Mantle Xenoliths From SE China And SE Australia:Nature And Evolution Of The Lithospheric Mantle

Volcanism,as a virtual deep drill-hole,brings up mantle-derived xenoliths?peridotite and pyroxenite?,which provide us with insights into the composition,structure and evolution of the ambient mantle.Peridotite,as the major rock type in the uppermost mantle,is the best sample to study the nature of the lithospheric mantle?partial melting,mantle metasomatism etc?and it is also the source for the growth of the continental crust.Subordinate pyroxenite?less than 10 vol%in the lithospheric mantle?plays a key role in mantle heterogeneity.Intergrating peridotite and pyroxenite xenoliths can effectively discuss the series of physical,chemical and geodynamic processes of the lithospheric mantle.Southeastern?SE?China and SE Australia underwent paleo-Pacific subduction during late Mesozoic-Cenozoic,and yielded abundant basalts.These Cenozoic basalts contain abundant mantle xenoliths and provide optimal targets for inverstigate mantle evolution in different tectonic settings.Peridotite xenoliths are widespread in southeastern China,but garnet-pyroxenite xenoliths in the southeast Australia are common.This thesis focuses on the peridotite and pyroxenite xenoliths to investigate the effects of subduction on the evolution and geodynamic process of the lithospheric mantle.This thesis investigated the peridotite xenoliths in the South China Craton?southeastern China?and garnet-pyroxenite xenoliths in the Lachlan Orogeny?southeastern Australia?,using petrography,microstructure,major-and trace-element analysis and Sr-Nd-Hf-O isotopic analysis of whole-rock samples and minerals.The aims are:?1?to understand the nature of the Phanozoic lithospheric mantle underneath the Archean Craton and the processes of the craton destabilization;?2?to discuss the multiple stages of mantle metasomatism and its significance on the destabilization;?3?define the origin?s?of the pyroxenite xenoliths;?4?compare the effects of subduction processes on the off-craton lithospheric mantle at different tectonic settings.?1?Peridotite xenoliths in southeastern China can be subdivided into two groups.Group 1 has olivine Mg#⁹0-92,?and is thus relatively refractory?and shows the trace-element geochemical signature of“old”carbonatitic metasomatism.Group 1 samples are similar to the Hebi peridotite xenoliths that represents the relic Archean lithospheric mantle in the North China during Cenozoic time;Group 2 is more fertile with olivine Mg#mainly⁸9–90,and represents fragments of recently newly accreted fertile lithospheric mantle,which is similar to the contemporary“oceanic-like”lithospheric mantle in the North China.This coexistence suggests that the residual refractory lithospheric mantle?i.e.,Group 1 peridotites?may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle?i.e.,Group 2 xenoliths?as the asthenosphere cooled.The lithospheric mantle replacement beneath the Archean crust is probably triggered by paleo-Pacific subduction and retreat during the late Mesozoic-Cenozoic and thus results in the extensive extension of lithosphere and the upwelling of the asthenospherie.?2?Peridotite xenoliths in southeastern China also record sequential infiltration by two compositionally distinct silicate melts?Na-rich and K-rich?after early-stage melting and metasomatic events.The Na-rich melts are enclosed in spongy clinopyroxene and spinel rims and are inferred to have triggered the reactions that formed the spongy rims,which have lower Al₂O₃,Na₂O and Mg#,but higher Fe O,Ti O₂and Cr#than the primary phases.The undersaturated Na-rich mafic melts were probably formed in the asthenosphere by low-degree melting.The infiltration of K-rich melts occurred after the percolation of Na-rich melts,and the melts occur mainly in reaction zones around orthopyroxene and in reaction patches.The melts have high contents of Si O₂,K₂O?mean 14.3 wt.%?,Rb,Ba,and LREE but very low Na₂O/K₂O?0.01–0.29?,positive anomalies in Eu and Sr,and variable HFSE anomalies.These compositional characteristics are consistent with an origin as low-degree partial melts of pre-existing phlogopite-bearing rocks.?3?Garnet pyroxenite xenoliths from Lakes Bullenmerri and Gnotuk in the Lachlan Orogen of southeastern Australia reveal a picture of continental arc magmatism at mantle depths.The Paleozoic subduction of the Proto-Pacific oceanic slab beneath the southeast Australia triggered melting of the convective mantle wedge at the initial stage due to slab dehydration and generated hydrous arc-tholeiitic melts that crystallized high-Mg O garnet pyroxenites at¹420-1460°C and 23-30 kb when the melt intruded into the overlying lithospheric mantle.Sm-Nd dating of the whole rocks suggests that this intrusion occurred³00-500 Ma ago.Subsequently,fluid-fluxed melting of mantle wedge by a mixture of recycled oceanic crust and its cover sediment generated the melts that precipated the low-Mg O garnet pyroxenites at 1280-1400°C and 16-20 kb.Microstructural evidence of the exsolution of garnet?±orthopyroxene±spinel±plagioclase±ilmenite?from complex clinopyroxene megacrysts suggest that all garnet pyroxenite xenoliths finally equilibrated at⁹50-1100°C and 12-18 kb.Thus,all garnet pyroxenites are uplifted by at least 10²0 km in Paleogene time?⁴4 Ma;determined by Cpx-Gt Sm-Nd isochron?during backarc lithosphere extension of southeastern Australia due to the Mesozoic-Cenozoic paleo-Pacific subduction.Our findings indicate the protoliths of garnet pyroxenites formed along a decompressional cooling path and underwent a long-term evolution of recycling in the mantle and led to a significant compositional and isotopic diversity.?4?Peridotite xenoliths in southeastern China suggest that the Phanerozoic lithospheric mantle is mainly fertile and minor refractory,although it was ever an Archean cratonic mantle.The processes of accretion and modification could be due to the subduction and roll-back of paleo-Pacific plate during late Mesozoic-Cenozoic,which triggered the extension of lithospheric mantle and the upwelling of asthenosphere.The upwelled asthenosphere erosed and reacted with the base of the lithospheric mantle,which causes the destability of the ancient lithosphere.Finally the ancient lithosphere would be totally or partly destructed.In addition,subduction acts as a more important role on the modification of lithospheric mantle beneath the orogenic setting.Garnet pyroxenite xenoliths in southeastern Australia formed in the sub-continental arc mantle wedge,and they records classical“Andean-type”plate convergence and magmatic dynamics during Paleozoic.They also record Mesozoic to Cenozoic paleo-Pacific subduction.The rapid subduction and retreat of the oceanic slab trigger complementary rapid upwelling of deep-mantle material;this process can uplift the deep pyroxenites to the shallow lithospheric mantle.Overall,subduction plays a critical role on the formation and evolution of the lithospheric mantle beneath the southeastern China and the southeastern Australia,which is a major source for Earth's dynamics.