Stratigraphic and Petrological Insights into the Late Jurassic– Early Cretaceous Tectonic Framework of the Northwest Pacific Margin

Late Jurassic–Early Cretaceous volcano‐sedimentary sequences in the Sorachi, Kumaneshiri, and Yezo groups are exposed in central Hokkaido. The sequences are considered to reflect the Late Mesozoic tectonic history of the northwest Pacific continental margin. Based on the stratigraphic and petrological characteristics of igneous and volcaniclastic rocks of the Sorachi, Yezo, and Kabato groups, Late Jurassic–Early Cretaceous tectonics in central Hokkaido can be divided into six stages. Stage I (Tithonian) is characterized by extensive eruption of tholeiitic basalt accompanied with andesitic volcaniclastic rocks and terrigenous deposits. Seafloor spreading or large igneous province formation occurred near an island arc and/or continent during this stage. In Stage II, island arc volcanic islands were constructed on the basaltic rocks formed during Stage I. Stage III (latest Berriasian‐Valanginian) is characterized by the formation of pull‐apart basins accompanied by seafloor spreading. Widespread upwelling of the asthenosphere below central Hokkaido may have occurred during this stage. After the cessation of in situ volcanism in Stage IV (Hauterivian), submarine island arc volcanism reoccurred in Stage V (Barremian). In Stage VI (Aptian–Campanian), typical active continental margin volcanism occurred and voluminous granitic batholiths were formed in western Hokkaido

Magnetite microexsolutions in silicate and magmatic flow fabric of the Goyozan granitoid (NE Japan): Significance of partial remanence anisotropy

金沢大学理工研究域地球社会基盤学系Anisotropy of magnetic susceptibility (AMS) has been widely used to infer magmatic flow patterns of granitoids where an appropriate AMS axis is parallel to an alignment of mafic minerals or magnetite. The magmatic flow fabric in Cretaceous granitic plutons from northeastern Japan was verified using an analysis of anisotropy of partial anhysteretic remanent magnetization (ApARM) which further isolates the magnetite subfabrics according to magnetite grain size. The preferred orientation of polysynthetic twins in plagioclase laths and clinopyroxene is discordant with the bulk AMS fabric along outer marginal zones of the granitoid, as shown by image analysis of microphotographs from thin sections cut in orthogonal planes. This suggests that the uncorroborated use of bulk AMS to detect flow fabric in granitoids has risks. Scanning electron microscopy (SEM) reveals that submicroscopic, needle-shaped magnetite inclusions exsolved in anhedral plagioclase and clinopyroxene may explain such anomalous exceptions to the validity of AMS fabrics. Our ApARM measurements show that the ApARM alignment of relatively high-coercive, submicroscopic magnetite inclusions is concordant to the linear orientation of anhedral plagioclase and clinopyroxene. The combination of SEM, AMS, and ApARM was required to confirm the magmatic and submagmatic flow pattern of granitoids in this study and is generally preferable to the use of AMS alone. Copyright 2006 by the American Geophysical Union

Primary Thymic Mucosa-Associated Lymphoid Tissue Lymphoma: Diagnostic Tips

AbstractMucosa-associated lymphoid tissue (MALT) lymphoma arising in the thymus is extremely rare and little is known regarding its clinicopathological features. This study examined the clinicopathological features of nine cases of thymic MALT lymphoma. Most patients had autoimmune disease or hyperglobulinemia, and they also had cysts in the tumors. Both increased serum autoantibody levels and polyclonal serum immunoglobulin levels remained essentially unchanged after total thymectomy in all patients. Thymic MALT lymphoma needs to be included in the differential diagnosis in Asian patients with a cystic thymic mass accompanied by autoimmune disease or hyperglobulinemia

Missing western half of the Pacific Plate: Geochemical nature of the Izanagi-Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles

The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous

Late Jurassic–Early Cretaceous intra-arc sedimentation and volcanism linked to plate motion change in northern Japan

The Sorachi Group, composed of Upper Jurassic ophiolite and Lower Cretaceous island-arc volcano-sedimentary cover, provides a record of Late Jurassic–Early Cretaceous sedimentation and volcanism in an island-arc setting off the eastern margin of the Asian continent. Stratigraphic changes in the nature and volume of the Sorachi Group volcanic and volcaniclastic rocks reveal four tectonic stages. These stages resulted from changes in the subduction direction of the Pacific oceanic plate. Stage I in the Late Jurassic was characterized by extensive submarine eruptions of tholeiitic basalt from the back-arc basin. Slab roll-back caused rifting and sea-floor spreading in the supra-subduction zone along the active Asian continental margin. Stage II corresponded to the Berriasian and featured localized trachyandesitic volcanism that formed volcanic islands with typical island-arc chemical compositions. At the beginning of this stage, movement of the Pacific oceanic plate shifted from northeastward to northwestward. During Stage III, in the Valanginian, submarine basaltic volcanism was followed by subsidence. The Pacific oceanic plate motion turned clockwise, and the plate boundary between the Asian continent and the Pacific oceanic plate changed from convergent to transform. During Stage IV in the Hauterivian–Barremian, in situ volcanism ceased in the Sorachi–Yezo basin, and the volcanic front migrated west of the Sorachi–Yezo basin

Geology, petrology and tectonic setting of the Late Jurassic ophiolite in Hokkaido, Japan

The Gokurakudaira Formation, which has a N–S zonal distribution within a latest Jurassic greenstone belt in Hokkaido Island, Japan, constitutes the uppermost ultramafic–mafic unit of the Horokanai Ophiolite. The following three hypotheses for the origin of the ophiolite have been proposed: (1) a mid-oceanic ridge; (2) an oceanic plateau; and (3) an island arc. The Gokurakudaira Formation can be subdivided into four zones extending NNW to SSE, from east (Zone I) to west (Zone IV), based on lithofacies and areal distribution. Zones I and III consist of aphyric tholeiite resembling back-arc basin basalt (BABB), while Zone II is characterized by the coexistence of BABB-like tholeiite along with high-Mg andesite. Zone IV has a different lithology from the other zones, and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite (IAT) type andesitic subaqueous pyroclastic deposits and terrigenous sediments. These stratigraphic and petrological characteristics of the Gokurakudaira Formation cannot be explained by the oceanic plateau or mid-oceanic ridge models, but they could correspond to the marginal sea model, as in the Lau Basin. Therefore, we conclude that the Horokanai Ophiolite was formed in the Late Jurassic in a marginal basin above a supra-subduction zone on the margin of the Asian continent