Hideo Ishizuka

Geochemical evidence for a mid-Cretaceous superplume

Basalt lavas with a high Nb/Y ratio for a given Nb/Zr ratio occur in the Polynesian “superswell” region of the South Pacific, which probably formed by upwelling of a deep-mantle superplume. The distinctive geochemical characteristics of the Polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic (ancient mid-oceanic-ridge basalt) component. Basalts displaying such chemical signatures have been found on Shatsky Rise, the Ontong Java Plateau, and greenstones from subduction-zone complexes of Sakhalin Island. The occurrence of Polynesian-type basalts, together with an estimate of their ages, suggests that the South Pacific superplume was active as long ago as 90–150 Ma. The superplume activity preceded the onset of the superchron, supporting an idea that the superplume acted as a trigger for such a global event.

Sr isotope geochemistry and hydrothermal alteration of the Oman ophiolite

The Sr isotopic behavior in hydrothermally altered rocks of a complete section through the Wadi Fizh oceanic crust in the Oman ophiolite was investigated in order to evaluate seawater-rock interaction through the oceanic crust. On the basis of rock-types, secondary mineral assemblage, alteration temperature, average 87Sr/86Sr of hydrothermal fluids, and degree of alteration, the section is divided into the following 5 sequences: (1) mainly basalts and clay minerals, low alteration temperature, average 87Sr/86Sr 0.70634, and 78% degree of alteration; (2) mainly basalts and chlorite, clay minerals and calcite, fairly low alteration temperature, average 87Sr/86Sr 0.70584, and 76% degree of alteration; (3) basalts and sheeted dike diabase and prehnite-actinolite facies, moderate alteration temperature, average 87Sr/86Sr 0.70519, and 85% degree of alteration; (4) dike diabase, plagiogranite, metagabbro and epidosite and prehnite-free greenschist facies, high alteration temperature, average 87Sr/86Sr 0.70514, and 65% degree of alteration; and (5) noncumulate and cumulate gabbro and amphibolite facies, very high alteration temperature, average 87Sr/86Sr 0.70413, and 85% degree of alteration. Alteration (metamorphic) grade deduced from secondary mineral assemblages generally increases versus stratigraphic depth. Only 15% of the rocks have escaped hydrothermal alteration. The estimated strontium isotopic compositions of hydrothermal fluids are much more water-dominated than those reported previously from modern and ophiolitic hydrothermal systems. On the basis of ideal model analysis we conclude that the whole sequence of oceanic crust in the Wadi Fizh section has experienced seawater recharge (downflow zone). The evidence suggests that the Wadi Fizh section was located close to a segment boundary of the oceanic crust along a spreading axis.

Activation of Pacific mantle plumes during the Carboniferous: Evidence from accretionary complexes in southwest Japan

The identification of ancient activity of large igneous provinces, which may be surface manifestations of giant mantle plumes, is of primary importance in understanding the evolution of the Earth system, because such major magmatic redistribution could have profound effects on the global environment. The immobile trace element compositions of metamorphosed basaltic rocks called greenstones found in accretionary complexes of southwest Japan suggest origins in accreted oceanic lavas at Pacific hotspots, rather than at mid-oceanic ridges. Furthermore, some of these greenstones have geochemical characteristics typical of hotspot lavas in the present superswell region of the South Pacific Ocean. Available age data show that the hotspot lavas formed at 350–300 Ma. Greenstones with eruption ages identical to those in southwest Japan are widely distributed in the circum-Pacific region. These observations provide evidence for activity of a giant Pacific mantle plume during the Carboniferous. The onset of such giant plume magmatism may have preceded that of the geomagnetic quiet zone.

