Study On Mantle Peridotitie Diagenetic Environment And Chomite Mineralization At Marmaris Of Sw Turkey

The Lycian ophiolites in SW Turkey are mainly exposed in Marmaris-K(?)yce(?)iz(Mu(?)la)and Ye(?)ilova(Burdur)regions.The mantle section is widely exposed around Marmaris,and is mainly composed of oceanic harzburgites with sparse lenses of dunites.The Marmaris harzburgite is characterized by a moderate Cr#[=100 Cr/(Cr+Al);?50]and very low Na2O content(<0.1 wt%)in clinopyroxene.However,many layered,sparse disseminated,dense disseminated and dense massive high-alumina chromite are developed in these harzburgite massifs.The Cr#of spinel in these chromtie is less than10,much lower than the reported lowest Cr#of spinel in podiform chromitites(17.7),due to sub-spinel group mineral formed under natural conditions.As to this view,a new study direction has been given out that how to clarify the metallogenic mechanism of this new unknown deposit is significant for prospecting at this area.Spinels of the high-Al chromite are in light green,dark green color,and exhibit euhedral to subhedral granular with homogeneous texture,which are magmatic crystallizations.The Al₂O₃ content of these spinels is upto 66.18 wt.%,with an average>60 wt.%,which is much higher than that in mantle melt.Therefore,it is necessary to make clear about the source of Al in high-Al chromite.It is found that the background value of Al in around peridotites is quite high.The Al₂O₃ content of accessory mineral spinel in peridotites is 23.85?41.80 wt.%.Moreover,there is an obvious compositional gap amongst the Al₂O₃ contents of clinopyroxenes in peridotites.The Al₂O₃ content of some clinopyroxenes is 1.76?3.05 wt.%,and that of the others is0.53?0.78 wt.%.Combined with petrography,it is found that clinopyroxenes with high Al₂O₃ content mostly are formed nearby orthopyroxenes with a harbor texture,belonging to primary euhedral-subhedral granular minerals.While,clinopyroxenes with low Al₂O₃ content are mostly a euhedral xenogenetic minerals formed by recrystallization.Moreover,orthopyroxenes also have the same compositional gap.Therefore,we preliminarily consider that Al in high-alumina chromite may be melted out from clinopyroxenes and orthopyroxenes of wallrock harzbugites.By the analysis of whole-rock geochemistry and mineral geochemistry analysis,it can be found that the Marmaris mantle peridotites may have experienced a multi-stage and complex magmatic evolution process,which can be roughly divided into three main stages:In the first stage,due to plate extension,the primitive mantle underwent a low degree of partial melting(0?15%)and melt out basaltic melts,the initial depleted oceanic mantle peridotite formed at a mid ocean ridge(MOR)setting.In the second stage,due to geodynamic mechanism change from extension to subduction,the subducted plate experienced a high degree(15?25%)remelting,and melt out basaltic melt enriched in light rare earth elements,or melt out boninitic melt with high Ti O2content.In the third stage,about 0.05?1.2%of the differentiated melts reacted with wallrock peridotites in situ or along permeability pores,reshaping mineral compositional heterogeneity in peridotites.Oxygen fugacity unit value of wallrock peridotitesfO2 is about 0.50?3.87,which is higher than the oxidation degree of FMQ curve of mid ocean ridge basalt,indicating that the wallrock mantle peridotites finally experienced the high oxygen fugacity environment in subduction setting.The diagenetic pressure range of wallrock mantle peridotites is 22?32kbar calculated by PNT clinopyroxene manometer.The diagenetic temperature range of wallrock mantle peridotites calculated by TNT clinopyroxene thermometer is 677?980°C,which is generally consistent with the result calculated by TBA olivine-spinel thermometer with 756?984°C.Thus,due to mantle heterogeneity and mantle melt differentiation,peridotites with high Al₂O₃ content can be formed at ocean ridge or back arc basin environment.The high-Al chromtites have a high PGE abundance of 422.90?889.50×10^-9,and exhibit a positive slope from Os-Pd,which is different from the typical Pt depletion PGE modal.This indicates that the metallogenic is not simple partial melting.A obvious melt differential crystallization trend is shown in Pt/Pt*vs.Pd/Ir.Moreover,according to the calculation,the Al₂O₃ content of its parent magma is 20.34?20.83wt.%,which is higher than that of the mid ocean ridge basalt(?16wt%)and that of the boninite melt(10.6?14.4 wt.%).Therefore neither the melt melted out from mid ocean ridge peridotite,nor the melt separated from the subducting depleted peridotite can directly crystallize such high alumina chromite.The Oxygen fugacity unit value of high alumina chromite?log is 2.16?4.68,and its metallogenic environment should be at a high oxidation setting.The parental melt could be Al₂O₃ enriched tholeiitic melt,which was formed during the initial stage of subduction or during the processes of ophiolite obducting emplacement.Therefore,the high-Al chromite is neither formed by mantle magma differentiation with cogenetic wallrock peridotites,nor formed by ascending melt reaction with wallrock peridotites.Instead,it is crystallized in channel form ascending melt.Such melt could have a limited weak reaction with wallrock peridotites to form the thin dunite shall.In order to make further chronology constrain on the mechanism of high-alumina,we selected 124 detrital zircons from 2 dense massive high-alumina chromites,and obtained 41 sets of valid test data.Among them,the U-Pb age of detrital zircon test point 56 is 67±4Ma,?_Hf(t)=-4.03;the U-Pb age of detrital zircon test point 53 is66±2Ma,?