The Volcanics Characteristics And Its Evaluation Of Prospecting Prospects Of Xiaodingxi Formation In South Lancang River Belt

The volcanic rocks of Xiaodingxi formation are an important part in the Lancang river volcanic arc in western Yunnan,and an important window for studying the geological evolution history of the south Lancang river tectonic belt.In this paper,through the study of petrological characteristics of Xiaodingxi formation volcanic rocks,rock geochemical characteristics,the chronology characteristic to analyze and discuss the genesis and tectonic setting and combined with the study area discovered mine(mineralization)make a brief comment on the prospecting prospect.Field profile data show that the lithology of volcanic rocks in Xiaodingxi formation is mainly basaltic basic lava with acid lava on top,the distribution of the volcanic rocks is about 5:1,and some intermediate volcanic lava and pyroclastic rocks are sandwiched in the middle.The analyzing results of major elements show that The Si O₂ content of Xiaodingxi formation volcanic rocks has obvious discontinuity between 62.48%and74.29%,showing the characteristics of the Si O₂ content of the volcanic rocks in Xiaodingxi formation has obvious discontinuity between 62.48%and 74.29%,showing the characteristics of bimodal volcanic rock assemblage.The mafic-intermediate end-member rock belongs to aluminium-oversaturated low-potassic alkali-calcic trachyte-broad-faced basalt,and the acidic end-member belongs to peraluminous low-calcium-calcium rhyolite.Both end-members have high dissimilarity.The crystallization index shows a negative correlation between Al₂O₃,Mg O,P₂O₅,Ti O₂and Si O₂ in the Huck plot.The standard mineral calculation results show Xiaodingxi formation volcanic rocks have higher albite.They all contain accessory minerals such as ilmenite,apatite and magnetite.The mafic-intermediate end-member rock generally contain olivine.The distribution of trace elements indicates that the mafic-intermediate end-member rocks in Xiaodingxi formation are enriched in Th,U,La,Zr and P compared with the the original mantle,depleting Nb,Ce,Nd,and Ti;the acidic end-members rocks are enriched in Th,U,La,Zr and Hf compared with the the original mantle,losing K,Nb,P,Ce and Ti.The REE pattern indicates that the mafic-intermediate end-member rocks are characterized by the relatively enriched and incline to the right of the LREE,the LREE and HREE have a high fractionation degree,which have weak Eu anomaly.The acidic end-members rocks also display the right-leaning feature of LREE enrichment,LREE fractionation degree is higher than mafic-intermediate end-member rocks,which have obvious Eu anomaly.The zircon U-Pb age of the basalt in the Xiaodingxi Formation is 219.1±1.6 Ma,which represents the diagenetic age of Xiaodingxi formation volcanic rocks.Mafic-intermediate end-member rocks in Xiaodingxi formation originate from the partial melting of the continental lithosphere,which is a product of the partial melting of the lossy spinel-olivine to the transitional phase of the garnet-olivine peridotite.During the process of ascending to the earth's surface,it is contaminated by a certain degree of crustal materials and the separation and crystallization of titanium-iron oxide and plagioclase.The acidic end-members rocks are formed in the partial melting of crustal sedimentary rocks or sedimentary metamorphic rocks,with separate crystallization during the process.According to the regional background analysis,it is believed that Xiaodingxi formation volcanic rocks formed in the stretching zone of the continental plate in continental stretch zone environment dominated by stretching.In this environment,the lithosphere mantle magma is ejected from the surface by decompression and partial melting uplift forms the mafic-intermediate end-member rocks in Xiaodingxi formation.The anomalously high temperature brought about by the upsurge of the mantle-derived magma caused the partial melting of the upper crust to form acidic magma and spewed out the surface to form the rhyolite in Xiaodingxi formation.Through the analysis of the mine(mineralization)points and metallogenic conditions found in the distribution area of the Xiaodingxi formation volcanic rocks,it is considered that it has good prospecting prospects.The mine(mineralization)points in this area are widely distributed,type mainly in copper,lead,zinc and iron ore(mineralization)points.It is worth carrying out further prospecting and exploration work.