Fingerprinting And Geodynamical Significance Of Precambrian Podiform Chromite In Zunhua And Miaowan Areas

Plate tectonics on earth greatly influence the formation of land and sea,distribution of mineral resources and geological disasters.Plate tectonics in Archean are thought to be different with Phanerozoic.Compared to plate tectonics in Phanerozoic,we know less about plate tectonics on the early earth plate?Precambrian?.This is because the early geological phenomena are seldom exposed and superimposed by later geological processes that the earlier geological evolution information was erased.Therefore,it is very difficult to study the early geological evolution of the earth.The main research controversies of the Precambrian plate tectonics include the onset time,operation style of plate tectonics and difference from Phanerzoic.Fortunately,geologists learn from typical tectonic units in the Phanerozoic convergent boundary,such as ophiolite,ultrahigh pressure?UHP?metamorphism,different orogenic types,accretionary mantle wedge and paired metamorphic belts,and to use them as constraints to infer tectonic evolution style on early Earth.A large number of studies proved that the ancient oceanic lithosphere fragments preserved in the orogenic belt?called ophiolite?and their unique podiform chromitite could contribute to reveal the characteristics of crust-mantle material cycling and plate tectonics in different geological periods,especially in the early stage of the earth.Similar to ophiolites,the ophiolitic mélange in the orogenic belt also indicates the position of ancient suture zone or subduction zone.The mélange often refers a set of rock accumulations with different sources,different times of the rock units due to subduction and accretion process at plate convergent margin.If the mélange component contains rock units from ophiolite suite completely or incompletely,the mélange is called ophiolitic mélange.The mineral-rock assemblage and structural geometry analysis in ophiolites or ophiolitic mélanges can be used to study the environment,origin and subduction polarity of plate tectonics.Compared with other minerals,podiform chromitite as a typical rock and mineral unit in ophiolites and ophiolitic mélanges can especially preserve more primary information about the origin and evolution of ophiolites and ophiolitic mélanges after later metamorphism and deformation process.The Phanerozoic ophiolites and podiform chromitites were paid more attention because they are younger and better preserved.Previous researches on Phanerozoic ophiolite and podiform chromitite obtain a large number of new findings about the formation environment of Phanerozoic ophiolites,origin of podiform chromitites and understanding of plate tectonics.Of particular concern among these researches is many in-situ ultrahigh pressure diamond,coesite and hundreds of mantle minerals recovered from the Phanerozoic Luobusa ophiolite of Tibet,together with many crustal minerals especially zircons showing ages older than the formation age of the Luobusa ophiolite.These findings represent that operation of material cycling?such as the carbon cycle?between crust and mantle and the plate deep subduction in Phanerozoic.However,the style of plate tectonic and material cycling process between the crust and mantle at Precambrian?especially Archean?have not yet been determined.Therefore,we can use the means in the researches of the Phanerozoic podiform chromitites to study podiform chromitite in Precambrian ophiolites or ophiolitic mélanges.Based on these research backgrounds,this PhD dissertation focus on podiform chromitites in Miaowan ophiolite complex and Zunhua ophiolitic mélanges.We carried out a comprehensive study of podiform chromitite including petrography,mineralogy,geochemistry,microscopic feature of minerals and mineral inclusions based on geological survey and comparison study of chromitite in different times.Proterozoic Miaowan ophiolite complex is located at the northern margin of the Yangtze Craton.There are many studies including field geology,mineralogy,geochemistry,geochronology and isotope geochemistry of rocks and minerals in this ophiolitic mélange.These researches suggest that it is a late Mesoproterozoic tectonic dismembered ophiolite from oceanic ridge environment firstly,then experienced SSZ environment.The chromitite is well exposed in host harzburgites and dunites,which provides a good opportunity to study podiform chromitite and then infer the formation and evolution of ophiolite complex.Since Miaowan podiform chromitite and its host peridotite suffered intense serpentinization,whole-rock geochemical data is not so accurate,but the characteristics of whole-rock platinum group elements is similar with other typical Phanerozoic ophiolite.In addition,the study on the chemical characteristics of primary chromite composition indicates that it has high Cr#value and low TiO₂ content,indicating that chromite may formed from subduction zone environment.Chromite grains in dunites show characteristics of island arc.Whereas the chromite grains in harzburgites also characterized by mixture of MORB and island arc,indicating that the host ophiolite complex has undergone two stages of evolution from middle ocean ridge to island arc environment.Chromite grains from both dunites and harzburgites contain large amounts of mineral inclusions?such as amphibole,olivine,clinopyroxene,base metal sulfide,platinum group minerals?,most of which are amphiboles and other hydrous minerals,indicating that chromite may be crystallized from hydrous melts.These results show that the Miaowan dunites and its podiform chromiteformed in the forearc environment by reaction of boninitic melt and MORB-type harzburgites in subduction initiation.Compared with Miaowan podiform chromitites,the Archean podiform chromitite in Zunhua has a good research basis and the formation age is older.There is a suit of exotic blocks of mafic-ultramafic rocks in the Zunhua ophiolitic mélange belt.The structural and geochemical analysis of these rocks suggest that