Geochemistry And Hf Isotopes Of Zircon Megacrysts In Cenozoic Basalts Along Eastern China And Its Implications For Crust-mantle Interaction Beneath Subcontinental Lithosphere Mantle

Zircon is the most important mineral probe in petrology and geochemistry,and the study on zircon has become a new subject,zirconology.The Cenozoic basalts and their xenoliths in southern,northern and northeastern tectonic units along eastern China occurred a set of zircon megacrysts.The research on the genesis and their deep geological process of zircon megacrysts is very significant for understanding the thinning of SCLM and its dynamics.The internal structures,inclusions,main and minor elements,U-Pb dating and hafnium isotopes of zircon megacrysts from several locations along eastern China were studied by using infrared spectroscopy,laser raman microprobe,electron microprobe analysis,LA-ICPMS and MC-ICPMS.Based on the previous research on petrology,geochemisty and isotopes on basalts in eastern China,this paper obtained the following results on the genesis of zircon megacrysts and their significance in revealing the thinning of subcontinental lithosphere mantle:1.In eastern China,zircon megacrysts are closely associated with alkaline basalts.The alkaline basalts are consistently characterized from south to north and northeast China by Si depletion,alkali enrichment,high Fe/Mn,low Nb/Ta,and occur a variety of mantle xenoliths in which the olivine exhibits the features of undergoing mantle metasomatism,e.g.an average Mg^# of 88.63,and enrichment in fusible components(such as FeO,TiO₂,Al₂O₃,CaO and K₂O+Na₂O).The zircon megacrysts, most having dominant {110} form,show high crystallinity,low metamict degree,and high ZrO₂/HfO₂ ratio.The oscillating internal zones in zircon megacrysts in cathodoluminescence image show that they were probably crystallized in the magma process during the uncontinuous drop of temperature and the obvious absorption between 3500-3700 cm^-1 in infrared images reflects the existence of structural water in zircons.2.All zircon megacrysts from the four locations along eastern China have an inclusion assemblage of mineral-fluid-melt,such as zircon-magnetite-CO₂,still have mineral inclusions of biotite,baddeleyite,apatite,magnesioferrite,calcite,orthoclase, hematite and REE(La,Ce)-enriched minerals(suspected monazite,epidote),silicate melt inclusions with abundant SiO₂(over 60%),alkaline and Al,Zn and Ca components,and a plenty of immiscible fluid-melt inclusions,indicating zircon megacrysts were formed fluid-melt environment being enriched in SiO₂,Fe,Zr, alkaline,H₂O and CO₂ at an average temperature between 657-692℃(calculated by Ti geothermometer).Furthermore,the similarity between the fluid-melt characteristics reflected from inclusions in zircon megacrysts and those of SiO₂ & alkali-rich mantle metasomatic fluid of mantle xenoliths show they both possibly possess similar genesis.3.LA-ICPMS analyses show that zircon megacrysts have total REE between 15-300 ppm and show concordant "right inclined N-Iike"REE distribution pattern with obvious HREE enrichment and positive anomaly in Ce,with or without weak negative anomaly in Eu.The irregular variation of REE and minor element contents from edge to center in every single zircon megacryst demonstrate that the zircons were not crystallized during simple temperature dropping process of magma.But the zircon megacrysts'high Th/U ratio(most above 0.4) is a feature of magmatic origin zircons.Meanwhile,apparent Ta enrichment and low Nb/Ta in zircon megacrysts indicate that they probably crystallized in strongly evolved magma source with low ratios of Nb/Ta and Nb/U(derived from possible crust material addition or Nb-Ta differentiation during zircon crystallization).That means zircon megacrysts were crystallized during metasomatism process of mantle fluid or melt on recycled crust residual of rutile ecologite.4.The U-Pb dating age of zircon megacrysts are basically the same as those of alkali basalts,e.g.the weighted average age of U-Pb dating of zircon megacrysts for Penglai of Hainan is 4.37±0.24Ma,Mingxi of Fujian 1.27±0.36 Ma and 3.75±0.63Ma,Muleng of Heilongjiang 8.82±0.27Ma,Changle of Shandong 5.00±0.66 Ma,15.98±0.58Ma,20.56±0.80Ma.5.¹⁷⁶Hf/¹⁷⁷Hf ratios range between 0.282883±14～0.283151±7 by using MC-ICPMS and theε_Hf ranges between 3.9±0.5～13.4±0.3,dropping in the depleted mantle area.Among of them,zircon megacrysts of Hainan have the smallest ¹⁷⁶Hf/¹⁷⁷Hf ratios and wide variation range,the zircons of Heilongjiang have narrow range of ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.283105～0.283151 andε_Hf=+11.8～+13.4,but a wide variation of ¹⁷⁶Lu/¹⁷⁷Hf.Hf isotopes dating(THfl) show subcontinental lithospere mantle along eastern China was possiblely metasomatized during Cenozoic to Mesozoic.Based on the results obtained,this paper proposed that the zircon megacryst closely associated with Cenozoic alkali basalts along eastern China were formed during the metasomatism process of depleted mantle fluid and/or melt on the recycled residual of lithosphere.Combined with the previous researches arguing that there existed thicken crust beneath south and north China during Mesozoic,and the present lithospheric mantle is apparently thinner than that in Paleozoic and its principal parts are juvenile,the zircon/corundum megacrysts generally occurred in Cenozoic basalts along eastern China demonstrate that there was a general SCLM thinning beneath eastern China during Mesozoic and Cenozoic,and these megacrysts were the products of the interaction between depleted mantle fluid-melt and the crust residual of rutile-being eclogite after partial melting during lithosphere delamination.