まえがき

金沢大学大学院自然科学研究科自然計

Basalts and Dolerites in the Sorachi-Yezo Belt, Central Hokkaido, Japan

Basalts and dolerites from the late Jurassic to early Cretaceous Sorachi Group and the Cretaceous melange complex in the central axial zone of Hokkaido are classified into the following three types: high K2O type alkaline basalts, characterized by high abundance in K2O, TiO2, P2O5 and normative nepheline, low K2O type alkaline basalts with high Na2O/K2O ratio and normative nepheline, and MORB type tholeiites, poor in K2O, TiO2, P2O5. The basaltic pillow lavas, dolerite dykes and sheets, observed as autochthonous members of the Upper Sorachi Group, correspond to the low K2O type alkaline basalts. The basaltic pillow lavas in the Lower Sorachi Group are of the MORB type tholeiites. Exotic blocks of basalt and dolerite in the Cretaceous accretionary melange complexes involve both the high K2O type alkaline basalts and the MORB type tholeiites

Petrology of the Horoman Ultramafic Rocks in the Hidaka Metamorphic Belt, Hokkaido, Japan

The Horoman ultramafic massif, covering more than 8×10 km in the Hidaka Metamorphic Belt, is an "alpine-type" peridotite which is a gently warped sheet about 3,000 m in thickness. The massif exhibits a conspicuous layered structure which consists of layers of dunite, lherzolite, plagioclase lherzolite, and a small amount of gabbro and pyroxenite. Coexisting minerals from all the rock types of the layered ultramafic to mafic sequence were analysed by EPMA. The ferromagnesian minerals show large compositional variations in accordance with lithological change in the series of dunite → lherzolite → plagioclase lherzolite → gabbro. Forsterite content of olivine varies successively from Fo92.5 to Fo64.5. Enstatite content of orthopyroxene also varies from En93.0 to En85.5. Large and continuous compositional variation was also obtained for clinopyroxenes, e.g. Ca46Mg51Fe3 from dunite, Ca48Mg48Fe4 from lherzolite, Ca49Mg46Fe5 from plagioclase lherzolite, Ca50Mg44Fe6 from the margin of gabbro, and Ca38Mg39Fe23 from the center of gabbro. Al and Ti contents of clinopyroxcncs and pargasitic amphiboles increase with decrease of the Mg/ Mg + Fe ratio. The mineralogical characteristics indicate that the Horoman layered sequence represents a magmatic series formed by fractional crystallization. The gabbroic seams in the plagioclase lherzolite and the layers of gabbro were probably formed by crystallization of residual liquid which were slightly alkaline in chemical nature. The Horoman ultramafic rocks are strongly modified by deep-seated deformation and recrystallization, and additionally by mylonitization during the up-thrusting intrusion into the Earth's crust. The primary composition of minerals, obtained by step-scanning EPMA analyses are recognized as a distinct and uniform compositional plateaus at the cores of large, porphyroclastic, primary grains. The equilibration temperatures were calculated for the orthopyroxene-clinopyroxene pairs of primary porphyroclasts, using the Opx-Cpx geothermometer. The temperatures range from 900℃ to 1,100℃. The estimates for the neoblastic pyroxene pairs are slightly lower, ranging between 850℃ and 1,000℃. The rocks might have re-equilibrated under the subsolidus conditions in the upper mantle

