Early Cretaceous paleogeography of Korea and Southwest Japan inferred from occurrence of detrital chromian spinels

The Sindong Group was deposited in the north–south trending half‐graben Nakdong Trough, southern Korean peninsula. The occurrence of detrital chromian spinels from the Jinju Formation of the Sindong Group in the Gyeongsang Basin means that the mafic to ultramafic rocks were exposed in its provenance. The chromian spinels from the Jinju Formation are characterized by extremely low TiO₂ and Fe³⁺. Moreover, their range of Cr# is from 0.45 to 0.80 and makes a single trend with Mg#. The chemistry of chromian spinels implies that the source rocks for chromian spinels were peridotites or serpentinites, which originated in the mantle wedge. To more narrowly constrain their source rocks, the Ulsan and Andong serpentinites exposed in the Gyeongsang Basin were examined petrographically. Chromian spinels in the Andong serpentinite differ from those of the Jinju Formation and those in the Ulsan serpentinite partly resemble them. Furthermore, the Jinju chromian spinel suite is similar to the detrital chromian spinels from the Mesozoic sediments in the Circum‐Hida Tectonic zone, which includes the Nagato Tectonic zone in Southwest Japan and the Joetsu Belt in Northeast Japan. This suggests that the basement rocks, which were located along the main fault bounding the eastern edge of the Nakdong Trough, had exposures of peridotite or serpentinite. It is possible that the Nakdong Trough was directly adjacent to the Circum‐Hida Tectonic zone before the opening of the Sea of Japan (East Sea).     

Chemical characteristics of chromian spinel in plutonic rocks: Implications for deep magma processes and discrimination of tectonic setting

We summarize chemical characteristics of chromian spinels from ultramafic to mafic plutonic rocks (lherzolites, harzburgites, dunites, wehrlites, troctolites, olivine gabbros) with regard to three tectonic settings (mid‐ocean ridge, arc, oceanic hotspot). The chemical range of spinels is distinguishable between the three settings in terms of Cr# (= Cr/(Cr + Al) atomic ratio) and Ti content. The relationships are almost parallel with those of chromian spinels in volcanic rocks, but the Ti content is slightly lower in plutonics than in volcanics at a given tectonic environment. The Cr# of spinels in plutonic rocks is highly diverse; its ranges overlap between the three settings, but extend to higher values (up to 0.8) in arc and oceanic hotspot environments. The Ti content of spinels in plutonics increases, for a given lithology, from the arc to oceanic hotspot settings by mid‐ocean ridge on average. This chemical diversity is consistent with that of erupted magmas from the three settings. If we systematically know the chemistry of chromian spinels from a series of plutonic rocks, we can estimate their tectonic environments of formation. The spinel chemistry is especially useful in dunitic rocks, in which chromian spinel is the only discriminating mineral. Applying this, discordant dunites cutting mantle peridotites were possibly precipitated from arc‐related magmas in the Oman ophiolite, and from an intraplate tholeiite in the Lizard ophiolite, Cornwall.     

Mineral chemistry and petrogenesis of chromitites from the Khoy ophiolite complex,Northwestern Iran: Implications for aggregation of two ophiolites

