A Geochemical Study Of Mantle Source And Fluid Metasomatism Recorded By Xigaze Ophiolites In Southern Tibet

Ophiolites are the remnants of ancient oceanic plates that were tectonically emplaced onto continental margins,so that they are an important indicator of the ancient suture zone.Ophiolites provide important information for the study of magmatic,tectonic and hydrothermal processes during the seafloor spreading of the ancient oceanic plate.It is also of great significance to the study of the subduction initiation and the tectonic reconstruction of ancient plates as well as to understanding of the metamorphism and fluid metasomatism at convergent plate boundaries.In this PhD thesis,a systematic study of petrology and geochemistry was carried out for the Xigaze ophiolite suite,which belongs to the Yarlung Zangbo ophiolite belt in southern Tibet.The results provide new constraints on the mantle source of the ophiolites and the partial melting process of the ophiolitic mantle,as well as on the serpentinization and rodingization at the ocean floor and fluid metasomatism during subduction initiation.These have helped our understanding of the origin of the ophiolites and their tectonic evolution.The study of gabbro and rodingite in the Xigaze ophiolite provides geochemical and geochronological data for the formation process as well as the nature of mantle source for these rocks.Both gabbro and rodingite show low contents of K2O,Na2O and MnO as well as variable contents of Al2O3,CaO,MgO and SiO2.They show relatively uniform REE contents with similar distribution patterns to normal mid-ocean ridge basalts(N-MORB)but lower REE contents than N-MORB.These observations indicate that these rocks are sourced from the depleted mantle that is more depleted than the mantle source of N-MORB.The gabbro and rodingite have high ENd(t)values of 7.3-8.5 for gabbro and 6.5-8.5 for rodingite,suggesting that they were originated from the depleted mantle.Zircon from these rocks can be devided into three groups according to its U-Pb ages and trace element characteristics.Group? and Group ? zircons have the similar U-Pb ages of 124.4 ± 1.2 Ma,which represent the crystallization age of these rocks.In REE patterns,Group ? zircons show steep HREE patterns whereas Group ? exhibit rather flat HREE patterns.Both groups of zircons have a wide range of ?HK(t)values,i.e.,0.07-20.6 for gabbros and-10.8 to 25.0 for rodingites,suggesting that their mantle source is very heterogeneous,including not only the depleted mantle but also the ancient crust components.In addition,both groups of zircons show relatively lower ?18O values,i.e.,4.5-5.6‰ for gabbros and 4.6-6.0‰ for rodingites.These ?18O values are mostly lower than the normal mantle zircon values,suggesting that they were originated from relatively low ?18O sources.The modelling suggests that the mantle sources of these rocks would experience metasomatism by crustal rocks-derived materials,containing about 30%high-T hydrothermally altered oceanic crust.Combined with whole-rock trace element and Nd isotope compositions as well as zircon Hf-O isotope compositions,this work has elucidtated the heterogeneous feature of the basic rocks in the Xigaze ophiolite mantle source,which is possibly related to the incorporation of crust components during oceanic subduction.The study of harzburgite from the Xigaze ophiolite provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic.The harzburgites show high spinel Cr#values and strong whole-rock depletion in CaO,Al2O3 and REE,suggesting high degrees of melt extraction(?10-20%)during seafloor spreading.The occurrence of tremolite and dolomite in some of the harzburgites as well as the LILE anomalies in whole-rock trace element distribution patterns are prominent,indicating metasomatism by subduction zone fluids.Based on the presence or absence of carbonate minerals,two types of metasomatic fluids are distinguished.One is the aqueous solutions,leading to the formation of tremolite and the enrichment of LILE in clinopyroxene.The other is the carbonate-bearing fluids,resulting in the formation of dolomites,tremolite and low Mg#(79-82)olivine.The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated ?18O values for the harzburgites.It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench.During this period of subduction initiation,the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths that would metasomatize the overlying nascent mantle wedge,leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites.Therefore,the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation.Furthermore,the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and then they were emplaced onto the southern margin of the Asian continent in the Cenozoic.There are many serpentinites in the Xigaze ophiolite suite.The study of these serpentinites indicates that they were produced by seawater hydrothermal alteration at the seafloor close the mid-ocean ridge,and afterwards they experienced fluid metasomatism during the subduction initiation,Thses rocks are mainly composed of lizardite,with low Na2O and CaO contents but high MgO and SiO2 contents.MgO contents are negatively correlated with CaO,Lu and Sr.The serpentinites are characterized by U-shaped patterns and weak enrichment of LREE in the REE distribution diagram.In the trace element spider diagram,they show positive anomalies in U and Sr.According to the O isotope composition of serpentine and magnetite as well as the O isotope fractionation factor under equilibrium conditions,the formation temperature of serpentinite is estimated to be 250 to 350?.The representative LILE contents and ratios of the serpentinites generally fall within the range of global serpentinties,suggesting that the serpentinites were formed at the ocean floor close to the mid-ocean ridge.When subduction was initiated,the serpentinites underwent fluid metasomatism at the forearc depth to increase the LREE contents of these rocks.Although the serpentinization process in general does not change the REE contents of the rocks,it can significantly modify the contents of fluid mobile elements in the serpentinite.Therefore,the geochemical compositions of serpentinite not only provide the fluid source and element mobility during the serpentinization,but also constrain the temperature and tectonic environment of serpentinization,thus providing constraints on the tectonic background for the ophiolite formation.