| Mariana Trough,a relative simple intra-oceanic back-arc basin,is one of the ideal areas for investigating magmatic processes and mantle-crust interaction in thesubduction factory.On the basis of detailedly identifying petrographic characteristics,this study carried out detailed study of mineral chemistry and whole-rock major and trace elements compositions on 26 lavas samples obtained from three locations(18°N?17°N?15°30?N)of back-arc spreading center of Mariana Trough.Our results,combined with previous data,providing some key clues to magmatic processes of Mariana back-arc spreading center and the influences of subduction components on the genesis of lavas.The main results and conclusions obtained in this study are as follows:Mariana Trough lavas are of porphyritic texture and vesicular structure,and their main phenocrysts or microlites are plagioclase,olivine and clinopyroxene,which is similar to those of sub-akali tholeiite series.Olivines,occurred in small idiomorphic crystals or hypidiomorphic crystals with slightly resorption,are chrysolite,.Plagioclase phenocrysts,occurred in idiomorphic crystals with zoned or twining structure,are mainly bytownite.Plagioclase microlites,occurred in acicular and skeletal crystals,are mainly labradorite.Pyroxenes,mostly intergrowth with Plagioclase phenocrysts are mainly augite.The average mantle potential temperature beneath Mariana Trough 18°N spreading center,calculated from olivine-melt equilibrium,is 1321.5?,which is less than that of North Fiji basin(1431±40?)North Lau basin(1449±23?)and Mid-Ocean Ridges(1453-1475?)by ~100?,and the temperature is consistent with those calculated from Na(Fo90)and Fe(Fo90)compositions of whole-rock or glass.Mantle temperature variations imply significant differences in the mantle dynamics between Mariana Trough and other regions mentioned above.Results from Plagioclase-melt equilibrium thermo-barometry indicate that there may exist three different depths of magma chamber under Mariana Trough 18°N spreading center: 10.6km,6.2-5.2km and 2.1km(correspond to magma chamber pressure 3.57 kbar,1.91~2.06 kbar and 0.69 kbar,respectively).Magma chambers under Mariana Trough 17°N and 15°30?N spreading center are about at depth of 7.5km(correspond to pressure 2.47kbar)and 10.4km(correspond to pressure 3.43kbar),respectively.Plagioclase phenocryst equilibrium temperatures vary from 1110? to 1156?.The quenching temperature of magma erupting on the surface,calculated from plagioclase microlites,is 979?-1104?.Clinopyroxene phenocryst equilibrium temperatures and pressures are 1152?-1207 ?and 0.42-5.11 kbar,respectively,which are higher than those calculated by plagioclase-melt equilibrium thermo-barometry.Thus,minerals in Mariana Trough Lavas crystallized in the order of olivine?clinopyroxene? plagioclase,which is consistent with general mineral crystallization order during magmatic evolution in back-arc basins.The major element compositions suggest that lavas from the Mariana Trough belongs to a sub-akali tholeiite series,and the lithologies of them cover from tholeiitic basalt to basaltic andesite,which can be basically ascribed to fractional crystallization of primary basaltic melts.Compared with MORB,Mariana Trough basalt is higher in Al2O3,Na2 O,and lower in TiO2,which reflect the differences in melting extent of and magmatic evolution processes between them.Trace element compositions show that the samples are transitional between typical normal mid-ocean ridge basalts(MORB)and island arc basalt(IAB).Primitive mantle(PM)normalized trace element spider diagrams show strong incompatible trace elements enrichment as well as right-leaned style,which are enriched in large ion lithophile elements(LILEs)and light rare earth elements(LREEs),depleted in high field strength elements(HFSE).N-MORB-normalized REE distributional patterns show right-leaned style((La/Yb)N=0.99-1.93),with slightly positive Eu anomaly(?Eu=1.01~1.22,mean value 1.067).The trace element ratios,e.g.,Ba/Th,Pb/Ce,Th/Nd,La/Sm,Th/Nb,Ba/Nb and Th/Nb,indicate that lavas derived from the mantle modified by the addition of multiple subduction components.Based on our new data,the distribution pattern of total subduction component(Ba/Nb),shallow subduction(Ba/Th)and deep subduction component(Th/Nb)in Mariana Trough not only lend strong support to previous conclusion(a latitudinal variation of addition of subduction components),but also find out a more complex and heterogeneous distribution of subduction components in Mariana back-arc region.Calculation results for the addition amount of subduction components show that the relative impact of hydrous fluid released from subducted slab on three studied locations of the Mariana Trough back-arc spreading center is 17°N>18°N?15.5°N,and the relative impact of sediment melts is 18°N>17°N?15.5°N.Combined with previous data,our new results show that the effect of subduction components on the composition of back-arc basin mantle is mainly controlled by the distance between the Mariana arc and back-arc spreading center,superimposed with geographic contrasts of the composition of subduction components.Thus,to a first approximation,the addition of subduction components in back-arc basin lavas can be used as an indictor of tectonic elvolution of arc-basin system. |
PDF Full Text Request