there is an incomplete sequence of ophiolite,which is called the Zunhua ophiolite mélange belt.The podiform chromitites is well preserved in exotic blocks of harzburgites and dunites.The petrochemistry of the mafic-ultramafic rocks and podiform chromitite in the Zunhua mélange belt indicates that it formed in intra-oceanic island arc and forearc environment.The Re-Os isotope isochron age of the podiform chromitites is2547±10 Ma and the whole-rock Lu-Hf isotope isochron age of the host peridotite of the Zunhua ophiolitic mélange belt is 2528±130 Ma.Furthermore,the ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from the host paragneiss range from 2522 to 2867 Ma.The youngest individual grain from these detrital zircons yields a Concordia age of2522±39 Ma,indicating that deposition of the sedimentary precursor of the paragneiss took place after 2.52 Ga.The metamorphic rims of these detrital zircons yield an age of 2467±27 Ma,and the ophiolitic mélange is cut by 2458±17 Ma,undeformed granitic dykes,therefore the age of formation of the mélange is constrained to be between 2.52 and 2.46 Ga.Thus,the formation time of the Zunhua ophiolitic mélange and podiform chromitites is confirmed to be late Archean.Zunhua Archean podiform chromite occurs in dunites,harzburgites and chromitites;it has characteristics of typical ophiolitic podiform chromitite with disseminated,nodular and massive textures.The mineral chemistry of chromite grains shows the characteristics of high chromium,low aluminum and low titanium.The whole-rock platinum group elements of chromitites show a typical negative slope after chondrite-normalized,which is a relative enrichment of IPGE and depleted in PPGE.There are many mineral inclusions in Zunhua podiform chromite grains.Different minerals including silicates,platinum group minerals,elementary minerals,base metal minerals,carbonates,phosphates,sulfates and oxides are founded.The inclusions can be divided into single mineral phase and multiple phase?from two phases to five phases?according to their occurrences.The crystal characteristics and mineral types of chromite and mineral inclusions in Zunhua were identified by transmission electron microscopy combined with sample preparation by focused ion beam.We found TiO₂????UHP phase of rutile?and chenmingite?UHP phase of chromite?.According to previous studies,the formation pressure of TiO₂???is 6 GPa,corresponding to the depth of 200 km underground.In addition,the formation pressure of chenmingite is more than 14 GPa,which corresponds to the depth of 400km underground,that is,the upper part of the mantle transition zone.The appearance of chenmingite and TiO₂???with rutile,amphibole and apatite as a multiphase inclusions appear in chenmingite indicate that crustal minerals can subduct to the depth of mantle transition zone at Archean eon.We believed that rutile,amphibole and apatite from the crust could subduct to the deep mantle along with subduction plates in the forms of single phase or multiphase minerals.When the depth is over200 km,rutile is partially recrystallized into TiO₂???.After subduction goes the mantle transition zone,some chromite are transformed into chenmingite,and these crustal mineral are incorporated in chenmingite grains.After that process,chenmingite is brought to the surface of the earth and get into the oceanic podiform chromitite of ophiolites.Subsequently,these podiform chromite grains were added to the accretionary-collisional mélange during a collision between arc and continent at the end of the Archean.In addition,the stable physicochemical properties of chromite and the abundant mineral inclusions in chromite from different ages suggest that chromite is similar to diamond,which is an important carrier in the process of crust-mantle material cycle,serving the material cycling process such as carbon cycle in Earth's evolution.In addition,by comparing the mineral chemistry and mineral inclusions in podiform chromitite with different formation ages,we infer that the crust-mantle circulation and deep subduction of plate tectonics can be traced back to late Archean.Based on detailed analysis of Archean Zunhua and Proterozoic Miaowan podiform chromitites and comparative study of chromite in different ages,the following conclusions are obtained:?1?The formation and evolution of podiform chromites and host peridotites in the Miaowan ophiolitic mélange and their tectonic origin are determined.We confirmed the Miaowan chromite is ophiolitic chromite by studying its mineralogy,geochemistry and mineral inclusions of chromite from Miaowan ophiolite complex at Yangtze Craton.Based on the characteristics of the parent melts?calculated?of chromites from dunites and harzburgites combined with mineral inclusions in chromite grains,we suggest that the Miaowan podiform chromite was formed in forearc environment under mantle wedge of subduction zone through the reaction between boninitic melts and MORB-type harzburgite.We also infer that chromite and host peridotites in Miaowan ophiolite complex experienced environment transformation from mid-ocean ridge to SSZ.?2?The chromites occurred in the Archean Zunhua ophiolitic mélange belt has the typical characteristics of podiform chromitite.In addition,the different types of podiform chromitites and mineral inclusions in them were systemic studied.The UHP chromite?chenmingite?and UHP TiO₂???inclusions in podiform chromitites are reported for the first time and we determined their occurrences and mineral crystal parameters.The presence of UHP chromite and mineral inclusions in Zunhua Archean ophiolitic mélange suggested that deep subduction similar with the Phanerozoic might operate at Archean and chromite grains can be used as a carrier during material exchanges between the crust and mantle.?3?Throughthechemicalcompositioncomparisonstudyofthe Phanerozoic-Proterozoic-Archean podiform chromitites and their mineral inclusions,we suggest that plate tectonic operated similar in different geological time and the earliest deep subduction started at least in the late Archean.