| At convergent plate margins,oceanic lithosphere followed by continental lithosphere,carrying crustal materials sinks into subduction channel and reacts with overlying mantle wedge in different ways and proportions,triggering mantle melting and incidentally creating arc magma.Therefore,arc-type magma and mantle peridotite play essential roles in crust-mantle reaction and understanding the operation between different layers in Earth and deepening the insight into our inhabitable planet.In this study,we investigate newly discovered Erlangping meta-troctolite enclave in Silurian Huanggang diorite,located 2.8 km southern away from Xingji county,Xinyang city,and the Qinling Liushuzhuang peridotite,approximately 56 km west of meta-troctolite.They represent arc magmatic rock and sub-arc lithospheric mantle in the Tongbai orogen,respectively.Through detailed petrology,geochemistry and geochronology analysis,combined with data of sub-lithospheric mantle beneath the southern margin of the North China Craton,we aim to:(1)investigate petrogenesis;(2)reveal the nature of lithospheric mantle,its formation age and metasomatic processes;(3)decipher recycling of paleo-oceanic crustal components in subduction zone and the process of crust-mantle interaction.(1)Meta-troctolites indirectly record crust-mantle interactionMeta-troctolite enclaves recorded the Erlangping mafic magma underplating and granulite-facies metamorphism,thus a significant indicator of crust-mantle interaction.Evidence for this granulite facies event is preserved in coronitic and symplectitic textures developed in the meta-troctolites enclaves.The primary mineral assemblage comprises olivine and plagioclase with minor orthopyroxene,clinopyroxene,spinel and amphibole.The late magmatic stage comprising an orthopyroxene(Opx1)corona around olivine and clinopyroxene corona around spinel formed by reaction of cumulus and intercumulus phases with trapped melt at around 1040-1060 ℃ near the solidus.During troctolite cooling,metamorphic solid-state replacement of primary olivine and plagioclase produced orthopyroxene(Opx1)/clinopyroxene(Cpx1)+ vermicular spinel(Sp1)symplectite,followed by an inclusion-free amphibole(Amp1)layer surrounded by amphibole + spinel symplectite(Amp2 + Sp2).Microstructural,pseudosection and mineral isopleth analysis coupled with prominent compositional zoning in the Opx1 corona and chrome-rich spinel(Cr-Sp)suggests that the symplectites formed by diffusion-controlled solid-state replacement at the interface of reactant minerals,probably during cooling to granulite-facies conditions of ~ 820–880 ℃ and 6.7–7.0 kbar.U-Pb dates of magmatic zircon core yielded a 460 Ma for formation age and rare metamorphic zircon clustered at ca.450 Ma for granulite-facies metamorphism.Metatroctolites show arc-like trace elemental characteristics with enrichment in large iron lithophile elements(LILE)and light rare earth element(LREE),depletion in high field strength element(HFSE),in combination with Tongbai granulite and coeval arc magma,suggesting these microtextures recorded the transition from late magma underplating to granulite facies metamorphism,and probably the caused by decompressed melting triggered by asthenosphere upwelling due to extension of Erlangping back-arc.The geochemistry characteristics of meta-troctolite can indirectly reveal crustmantle interaction.Erlangping meta-troctolite has high Mg O(10.27–14.97 wt.%;Mg#up to 77.4),Cr(up to 265 ppm)and Ni(294 ppm)contents,low Si O2(42.08–45.31 wt.%)contents,suggesting they are derived from mantle source after partial melting.They show enrichment in LILE,LREE,Sr and Pb,depletion in HFSE,indicating their mantle source has modified by crustal materials from subducted slab.Furthermore,their high Ba/Th and Rb/Y ratios,low(La/Yb)N,Th/Nb and Nb/Y ratios imply that the metasomatic agent is dominated by aqueous fluids released from Erlangping oceanic slab.In addition,the low Rb/Sr and high Ba/Rb ratios suggest an amphibole-bearing mantle origin.Thus,we proposed that the aqueous fluids released from subducted Erlangping ocean react with overlying mantle wedge peridotite and form amphibolebearing mantle source,and subsequently at ~ 460 experienced decompressed partial melting to form Erlangping arc-like meta-troctolites triggered by asthenosphere upwelling in response to the opening of Erlangping back-arc.(2)Liushuzhuang peridotites: refractory residue from Neoproterozoic sub-arc lithospheric mantleLiushuzhuang peridotite in the Qinling unit is mainly composed of spinel facies harzburgite and dunite,showing prophyroblastic or mesh texture with or without clinopyroxene.Although Liuzhuzhuang peridotites have variable Al2O3(normalised to100%;1.34–4.18 wt.%),they have high Ni(1845–2389 ppm),low Ca O(0.21–1.23 wt.%)and Mn O(0.14–0.16 wt.