Magma Mixing Mechanism About The Late-Triassic Dongjiangkou Pluton,South Qinling Orogenic Belt

Magma mixing cause the geochemical variations in granites.Many I-type granites have the geochemical characteristics of crust-mantle mixture and contain a large number of mafic enclaves,suggesting mantle magma plays an important role in the formation of granite.Therefore,the study of mantle-derived magma properties in magma mixing and magma mixing mechanism is the key to reveal the genetic mechanism and tectonic implication of granite.The Dongjiangkou pluton in the south Qinling orogenic belt is characterized by distinct lithofacies zones and extensive mafic enclaves,which is a typical object to study the magma mixing mechanism.Therefore,this thesis presents petrography,mineral chemistry,zircon U-Pb age,zircon Lu-Hf isotopic compositions and whole-rock geochemistry of the monzogranites,mafic enclaves and lamprophyres in Dongjiangkou pluton,and discusses the mineral temperature and pressure conditions,properties of magma source,and the magma mixing mechanism.The main achievements are as follows.?1?The lamprophyres in Yaowangtang unit display low Si O₂ content?56.54%to 61.19%?and K₂O content?1.64%to 3.45%?,belong to calc-alkaline lamprophyre.It has zircon U-Pb weighted average ages of ²⁰⁶Pb/²³⁸U of 220±2Ma,which are consistent with the age of the host granites studied,indicating that they are the products of magmatic crystallization in the same period.It is enriched with LREE and LILE,and depleted in HFSE such as Nb and Ta,with low Nb/U value?with average of?4.11?and Ba/Rb value?with average of?15.9?.Combined with its zircon?_Hf?t?value?-7.73?+0.62?and the two-stage Hf model age?999?1425Ma?,it indicates that the lamprophyre originated from the partial melting of phlogopite-bearing garnet lherzolite in the Mesoproterozoic lithospheric mantle enriched by subduction fluid metasomatism.In addition,most of zircons in the lamprophyre have low?REE??659ppm?,and there are mantle-derived amphiboles?Ti O₂=2.31%to 3.30%?in the lamprophyre,indicating that the lamprophyre can represent the primary mantle-derived magma.?2?The monzogranite in the Dongjiangkou pluton has high content of Si O₂??68%?,Na₂O?4.13%?4.19%?and K₂O?3.15%?3.48%?,A/CNK=0.90?0.94,which belongs to the metaluminous-high K calc-alkaline series and shows the features of the I-type granite.Combined with its high Sr?689?739ppm?contents and Sr/Y ratios?53-89?,low Y?8.33?12.9ppm?and Yb?0.83?1.19ppm?contents,it indicates that the monzogranite originated from the partial melting of eclogite and garnet-amphibolite in the lower crust.In addition,the monzogranite has high Mg#?61-63?values,and amphiboles of mixed crust-mantle derivation with low Ti O₂?0.61%?0.79%?contents and biotites of mixed crust-mantle derivation with high Mg O?13.08%?13.64%?contents in the monzogranite,indicating that the monzogranite is the product of crust-mantle magma mixing.?3?The mafic enclaves in the Shaluozhang unit are intermediate rocks which belong to the metaluminous,shoshonite to calc-alkaline series.It has low Si O₂?50%?62.96%?contents,high Cr??306ppm?and Ni??134ppm?contents,and high Mg#?57-78?values,showing the characteristics of mantle source.In addition,with the increase of Si O₂ content in the mafic enclaves,the content of Fe₂O₃T,Mg O and Ca O decreased.The mafic enclaves have disequilibrium textures,such as augen-shaped quartz,amphibole clump,needle-like apatite,indicating the process of mingling.The plagioclases in the mafic enclaves are oligoclase-andesine?An=15?45?,and the rhythmic zoning of plagioclases shows a changing crystalline environment.Amphiboles and biotites in the mafic enclaves belong to that of mixed crust-mantle derivation with high Mg O contents,showing the characteristics of magma mixing.?4?Magma mixing can be generally divided into three stages:The first stage is dominated by mingling.The mantle-derived basic magma crystallized about 19km depth,producing the mantle-derived amphiboles.With the continuous intrusion of mantle-derived magma and further magma mixing with granitic magma,chemical mixing in the second stage.At a depth of about 5?10km,the mixed magma produces amphiboles of mixed crust-mantle derivation,and at a depth of about 3?5km,the mixed magma produces biotites of mixed crust-mantle derivation.Moreover,with the increasing mixing degree,the components of enclaves gradually change into intermediate.And with the increase of Si contents in the mafic enclaves,the contents of Ti,Ca,Mg and Fe will gradually decrease.In the third stage,there is almost no mingling but only mixing,in which the monzogranite is the representative product.