Ages Of Peridotitic Xenoliths From The Central And Eastern Areas Of North China Craton And Mantle Heterogeneity

The Phanerozoic considerable losing of the Archean keels of the North China lithospherehave attracted many attentions in the past few years.The nature and age of the North Chinalithospheric mantle are especially important to understand the thinning mechanism and evolutionof the lithosphere.Peridotite xenoliths entrained in the Cenozoic basalts from the North ChinaCraton are ideal samples for investigating the nature and evolution of the lithospheric mantle.Inthis paper,we present detailed petrologic,geochemical and Re-Os geochronologicalinvestigations for the peridotite xenoliths captured in the Cenozoic alkaline basalts fromHannuoba (northern edge of the Trans-North China Orogen within the craton),Changle-Shanwang and Hebi (interior of the craton),with the aim of constraining the age andnature of lithospheric mantle beneath these areas.Our main conclusions are listed as below:1.The Hannuoba peridotite xenoliths are mainly lherzolites with minor harzburgites (withClinopyroxene content less than 5%);the Changle-Shanwang mantle-derived xenoliths are allmost lherzolites and characterized by high proportion of Clinopyroxene;the Hebi xenoliths aremainly comprised of coarse-grained harzburgites and minor lherzolites.The lherzolites xenolithshave mineral assemblage of olivine+orthopyroxene+Clinopyroxene±Spinel,whereas theharzburgites are characterized by occurrence of olivine+ orthopyroxene±Clinopyroxene±Spinel.2.The olivine Mg^# of the Harmuoba xenoliths range from 86.4 to 92.8,which are slightlywider than the published data for this area.The highest olivine Mg^# of the Hannuoba samples issimilar to that of the Hebi refractory peridotites,which are interpreted as shallow relics of theArchean cratonic mantle;Mg^# of the harzburgites is higher than the lherzolites in the Hannuobaxenoliths.The olivine Mg^# of the Shanwang xenoliths is mainly concentrated in 80.2-90.9.However,the Mg^# of most Hebi olivine is higher than 92.The Mg^# and Cr^# correlation in theclinopyroxene and spinel indicate that the Hebi peridotites are the most refractory and theChangle-Shanwang samples exhibit more fertile characteristics.The Hannuoba peridotites,however,display heterogeneous characteristics,suggesting the co-existence of both refractoryand fertile mantle components underneath this region.The trace element for the studied xenolithshave differences between each other,however,their clinopyroxenes are all characterized byenrichment of Th,U,and depletion of HFSE such as Nb,Ti,Zr and Hf.Xenoliths from all of the studied areas have two distinct REE distribution patterns including LREE enrich type and LREEdepleted type,further indicating a comprehensive melts/fluids metasomatism within thelithospheric mantle beneath the studied areas.The balance temperatures of the peridotites havebeen calculated,which have no relevance with the Mg^# value in olivine.3.Some major element,such as MgO,Mg^# and (Al₂O₃+CaO) which are commonly regardedas indicator of lithospheric mantle refractory,also reveal that the Hannuoba peridotitic xenolithsdisplay both refractory and fertile characteristics.Among the Hannuoba peridotites,the much morerefractory samples have similar geochemical features to those of the Hebi peridotites,and therelatively fertile xenoliths exhibit nearly consistent characteristics with those of theChangle-Shanwang.The peridotites whole-rock trace element composition is in agreement withthat of the clinopyroxenes,both are characterized by U,Th enrichment and HFSE depletion.Meanwhile,the Shanwang and minor Hannuoba peridotites have positive Sr anomaly.The MgOexhibits excellent liner correlation with the compatible element but relatively weak linercorrelation with the incompatible element,suggesting that the peridotites may have undergonecomplicated modification.4.The lithospheric mantle beneath Shanwang area is characterized by enriched feature andhas undergone silicate melts metasomatism,such mantle represents