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The composition of trench sediment at convergent margins exhibits strong compositional links to the corresponding arc magmas. To test for the existence of such links at the Hikurangi margin, we provide a first geochemical characterisation of the IODP sites U1518 and U1520 drilled at the upper and incoming plate respectively. The identification of a décollement in the trench sediments allows distinction of the accreting clastic sediment above the décollement and the subducting pelagic and volcaniclastic material below it. The accreted material and that located above the décollement show homogenous Pb, Sr and Nd isotope ratios with bulk compositions of 206Pb/204Pb: 18.8953, 207Pb/204Pb: 15.6552, 208Pb/204Pb: 38.8371, 87Sr/86Sr: 0.70935, 143Nd/144Nd: 0.51241. In contrast, the material located below the décollement shows more heterogeneous Pb-Sr-Nd isotope ratios and bulk compositions of 206Pb/204Pb: 20.4826, 207Pb/204Pb: 15.7355, 208Pb/204Pb: 39.9833, 87Sr/86Sr: 0.70752, and 143Nd/144Nd: 0.51286. Tectonic erosion in the Hikurangi margin implies that accreted material could subsequently be subducted from the underside of the upper plate. Therefore, this characterisation is of prime importance for future work to constrain the role that subducted materials play in the compositional variations of the arc magmatism associated with subduction along the Hikurangi margin. [ABSTRACT FROM AUTHOR]

Monogenetic volcanism in the Maniototo Basin in the Dunedin Volcanic Group was unusual because the eroded lavas, plugs and dikes comprise spatially and temporally restricted silica-saturated and silica-undersaturated magmas. 40Ar/39Ar dating coupled with existing data and field relationships indicate that volcanism was focused at ∼11 Ma. Major and trace element data show the volcanic rocks to have mafic compositions and little evidence for fractionation and/or crustal contamination, but to have been variably altered. The volcanic rocks comprise an alkaline group and an alkaline to subalkaline transitional group, with one intermediate unit. With the exception of one flow, the transitional group has more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb than the alkaline group, as well as smaller Ti and K primitive mantle-normalised anomalies. Overall, the different magma compositions cannot be related to varying degrees of melting of equivalent mantle sources. Trace element ratios indicate that residual Ti and K-bearing mantle phases (amphibole, phlogopite) probably played a major role in the source of alkaline group but a smaller role in the transitional group. Therefore, the varying compositions of the Maniototo volcanic rocks relates to melting of a heterogeneous lithospheric mantle, or/and varying degrees of interaction with mantle hydrous metasomes as the magmas ascended. [ABSTRACT FROM AUTHOR]

The Sungun copper–molybdenum porphyry deposit is located in the north of Varzaghan, northwestern Iran. The Sungun quartz-monzonite is the oldest mineralized intrusive body in the region and was emplaced during the Early Miocene. Eight categories of the late and unmineralized dykes, which include quartz diorite, gabbrodiorite, diorite, dacite, microdiorite and lamprophyre (LAM), intrude the ore deposit. The main mineral phases in the dykes include plagioclase, amphibole and biotite, with minor quartz and apatite and secondary chlorite, epidote, muscovite and sericite. The composition of plagioclase in the quartz diorite dykes (DK1a, DK1b and DK1c) varies from albite-oligoclase to andesine and oligoclase to andesine; in the diorite, it varies from andesine to labradorite; in the LAM, from albite to oligoclase; and in the microdiorite (MDI), it occurs as albite. Amphibole compositions are consistent with classification as hornblende or calcic amphibole. Based on their AlIV value (less than 1.5), amphibole compositions are consistent with an active continental margin affinity. The average percentage of pistacite (Ps) in epidotes formed from alteration of plagioclase and ferromagnesian minerals is 27–23% and 25–30%, respectively. Thermobarometric studies based on amphibole and biotite indicate approximate dyke crystallization temperature of 850–750℃, pressure of 231–336 MPa and high fO2 (>nickel-nickel-oxide buffer). The range of mineral compositions in the postmineralization dyke suite is consistent with a genetic relationship with the subduction of the Neotethys oceanic crust beneath the continental crust of the northwest part of the Central Iranian Structural Zone. Despite the change from calc-alkaline to alkaline magmatism, the dykes are likely related to the late stages of magmatic activity in the subduction system that also generated the porphyry deposit.

