Composition, Structure And Evolution Of Oceanic Crust-Mantle Transition Zone: Insights From The Xigaze Ophiolite, South Tibet

The crust-mantle transition zone（CMTZ）is a critical interface for the exchange of material and energy between crust and mantle,where fractional crystallization and mantle–crust interaction take place.It plays an important role in the research of crustal growth process and the exploration of mineral resources.Ophiolites,representing fragments of ancient oceanic lithosphere,could provide natural macroscopic examples to investigate the formation and evolution of the oceanic CMTZ.Here,we present a systematic study of field geology,petrology and geochemistry for the Dazhuka massif of the eastern Xigaze ophiolite（south Tibet）,aiming to:（1）characterize the architecture and composition of the CMTZ,（2）explore the petrogenesis of different types of rocks in the CMTZ,and（3）reveal the formation,evolution process and mechanism of the CMTZ under oceanic slow-ultraslow spreading centers.The basic conclusions are:（1）Lithological architecture of the CMTZ in the Dazhuka ophiolite.The Dazhuka ophiolite CMTZ is dominated by harzburgite,and mafic rocks of different occurrence such as gabbro and diabase cutting through it and forming complex rock assemblages in the transition zone.The CMTZ is about 1800 m thick,contains complex rock types,including harzburgite,dunite,troctolite,pyroxenite,gabbro and dolerite.The CMTZ can be classified into four zones from top to bottom based on the field geological relationship,rock assemblages and the geochemistry characteristics of harzburgites.Zone 4 is located at the top of the CMTZ and dominated by the mafic rocks.The lithology of Zone 4 includes layered gabbro,sheeted diabase,troctolite and plagioclase-bearing harzburite.The plagioclase-bearing peridotites and troctolites appear as lenticles within the layered gabbros and dolerite sills.Zone 3-Zone 1 are dominated by harzburgite,which contain a certain amount dubites,they are both invaded by a large number of gabbro and diabase dikes.Layered gabbro and gabbro dikes have different mineral assemblages and geochemistry characteristics.Diabase occurs in all parts of the crust-mantle transition zone,and there is no obvious difference in lithofacies and geochemistry between diabase which intrudes the harzburgite and layered gabbro.（2）Harzburgite records complex melt/fluid-peridotite interaction in the CMTZIn the CMTZ of Dazhuka ophiolite,the peridotite from different subzones has different mineral assemblages and chemical compositions.They range successively from clinopyroxene-rich harzburgite without amphibole in Zone 1（the bottom）to clinopyroxene-poor harzburgite with more amphibole in Zone 3（the upper part,with Zone 2 transitional harzburgite in between）.The Al₂O₃contents decrease consistently in whole-rock（2.97-0.57 wt%）,orthopyroxene（4.99-0.85 wt%）and clinopyroxene（5.25-1.22 wt%）for the harzburgites from Zones 1-3,and increase in the trends for the spinel Cr#,clinopyroxene Li/Y,bulk Mg O（39.02-45.22 wt%）and bulk rare earth element（REE）,Pb and Sr contents.The plagioclase-bearing peridotite（commonly enclosed in layered gabbro）in Zone 4（the top）has remarkably elevated Ti O₂contents（0.12-0.69 wt%）and Cr#values（0.45-0.53）in spinel,suggesting equilibration and strong interaction with MORB-like melts inside the CMTZ.（3）Origin of mafic rocks in the Dazhuka CMTZMafic rocks in the Dazhuka CMTZ can be classified into four types:layered gabbros,troctolites,gabbro dykes and diabase dykes.The layered gabbros and troctolites mostly show high Mg#（82.2-89.9）,low total REE concentrations and evident positive anomalies in Eu,suggesting accumulation of plagioclase.Clinopyroxenes in layered gabbros have high Mg#（88.1-91.5）,Cr₂O₃（0.73-1.28 wt%）and low Ti O₂（0.15-0.53 wt%）,which are different to those clinopyroxenes formed in low-pressure,MORB crystallization experiments（Mg#<84）.Reactions between preexisting olivines and porous flow migrating melts may intensely modify mineralogy and geochemistry of the lower oceanic crust to form high Mg#clinopyroxenes at the expense of magnesian olivines.Serpentinized olivines in troctolite are always enclosed in poikilitic orthopyroxenes,indicating that the troctolite has firstly undergone low temperature hydrothermal metasomatism and then experienced metasomatic overgrowth of pyroxene.With respect to the chondrite normalized REE patterns of the gabbro dykes and diabase dykes,the slight depleted LREE of these rocks are similar to the N-MORB signature.In the Sm/Yb vs.La/Sm diagram,the mafic rocks plot along with the spinel lherzolite mantle source,indicating that their parental melts have been generated by 5%–20%partial melting of the depleted MORB mantle at shallow depths.The presence of high temperature magmatic hornblendes in these rocks suggests that these mafic rocks are formed from hydrous basaltic magma.（4）The formation and evolution mechanism of the CMTZ under oceanic slow-ultraslow spreading centersThese vertical variations collectively suggest the complex melt/fluid-peridotite interaction during the upward movement of deeply sourced melts and the downward penetration of seawater.This may be an inherent feature of CMTZ formed in the slow-ultraslow spreading centers.The CMTZ in slow-ultraslow spreading center is characterized by serpentinized harzburgite-dominated lithologies with ubiquitous hydrothermal fluid metasomatism across the CTMZ,while the fast-spreading setting has commonly formed dunite-dominated CMTZ with hydrothermal fluid metasomatism constrained to the fault zones.These differences are interpreted to be controlled by the varying melt production rates and fluxes in these settings.The low melt supply at the slow-ultraslow spreading centers would suppress the formation of voluminous dunite which demands intensive interaction at high melt/rock ratios.On the other hand,the very thin or even missing igneous oceanic crust due to the low melt supply can facilitate the circulation of seawater deep into the lithosphere and enhance pervasive metasomatism throughout the CMTZ.In short,the formation of the CMTZ of the Dazhuka ophiolite is mainly controlled by the bottom-up melt system and the top-down fluid system.Their complex interactions lead to the diversity of petrology and geochemistry in the oceanic crust-mantle transition zone,which is an inherent property in the slow-ultraslow spreading center.