam, and intercalates the Devonian and Carbonif-erous (D-C) boundary (Ta and Doan, 2007; Komatsu et al., 2012). The D-C boundary section consists mainly of limestone beds, numbered from 1 to 167, interca-lated with alternating black organic-rich shales. The limestone yields abundant brachiopods, crinoid-stems and conodonts. Preliminary investigations on strati-graphy (conodont biostratigraphy and δ13C) and sedi-mentology of beds 113-133 were undertaken in this study.     

DETERMINATION OF RARE EARTH ELEMENTS IN SIXTEEN SILICATE REFERENCE SAMPLES BY ICP-MS AFTER TM ADDITION AND ION EXCHANGE SEPARATION

This study reports the results of thirteen rare earth elements (REE) in sixteen geochemical reference samples. The analytical procedure involved dissolution of a whole rock or mineral separate, spiking with Tm, and separation of the REE using a simple ion-exchange chromatography procedure. The resulting REE solutions were analysed by ICP-MS. The results are compared with literature values. The agreement between our data and recommended or ID-TIMS values is very favorable. The precision of the technique is better than 5% (2) for all the REE.     

Geochemical Constraints for the Genesis of Post-collisional Magmatism and the Geodynamic Evolution of the Northern Taiwan Region

Taiwan is an active mountain belt created by the oblique collisionof the northern Luzon arc with Asia. Late Pliocene extensionalcollapse of the northern Taiwan mountain belt (NTMB) was accompaniedby magmatism that formed the Northern Taiwan Volcanic Zone (NTVZ;2·8–0·2 Ma). The geochemical characteristicsof the NTVZ magmas can thus provide constraints both for themantle source composition and the geodynamic processes operatingin the late orogenic stage of the region. The NTVZ volcanicrocks consist dominantly of calc-alkaline andesites and basalts,along with subordinate but heterogeneous lavas including low-K,shoshonitic and ultrapotassic magmas. From the NE to the SWin the NTVZ, the magmas show systematic compositional variationsfrom low-K to calc-alkaline and then shoshonitic. This spatialgeochemical variation, characterized by southwesterly increasein potassium and incompatible trace elements, appears to besubparallel to the southwestern part of the modern Ryukyu subductionsystem. Sr–Nd isotope ratios of the NTVZ volcanic rocks(⁸⁷Sr/⁸⁶Sr     

Characterizing and navigating small bodies with imaging data

Abstract— Recent advances in the characterization of small body surfaces with stereophotoclinometry are discussed. The principal data output is an ensemble of landmark maps (L‐maps), high‐resolution topography/albedo maps of varying resolution that tile the surface of the body. Because they can have a resolution comparable to the best images, and can be located on a global reference frame to high accuracy, L‐maps provide a significant improvement in discriminatory power for studies of small bodies, ranging from regolith processes to interior structure. These techniques are now being used to map larger bodies such as the Moon and Mercury. L‐maps are combined to produce a standard global topography model (GTM) with about 1.57 million vectors and having a wide variety of applications. They can also be combined to produce high‐resolution topography maps that describe local areas with much greater detail than the GTM. When combined with nominal predictions from other data sources and available data from other instruments such as LIDAR or RADAR, solutions for the spacecraft position and camera pointing are the most accurate available. Examples are drawn from studies of Phobos, Eros, and Itokawa, including surface characterization, gravity analysis, spacecraft navigation, and incorporation of LIDAR or RADAR data. This work has important implications for potential future missions such as Deep Interior and the level of navigation and science that can be achieved.