Petrogenesis Of Triassic Alkaline Rocks In The Northern North China Craton And Its Insight Into The Deep Carbon Cycle Induced By Paleo-Asian Oceanic Slab Subduction

Deep carbon cycle controls the exchange of material among reservoirs of carbon,which in turn influences the climate system over geological timescales,evolution of life,composition and differentiation of crust and mantle,and mineralization.The subducted slab carried carbonated sediments,sedimentary carbonate rocks,and carbonaed oceanic crust back to the mantle and induced mantle metasomatism.This process mofidies the physical property and chemical composition of mantle,and then affects the later magmatic activity.Potassic alkaline rocks commonly occur in post-orogenic,rift,or intraplate tectonic settings and are formed by the low-degree melting of metasomatized mantle,which are significant carriers for us dephier deep mantle processes.In this study,we present petrological and geochemical studies of Triassic alkaline rock in the northern North China Craton.In particular,the application of in situ analyses and non-traditional stable isotope in investigating the petrogenesis of alkaline rocks and the role of Paleo-Asian oceanic slab subduction related deep mantle recycling of sedimentary carbonate in modifying lithospheric mantle.The petrogenesis of alkaline rocks is a key to our understanding the Paleo-Asian oceanic slab subduction related deep carbon recycling.The alkaline rocks occur in an east–west-trending belt close to the northern edge of the northern North China Craton.In this study,consistent zircon and titanite U–Pb age of?234 Ma is presented for the Baotoudong alkaline rocks and of?235 Ma for the Wujiashan alkaline rocks from the alkaline rock belt.The combination of previous zircon,baddeleyite and eudialyte U–Pb ages shows the emplacement of alkaline rock belt during the Middle-Late Triassic at around 215–235 Ma,which is associated with the extensional event since final closure of the Paleo-Asian Ocean.Based on chemical composition,two types of alkaline rocks were identified:ultramafic-intermediate(e.g.,Baotoudong and Fanshan alkaline rocks)and evolved intermediate alkaline rock(e.g.,Wujiashan alkaline rocks).The Baotoudong alkaline complex comprises biotite pyroxenites,orthoclase pyroxenites,pyroxene syenites,and alkali-feldspar syenites with similar mineral assemblages and trace element patterns,which may be related with the differentiation of primitive magma.Their high large ion lithophile elements content,negative high field strength element anomalies,evolved Sr–Nd isotopic composition and magmatic calcite suggest a metasomatized mantle origin by recycled carbonated melt.Furthermore,the Fanshan alkaline rocks show similar chemical and mineral features with the Baotoudong alkaline rocks,with occurrence of cogenetic carbonatite,indicating that they are derived from a similar mantle source.Moreover,mineral archive provided by in situ analyses of evolved Wujiashan alkaline rocks from the North China Craton are reported to decipher their origin via reconstruction of primary magma composition.The alkaline rocks are nepheline syenites.They are highly evolved(Si O2=58.1–60.0 wt.%)with negative anomalies for Sr,Ba,Ti,P and Eu,and relative depletion of MREE,indicating fractional crystallization of feldspar,apatite,titanite and amphibole.Core-rim variation of ⁸⁷Sr/⁸⁶Sr(0.7052–0.7092)of late-crystallized low Mg#clinopyroxene and feldspar are attributed to crustal assimilation.The origin of primary alkaline magma is recorded in early high Mg#(70–84)clinopyroxenes with weak Eu anomalies(Eu/Eu*=0.94–1.29),suggesting that they crystallized from a little-differentiated mantle-derived magma(Mg#>60).Their variable ⁸⁷Sr/⁸⁶Sr ratios(0.7031–0.7060)correlate positively with Sr/Y and(La/Yb)N and negatively with Ti/Eu ratios,but are not correlated with Mg#or Eu/Eu*.Moreover,the REE patterns of melt equilibrated with high ⁸⁷Sr/⁸⁶Sr clinopyroxene is similar to coeval alkaline rocks from adjacent areas.These features indicate that the primary alkaline magma was derived from the mixing of depleted mantle and enriched mantle metasomatized by carbonatite melts from subducted crustal materials.Low zircon?Hf(t)values(-1.4 to-2.8)imply that the subducted crustal material was carbonated sediments.Calcite and CO2 inclusions occur in early-crystallized olivine,clinopyroxene and zircon,and amphibole and fluorapatite both reveal a volatile-rich(H2O,F and CO2)alkaline magma derived from mantle enriched by carbonate metasomatism.Deep carbon cycle induced by Paleo-Asian oceanic slab subduction not only modifies the chemical composition of mantle,but also affects its oxygen fugacity,which control element distribution and mobility.Triassic ultramafic to intermediate alkaline rocks in the northern North China Craton are characterized by high ⁸⁷Sr/⁸⁶Sr,low?