Stratum Characteristics And Tectonic Environment Analysis Of The Devonian Maoniushan Formation In Northern Margin Of The Qaidam Basin

Maoniushan Formation layer is this paper’s object of study in Maoniushan area, WulanCounty, Qinghai. It is studied from the stratigraphic distribution characteristics, depositionalsetting, petrology, petrography, geochemistry and other aspects, comprehensive researched onDevonian layer and Volcanic rocks in Maoniushan area. This disquisition is filled in the lackof previous research on the Maoniushan Formation volcanic rocks in the north margin of theQaidamu Basin. Combine the characteristics of rock association. geochemical characteristicsand the characteristics of regional tectonic evolution, preliminarily study the Palaeozoic Erageological evolution in Mmaoniushan area, and obtained that Maoniushan Formation’svolcanic rocks formed in continental rift setting which is exist in the extensional environmentof the intraplate.Firstly, according to geological line and profiling in field, Inentified, the layer ofMaoniushan formation divided into four members, rather than the previous view that it is twodivided. There are new discoveries which have a detrital rock member and volcanic rocksmember at the top of Maoniushan formation, on the basis of the original two lithologicalcharacter members. And for the first time, Maoniushan formation detrital rock gradinganalysis is test the grading analysis. And there is founded piedmont rollsteinfluten deposits atthe top of the first lithological character member. Last, according to the stratigraphiccorrelation and division, drawing Maoniushan was existed two volcanic activities inDevonian.Secondly, according to its rock characteristic of Maoniushan formation, this studysuggest that the rock combination of Maoniushan formation include andesitic breccia,almond-shaped andesite, hornblende andesite and quartz andesite. It Generally belong to theShoshonitic series, The geochemical characteristics are as follows, The main elements: thecontent of SiO2is58-66%, Na₂O+K₂O is8.56-10.31%, A/CNK＞1, Na₂O/K₂O is0.7-1andthe poor Ti; Trace elements with enrichment in LILEs and HFSEs, depletion in Sr、Y、Yb,obviously Eu anomalies.In conclusion, the evidence of Geochemistry and evolution of regional geology suggestthat the depositional setting of Maoniushan formation Stratigraphy is in continental rift of theintraplate extension system.