_Hf(t)=-4.15;The U-Pb age of detrital zircon test point 51 is 64±1Ma,?_Hf(t)=-4.57,and the U-Pb age of detrital zircon test point 48 is 56±2Ma,?_Hf(t)=-0.97.Combined with the zircon CL image,it can be seen that these zircons have clear magmatic zircon oscillationitc testure and typical magmatic zircon rare earth element pattern.It is shown that these 4 zircons are the fragment of recrystallized magmatic zircons from the Late Cretaceous to the Paleocene.Therefore,it can be inferred that the crystallization time of the four zircons captured by the high-aluminum chromite ores in this thesis is later than the known nappe emplacement time(91-94 Ma)of the Lycian ophiolitic nappe.It may represent a relatively late phase of magmatic hydrothermal event in this area,which proves that the crystallization time of high-aluminum chromite is no earlier than 67±4Ma?56±2Ma.Combined with the whole-rock geochemistry and mineral geochemistry characteristics of high-aluminum chromites,it can be further inferred that the metallogenic environment of high-aluminum chromite is not only different from the decompression melting environment at the mid-ocean ridge,but also different from the subduction melting environment at the subduction zone.It is possibally formed due to weakening of the thrusting force,slowing down of the speed of the thrusting front,and locally thickening of the thrusting sheets,or the multi-slab overlaying of the thrusting front,resulting in thickening of the stratum/rock,and the Al melted into melt from clinopyroxene and orthopyroxene of peridotite by pressurized melting which was migrated and enriched and finally crystallied in ascending chennals During the pressurized melting process,the main minerals(clinopyroxene+orthopyroxene+spinel)in wallrock peridotites continued melting,and the melt captured the old crust zircons from the wallrocks.Locally,the Zr in melt became saturated and magmatic zircons recrystallized.For the other 36 sets of valid data from detrital zircons,it is found that the ages vary in a large spectrum(95?2280Ma),including ages from Early Proterozoic,Neoproterozoic,Paleozoic,Triassic,Jurassic to Cretaceous,corresponding to old zircons had been carried by the processes of cracking,collision and amalgamation of Gondwana ancient continent,opening and closing of Para-Tethys and Neotethys,and lithospheric delamination and detachment,and then been transported to the Neotethys mantle,and finally captured and brought into the high-Al chromitite in late Cretaceous.The Hf isotope characteristics of the test zircons indicate shall derived materials,which is consistent with shall zircons recycled to mantle in other places wordwidely.Interstitial magnetite is also widely developed between high-Al spinel.Although it is another phase of fluid metasomatic crystallization and will not have a significant impact on the genesis of high-Al chromite,it is of makeable significance for magnetic exploration and prospecting.In this regard,we have carried out 1:10000 ground magnetic exploration,and the test results show that the magnetic susceptibility of chromite is 8×10^-5?1231×10^-5SI,the residual magnetization is 15×10^-3?1943×10^-3A/m,which is the main factor causing local strong magnetic anomaly in the south of the mining area.Harzburgite takes the second place,and the magnetic susceptibility is12×10^-5?1088×10^-5 SI,the residual magnetization is 20×10^-3?5145×10^-3 A/m.Dunite is the smallest with magnetic susceptibility of 2×10^-5?1868×10^-5 SI,the residual magnetization is 22×10^-3?1760×10^-3 A/m.The magnetic difference between chromitite and wallrock is very significant.Therefore,an effective prospecting method for such high-Al chromite is formed,which makes the measured magnetic anomaly interpretation very consistent with the actual prospecting result.There is high-Cr chromites with pod texture developed in the west of our study area.The Cr2O3 content of the chromium spinel is 51.63?57.61,and the Al₂O₃ content in the parent magma is 12.41?14.06 wt.%,which is similar to the boninitic melt.It reacted with arrounding harzburgites in channel during its ascending at a SSZ environment,causing Cr enriched and saturated in parental melt,and finally crystallized.The Cr in the melt is continuously enriched,saturated,crystallized and mineralized.However,Cr#(65.79?74.44)value of high-Cr chromite spinel is different from that of accessory mineral spinel(36.18?53.41)in peridotite.It may be that other Cr saturated magmatic systems crystallize in a new harzburgite or dunite channel.Alternatively after the crystallizaiton of high-Cr chromites,the composition of accessory spinels has been modified due to poric infiltration melt.Twenty-one detrital zircons were also selected from high-Cr chromite,and 5 sets of valid data were obtained with U-Pb ages of1767±28Ma,1732±34Ma,5558±9Ma,416±8Ma,162±3Ma respectively,which is consistent with the U-Pb age range of zircons separated from high-Al chromitite.In summary,the genesis of the high-aluminum chromite in this thesis is due to thickening of thrust sheet and local melting;new melt recrystallized in ascending channel of wall-rock peridotites.Correspondingly,the genesis of the high-chromium chromite in this thesis is due to boninitic melt from subduction slab reacting with wall-rock peiridotite to form Cr saturated melt and recrystallized when secondly entering into ascending channel.Therefore,the two types of high-aluminum and high-chromium chromites in this thesis are formed due to two sets of magma systems formed in two different geological environments,which were crystallized in adjacent space channels in the ophiolitic mantle peridotite at the Marmaris area.