The Khoy ophiolitic complex in Northwestern Iran is a part of the Tethyan ophiolite belt, and is divided into two sections: the Eastern ophiolite in Qeshlaq and Kalavanes (Jurassic–Cretaceous) and the Western ophiolite in Barajouk, Chuchak and Hessar (Late Cretaceous). Our chromitites can be clearly classified into two groups: high‐Al chromitites (Cr# = 0.38–0.44) from the Eastern ophiolite, and high‐Cr chromitites (Cr# = 0.54–0.72) from the Western ophiolite. The chromian spinels in high‐Al chromitite include primary mineral inclusions mainly as Na‐bearing diopside and pargasite with subordinate rutile and their formation was probably related to reaction between a MORB (mid‐ocean‐ridge basalt)‐like melt with depleted harzburgite, possibly in a back‐arc setting. Their host harzburgites contain clinopyroxene with higher contents of Al₂O₃, Na₂O, Cr₂O₃, and TiO₂ relative to Western harzburgites and are possibly residue after moderate partial melting (~15 %) whereas the Western harzburgite is residue after high partial melting (~25 %). The chromian spinel in the Western Khoy chromitites contains inclusions such as clinopyroxene, olivine and platinum group mineral‐bearing sulfides. These Western chromitites were possibly formed at two stages during arc growth and are divided into the moderately high‐Cr# chromitites (Barajouk and Hessar) and the high‐Cr# chromitites (Chuchak A and C). The former crystallized from island‐arc‐tholeiite (IAT) melts during reaction with the host depleted harzburgites, whereas the latter crystallized from boninitic melts (second stage melt) during reaction with highly depleted harzburgite in a supra‐subduction‐zone environment. Based on the mineral chemistry of chromian spinels, pyroxenes, and mineral inclusions, the chromitites and the host peridotites from the Eastern and Western Khoy ophiolites were formed in a back‐arc basin and arc‐related setting, respectively. The Khoy ophiolitic complex is a tectonic aggregate of the two different ophiolites formed in two different tectonic settings at different ages.     

Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence

Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO^*/FeO^* + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.     

Chrome spinel in normal MORB‐type greenstones from the Paleozoic–Mesozoic Mino terrane,East Takayama area,central Japan: Crystallization course with a U‐turn

Phenocrystic chrome spinel crystallized in normal MORB‐type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe²⁺) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe³⁺ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe²⁺ and Fe³⁺. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe²⁺‐ and Fe³⁺‐rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe²⁺‐ and Fe³⁺‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr‐ and Fe²⁺‐rich to Al‐ and Mg‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe²⁺) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe²⁺)–Cr/(Cr+Al) diagram, exhibits a U‐turn, and appears to be set on a double‐lane route. The U‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.     

Coda Q Estimates in the Andaman Islands Using Local Earthquakes

The attenuation property of Andaman Island has been investigated analyzing coda waves from 57 local earthquakes in the magnitude range of 2.0–4.9, using the single backscattering model. These earthquakes waveforms, recorded on five broadband seismographs sited over the island from north to south during Nov. 2003 to March 2004, have been used to calculate the frequency dependent Coda Q (Q _c ) applying the time domain coda-decay method. The Coda Q, computed at central frequencies from (0.5–12) Hz and five-lapse time windows from 40 to 80 s, progressively increases from 105 f ^0.88 in the north Andaman to 135 f ^0.79 in the south Andaman with an average of 119 f ^0.80. The average Q _c values vary from 75 ± 42 at 0.5 Hz to 697 ± 54 at 12 Hz central frequency for 40 s lapse time window, while for 80 s lapse time window its variation is from 117 ± 38 at 0.5 Hz to 1256 ± 115 at 12 Hz. The Q _c estimated at different lapse times manifests a significant variation from 122f ^0.75 to 174f ^0.73, corresponding to lapse time window lengths of 40 and 80 s, respectively. The variation of Q _c with frequency, lapse time and also with the location of seismograph reflects the marked structural and compositional inhomogeneity with depth along the Andaman Islands. These observations are well correlated with the seismicity pattern and distinct high angle subduction beneath the island.     

Podiform chromitites in the lherzolite-dominant mantle section of the Isabela ophiolite, the Philippines

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr? (Cr/(Cr + Al) atomic ratio = ～0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr? chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H₂O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr? of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.     

Occurrence of chromian,hercynitic spinel (“pleonaste”) in Apollo-14 samples and its petrologic implications

Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An₈₀-An₉₄), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide.     

Morphology and composition of spinel in Pu’u ’O’o lava (1996–1998), Kilauea volcano, Hawaii

