Analysis Of Volcano Rock Geochemistry And Tectonic Background From Late Permian To Middle Triassic In The Nanpanjiang Basin

The Nanpanjiang Basin(area)at the junction of Yunnan,Guizhou and Guangxi provinces,wich is located in the Pacific structural domain,Tethys Sea structural domain and Southern China Caledonian fold belt.It contains the whole Yangtze continental margin.Shizong Maitreya fault,Ziyun-yadu-Luodian-Du’an fault,Maguan Xidamingshan-Dayaoshan uplift as its boundary.In the study area,the crust formed the basement under the rifting of the late Paleozoic,and the basin body was formed by tectonic action on the basement,and the tectonic activity lasted until the Middle Three Triassi.In this paper,based on the study of t he existing achievements and detailed field geological work,the regional geological evolution is analyzed.It is considered that during this period,the basin underwent 4 different stages of evolution: basement formation,continental rift,post arc basin and foreland basin.The rocks include volcano lava and tuff rocks from Late Permian-early Triassic in the basin.The SiO2 content of lava varies from 47.46-56.40%,with an average content of 52.28%,mainly belonging to intermediate rock-acid rock,and the content of TiO2 is higher to 1%,with an average of 2.85%.The ratio of K2O/Na2 O varies greatly.The Mg# value is higher,which is 25.16-46.08(average 33.60),which belongs to the mantle derived magma range.The content of tuff SiO2 varies from 48.84%-78.52%(average 63.04%),which is mainly intermediate acidity.The Mg# value is higher than lava,31.49-68.49,with an average value of 48.39.The middle Triassic samples are divided into two parts: the north and south of the basin.The southern part is mainly intermediate-basic volcanic rock and the northern part is acidic tuff(green-bean rocks).In the northern basin,tuff SiO2 varies between 57.37-75.9%,with an average of 66.63%.It mainly belongs intermediate acid volcanic rocks.TiO2 content was low and varied between 0.22-0.36,with an average 0.28.Because of the late alteration of samples,the K2O/Na2 O ratio is higher,with an average of 52.The southern volcanic rock SiO2 varies between 55.04-57.49%,an average of 56.26%,and is more basic than the north.The content of TiO2 changed from 0.89-3.11%,which was higher than that in northern basin,and the ratio of K2O/Na2 O was low,an average of 0.92.The characteristics of trace elements in the Late Permian and Triassic volcano rocks,Late Permian-Early Triassic volcano rock samples,Rb is relatively enriched in K,Ta,Nb and other large ion lithophile elements,Hf,Y loss of high field strength elements.Among them,Sr,P and Ti in tuff have strong losses,while from intermediate to acid basalt do not show similar losses,and conform to the characteristics of intraplate basalt.In the Middle Triassic northern samples showed obvious negative anomalies of Ce,Ti and P showed obvious losses,and the contents of other elements were not obvious.The middle Triassic samples in the northern basin have smaller Eu negative anomalies(δEu=0.18-0.29,average δEu=0.24).Middle Triassic samples in the southern part of the basin: obvious difference between LREE and HREE(LREE/HREE=1.36-1.48);LaN/YbN ratio is 3.09-3.62(average 3.35).Has a obvious negative Eu anomaly(δ Eu=0.61-0.71,average δEu=0.66).There was no obvious abnormality in the content of Ce.The northern volcanic rocks have more obvious negative Eu anomalies than the southern volcanic rocks.