Geochemistry And Genesis Of Maoniushan Formation Volcanic Rocks In Amunick,Qinghai Province

The suture zone of northern margin of Qaidam Basin belongs to the Qin-Qi-kun(Qiling-Qilian-Kulun Mts.)orogenic system,which experienced the closure of the original Proto-Tethys Ocean,the ocean-continent subduction and the late collision orogeny during the early Paleozoic.The arc-basin system is plenty.In the late Silurian to early Devonian period,the molasse formation of Maoniushan Formation,which widely developed along the magin of Qaidam Basin,represents the basic end of orogeny.Many geochemical studies in recent years show its lower limit of age,but lack of the upper limit of age.The Amunick area is in the northern part of the northern margin of the Qaidam Basin,where a large number of volcanic rocks of Maoniushan Formation outcropped.It is helpful to know the tectonic evolution of the northern margin of the Qaidam Basin after the closure of the oceanic basin in early Paleozoic and the basin began its continental evolution stage by the comprehensive study of petrology and geochemistry characteristics.And through the study of the upper volcanic chronology of Maoniushan Formation,the upper limit of age of Maouniushan Formation is restrained.In this paper,petrological and petrological geochemical characteristics of the volcanic rocks were studied by means of field geological survey and laboratory identification and geochemical analysis.The tectonic environment identification,magmatic source area,genesis and evolution were discussed.The types of volcanic rocks of Maoniushan Formation are pyroclastic rocks and lava,rhythm and cycle characteristics are obvious.The petrographic features are characterized by eruptive facies,burst facies,effusive facies,and a small amount of eruption-deposition facies in local area.Ancient volcanic institutions are well preserved,of which the Amunick mountain and Dadakenwulashan mountain two preserved the most complete.In addition,lava volcanic rocks show the characteristics of continental overflow,with a typical columnar joint,which cross-sections are mostly quadrilateral.According to the content of SiO2,the volcanic rocks of Maoniushan Formation in the study area are divided into two types: one is intermediate volcanic rocks,SiO2 content is 53.94 ~ 61.88%;one is intermediate-felsic volcanic rocks,SiO2 content is 62.99 ~ 76.61%.In contrast,the former oxide content than the latter,the latter showed high K characteristics.Both showed low Ti,TiO2 content is 0.1~1.12%,an average of 0.41%,the content of low P2O5 is 0.019% ~ 0.33%,the average value is 0.11%,the content of FeO* is 0.70 ~ 7.16%,the average is 2.81%,the range of ALK is 5.38 ~ 9.86%,Rittman index ?: 1.45 ~ 4.54,basically is concentrated in the calcium alkaline range.The trace elements are rich in large ion lithophile elements(LILE),loss of high field strength elements(HFSE),and two volcanic rocks are negative anomalies of Nb and Sr.The negative anomalies of P,Ti and Ba in the intermediate-felsic volcanic rocks are obvious,and both show the relative positive anomalies of Th,La,Ce,Nd,Zr,Hf and Y.The rare earth elements are characterized by light rare earth elements enrichment,heavy rare earth elements loss,and intermediate-felsic volcanic rocks are significantly negative anomalies of Eu.The rare earth element normalization curve shows the characteristics of the Andean-type volcanic rocks.The results of Zircon LA-ICP-MS U-Pb dating of dacite and rhyolite in the upper Maoniushan Formation in the study area show that its age is between 369.2 ± 3.3Ma and 392.4 ± 3.3Ma,which belongs to the Middle-late Devonian.The petrographic and geochemical characteristics of the volcanic rocks show that the volcanic rocks of Maoniushan Formation in the study area are formed in the intraplate extensional tectonic environment and have the characteristics of island arc volcanic rocks,and it is judged to be formed in the back-arc basin extension.The magmatic source region is an enriched wedge-shaped mantle with the hydrous fluid of subdducted oceanic crust metasomatic.During the ascent process of magma,there is a strong assimilation and contamination with the salic crust.