The morphology and composition of spinel in rapidly quenched Pu’u ’O’o vent and lava tube samples are described. These samples contain glass, olivine phenocrysts (3–5 vol.%) and microphenocrysts of spinel (0.05 vol.%). The spinel surrounded by glass occurs as idiomorphic octahedra 5–50 μm in diameter and as chains of octahedra that are oriented with respect to each other. Spinel enclosed by olivine phenocrysts is sometimes rounded and does not generally form chains. The temperature before quenching was calculated from the MgO content of the glass and ranges from 1150°C to 1180°C. The oxygen fugacity before quenching was calculated by two independent methods and the log f_O2 ranged from −9.2 to −9.9 (delta QFM=−1). The spinel in the Pu’u ’O’o samples has a narrow range in composition with Cr/(Cr+Al)=0.61 to 0.73 and Fe²⁺/(Fe²⁺+Mg)=0.46 to 0.56. The lower the calculated temperature for the samples, the higher the average Fe²⁺/(Fe²⁺+Mg), Fe³⁺ and Ti in the spinel. Most zoned spinel crystals decrease in Cr/(Cr+Al) from core to rim and, in the chains, the Cr/(Cr+Al) is greater in the core of larger crystals than in the core of smaller crystals. The occurrence of chains and hopper crystals and the presence of Cr/(Cr+Al) zoning from core to rim of the spinel suggest diffusion-controlled growth of the crystals. Some of the spinel crystals may have grown rapidly under the turbulent conditions of the summit reservoir and in the flowing lava, and the crystals may have remained in suspension for a considerable period. The rapid growth may have caused very local (μm) gradients of Cr in the melt ahead of the spinel crystal faces. The crystals seem to have retained the Cr/(Cr+Al) ratio that developed during the original growth of the crystal, but the Fe²⁺/(Fe²⁺+Mg) ratio may have equilibrated fairly rapidly with the changing melt composition due to olivine crystallization. Six of the samples were collected on the same day at various locations along a 10-km lava tube and the calculated pre-collection temperatures of the samples show a 5°C drop with distance from the vent. The average Fe²⁺/(Fe²⁺+Mg) of the spinel in these samples shows a weak positive correlation with decreasing MgO in the glass of these samples. The range in Cr₂O₃ (0.041–0.045 wt.%) of the glass for these six samples is too small to distinguish a consistent change along the lava tube. The spinel in the Pu’u ’O’o samples shows a zoning trend in a Cr–Al–Fe³⁺ diagram almost directly away from the Cr apex. This compares with a zoning trend in rapidly quenched MORB samples away from Cr coupled with decreasing Fe³⁺. The trend away from Cr displayed by spinel in rapidly quenched samples is in marked contrast to the trend of increasing Fe³⁺ shown by spinel in slowly cooled lava.     

Chromitite and peridotite from Rayat, northeastern Iraq, as fragments of a Tethyan ophiolite

Ophiolitic rocks (chromitites and serpentinized peridotites) were petrologically examined in detail for the first time from Rayat, in the Iraqi part of the Zagros thrust zone, an ophiolitic belt. Almost all the primary silicates have been altered out, but chromian spinel has survived from alteration and gives information about the primary petrological characteristics. The protolith of the serpentinite was clinopyroxene-free harzburgite with chromian spinel of intermediate Cr# (= Cr/[Cr + Al] atomic ratio) of 0.5 to 0.6. The harzburgite with that signature is the most common in the mantle section of the Tethyan ophiolites such as the Oman ophiolite, and is the most suitable host for chromitite genesis. Except for one sample, which has Cr# = 0.6 for spinel, the Cr# of spinel is high, around 0.7, in chromitite. The variation in Cr# of spinel in chromitite observed here has been also reported in the Oman ophiolite. The peridotite with chromitite pods exposed at Rayat was derived from an ophiolite similar in petrological character to the Oman ophiolite, one of the typical Tethyan ophiolites (fragments of Tethyan oceanic lithosphere). This result is consistent with the previous interpretation based on geological analysis.     

Petrochemical evidence for off-ridge magmatism in a back-arc setting from the Yakuno ophiolite, Japan

Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]_N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe³⁺? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]_N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO₂ contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin.     

Possible sub-arc origin of podiform chromitites

Abstract The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc-related primitive magmas. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high-field strength elements (HFSE) to large-ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.     

Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Abstract A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse‐grained and fine‐grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high X_Mg and low Al₂O₃ contents, whereas clinopyroxenes have low TiO₂ and Al₂O₃ contents. Amphiboles and biotites have a high Mg/(Mg + Fe²⁺) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al₂O₃, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid‐oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc‐alkaline basalt and andesite with an island‐arc affinity. The rock has a high ¹⁴³Nd/¹⁴⁴Nd ratio with ?Nd(0) > 0, indicating derivation from a mantle‐depleted source. In the present study, a two‐stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc‐alkaline basalt and andesite with island‐arc affinity were subducted into a deeper level of the crust and subjected to granulite‐facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.     

