Adakitic Andesites Research Articles

Dredge petrology of the boninite‐ and adakite‐bearing Hahajima Seamount of the Ogasawara (Bonin) forearc: An ophiolite or a serpentinite seamount?§

Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr♯ 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body.

Geochemistry of late Mesozoic adakites from the Sulu belt, eastern China: magma genesis and implications for crustal recycling beneath continental collisional orogens

Both low-Al and high-Al adakitic andesites erupted at ∼ 114 Ma in the Sulu collisional belt, eastern China, provide evidence for recycling of continental crust into the mantle more than 100 million years after the Triassic (∼ 240 Ma) collision between the North China and Yangtze blocks. These rocks display similar normalized trace element patterns, with enrichments in large ion lithophile elements (LILE), light rare earth elements (LREE) and depletions in Nb, Ta and Ti, and have highly radiogenic Sr and non-radiogenic Nd isotopic compositions (high-Al: 87Sr/86Sr(i)=0.70645–0.70715 and εNd(t)=−20.1 to −19.1; low-Al: 87Sr/86Sr(i)=0.70593–0.70598 and εNd(t)=−17.1 to −15.8). The high-Al (Al2O3 > 15 %) adakitic andesites are compositionally comparable with experimental slab melts, whereas the low-Al series (Al2O3 ∼ 13 %) have higher MgO, Cr and Ni, and higher Sr/Y ratios, and are compositionally comparable with slab melts hybridized by mantle peridotites. Combined major- and trace-element and Sr–Nd isotope data indicate that the two types of adakitic andesites have been derived from a LILE- and LREE-enriched eclogitic lower continental crust; in the case of the high-Al adakitic andesites, the melts underwent insignificant mantle contamination, whereas the low-Al magmas reacted with peridotites. Generation of the two types of late Mesozoic adakitic andesites favours a model of lithospheric delamination, leading to asthenospheric upwelling and extensive melting of lower continental crust, including a delaminated block, in the Sulu belt.

Geochemistry of high-Mg andesites and adakitic andesite from the Sanchazi block of the Mian-Lue ophiolitic melange in the Qinling Mountains, central China: Evidence of partial melting of the subducted Paleo-Tethyan crust.

Major and trace element and Nd and Pb isotopic compositions of volcanic rocks from the Sanchazi (SCZ) block of the Mian-Lue ophiolitic melange in the Qinling Mountains, central China were analyzed to provide insights into the subduction processes during the Late Paleozoic along the margins of the paleo-Tethyan ocean. All igneous rocks from the SCZ block show characteristics typical of arc volcanic rocks such as depletion in high field-strength elements (HFSE) and enrichment in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalts (N-MORB). Included among these arc rocks are high-Mg andesites (HMAs) and an adakitic andesite. The adakitic andesite has a steep REE pattern, low Y and Yb, and high La/Yb values, which is similar to those of typical adakites derived by partial melting of subducted basaltic crust. The HMAs from the SCZ block also have adakitic affinities except that they have lower La/Yb (∼10) and Al2O3 (<15 wt.%). Both the adakitic andesite and HMAs have higher MgO, Ni, and Cr contents than the normal andesites and basalts. The age-corrected Nd-Pb isotopic compositions of the HMAs and adakitic andesite show an enriched isotopic signature relative to the MORB-type rocks in the Mianlue area, suggesting that the former probably contain components from sediments or continental crust materials. The data reveal that the adakitic andesite and HMAs in the SCZ block were most probably produced by partial melting of an eclogitic oceanic crust and/or subducted sediments, followed by interaction of the melt with the overlying mantle wedge, and finally by a possible contamination of the melt by the crustal materials. Our result infers that seafloor spreading was active in the paleo-Tethyan ocean, and an ancient island arc system was present along the proto-Qinling area in the Late Paleozoic. The paleo-Tethys ocean separated the South China and North China blocks in Late Paleozoic. The final integration between above two blocks is suggested to have occurred in the Triassic.

富山県南部の中新統岩稲累層のアダカイト質安山岩，高マグネシア安山岩，カルクアルカリ系列安山岩およびソレアイト系列安山岩の成因

富山県南部の中新統岩稲累層のアダカイト質安山岩，高マグネシア安山岩，カルクアルカリ系列安山岩およびソレアイト系列安山岩の成因

Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone

All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49–54°S) have erupted exclusively adakitic andesites and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively thin crust ( 1), followed by limited interaction of this melt with the overlying mantle (≥90% MORB melt, ≤10% mantle), but only very little (≤1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of Burney (52°S), Reclus (51°S) and northernmost AVZ (49–50°S) andesites and dacites require melting of a mixture of MORB and subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant northwards in the AVZ as the angle of convergence becomes more orthogonal.

Lead isotopic compositions of Neogene volcanic rocks from the Aegean extensional area

Lead isotope composition has been determined for a diverse suite of Cenozoic igneous rocks in the central and north Aegean Sea. These include voluminous Miocene shoshonitic and calc-alkali volcanic rocks and correlative plutonic phases; middle Miocene adakitic (high Mg) andesite and associated rhyolite; and middle Miocene to Quaternary ne-normative basalt, trachyte and K-rich andesite. The trace-element contents of most of the studied rocks suggests that Pb-isotopic composition is not a result of upper-crustal contamination, but rather is a feature of the mantle source for K-rich rocks. This mantle source is represented by primitive mafic to intermediate volcanic rocks with 87Sr86Sr ≈ 0.7095, high 207Pb204Pb and 208Pb204Pb, >1000 ppm Sr and Ba, and >35 ppm Pb. The high 207Pb204Pb requires a metasomatic origin that lowered U/Pb and increased Th/Pb, followed by a second-stage residence time of >1 Ga. This K-rich magma was generated as a result of late Tertiary extension of the Aegean. In the South Aegean Arc, there is more evidence for crustal contamination, either from present crust or from subduction of Nile-derived sediment.