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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Zinc isotopic systematics of the Mt. Baekdu and Jeju Island intraplate basalts in Korea, and implications for mantle source lithologies

    Sung Hi ChoiSheng-Ao Liu
    11页
    查看更多>>摘要：Zinc isotope data for late Cenozoic intraplate basalts from Mt. Baekdu (North Korea) and Jeju Island (South Korea) were determined to characterize their mantle source lithologies. The average d~(66)Zn values (relative to JMC-Lyon) are 0.34‰±0.08‰(2SD; N = 13) for the Mt. Baekdu basalts and 0.37‰±0.08‰(2SD; N = 12) for the Jeju basalts, which are higher than values for refractory peridotites (0.18‰±0.06‰ from the lithospheric mantle beneath the North China Craton. Given that Zn isotopic fractionation is limited during lithospheric processes, such as crustal contamination, magmatic differentiation, degassing, or thermal/chemical diffusion, the elevated d~(66)Zn values indicate the incorporation of recycled carbonates into the sub-lithospheric mantle source. Plots of Zn/Fe versus d~(66)Zn or d~(26)Mg for the Korean basalts can be explained by mixing of at least three source lithologies: (1) carbonated peridotite; (2) recycled siliciclastic sediments; and (3) eclogite. Strontium-Nd-Pb isotopic systematics show that the mantle source follows the DMM-EM1 array for the Mt. Baekdu basalts and a DMM-EM2 array for the Jeju basalts. Both the enriched components have isotopically light Zn and heavy Mg, which are typical of recycled siliciclastic sediments. Combined with the Pb isotope and trace element data, the Zn isotopic compositions suggest that the EMI component could be ancient K-hollandite-bearing siliciclastic sediments and the EM2 component could be recently recycled clay-rich pelagic sediments.
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    • NSTL

    Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

    Peng YangQiang-tai HuangKai-Jun Zhang
    18页
    查看更多>>摘要：The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong-Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high e_(Nd)(t) values (+6.5 to +9.5) and low initial ~(87)Sr/~(86)Sr ratios (0.7033-0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable e_(Nd)(t) values of -3.7 to +7.3, and low initial ~(87)Sr/~(86)Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded U-Pb ages of 104-100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young U-Pb ages of 100-86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104-100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong-Nujiang Tethys Ocean most likely at 100-86 Ma.
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    • NSTL

    Petrogenesis of Middle Miocene to Early Quaternary basalts from the Karayazi-Goksu plateau (Eastern Anatolia, Turkey): Implication for the role of pyroxenite and lithospheric thickness

    Yavuz OzdemirVural OyanFred Jourdan
    17页
    查看更多>>摘要：In Eastern Anatolia, the Karayazi-Goksu basaltic plateau is one of the basaltic eruption centers of post-collisional volcanism, located between the convergence zone of the Arabian and Anatolian plates. The geological and geochemical evolution of the plateau is documented using new ~(40)Ar/~(39)Ar age data, major-and trace element abundances, mineral geochemistry, and isotope compositions (Sr-Nd-Hf) for better understanding the petrogenesis and mantle source characteristics. Samples yield whole-rock ~(40)Ar/~(39)Ar ages of 12.53 ±0.05 and 12.68 ±0.05 Ma for transitional basalts (Middle Miocene), 5.88 ±0.20 Ma for tholeiitic lavas (Late Miocene), and 2.28 ±0.03 and 2.48 ±0.12 Ma for alkali members (Early Quaternary). The results suggest that the fractional crystallization of olivine and clinopyroxene in transitional, tholeiitic and lower alkaline lavas, and clinopyroxene in upper alkaline lavas, played a crucial role in their magmatic evolution without significant crustal contamination. Clinopyroxene-liquid equilibria indicate that the crystallization of transitional basalts occurred at the Moho and lower crust (temperature range 1157-1215 °C), tholeiitic basalts at the lower crust (temperature range 1154-1195 °C), alkaline basalts at lower to mid-crustal depths (temperature range 1083-1195 °C). The subduction-related metasomatism is in the form of melts derived from subducted sediments and decrease from Middle Miocene transitional to alkaline Early Quaternary basalts, as indicated by the Sr-Nd-Hf isotopes and Ba/ La and Th/Yb contents. The Early Quaternary alkaline basalts contain contributions from isotopically depleted but chemically enriched (enriched-MORB-like) asthenospheric mantle domain. Major elements-based parameters and olivine mineral chemistry yield that Karayazi-Goksu basalts could not be derived from a sole peridotite mantle source; the pyroxenite/eclogite-like mantle component would have contributed to the members of the plateau. Three-component mixing results using Sr-Nd isotopes indicate that the altered oceanic crust contributes to 0.5%-1% of the melt (crust vs. sediment about 65:35) for tholeiitic and alkaline basalts and reach up to 4% for transitional basalts (crust vs. sediment about 55:45). Melt equilibration depths of the basalts indicate that the lithospheric thickness of the region has not been changed on a large scale since the Middle Miocene to the Early Quaternary.
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    • NSTL

    Zircon mineral chemistry and implications for magmatic-hydrothermal evolution of the granite-hosted Zaaiplaats Sn deposit, Bushveld Large Igneous Province, South Africa

    Teimoor Nazari-DehkordiLaurence Robb
    16页
    查看更多>>摘要：This study presents trace element compositions for zircons from a series of co-genetic granitic rocks of the Province (LIP). In this area, the LGS comprises the coarse-grained Nebo and Bobbejaankop granites, and the finegrained Lease granite, of which the two latter are variably altered and host Sn mineralisation. Under cathodoluminescence, zircons of the Nebo granite display long prismatic crystals and well-developed internal oscillatory zoning, whereas those of the Bobbejaankop and Lease granites are porous with faint zoning and vermicular to spongy internal textures. Trace element contents of zircons in the Bobbejaankop and Lease granites show significant enrichments in all the trace elements (e.g., Hf, Nb, Ta, U, Th, Ti, and REE) suggesting a hydrothermal origin for the zircons of the Bobbejaankop and Lease granites, that are different to those for the magmatic zircons of the Nebo granite. Moreover, the inclusion-free hydrothermal zircons of the Bobbejaankop and Lease granites point to lower Ti-in-zircon derived crystallisation temperatures (450-600 °C) and oxygen fugacities [as log (fO2) values in the range - 20 to -26] than those of the Nebo granite [where T = 570-625 °C and log (fO2) values = -19 to -21]. Collectively, the data record an apparent magmatic differentiation from a single magma, showing derivation of the hydrothermal zircons of the Bobbejaankop and Lease granites from a highly-fractionated late-magmatic-hydrothermal fluid enriched in F, CI and B, as indicated by non-chondritic Y/ Ho and Zr/Hf ratios under reducing conditions, as required typically for the formation of the granite-hosted Sn deposits.
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    Complex magmatic processes recorded by clinopyroxene phenocrysts in a magmatic plumbing system: A case study of mafic volcanic rocks from the Laiyang Basin, southeastern North China Craton

    Guangyue CaoHuaimin XueYing Tong
    13页
    查看更多>>摘要：Understanding the evolution of magmatic plumbing systems requires an understanding of the mechanisms of magma ascent from the magma source to the surface. We undertook textural and in situ geochemical studies of clinopyroxene phenocrysts in Early Cretaceous mafic volcanic rocks from the Laiyang Basin, China. Four distinct clinopyroxene populations were recognized based on their textures and zoning patterns in the mafic volcanic rocks (types 1, 2, 3, and 4). Type 1 is reversely zoned and contains rounded, irregularly resorbed, low-Mg cores (Mg# = 60-75) surrounded by clear, high-Mg mantles (Mg# = 75-90). Type 2 phenocrysts are typically normally zoned and characterized by relatively high-Mg cores (Mg# > 85) and thin lower Mg# rims (Mg# = 70-85). Type 3 comprises strong oscillatory zoned crystals (Mg# = 72-85) that are generally normally zoned and, in some cases, exhibit substantial sector zoning. Type 4 crystals are euhedral and unzoned phenocrysts. The textural and compositional features of the clinopyroxenes record complex magmatic processes including fractional crystallization, recycling of early formed crystals, magma convection, and magma recharge and subsequent mixing. Clinopyroxene-melt equilibrium pairs in the volcanic rocks yielded a relatively wide range of temperatures (984-1264°C) and pressures (0.1-10.1 kbar). The P-T array defines two main magma storage zones: (1) a deep, high-T, magma reservoir (～22-28 km), and (2) a shallow, low-T, magma reservoir (～8-19 km). At pressures of >3 kbar, the amount of water in melts in equilibrium with the clinopyroxene is in the range 1.5-3.0 wt.%, but at pressures of <3 kbar, the melt water contents decrease to 0.1 wt.%, indicative of extensive magma degassing. The clinopyroxene phenocrysts have a wide range of ~(87)Sr/~(86)Sr ratios from 0.70683 to 0.71366. The broad range of ~(87)Sr/~(86)Sr ratios (0.70796-0.71171) for the most primitive clinopyroxene (Mg# > 89) might have been inherited from a heterogeneous mande source. However, some type 1 crystals with more radiogenic Sr isotopic compositions may record assimilation of crustal material during evolution of the mantle-derived magma. The textural and compositional features of the studied clinopyroxene phenocrysts suggest that Laiyang volcano is fed by a vertically extensive magmatic plumbing system that extends from shallow to deep levels, in which geochemically distinct magmas crystallized and mixed under variable physicochemical conditions.
      原文链接:
    • NSTL