The Characteristics And Petrogenesis Of Late Triassic Porphyritic Granite In Bo Shang,Southwest Yunnan Province

The porphyritic biotite monzonite granite is the maximum granite mapping unit in Bo Shang area.and is the important component part of Lincang granitic batholith.Further research on it can expand the understanding of Lincang granitic batholith.and play an important role in explaining magmatism in the tectonism process of Sanjiang area.On the basis of field investigation,the Boshang porphyraceous granite is systematically researched in this study,including petrology,elemental geochemistry and U-Pb isotope geochronology.The aim is to reconstruct the petrogenesis of Boshang Porphyraceous granite and regional magma evolution model.(1)Petrochemistry and petrogeochemistry of representative porphyritic bio-monzonitic granite samples are studied in this paper.The result shows that the Si O₂content is 66.52%to 74.84%with an average of 70.1%,rich in silicon;K₂O content was 3.01%?5.91%,averagely 4.3%;Na₂O content equals to 2.22%?3.12%,averagely 2.7%;K₂O/Na₂O was 0.99?2.43,averagely 1.65,more than 1;A/CNK=1.07?1.31,NK/A=0.55?0.79,less than 1.The content of corundum in the CIPW standard minerals is more than 1%,and the rittman index(1.42?2.57)is less than 3.3,So Boshang porphyritic bio-monzonitic granite are high K calc-alkaline and peraluminous series.(2)The content of MgO was higher than 1 except the sample PM0209H1,with an average of 1.74 and a higher content of Mg#(43?51).The average contents of MgO and Ca O was higher than the average 0.9%of the granites in China and the world.According to the main element standard minerals,There was no diopside(Di)in the samples,hypersthene(Hy)content is high.The content of Ni,Co,Ba and V is higher,At the same time,Th elements were enriched,Sr elements were depleted,and Nb and Zr were depleted to varying degrees.At the same time,a large number of mafic inclusions are found in this type of granite.Combined with Zr-Ti O2 and A-C-F diagrams,it is determined that the main body is S type granite,but has significant characteristics of mantle derived material contribution.(3)The U-Pb ages of two zircon samples are 208.1±1 Ma and 209.2±1.1 Ma respectively,The age data indicate that the porphyritic bio-monzonitic granite was formed in the Late Triassic and later than the previous age data.And the age data obtained later than those obtained by the predecessors.(4)The zircon saturation thermometer is used to calculate the diagenetic temperature of 754??830?,indicating the characteristics of mantle source heating.The analysis of trace and rare earth elements determined that the rocks formed during the post collisional tectonic extension stage.Therefore,we believe that during the late Triassic lithospheric extension and thinning,and in the rising process of the bassal magma associated with upwelling of the asthenospere,magmatic heating coupled with the effect of thermal relaxation induces partial melting of sandstone in the upper crust.which leads to the formation of acid magma with crust-mantle mixing characteristics.The magma experienced cooling and crystallization processes from heat to cold,and the porphyritic bio-monzonitic granite were formed in the late stage of magma evolution.