| Kudi-Qimanyute ophiolite belt is the northernmost suture zone of the Tibetan plateau.Most of researchers suggested that this belt is the relic of Proto-Tethys.Along this belt,there are Oytag,Kegang,Kudi and Qimanyute ophiolites.Among these ophiolites,the Kudi ophiolite is well-preserved and mainly consists of mafic-ultramfic rocks in Buziwan valley,and volcanic and volcaniclastic rocks in Yixieke valley.Mafic-ultramfic rocks are oceanic and mantle rocks,and consist of harzburgite,dunite,pyroxenite and gabbro.During these years,chromitite bodies are found within ultramafic bodies.These bodies are podiform,stratiform and stratoid,distributed in harzburgite and dunite.Chromitite bodies and wall rock show concordant or disconcordant contact.Volcanic and volcaniclastic rocks are massive and pillow lavas,boninite,tuff,andesitic breccia and agglomerate.Previous researches are mainly about volcanic rocks,few about mafic-ultramafic rocks.During these years,mafic-ultramafic rocks are becoming more and more important because of the discovery of chromitite bodies hosted in these rocks.In this study,we select representative dunite,harzburgite and chromitite as research objects to study their origins and the tectonic environment that they formed.For chromitites,we also study the origin of mineral inclusions in chromite and nature of parental magma of chromitites.Dunite mainly has two occurrences.One(Type I dunite)is distributed in the upper part of harzburgite and close to cumulate gabbro.It is stratiform and stratoid,and widely distributed.The other one(Type II dunite)is lenticular,and distributed within harzburgite.Such dunite and host harzburgite show sharp or gradual contact.Type I dunite has relatively high TiO2,Mn O,PPGE(Pt,Pd),PPGE/IPGE(IPGE: Os,Ir,Ru)ratios,lower Mg# and CaO/Al2O3 in bulk rock.Olivine in type I dunite has low Fo,NiO,high Mn O.Chromite in type I dunite has low Cr#,high TiO2.All lines of evidence suggest that type I dunite is a cumulate.Type II dunite contains low contents of incompatible components(such as Al2O3?CaO?PPGE),high contents of compatible components(such as MgO?IPGE).Besides,it has negatively sloped primitive-normalized PGE patterns.Olivine in type II dunite has high Fo,NiO,and low MnO.Chromite has relatively high Cr#,low TiO2.All of these suggest that type II dunite is a residual mantle peridotite.Harzburgite has olivine with high Fo,clinopyroxene and orthopyroxene with high Mg#,chromite with moderate to high Cr#.Besides,harzburgite contains high IPGE,low PPGE,and displays flat to slightly negatively sloped primitive mantle-normalized PGE patterns.In some harzburgite samples,some chromite grains are coarse-grained.Such chromite contains silicate mineral inclusions,such as olivine,clinopyroxene,orthopyroxene and amphibole,and displays high TiO2 contents(up to 0.22 wt.%).Chromite with high TiO2 contents has been reported in fore-arc harzburgite from elsewhere,and is interpreted to be a product of interaction between harzburgite with melt during melt percolation.Interaction between harzburgite and melt is also demonstrated by trace-element characteristics.Therefore,harzbutgite is a residual mantle peridotite after partial melting,and experienced melt percolation.Chromitite has three types: massive chromitite,disseminated chromitite and banded chromitite.Massive chromitite mainly consists of chromite,olivine and clinopyroxene.Chromite is euhedral,with high TiO2 contents(> 0.3 wt.%),and is similar to that of cumulate chromite.Massive chromitite contains high PPGE,low IPGE,displaying positively sloped normalized PGE pattern,and can be comparable to cumulate chromitites from elsewhere.Disseminated and banded chromitites mainly consist of chromite and olivine.Chromite is irregular in shape,and contacts each other along olivine boundaries.TiO2 content is low in chromite.Besides,these chromitites contain high IPGE,low PPGE,and different from massive chromitite.They are interaction products between dunite and chromite-saturated melt.Chromite in chromitite contains aboundant mineral inclusions,including anhydrous silicate minerals(such as olivine,clinopyroxene),hydrous silicate minerals(such as amphibole,phlogopite),oxide(such as chromite),carbonate minerals(calcite),phosphate minerals(apatite),base-metal sulfides(such as pentlandite,chalcopyrite and cubanite),and so on.Inclusions such as olivine and clinopyroxene,occur as monophase,and formed before chromite crystallization.They are trapped by chromite in magma stage.Minerals such as amphibole and phlogopite,common coexisting with other minerals,formed from complex interaction between melt with early-crystallized minerals.Such minerals are entrapped at the transition from magmatic to hydrothermal conditions by recrystallization of chromite in the presence of hydrous fluids.Sulfide inclusions have two occurrences,coexisting with silicate minerals and occurring as monophase.They formed from the separation of sulfide-bearing fluids from magma.Parental magma of chromitites is similar to tholeiitic melt,and there are abundant hydrous mineral inclusions in chromite.Therefore,parental magma of Kudi chromitites is H2O-bearing tholeiitic melt.Oxygen fugacity of samples and mineral compositions of mantle rocks of the Kudi ophiolite sugest an arc-related environment: sub-arc and fore-arc.Volcanic rocks of Kudi ophiolite are tholeiitic to boninitic lavas from the bottom up,and suggest they formed during subduction initiation.Therefore,we suggest these ultramafic rocks and chromitites are also formed during subduction initiation of the Proto-Tethys.They were originally formed beneath arc front,and then were dragged by the mantle flow toward fore-arc basin. |
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