The elemental and isotopic properties of garnet pyroxenites can yield information on lithospheric mantle composition, thermal state, and evolution. The 34Ma Kakanui Mineral Breccia in New Zealand contains spectacular but little-studied mantle peridotite and pyroxenite xenoliths that yield new insights into the evolution of a portion of the underlying mantle lithosphere of a former Gondwana margin. The moderately depleted and metasomatized spinel peridotites, as judged from spinel and olivine compositions and bulk rock major and platinum group element abundances, give mineral equilibration temperatures <1020°C and are derived from the middle to shallow (~35 to 50 km) lithospheric mantle when projected onto a 70 mW∙m−2 geotherm. These residues have low Re/Os and Re-depletion 187Os/188Os model ages that range from Eocene (0.05 Ga) to Paleoproterozoic (1.9 Ga), consistent with extraction from a lithospheric mantle comprising fragments with complex depletion histories. Although the peridotites have restricted δ18O (olivine +5.2 to 6.2), evidence for an isotopically heterogeneous mantle column in addition to the 187Os/188Os is seen in clinopyroxene 87Sr/86Sr (0.70244 to 0.70292), εNd (+4.1 to 18.8), 206Pb/204Pb (17.8 to 20.3), and εHf (+10 to +101). Higher metamorphic equilibrium temperatures of the garnet pyroxenites (Fe–Mg exchange of >1150°C) compared to the peridotites indicate their Eocene extraction was from towards the base of this isotopically heterogeneous mantle lithosphere. Pyroxenite bulk compositions point to cumulate origins, and the mineral isotope ratios of 87Sr/86Sr (0.70282 to 0.70294), εNd (+5.5 to 8.0) and 206Pb/204Pb (18.1 to 19.3) match many of the Zealandia metasomatized mantle peridotite xenoliths as well as the primitive intraplate basalts but not the garnet pyroxenite host magmas. In contrast to many global pyroxenite studies, the garnet pyroxenite 87Sr/86Sr and δ18O (+5.2 to 5.8) data provide no evidence for subducted crustal material in the primary magma source region, and Sm–Nd and Lu–Hf isotope data yield mid-Cenozoic ages that are probably related to isotope closure during eruption. An exception is one sample that yields a Lu–Hf isochron age of 111.9 ± 9.1 Ma, which corresponds to the convergence of the Lu–Hf isotope evolution curves of three other samples. Liquids calculated to have been in equilibrium with these cumulates have trace element compositions comparable to primitive alkaline intraplate basalts like those found at the surface of Zealandia. The new data, therefore, indicate that a pulse of intraplate magmatism occurred during or directly after the cessation of long-lived subduction on the former Zealandia Early Cretaceous forearc Gondwana margin, despite any volcanic surface exposure having been long eroded away. The lower lithospheric mantle emplacement of the garnet pyroxenites suggests that the source of the alkaline parent magmas was probably the convecting mantle, which supports conclusions that intraplate magmas in Zealandia have asthenospheric and lithospheric mantle sources. [ABSTRACT FROM AUTHOR]

Heavy rare earth elements (HREEs) are critical metals in advanced technologies. Here, we report unusual REE patterns and O–Hf–Nd isotopic compositions of composite granites from the Zhaibei HREE deposit. The complex consists of two Mesozoic granites with ages of 188 and 170 Ma. The older granites are characterized by higher HREE contents and negative Ce anomaly and contain cerianite veinlets. Their REE-rich minerals also show strong Ce depletion, indicating a high oxidation state. Magmatic zircons have variables δ18O (6.1–9.4‰) and εHf(t) (−6 to 0), towards a depleted mantle contribution. Lower δ18O (~5.4 vs 6.6‰) and higher εNd(t) (approximately −3 vs −11) values in the altered apatite rim compared to the core and primary phase result from subduction-derived fluids. In contrast, the younger granites have lower REE contents and lack a Ce anomaly. They show mantle-derived Nd (apatite εNd(t) = 1–2), Hf (zircon εHf(t) = 3–8), and O (zircon and apatite δ18O = 4.6–5.7‰) isotopic compositions. Therefore, we infer that a REE-rich fluid migrated from the flat subducting slab and flushed the subsolidus granitic mush in the Early Jurassic. This caused volatile saturation and degassing, leading to the generation of highly oxidized fluids. These processes may cause temporary oxidizing conditions in post-magmatic hydrothermal stages, resulting in the oxidation of Ce3+ to Ce4+. The resulting Ce fractionation from other REE3+ promoted HREE enrichment in the granites. [ABSTRACT FROM AUTHOR]

Jeju Island is an intraplate volcanic island with enigmatic origins, located on the continental shelf south of the Korean Peninsula. A dense temporary seismic array, operated on Jeju Island from 2013 to 2015, revealed several important constraints on the magma plumbing system of Jeju Island. In this study, we determined the deep crustal seismic structure beneath Jeju Island from the teleseismic P -to- S converted phases (receiver functions) recorded from 20 temporary and three permanent stations. We removed the contribution of near-surface reverberations in the resulting receiver functions by applying a resonance removal filter. We estimated crustal P -to- S velocity ratio (VP / VS) and discontinuity depth to provide teleseismic constraints on the composition and structure. We observed two major seismic discontinuities, which are the upper boundaries of a mid-to-lower crustal low-velocity zone (LVZ) and the Moho transition zone. The depth to the upper boundary of the LVZ is deep in the west and southeast (24–30 km) and shallow in the northeast (8–11 km). The LVZ can be interpreted as an extensively distributed residual magma plumbing system, with magma batches stalled at various levels and at various degrees of crystallization, consistent with the chemical diversity of Jeju magmas. The seismic characteristics of the Moho transition zone vary greatly among regions. The top interface of the Moho transition zone is at a wide range of depth (26–40 km), and is shallow at 26–29 km depths beneath central Jeju, suggesting a complex Moho topography. The presence of mafic cumulates and partially molten mushes may contribute to the observed shallow seismic discontinuity at a depth of 26–29 km. The lack of obvious crustal thickening below the shield volcano, Mt Halla, may be associated with mantle upwelling or presence of mafic underplating and cumulates below Jeju. Spatial variations of crustal VP / VS represent highly heterogeneous crustal composition, resulting from magma differentiation during the evolution of the island. [ABSTRACT FROM AUTHOR]