Hirokazu Maekawa

Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific

In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, and antigorite seems to coexist stably with them. Iron-rich secondary olivine (Fo 86–90 ) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo 90–92 ). The assemblage shows high-temperature conditions of serpentinization at ~450–550 °C, whereas chrysotile- and/or lizardite-bearing assemblages occur at ~200–300 °C. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may reflect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.

Significance of Serpentinites and Related Rocks in the High-Pressure Metamorphic Terranes, Circum-Pacific Regions

A large number of serpentinite seamounts in the Izu-Ogasawara-Mariana forearc reflect serpentinite diapirism under a tensional stress field. The Franciscan Complex, California, and the Kamuikotan metamorphic belt, Japan, are accompanied by serpentinite mélange including deep, high-grade metamorphic rocks, whereas the Sanbagawa metamorphic belt, Japan, contains lesser amounts of serpentinite and is not accompanied by serpentinite mélange. The former were probably formed under a tensional stress regime, and the latter under a compressional one. The restricted occurrence of serpentinite to high-grade pelitic schists in the Sanbagawa belt suggests that the serpentinites were transported from the mantle wedge into pelagic sediments on the top of the subducting plate at the initiation of accretion. Tremolite rocks and phengite-chlorite schists near serpentinite in the Sanbagawa belt and from Chamorro Seamount, Mariana forearc, may suggest widespread metasomatism along the boundary between the mantle-wedge serpentinite and pelagic sediments.

Mineralogy and origin of Mlakawa albitite from Kurdistan region, northeastern Iraq

For the first time, albitite was found in the Iraq Zagros thrust zone near the village of Mlakawa, 60 km northeast of Sulaimani City, Kurdistan region, northeastern Iraq. It occurs as a white pod within the massive tectonized and serpentinized part of the Penjwin ophiolite sequence. Based on the preserved texture and mineralogical, petrological, and geochemical data from the core of the albitite pod, a plagiogranite protolith of Mlakawa albitite was inferred. It has undergone rodingitization and blackwall formation along its rim. The occurrence of barium aluminosilicate (celsian), cymrite, barium muscovite, and a high Na2O concentration (11 wt%) of albitite suggests that barium-sodium–rich fluid was involved during the albitization process of plagiogranite. Evidence of the progressive albitization includes the metasomatic replacement of Caplagioclase to albite and grossular, celsian to cymrite, replacement of tremolite by edenite, and newly formed sheaf-like barium muscovite. The presence of analcime and multiple generations of chlorite suggests that the albitite protolith was accompanied by chloritization and retrograde metamorphism before and after the albitization process. Ca-amphibole thermobarometry and the occurrence of strontium apatite and cymrite suggest that the albitization of plagiogranite occurred at <650 °C and 1.5 GPa.

Origin of titanite in metarodingite from the Zagros Thrust Zone, Iraq

Abstract Three types of metamorphic titanite have been noted in a metarodingite pod within the tectonized and serpentinized peridotite portion of the Penjwin ophiolite sequence within the Iraqi Zagros Thrust Zone (IZTZ), northeastern Iraq. Type I metamorphic titanite occurs as subhedral to anhedral finegrained disseminated crystals within chlorite that was formed during chloritization of biotite as a result of low-temperature (T = 330-340 °C) ocean-floor metamorphism or rodingitization of plagiogranite under a reducing environment. It is characterized by intermediate Al2O3 (Avg: 3.61 wt%), high FeOT (Avg: 0.89 wt%), and intermediate TiO2 (Avg: 34.7 wt%). Type II metamorphic titanite occurs as a thin rim around ilmenite and has high Al2O3 (Avg: 4.8 wt%), intermediate FeOT (Avg: 0.6 wt%), and low TiO2 (Avg: 33.7 wt%); it represents a reaction product between grossular and ilmenite, which resulted from an oxidizing high-pressure-high-temperature (P = 1.4-1.6 GPa and T > 750 °C) metamorphic event involving plagiogranite. Type III metamorphic titanite occurs as very coarse, highly fractured grains up to 0.5 mm, with inclusions of ilmenite, and surrounded by albitic plagioclase, analcime, and chlorite. This titanite is characterized by low Al2O3 (Avg: 1.23 wt%), low FeOT (Avg: 0.30 wt%), and high TiO2 (Avg: 36.98 wt%). It formed during extensive titanitization of ilmenite by a reaction with Ca-plagioclase during moderate pressure and temperature (P < 1.6 GPa and T < 750 °C) conditions, as a result of albitization of Ca-plagioclase in plagiogranite.

Sheath folds found in the Sanbagawa metamorphic rocks at Nushima, the southern end of Hyogo Prefecture

兵庫県三原郡南淡町沼島は, 淡路島とは中央構造線で境されており, 兵庫県で唯一, 三波川変成岩類が分布する島である(藤本ほか, 1976). 島全域が三波川変成岩類によって占められており, 北部で緑色片岩, 南部で泥質片岩が卓越する. 全域にわたって泥質片岩中に黒雲母と曹長石が含まれることから, 曹長石・黒雲母帯に属すると考えられる. 最近, 筆者等は, 沼島北瑞部黒崎東方で, さや状褶曲(sheath fold)の良好な露頭を発見した. さや状褶曲の伸びの方向は, 引き伸ばし線構造の方向(N60-80°E 10-30°E)に一致する. さや状褶曲をなす岩石は珪質の泥質片岩～石英片岩で, 藍閃石, バロア閃石, アルカリ輝石, 黒雲母, 白雲母, ザクロ石, 緑泥石, 方解石, 石英, 曹長石, 石墨からなる. さや状褶曲は, 今回報告する露頭周辺にも多数認められる.

Geochemistry of peridotites from ODP Leg 125 diapiric serpentinites

Refractory spinel peridotites were drilled during Leg 125 from two diapiric serpentinite seamounts: Conical Seamount in the Mariana forearc (Sites 778-780) and Torishima Forearc Seamount (Sites 783-784) in the Izu-Ogasawara forearc. Harzburgite is the predominant rock type in the recovered samples, with subordinate dunite; no lherzolite was found.The harzburgite is diopside-free to sparsely diopside-bearing, with modal percentages of diopside that range from 0% to 2%. Spinels in the harzburgites are chrome-rich (Cr/[Cr + Al] = 0.38-0.83; Fe3+/[Fe3+ + Cr + Al] = 0.01-0.07). Olivine and orthopyroxene are magnesian (Mg# = 0.92). Discrete diopsides reveal extreme depletion of light rare earth elements. Primary hornblende is rare. The bulk major-element chemistry shows low average values of TiO2 (trace), Al2O3 (0.55%) and CaO (0.60%), but high Mg# (0.90).These rocks are more depleted than the abyssal peridotites from the mid-oceanic ridge. They are interpreted as residues of extensive partial melting (= 30%), of which the last episode was in the mantle wedge, probably associated with the generation of incipient island-arc magma, including boninite and/or arc-tholeiite. These depleted peridotites probably represent the residues of melting within mantle diapirs that developed within the mantle wedge.