The Accumulation Sequence Of The Mesozoic Volcanic Rocks And The Structure Features In Yakeshi Area,Inner Mongolia

Mesozoic rocks extensively outcropped in Yakeshi area,Inner Mongolia,but no systematic geochronology studies have been taken out,which significantly constrains the strata division and age definition.This paper focuses on the Mesozoic volcanic rocks in Yakeshi area,and discuses the tectonic setting of the extensive Mesozoic volcanic rock in Great Khingan combining the geochemical and regional tectonic features.Based on the rock assemblages,fossils,stratigraphic correlation,isotopic dating and volcanic-eruptive sedimentary cycles,the Mesozoic volcanic rocks in Yakeshi area can be subdivide into five formations from bottom to top:Tamulangou Fm,Manketouebo Fm,Manitu Fm,Baiyingaolao Fm and Meiletu Fm.According to numerous cross-section surveys and fossil analysis in sedimentary beddings,the Manitu Fm,Baiyingaolao Fm and Meiletu Fm are classified into Early Cretaceous,while the Manketouebo Fm is classified into Late Jurassic.The geochemical analytical data indicate that the rock assemblages and geochemical features of the Mesozoic volcanic rocks in Yakeshi area are extremely complex,and multiple rock genesis.The Tamulan Fm is composed of calc-alkali intermediate and basic lavas and clastic rocks;Manitu Fm consists of continental intermediate,intermediate-acidic volcanics interbedding with minor acid volcanics and volcanic clasitic sedimentary.Baiyingaolao Fm is composed of acid nodular rhyolite,rhyolite,perlite and rhyolitic volcanic clastics;Meiletu Fm consists of intermediate-basic,basic rocks,with main lithology of basalt,andesite and andesitic basalt.Based on the geochemical analysis results,we have detailed reviewed the previous literature about the lithology and tectonic settings of the Mesozoic rocks in Great Khingan and found that this area has undergone through the post-orogenic extensional stage of the Mongolia-Ehuozike Ocean in Middle-Late Jurassic,and then developed strike-slip faulted and extensional basins in Late Jurassic-Early Cretaceous,characterizing with enhancing extension.The end stage is Late Cretaceous-Quaternary,in which the tectonic movement gradually weakened with elevating movement.