_Nd(t)and zircon?Hf(t),which implies the lithospheric mantle was metasomatized by subducted crustal materials.In addition,Abundant hydrous minerals(e.g.,biotite/amphibole),primary carbonate minerals and cogenetic carbonatite are found in alkaline rocks in the alkaline magma belt,which indicates that these alkaline magmas were enriched in volatile components(H2O and CO2).Compared with MORB,the ultramafic and mafic end-member of alkaline rocks and early-crystallized clinopyroxenes are featured by high(La/Yb)N and low Ti/Eu ratios.These features suggest that the recycled carbonates are widely distributed in the lithospheric mantle beneath the northern North China Craton.This is consistent with the studies of peridotite and carbonatite xenolith from adjacent areas,showing that the subduction of Paleo-Asian oceanic slab carried sedimentary carbonate into deep mantle.The oxidizing sedimentary carbonate may promote the oxidation of the peridotite mantle(e.g.,hematite exsolution in olivine from peridotite)and thus form oxidized alkaline magma via melting.Clinopyroxene from the Baotoudong and Wujiashan alkaline rocks are featured by diopside core,aegirine-augite mantle and aegirine rim.This crystallization trends of clinopyroxene indicate that these alkaline magmas are oxidized.This conclusion is consistent with results from the zircon Ce oxybarometer,suggestive of high oxygen fugacity(?FMQ=+3.1–+4.1).Furthermore,Magnetite and hematite inclusions were found in the early-crystallized minerals from the Baotoudong alkaline rocks including clinopyroxene and apatite.Fluorapatites are characterized by high S(SO3 up to 0.64 wt.%)and low Mn contents,with estimated?FMQ higher than+3.These features together indicate that the primary oxidized magmas were inherited from the metasomatized mantle by recycled carbonate.High?⁶⁶Zn(>0.3‰)of mantle-derived magma reflect the involvement of recycled carbonate(average?⁶⁶Zn of 0.91‰).In order to better identify the contribution of Paleo-Asian oceanic slab subduction related recycled carbonate in modifying lithospheric mantle,we present Zn isotopic composition of the Fanshan and Baotoudong alkaline rocks.In the early-stage magmatic differentiation,the Baotoudong alkaline rocks show limited variation of?⁶⁶Zn(0.244–0.281‰)with dcreasing Mg O and Zn content.This feature implies that the fractional crystallization of clinopyroxene will not induced Zn isotopic fractionation and represent the Zn isotope of mantle source.However,large Zn isotopic variation(0.208–0.392‰)was observed in the late-stage evolution of alkaline magma,correlated with Fe and Zn content,which may be related to fractional crystallization and accumulation of Fe–Ti oxides.Moreover,Zn isotopes in the early-stage evolution of Fanshan alkaline rocks are also homogeneous(?⁶⁶Zn=0.248–0.261‰).A binary mixing model using Zn–Sr isotopes shows that the incorporation of magnesite(high Zn content of 280 ppm)into the mantle will caused heavy Zn isotope,while<40%recycled calcite(low Zn content of 20 ppm)may not induce obvious Zn isotopic variation in mantle-derived magma.Petrological(magmatic calcite and cogenetic carbonatite)and geochemical(crust-like Sr–Nd isotopes,high(La/Yb)N and low Ti/Eu ratios)features of the Fanshan and Baotoudong alkaline rocks are attributed to the recycled carbonate in the mantle.However,the Fanshan and Baotoudong alkaline rocks during early-stage evolution show similar Zn isotope with MORB(?⁶⁶Zn=0.27±0.05‰),which suggest the recycled carbonate in mantle source is limestone or calcite-bearing silicate sediment.In summary,this study reveals the the application of in situ analyses in investigating the origin and magmatic process of alkaline rocks and the role of recycled sedimentary carbonate in modifying the chemical composition and oxygen fugacity of lithospheric mantle.Meanwhile,this study shows the application of Zn isotopes on alkaline magma and deep carbon cycle.These results in this thesis provide more referential meanings for the studies of crust-mantle interaction and deep carbon cycle.