Geodynamic evolution of Mt. Riiser-Larsen, Napier Complex, East Antarctica, with reference to the UHT mineral associations and their reaction relations

Abstract Mt. Riiser-Larsen is the largest outcrop in the Archaean–early Proterozoic Napier Complex, East Antarctica. The area is structurally divided into the Main and the Western Blocks by the subvertical Riiser-Larsen Main Shear Zone (RLMSZ) of about 200 m width composed of mylonite and pseudotachylite. Mineral parageneses including sapphirine+quartz and osumilite, diagnostic of ultrahigh-temperature (UHT) metamorphism, are found in Mg-rich aluminous, siliceous and quartzo-feldspathic gneiss layers in both the Main and the Western Blocks of the Mt. Riiser-Larsen area. Some of the sapphirine–quartz associations are accompanied by retrograde reaction textures, which include growth of cordierite and/or garnet between sapphirine and quartz in the Main Block, and of orthopyroxene+sillimanite in the Western Block. These textures indicate the reaction 1 and 2 in the Main Block and 3 in the Western Block. Phase equilibria and P–T pseudosections for sapphirine+quartz-bearing associations suggest that these three reactions took place during a temperature drop from 1100 °C to 1000 °C at pressures of 0.6–0.8 GPa in the Main Block and 0.8–0.9 GPa in the Western Block. The geological structure and distribution of the UHT rocks provide an insight into the vertical extent of the>1000 °C UHT metamorphic zone: a minimum thickness of 4–5 km of the UHT-metamorphosed layers, which become deeper towards the west in the Main Block. The Western Block represents a c. 0.1–0.3 GPa (c. 3–10 km) deeper structural level than the Main Block. In addition to the extent of the horizontal distribution of UHT metamorphism in the Napier Complex, our results on the vertical component provide new constraints for modelling the heat source and tectonic process of the unusually high-temperature regional metamorphism in the late Archaean–early Proterozoic. Electron microprobe monazite U–Th–Pb dating for hydrated and mylonitized sapphirine–quartz gneiss gave a wide spectrum of monazite age distribution between 2300 and 800 Ma, suggesting the tectonic uplift and juxtaposition of the two blocks in the Mt. Riiser-Larsen area later than the mid–late Proterozoic.

Pumpellyite from zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan

Pumpellyite occures in zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan, filling veins or amygdules, replacing igneous plagioclase or clinopyroxene or olivine, and occupying the matrix. Its composition and pleochroism vary greatly even within a single sample, but appear to be related to its mode of occurrence. Thus, the most Al-rich pumpellyite with pale green to green pleochroism develops in pseudomorphs after plagioclase, whereas the most Fe*-rich variety with deep green to brown pleochroism occurs in the matrix. In low-grade metamorphic rocks which commonly contain relict minerals, chemical equilibrium is attained only locally. This results in the correlation of the composition of pumpellyite with its mode of occurrence, such as the precursor phases which are replaced by pumpellyite. On the other hand, among pumpellyites occurring in similar mode and coexisting with Ca-zeolite (laumontite or wairakite), epidote, chlorite and quartz, the Al content tends to be enriched in the wairakite-bearing metabasites over the laumontite-bearing metabasites. It follows that the composition of pumpellyite is also dependent upon the temperature of metamorphism.

Early to middle Proterozoic dykes in the Mt. Riiser-Larsen area of the Napier Complex, East Antarctica: tectonic implications as deduced from geochemical studies

