Petrology,Geochronology,and Geochemistry Of The Magmatic Rocks From The Madeng Cenozoic Volcanic Basin,the Sanjiang Orogenic Belt,Western Yunnan,and Geological Significance

The Sanjiang Orogenic Belt is situated to the southeast of Tibet Plateau where Cenozoic magmatic rocks are widespread.The Cenozoic magmatic rocks can be broadly divided into two belts,(1)the eastern one developed along the Xuelongshan-Diancangshan-Ailaoshan shear zone mainly composing of ? 35 Ma intermediate-acid alkali-rich porphyry and granite with a small amount of ? 26 Ma leucogranite.(2)The western belt is located in the Chongshan-Biluo shear belt.It is dominated by leucogranite and felsic dike with age ranging from 60 to 17 Ma.The spatial and temporal distribution patterns of the Cenozoic magmatic rocks indicate that the magmatic rocks of the eastern and western belts have distinct diagenetic processes suggesting different deep processes.During 1/50,000 geological mapping,we identified a small Cenozoic volcanic basin in Madeng area,which is located in region between the Diancangshan and Xuelongshan Mountains.Our geochronology demonstrates that the ages of the volcanic rocks are about 35 Ma rather than the previously thought Late Tertiary.This study focused on the magmatic rocks of the Madeng Cenozoic volcanic basin trying to reveal(1)occurrence of the volcanic-sedimentary sequence,(2)lithology according to microtextural observations and bulk geochemistry,(3)the crystalized age of the volcanic rocks by zircon U-Pb dating,and(4)the petrogenesis and tectonic implications.The Madeng Cenozoic volcanic basin consists of at least three volcano-sedimentary sequences.In upwards,they are(1)trachy-andesite,volcanic breccia,trachy-basalt,volcanic breccia intercalated with crystal tuff,(2)trachy-andesite and/then crystal tuff,and(3)volcanic breccia,crystal tuff,fine sandstone,and trachy-andesite.Our U-Pb zircons analyses upon six volcanic samples demonstrated that the volcanism took place at 35 Ma.Obviously,the volcanics are of post-collision.Geochemical results indicate that the volcanic rocks are mainly shoshonite while a few belong to high potassium calc-alkaline series.They are enriched in large ion lithophile elements(LILE)and depleted in high field strength elements(HFSE)withhigh HREE content.Chondrite-and Primitive-Mantle-normalization revealed the rocks are featured by light rare earth elements(LREE)enrichment and lower fraction of heavy rare earth elements(HREE)with obvious depletions in Nb,Ta and Ti.Besides,the volcanics have ?Nd(t)< 0 on the basis of Sr-Nd isotopic analyses.In the Sr-Nd isotopic diagram,the samples located in an area between the type I and II Enriched Mantle regions(EM I and EM II).Synthesizing available data,the basaltic magma is primary one originating from the enriched mantle by low degree partial melting.The trachy-andesites likely have resulted from assimilation of crustal materials upon the differentiated and/or undifferentiated primary basaltic magmas,respectively.