Geochemistry and petrogenesis of ultramafic and mafic plutonic rocks of the Dras ophiolitic melange, Indus suture (northwest Himalaya)

Geochemical data are presented for a suite of ultramafic and related rocks from the Dras ophiolitic melange of the Indus suture zone in the western Himalaya. Harzburgites from the suite have highly refractory chemistry. Lherzolites from the suite represent modified mantle material and are comparable to potential source rocks for MORB and to the lherzolites of the Bay of Islands ophiolites. Cumulus dunite, pyroxenite and gabbro units contain olivine, chromite, clinopyroxene and plagioclase as the major cumulus phases. Orthopyroxene is absent as a cumulus phase and in this respect the Dras ophiolite differs from the Marum, Betts Cove and Troodos ophiolites. However, the Dras cumulates are similar to the Vourinos and Bay of Islands cumulate sequence and are consistent with accumulation of low-pressure liquidus phases of mid-oceanic ridge-type magmas. Magmas parental to the Dras cumulate rocks contained high 100 Mg/(Mg + Fe²⁺) ratios of 77–79, high Ni, Cr and possibly Ca, low Ti and depleted LREE. Parent magmas were probably similar to those of normal MORB formed by two-stage (or dynamic) melting processes in the mantle. Peridotite fabrics suggest high-temperature plastic (mantle) deformation. Disruption, serpentinisation and melange formation were probably produced during emplacement in the Indus suture zone.     

Oceanic mantle beneath the southern Rio Grande rift

Major element and modal trends in spinel lherzolites from Salt Lake Crater, Hawaii, and Kilbourne Hole, New Mexico, are essentially identical. These trends have developed through 5–30% partial melting of the same pyrolite-like parent composition. The melting events probably took place at mid-ocean ridges.Using Cr/(Cr + Al + Fe³⁺) in spinel as an index of the degree of depletion in fusible components in each of a large number of lherzolite inclusions, we have subdivided both suites into relatively parental, intermediate, and refractory groups, and computed an average bulk composition for each group. Compositions of hypothetical melts generated from these rocks are characterized by high MgO (about 18%), low Ti and K, and CaO/Al₂O₃ around 1.0.The strongly similarity between the two xenolith suites suggests that south central New Mexico is underlain by oceanic mantle. We propose that the Kilbourne Hole lherzolites represent oceanic lithosphere which was buried beneath offshore sediments during a Precambrian ocean basin formation, later to be preserved when subduction further offshore created new additions to the continent above that near-shore oceanic lithosphere. In addition, we propose that the Mg-rich, Ti- and K-poor calculated melts reported here are the primitive precursors to mid-ocean ridge basalts. Early dunite and wehrlite cumulates from some ophiolite sequences may record the fractionation histories these liquids follow prior to eruption at the surface.     

Slab-fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites,South Kitakami Belt,northeast Japan

The present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto-East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine-grained antigorite and lack of olivine, and the latter have coarse-grained antigorite and inclusion-rich olivine. Ca-amphibole occurs as isolated crystals or vein-like aggregates in the Type 1 serpentinites and as needle-shaped minerals in the Type 2 serpentinites. Ca-amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca-amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca-amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto-East Asian continental margin.     

Review of the source characteristics of the Great Sumatra–Andaman Islands earthquake of 2004

The December 26, 2004 Sumatra–Andaman Island earthquake, which ruptured the Sunda Trench subduction zone, is one of the three largest earthquakes to occur since global monitoring began in the 1890s. Its seismic moment was M ₀ = 1.00 × 10²³–1.15 × 10²³ Nm, corresponding to a moment-magnitude of M _w = 9.3. The rupture propagated from south to north, with the southerly part of fault rupturing at a speed of 2.8 km/s. Rupture propagation appears to have slowed in the northern section, possibly to ∼2.1 km/s, although published estimates have considerable scatter. The average slip is ∼5 m along a shallowly dipping (8°), N31°W striking thrust fault. The majority of slip and moment release appears to have been concentrated in the southern part of the rupture zone, where slip locally exceeded 30 m. Stress loading from this earthquake caused the section of the plate boundary immediately to the south to rupture in a second, somewhat smaller earthquake. This second earthquake occurred on March 28, 2005 and had a moment-magnitude of M _w = 8.5.     

Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay

Textural-petrographic features, major and trace elements and mineral chemistry are reported for spinel-peridotite xenoliths from northeastern Brazil (NE-suite) and Paraguay (PY-suite).Variation trends defined by bulk rock and mineral chemistry are consistent with the effects of variable degree of high-pressure melting for both suites.The PY-suite strongly differs from the NE-suite by its very high contents of incompatible elements. This different whole-rock chemistry is related to the presence of glassy-microcrystalline blebs which are interpreted as remains of pre-existing hydrous phases of metasomatic origin. The difference between the two suites is also recorded in the chemistry of pyroxenes and spinels, which develop along different variation patterns.Although the suites partially overlap in their ranges ofmg_opx (Mg/(Fe²⁺ + Fe³⁺) ratio in orthopyroxenes), the PY-suite is, on the average, moremg_opx rich (residual) than the NE-suite. Within the PY-suite a rough positive correlation exists between degree of residuality and degree of metasomatic effects.The sharp differences between the NE-suite and the PY-suite imply mantle heterogeneity on regional scale, whereas the variability within each suite is essentially related to different degrees of melting and/or metasomatism and imply mantle heterogeneity on local scale.     

Shallow seismicity, stress distribution and crustal deformation pattern in the Andaman-West Sunda arc and Andaman Sea, northeastern Indian Ocean

Shallow seismicity and available source mechanisms in the Andaman–westSunda arc and Andaman sea region suggest distinct variation in stressdistribution pattern both along and across the arc in the overriding plate.Seismotectonic regionalisation indicates that the region could be dividedinto eight broad seismogenic sources of relatively homogeneousdeformation. Crustal deformation rates have been determined for each oneof these sources based on the summation of moment tensors. The analysisshowed that the entire fore arc region is dominated by compressive stresseswith compression in a mean direction of N23^°, and the rates ofseismic deformation velocities in this belt decrease northward from 5.2± 0.65 mm/yr near Nias island off Sumatra and 1.12 ±0.13 mm/yr near Great Nicobar islands to as much as 0.4 ±0.04 mm/yr north of 8^°N along Andaman–Nicobar islandsregion. The deformation velocities indicate, extension of 0.83 ±0.05 mm/yr along N343^° and compression of 0.19 ±0.01 mm/yr along N73^° in the Andaman back arc spreadingregion, extension of 0.18 ± 0.01 mm/yr along N125^° andcompression of 0.16 ± 0.01 mm/yr along N35^° in NicobarDeep and west Andaman fault zone, compression of 0.84 ±0.12 mm/yr N341^° and extension of 0.77 ± 0.11 mm/yralong N72^° within the transverse tectonic zone in the Andamantrench, N-S compression of 3.19 ± 0.29 mm/yr and an E-Wextension of 1.24 ± 0.11 mm/yr in the Semangko fault zone ofnorth Sumatra. The vertical deformation suggests crustal thinning in theAndaman sea and crustal thickening in the fore arc and Semangko faultzones. The apparent stresses calculated for all major events range between0.1–10 bars and the values increase with increasing seismic moment.However, the apparent stress estimates neither indicate any significantvariation with faulting type nor display any variation across the arc, incontrast to the general observation that the fore arc thrust events showhigher stress levels in the shallow subduction zones. It is inferred that theoblique plate convergence, partial subduction of 90^°E Ridge innorth below the Andaman trench and the active back arc spreading are themain contributing factors for the observed stress field within the overridingplate in this region.     

Chromite in the Paricutin lava flows (1943–1952)

Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo_80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe²⁺/(Fe²⁺+Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe³⁺/(Fe³⁺+Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe³⁺/(Fe³⁺+Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H₂O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H₂O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H₂O>1 wt.%, and the log f_O2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H₂O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites.