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Serpentinite plays a crucial role in element mobility during seawater hydrothermal alteration of mid-ocean ridge peridotite on the seafloor and fluid metasomatism of peridotite in the subduction zone. However, element mobility and Mg isotope fractionation during serpentinization of peridotites are still uncertain. In order to address this issue, we present a geochemical study of serpentinites and their associated peridotites in the Xigaze ophiolite from southern Tibet. The serpentinites have lower MgO contents and Mg/Si ratios than the peridotites. The low Mg/Si ratios in serpentinites, along with the Isocon calculation results, indicate the Mg loss during serpentinization. In terms of trace elements, the serpentinites have higher contents of Ba, U, Sr and Pb than the peridotites, though both types of ultramafic rocks exhibit similar REE distribution patterns and HFSE contents. The serpentine-magnetite O isotope thermometry yields a relatively low serpentinization temperatures of 200–300 °C. The trace element composition of the serpentinites is distinct from that of forearc serpentinites but consistent with that of mid-ocean ridge ones. The peridotites show mantle-like δ26Mg values of –0.27 to –0.23‰ with an average of –0.25 ± 0.03‰ (2SD, n = 6). In contrast, the serpentinites have systematically higher δ26Mg values of –0.21 to –0.09‰ with an average of –0.16 ± 0.09‰ (2SD, n = 8). The Mg isotope composition of serpentinites shows no correlations with either the chemical weathering proxy or the talc modal content, indicating that the Mg isotope fractionation between serpentinite and peridotite is not caused by chemical weathering of the serpentinites. Instead, it probably resulted from the serpentinization of peridotite on the seafloor. Thus, there is the loss of not only element Mg but also isotopically light Mg during the serpentinization of peridotites. The isotopically heavy Mg serpentinites can be carried to oceanic subduction zones, giving rise to higher δ26Mg mantle sources for various basalts. The isotopically light Mg can be released into the seawater, regulating the Mg isotope composition of seawater in geological history. Taking into account the sources and sinks of Mg in subduction zone fluids, the formation of isotopically heavy Mg serpentinites during seafloor serpentinization can be probably an important process in changing the Mg isotope composition of mantle sources. [ABSTRACT FROM AUTHOR]

Rodingite is produced by Ca-rich fluid metasomatism of mafic igneous rocks in oceanic lithosphere. Despite its importance for the composition of subduction zone magmas, its formation mechanism still remains obscure. To address this issue, we investigated the major-trace element and Mg–O–Sr–Nd isotope compositions of rodingites in serpentinites from the Xigaze ophiolite, southern Tibet. Coexisting gabbros and rodingites show consistently depleted Sr Nd isotope compositions, suggesting their formation in a mid-ocean ridge setting. The rodingites exhibit variable δ26Mg values from −0.61‰ to −0.14‰, with an average of −0.37‰ ± 0.28‰ (n = 19; 2 SD), mostly lower than those of −0.26‰ to −0.13‰ for the protolith gabbros. Compared to the relatively homogeneous δ18O values of 6.1‰–6.7‰ for the gabbros, the rodingites show highly variable δ18O values from 3.8‰ to 9.4‰. The rodingites can be subdivided into two groups based on mineral assemblages and formation conditions. Group I rodingite is prehnite-dominated and hydrogarnet-free, formed at temperatures of 200–300 °C, and has higher δ18O values of 8.1‰ ± 1.4‰ (n = 15) and δ26Mg values of −0.32‰ ± 0.19‰ (n = 12). Group II rodingite consists mainly of hydrogarnet and clinopyroxene, formed at temperatures of 300–400 °C, and has lower δ18O values of 5.4‰ ± 2.5‰ (n = 7) and δ26Mg values of −0.44‰ ± 0.35‰ (n = 7). Modeling of the fluid–rock interactions between the protolith gabbro and fluid was undertaken using the Mg O isotope compositions of the serpentinizing fluid inferred from the associated serpentinites. Based on the modeling results, we propose that the metasomatism of gabbros by Ca-rich, low-δ26Mg fluids produced during serpentinization would result in the Mg O isotope compositions of the rodingites. Such rodingites with low δ26Mg values would generate the geochemical heterogeneity in the mantle wedge overlying the subducting oceanic lithosphere and eventually give rise to Ca-rich, low-δ26Mg basaltic magmas. The results indicate that low δ26Mg values of basaltic rocks cannot be unambiguously ascribed to carbonated mantle sources. • The Xigaze rodingites show low δ26Mg and variable δ18O values. • The rodingites were formed through metasomatism of gabbros by serpentinizing fluids under seafloor. • The subduction of rodingites in subduction zones can greatly affect mantle Mg isotopic composition. [ABSTRACT FROM AUTHOR]

The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher 176Hf/177Hf ratios than their protolith zircons. More interestingly, the 176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated 176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution. • Peritectic zircon have higher Lu/Hf and Hf isotopic ratios than protolith zircons. • Peritectic zircon also have higher HFSE contents than protolith zircons. • Minerals with high Lu/Hf ratios than zircon were decomposed during crustal anatexis. [ABSTRACT FROM AUTHOR]

Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al 2 O 3 and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic. • The harzburgite experienced melt extraction at the mid-ocean ridge; • The harzburgite was metasomatized by two types of subduction zone fluids; • The fluid metasomatism is closely associated with subduction initiation; • The Xigaze ophiolites were generated through the ridge-trench conversion. [ABSTRACT FROM AUTHOR]

• Triassic rocks in northern Qiangtang Block include basalts-rhyolites (237–235 Ma). • Basalts were originated from metasomatized mantle sources. • Rhyolites were generated from newly formed basaltic lower crust. • The formation of studied suite was related to Paleo-Tethys oceanic slab subduction. Triassic magmatic rocks are widely exposed in the northern Qiangtang Block in Tibet. However, their petrogenesis and relationship to the evolution of Tethys Ocean are matters of debate. Here we report zircon U-Pb ages, mineral compositions, whole-rock major and trace elements, and Nd isotope for newly reported Triassic volcanics in order to provide constraints on the formation of these rocks in Tethys ocean's evolution. The volcanic suite consists of grey-porphyritic basalts (B1), black-aphyric basalts (B2), and rhyolites. SIMS, LA-ICPMS zircon U-Pb age dating shows that they were erupted at 237–235 Ma. All rocks show no continuous trends in major-element compositions, and are enriched in Light Rare Earth Elements (LREE) relative to heavy REE (HREE) with various Eu-anomalies. Basalts have narrow εNd(t) range (−0.90 to −0.01), slightly higher than those of rhyolites (−4.36 to −0.66). Given similar Nd-isotope and REE patterns, B1 and B2 basalts were probably produced from a common parental magma by mineral accumulation and fractional crystallization during late- and early-stages of magma evolution, respectively. Based on indicative element-ratios of slab-derived materials, we suggest that parental magmas of these basalts were derived by partial melting of metasomatized mantle source with subducted oceanic slab component. Rhyolites were generated by partial melting of basaltic lower-crust as a result of underplating by magma that corresponds to the studied basalts. Given spatial–temporal distribution of Triassic magmatic-metamorphic rocks, and geological background, we suggest that magmatism of studied suite was triggered by northward subduction of the Paleo-Tethys ocean, and recorded late-stage of Paleo-Tethys Wilson cycle. [ABSTRACT FROM AUTHOR]