METASOMATIC VEINS AND MINERALS IN MANTLE-DERIVED XENOLITHS, ANTARCTICA

Late Cenozoic basanites of the McMurdo Volcanic Group, Antarctica, contain numerous ultramafic xenoliths derived from the upper mantle. The xenoliths show multiple-episodes of metasomatism resulting in formation of the veined metasomites and the equilibrated metasomites. The metasomatized xenoliths are characterized texturally and chemically by co-existence of Ti-augite, phlogopitic mica, pargasitic and kaersutitic amphiboles, Al-spinel and apatite with the upper mantle olivine, pyroxenes, and Cr-spinel. Ti-, Al-, Fe- and Ca- enrichments and Si-, Cr- and Mg-depletions observed in the metasomatic clinopyroxenes indicate that the metasomatism was probably generated by an alkaline silicate melt reacting with solid phases of the upper mantle. Ti- and Fe-enrichments and Al-depletion in the metasomatic clinopyroxenes can be also detected as a separate episode of metasomatism different from the above veined-type reactions. Texturally and chemically equilibrated phlogopites and pargasites in the mantle-derived xenoliths provide a different type metasomatism, showing conspicuous depletions in TiO_2,Al_2O_3,Cr_2O_3,and K_2O, and enrichments in SiO_2 and Na_2O

Metamorphosed Dolerite Intrusives from the Western Zone of the Hidaka Metamorphic Belt, Hokkaido

Metamorphosed dolerite intrusives in ultramafic tectonites, cumulates, and metagabbros are first described from the Western Zone of the Hidaka Metamorphic Belt. The dolerite intrusives ranging from a few cm to 30m in thickness occur as a dyke or sheet. Most of the dolerite intrusives are metamorphosed into amphibolite or metadolerite in various degrees. The metamorphic grade attains generally up to the amphibolite facies, and partly to the hornblende granulite facies in the ultramafic tectonites. Primary textures and minerals are well preserved in some metamorphosed intrusives. After mylonitization of the ultramafic tectonite the dolerite magma is considered to have intruded into the tectonites and into the already consolidated cumulate and gabbro complexes. Petrochemical and mineralogical data indicate that all of the dolerite intrusives are co-magmatic and are formed by crystallization of an olivine tholeiite magma which is highly undersaturated in silica and less differentiated

Rodingites from the Kamuikotan Tectonic Belt, Hokkaido

A large number of rodingites were found in and around serpentinite masses of the Kamuikotan Tectonic Belt. The original rocks of the rodingite were also found. They are microdiorite-microgabbro, olivine clinopyroxenite, and slate. Reaction zones between the serpentinite and the original rock are divided into two zones; one is rodingite zone composed mainly of diopside, hydrogrossular, wollastonite, chlorite, prehnite, and tremolite, and the other is chlorite zone which contains large amounts of chlorite. The rodingites are characterized by the higher contents of CaO, Al2O3, TiO2, and H2O and the lower contents of SiO2, Na2O and K2O than those in the original rocks. Compositional variation observed in the reaction zones suggests that the rodingites were formed through the following metasomatic process; (1) Ca, Al, Ti, and H2O migrated from serpentinite into the reaction zone, (2) Si and alkali moved from the original rock towards serpentinite through the reaction zone, and (3) a considerable amount of Mg, Fe, and Mn as well as Ca, Al, Ti, and H2O fixed within the chlorite zone. On the basis of hydrothermal experiments on stability ranges of xonotlite-wollastonite, vesuvianite, diopside, and hydrogrossular, most of the rodingites from the Kamuikotan Tectonic Belt are considered to have been formed at the temperature higher than 350℃

GLASS IN MANTLE-DERIVED PERIDOTITE XENOLITHS FROM THE McMURDO VOLCANIC GROUP, ANTARCTICA

Glass-bearing, mantle-derived peridotite xenoliths have been found in Late Cenozoic basanites at Turtle Rock and McMurdo Station, Ross Island, Antarctica. Glass in these xenoliths occurs as veinlets, interstitial pools, and thin films surrounding the primary xenolith minerals. The glass analyzed has three different compositions. Low-silica high-alkali glasses (42-48wt% SiO_2,4-6wt% Na_2O+K_2O) are likely to represent volatile-rich melts which were incorporated from the host basanite magma. High-silica low alkali glasses (53.5-56wt% SiO_2,4-5wt% Na_2O+K_2O) are non-alkaline and were possibly generated by decompressional breakdown of pargasitic amphibole. High-silica high-alkali glasses (55.5-57 wt% SiO_2,14-15 wt% Na_2O+K_2O) are unique and resemble the chemical composition of the most evolved nepheline-trachyte