Hideki Masago

Ultrahigh-pressure mineral inclusions in zircons from gneissic core samples of the Chinese Continental Scientific Drilling Site in eastern China

The Chinese drillhole CCSD-PP2 penetrated to a depth of 1028.28 m near Donghai in the southwestern Sulu terrane. Core samples consist mainly of para- and orthogneisses with minor intercalated layers of amphibolite and eclogite. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified using Raman spectroscopy and electron microprobe analysis as inclusions in zircon separates from para- and orthogneiss samples. Matrix minerals in the coesite-bearing para- and orthogneiss samples are amphibolite-facies assemblages of quartz + feldspar + epidote + biotite ± amphibole ± garnet ± titanite. In most cases, zircons, however, preserve multi-stage assemblages in different domains; the parageneses and compositions of these included minerals yielded P-T paths consistent with those deduced previously from eclogite and garnet peridotite outcrops in Donghai. In generally, quartz inclusions were identified in zircon cores and rims, whereas coesite and other UHP mineral inclusions occur in the mantles (intermediate zircon growth zone between core and rim) of the same zircons. The occurrence of coesite inclusions in zircon indicates that eclogites together with their country rock gneisses experienced UHP metamorphism and retrogression. Coesite inclusions are ubiquitous in paragneiss and orthogneiss core samples in CCSD-PP2 drillhole and in outcrop samples from the southwestern Sulu terrane, indicating that coesite-bearing lithologies are widespread in the Sulu terrane, requiring subduction of a large terrane to mantle depths. Cathodoluminescence images of zircons from gneissic rocks display distinct zoning from core to rim, that can be genetically correlated with inherited (especially in the orthogneisses) or prograde, UHP, and retrograde mineral inclusion assemblages. These images reveal irregular zoning patterns and various thickness of cores, mantles, and rims. The abundance of inclusions complicates conventional U-Pb age dating, therefore the SHRIMP microspot U-Pb analyses are essential for protolith and metamorphic age dating.

Exhumation Tectonics of the Sanbagawa High-Pressure Metamorphic Belt, Southwest Japan—Constraints from the Upper and Lower Boundary Faults

We have determined the exhumation process of the Sanbagawa Belt based on kinematic analyses of the upper and the lower boundary faults. The Sanbagawa Belt is tectonically intercalated as a thin subhorizontal sheet between overlying, weakly metamorphosed Jurassic and underlying Cretaceous accretionary complexes (e.g., Maruyama et al., 1996). On the lower boundary in Kii Peninsula, pumpellyite—actinolite facies metabasites have undergone semibrittle deformation, indicating a top-to-the south sense of thrusting. The upper boundary in central Shikoku is north-vergent, and indicates a top-to-the north sense of shear; this suggests that the original normal fault on the boundary was warped by later doming. These results support a model of selective exhumation of the Sanbagawa Belt from a depth of 30 km, and its juxtaposition against the over-and underlying accretionary complexes by orogen-orthogonal movements.

Mineral inclusions of zircon and UHP metamorphic evidence from paragneiss and orthogneiss of pre-pilot drillhole CCSD-PP2 in north Jiangsu Province, China

Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectroscopy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and orthogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.

Age‐calibrated three‐dimensional views of the crust

Scientific drilling provides detailed, highresolution, direct observations of the Earths crust. Ocean drilling science has a nearly 40-year history, starting with the Deep Sea Drilling Project (1968–1983), running through the Ocean Drilling Program (1985–2003), and continuing as the Integrated Ocean Drilling Program (IODP2003 to present). Ocean drilling science also has growing ties to the International Continental Drilling Project (1996 to present). Over the course of these scientific drilling programs, intensive analysis of a wide array of global geological and geophysical systems has been accomplished, including oceanic crustal architecture; tectonics and fluid flow along convergent and divergent plate margins; oceanic plateau composition, structure, age, and genesis; volcanic and nonvolcanic rifted margin tectonics, structure, and evolution; sedimentary records of ocean circulation, climate change, and evolution; fault zone geometry and dynamics.