%).The high Mg#(90.1–92.2)and Mg/Si(1.35–1.62)are similar with those from the southern margin of the North China Craton,while significantly distinguished from those of cumulate.The whole-rock heavy rare earth elements(HREE)indicate Liushuzhuang peridotite experiences at least 15–20% melt extraction.In addition,most peridotites show PPGE depleted relative to IPGE,and low CI chondrite normalised(Pt/Ir)N = 0.23–0.77,which also suggest they experienced high degree partial melting.Furthermore,the high Ol-Mg#(up to 92.4)and Sp-Cr#(74–87)overlap typical arc peridotite,suggesting Liushuzhuang peridotite represents refractory sub-arc lithospheric mantle beneath the Tongbai orogen.187Os/188 Os values show positive correction with depleted index of bulk rock Al2O3,and negative correlatives Ol-Mg# and Sp-Cr#,implying that the Os isotope of Liushuzhuang peridotite is dominated by partial melting yet has not been significantly modified by later metasomatism.The majority of Liushuzhuang peridotite have lower radiogenic187Os/188 Os values than primitive upper mantle(187Os/188 Os = 0.1296).Among them,three show lower Al2O3,higher Ol-Mg#,Sp-Cr# and lower(Pd/Ir)N and 187Os/188 Os,therefore their Re depleted Os model ages late Neoproterozoic probably represent the formation age of sub-arc lithospheric mantle in the studied area.(3)Liushuzhuang peridotites record complex crust-mantle interaction in supra subduction zoneCompared with meta-troctolite,peridotites are more capable to reveal the process of crust-mantle interaction in more details.Petrologically,Liushuzhuang peridotite have abundant hydrous and volatile minerals,such as amphibole,phlogopite,apatite,calcite and zircon.In addition,amphibole and phlogopite occur in olivine,as well as euhedral spinel,suggesting Liushuzhuang peridotites were intensively metasomatized by complex fluids/melts after partial melting.Amphibole has high Si O2(47.7–53.9 wt.%)and Mg#(90.5–93.7),but low Na2O(1.4–2.3 wt.%),Ti O2(0.3–0.5 wt.%)and Nb(1.2–6.7 ppm)concentrations,as well as high Zr/Nb(2.4–30.1)and Ti/Nb(276–2026)in amphibole and phlogopite,which are consistent with those are related to subduction setting.Apatite has high Cl(1.2–2.7 wt.%),low F(0.1–0.7 wt.%)and high Th and U concentrations(up to 92 and 58 ppm,respectively),which is similar with those was metasomatized by CO2+H2O-rich fluid.Furthermore,the high Ti O2(0.3–1.4 wt.%)abundances in spinel suggest the metasomatic fluids responsible for the formation of Liushuzhuang peridotite should also be Ti-rich.Thus the metasomatic fluids are rich in H2 O,CO2,Si,Zr and Ti.Amphibole and apatite are strongly enriched in LILE and LREE,while depleted in HFSE,which suggests their formation is related to fluids release from subducted slab.The high oxygen fugacity(△logf O2(FMQ)> 2.5)and Sp-Cr# is the robust evidence of subduction setting.Mantle modification by fluids from a subducted slab is partially supported by the positive Eu coupled with Sr anomalies in orthopyroxene and phlogopite.Such positive Eu anomalies in plagioclase-absent rocks may reflect a “ghost plagioclase” effect from recycled oceanic materials on peridotite.Isotope data also support a sub-arc origin to the metasomatic fluids.In situ amphibole and apatite87Sr/86 Sri data spans a range of 0.70323–0.70896 with the least radiogenic being close to MORB mantle but most of the data being more radiogenic(> 0.706)and thus requiring a crustal component.The 87Sr/86 Sri range of amphibole is similar to the bulk compositions of Tongbai arc magmas,which further confirms the modification of Liushuzhuang peridotite by fluids from subducted Shangdan ocean.The Hf isotopes values(εHf(t)= +0.5 to +10.7)of metasomatic zircons also show the mixture of depleted mantle and minor oceanic crust-derived components.Metasomatic zircons have low Th/U ratios,steep MREE/HREE pattern,low HREE abundances but high Yb/Gb N and contain abundance amphibole and phlogopite inclusions,thus the weighted mean 206Pb/238 U of 420 Ma represents an intensive mantle metasomatic episode.In addition,the phlogopite Ar-Ar dates at 395 Ma and in situ UPb dating of apatite in the peridotite,with an age of ca.340 Ma is most likely the cooling event.The country rock gneiss yielded a metamorphic age of 410 Ma,which means Liushuzhuang peridotite has been metasomatized before being incorporated and entrained to shallow crust by gneiss.Inherited cores show dark CL,high Th/U ratios and contain quartz inclusions.Giving that they show similar trace element with those grown in arc magma,as well as variable εHf(t)of-9.81 to +1.7,it is suggested that they have been subducted and incorporated into the mantle wedge beneath the southern part of North China Craton in association with the metasomatic melts at 420 Ma.Thus,not only is there newly grown zircon,but there appear to be components that have been physically incorporated into the peridotites. |
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