Nature And Deep Process Of The Lithospheric Mantle Beneath The Xing'an-Mongolian Orogenic Belt

This thesis studies petrological,mineralogical and geochemical data as well as whole rock Re-Os,in-situ clinopyroxene Sr and olivine O isotopic compositions of the mantle xenoliths in the Cenozoic basalts from Sviyagino,Shuangliao and Jiaohe regions in the Xing' an-Mongolian Orogenic Belt(XMOB),with the aim of constraining the compositions,formation ages and thermal state as well as deep processes of the lithospheric mantle beneath the XMOB.Finally,a model is set up for explaining the relationship between the subduction of the Pacific plate and the evolution of the lithospheric mantle beneath the XMOB.Main contents of this thesis are as follows:1.Petrogenesis of mantle xenoliths in the Cenozoic basaltsMantle xenoliths in the Sviyagino Cenozoic basalts in the Khanka Massif consist mainly of lherzolites and minor harzburgites.These peridotites have Al2O3 contents of 1.92-4.08 wt.%that correlated well with other major and heavy rare earth elements,olivine Mg#,spinel Cr#and modal clinopyroxene abundances.Combined with the LREE-depleted REE patterns for their clinopyroxenes,we suggest that the Sviyagino peridotites are residues of low to moderate degrees of melt extraction from fertile mantle with negligible post-melting enrichment.Mantle xenoliths from Shuangliao and Jiaohe show similar mineralogical and petrological characteristics,and comprise lherzolites,harzburgites,wehrlites,garnet pyroxenites and spinel pyroxenites.The lherzolites and harzburgites have low Al2O3 contents(1.16-2.88 wt.%)and were formed by moderate degrees of melt extraction from fertile mantle.Clinopyroxenes in these peridotites display LREE-enriched or convex-upward REE patterns and record multiple stages of metasomatism.The wehrlite is porphyroclastic and contains zoned clinopyroxenes,and was formed by melt-peridotite reaction.The garnet pyroxenites have high Al2O3 contents,positive Eu and Sr anomalies,and olivine ?180 of 4.7-5.1%?,which we attribute to reaction of recycled oceanic crustal components with peridotites.The spinel pyroxenites have peridotite-like pyroxene compositions,and were formed by reactions of lithospheric peridotites with silicate melts.2.Compositions,formation ages and thermal state of the lithospheric mantle beneath the XMOBThe lithospheric mantle beneath the Khanka Massif is composed mainly of fertile lherzolites with minor harzburgites,while the lithospheric mantle beneath the Songnen Massif is composed of lherzolites,harzburgites,wehrlites and pyroxenites.Compared to the Khanka Massif,the lithospheric mantle beneath the Songnen Massif contain larger percentage of refractory harzburgites and pyroxenites,implying that it has been modified by metasomatism more intensely.The Sviyagino peridotites show an 187Os/188Os vs.187Re/188Os correlation with an apparent 1.9 Ga age;the 187Os/188Os ratios are positively correlated with A12O3 and other melt extraction indices.The 1.9 Ga isochron age represents the first robust formation age of lithospheric mantle beneath the Khanka Massif.The synthesis of available Re-Os isotopic data for peridotites and sulfides in xenoliths,combined with the widespread occurrences of Precambrian basement,suggesting the prior existence of a common Paleoproterozoic lithospheric mantle beneath the Xing'an,Songnen and Khanka massifs in the XMOB.The lithospheric mantle and overlying crust were temporal coupled and kept genetic links during the evolution of the XMOB.During the Phanerozoic,most of the ancient lithospheric mantle beneath the Songnen Massif has been replaced by juvenile lithospheric mantle.Based on equilibrium temperatures and pressures for garnet pyroxenites,we propose that the Jiaohe region has a high geothermal gradient of?80 mW/m2,similar to those of eastern North China Craton.This result further confirms the existence of hot Phanerozoic lithospheric mantle beneath the Songnen Massif.3.Silicate and carbonatite metasomatisms in the lithospheric mantle beneath the XMOBSome peridotites from Shuangliao and Jiaohe have high modal orthopyroxene abundances and FeO contents,their clinopyroxenes display convex-upward REE patterns and have high Ti/Eu ratios without significant Nb,Ta,Zr and Hf depletions,implying these peridotites experienced silicate metasomatism.Silicate metasomatism formed the Shuangliao and Jiaohe spinel pyroxenites.One spinel pyroxenite has more spinel than other pyroxenites(2.2%vs.?0.5%)and contains orthopyroxenes surrounded by fine-grained olivines and low-Al clinopyroxenes,suggesting the infiltration of a carbonatite melt.Clinopyroxenes in the wehrlite have high Mg#,Na2O and K2O,and are zoned with increasing(La/Yb)N and Sr and decreasing Ti/Eu from core to rim.Some other peridotites have high Ca/Al,their clinopyroxenes are enriched in Sr and LREE and depleted in Nb,Zr,Hf and Ti.These results are consistent with a recent carbonatite metasomatism.4.The sources of metasomatic meltsSpinel pyroxenites and peridotites that only experienced silicate metasomatism have low 87Sr/86Sr ratios in clinopyroxenes(0.7020-0.7025)and mantle-derived ?18O in olivine(5.18-5.38‰).This suggests that the silicate melts were originated from the normal asthenosphere.Clinopyroxenes in samples metasomatized by carbonatite melts show gradual increases in 87Sr/86Sr ratios from cores(0.7019-0.7026)to rims(?0.7034).The feldspar formed by carbonatite metasomatism has an 87Sr/86Sr ratio of 0.7042.These results suggest that the metasomatic carbonatite melt had an 87Sr/86Sr ratio of>0.7042,which is distinct from the values for normal depleted mantle,but falls in the range for altered oceanic slab.In addition,samples metasomatized by carbonatite melts have mantle-like ?180 in olivine(5.07-5.35‰),suggesting that the carbonatite melts were originated from carbonated asthenosphere.5.The relationship between the subduction of the Pacific plate and evolution of lithospheric mantle beneath the XMOBThe lithospheric mantle beneath the Khanka and Songnen massifs have distinct compositions and experienced different deep processes.The former show little metasomatism in spite of the proximity to modern Pacific subduction zone,whereas the latter has been modified strongly.These results,together with the subduction history of the Pacific slab,geochemical data of the host basalts and geophysical data,suggest that crustal materials in the Pacific slab were initially subducted to mantle transition zone,then incorporated to the asthenosphere beneath the central XMOB to produce CO2 and H2O-enriched domains.Upwelling of the carbonated asthenosphere produced carbonatite melts that migrated upwards and metasomatized the overlying lithospheric mantle,formed the wehrlites and other clinopyroxene-enriched mantle rocks.