a relatively lower partialmelting (1%-5%) residue and mainly comprises of lherzolites.Most of the Hebi peridotitesrepresent refractory Archean relics that have undergone high proportion of melts exaction(15%-20%) and a later stage carbonatitic melts modification.Lithospheric mantle underneath theHannuoba area,however,exhibits heterogeneous feature,that is,with the coexistence of fertilecomponents (＜5%) and refractory mantle(15%-20%) of different melts exaction proportion,andmainly undergone silicate melts metasomatism with minor carbonatitic metasomatism.5.Re-Os isotope of the sulfides in peridotites and the whole rocks have been analyzed andcontrasted in this paper.The wider range of Re-Os isotopic ratios in sulfides than those in thewhole rock indicates that the Re-Os isotopes have mixed those in sulfides with differentoccurrence and origin.The maximum Re depleted model age (TRD) of the enclosed sulfides inHannuoba peridotitic xenoliths is 2.1 Ga.The enclosed sulfides,with Re-Os isotopic ratios lowerthan primitive mantle,give an isochron age of 2.3±1.2 Ga.However it is Archean era when theisochron line is fitted without the primitive mantle.All of the interstitial sulfides and some of theenclosed sulfides,with higher Re-Os isotopic ratios than primitive mantle,yield an isochron age of645±225 Ma,which might record mantle melt/fluid activity in SCLM in Neoproterozoic.1.3Gaobtained from an interstitial sulfide is interpreted as a thermal event in Mesoproterozoic.Therefore,several periods of the sulfide Re-Os ages in the Cenozoic Hannuoba peridotitic xenolithssuccessfully reveal the frequently-occurring mantle events and thus complex evolution oflithosphere beneath the North China Block.The oldest TRD age calculated by the Re-Os isotopes inperidotitic xenoliths from Changle is 1.2Ga and the Os proxy isochron is 2.1Ga.Therefore,themantle heterogeneity probably is common beneath the North China Craton according to the fertilecharacters of peridotites from Changle.The Archean age (2.5Ga and 3.0Ga) acquired by the Re-Osisotopes of sulfides in Hebi peridotitic xenoliths implies that the present mantle beneath Hebi is the relic refractory mantle.This mantle heterogeneity is mostly the result of the asthenospheric mantlereplacing the aged craton mantle through erosion,intermingling and modification.6.The heterogeneous nature of the lithospheric mantle beneath the central and eastern NorthChina Craton has been discussed,and the simply evolutional processes of the lithospheric mantleduring the Cenozoic period have been proposed in this paper.Such lithospheric mantle architectureis consistent with the mantle evolution model that the eastern North China lithospheric mantle hasundergone heterogeneous asthenospheric materials replacement via erosion and modification.Thepresent age data also indicate that the ancient Archean refractory lithospheric mantle beneath theNorth China has undergone a large scale replacement during 2.3-1.9 Ga in the Paleoprotozoic,with shallow relics preserved in Hebi of the central craton and Hannuoba of the northern edge ofthe Trans-North China Orogen within the craton.This is supported by the geochemical data of theperidotites xenoliths.The Hannuoba lithospheric mantle have undergone frequently occurred deepmantle thermal modification at 1.4-1.2 Ga and 0.9-0.6 Ga in the follow Mesoproterozoic andNeoproterozoic periods,but not affected by the considerable lithospheric mantle replacementunderneath the eastern North China Craton during Phanerozoic.Lithospheric mantle beneathChangle-Shanwang area within the Tanlu big fault has undergone at least two stages of mantlereplacement happened the Paleoprotozoic and Mesozoic.The Tanlu big fault provided a prefectway for the asthenospheric materials upwelling,through which finally replaced the Paleoprotozoiclithospheric mantle underneath Changle-Shanwang area and given formation of a newly accretedfertile mantle,with only minor Proterozoic mantle components preserved.This conclusion ispartly supported by the whole-rock Re-Os isotopic data.