Abstract NE–SW- and north–south-striking dykes were emplaced into ultrahigh-temperature (UHT) granulites apparently after UHT metamorphism in the Mt. Riiser-Larsen area of the Archaean Napier Complex, East Antarctica, of which the north–south-striking dykes interrupt the NE–SW-striking ones. The NE–SW-striking dykes are tholeiite basalt (THB) and high-magnesian andesite (HMA) in composition. The THB dykes display relict doleritic textures, whereas the HMA dykes shows blastoporphyritic textures characterized by phenocrysts of clinopyroxene and plagioclase. Both sets of dykes exhibit large ion lithophile element and light rare earth element enrichment and negative anomalies of Nb, Ti and/or P in a spider diagram normalized to primitive mantle, which is reminiscent of modern subduction-related arc volcanism or continental flood volcanism. The isotope ratios of the THB dykes define isochron ages of 2.0–1.9 Ga: 1979±80 Ma in the Rb–Sr system (initial ratio (I0): 0.70239±0.00035) and 2078±104 Ma in the Sm–Nd system (I0: 0.50964±0.00012). Such moderate 87Sr/86Sr and low 143Nd/144Nd initial ratios may represent source materials closely related to the mantle wedge of a subduction zone. The north–south-striking dykes are compositionally divided into two basalt types. One is an alkaline basalt (AL) showing intergranular texture and characterized by high concentrations of incompatible elements, similar to those of ocean island basalt. They yield an isochron age of c. 1.2 Ga: 1161±238 Ma in the Rb–Sr system (I0: 0.7047±0.0012). The other type (THB-m) is doleritic (ophitic) in texture, and has a tholeiitic affinity with a flat chondrite-normalized REE pattern, which is comparable with that of enriched mid-ocean ridge basalt. A comparison with dykes reported from other areas of the Napier Complex suggests that the north–south-striking dykes occur in restricted areas, whereas the NE–SW-striking dykes are more regional in occurrence. The 2.0–1.9 Ga magmatism of the NE–SW-striking dykes may have been related to the formation of continental crust of the Rayner Complex.

An overview of geological studies of JARE in the Napier Complex, Enderby Land, East Antarctica

Abstract Subsequent to the reconnaissance fieldwork in 1982, the Japanese Antarctic Research Expedition (JARE) carried out extensive geological studies that focused on structural and tectonic aspects, petrology, geochemistry and geochronology of the Napier Complex in Enderby Land, East Antarctica. Detailed field investigations in several key areas, including geological mapping of the Mt. Riiser-Larsen area and Tonagh Island, revealed that the Napier Complex comprises layered and massive gneiss units, of which the layered unit is composed of garnet felsic gneiss, orthopyroxene felsic gneiss, pelitic and basic gneisses, impure quartzite, and minor metamorphosed banded iron formation, whereas the massive unit consists mainly of orthopyroxene felsic gneiss. The boundary between the units is transitional in the Mt. Riiser-Larsen area, in which metamorphosed anorthosite and ultramafic rocks occur as thin layers, or blocks or pods, but on Tonagh Island the boundary is closely associated with the shear zone. Nine deformation episodes (D1–D9) were suggested for Tonagh Island. These results of fieldwork were presented in detail in two geological maps. Geochemical studies showed that (1) garnet–sillimanite gneisses and garnet-rich felsic gneisses were derived from mudstone and sandstone, respectively, both enriched in MgO, Cr and Ni; (2) orthopyroxene felsic gneisses have a close REE affinity with Archaean tonalite–trondhjemite–granodiorite (TTG); (3) basic gneisses were derived from light rare earth element (LREE)-enriched or -depleted basalts; (4) meta-ultramafic rocks are comparable with komatiite and related depleted mantle peridotite. This suite of protoliths is reminiscent of Archaean greenstone–granite belts. Precise analyses of physical conditions of metamorphism were carried out by using reliable approaches such as feldspar thermometry, alumina content of orthopyroxene, inverted pigeonite and bulk-rock compositions, and clino- and orthopyroxene compositions with different textures (porphyroblastic and neoblastic), and the results suggested that the maximum metamorphic temperature might have reached 1130 °C (i.e. ultrahigh-temperature (UHT) metamorphism). P–T evolution of the Napier UHT metamorphism was examined by analyses of reaction textures combined with fluid inclusion studies, suggesting both clockwise (Bunt Island) and counterclockwise (Mt. Riiser-Larsen and Tonagh Island) P–T–t paths. U–Pb sensitive high-resolution ion microprobe and secondary ionization mass spectrometry zircon ages from the Mt. Riiser-Larsen area and Tonagh Island indicate three stages of protolith formation at around 3.28–3.23, 3.07 and 2.68–2.63 Ga, and two contrasting ages for the timing of peak UHT metamorphism at either c. 2.55 or c. 2.51–2.45 Ga. On the basis of these results, more comprehensive studies on the Napier Complex are essential in the future for understanding (1) the role and age of TTG protolith and (2) the origin and timing of UHT metamorphism.