Zircon U-Pb Age And Geochemistry Of The Daohugou Volcanic Rocks In Ningcheng Area, Inner Mongolia

High-Mg andesites (HMAs) are distinct from normal andesites that typify arc magmatism by their elevated MgO contents and Mg#. Although HMAs are volumetrically rare on the modern Earth, their petrogenesis and tectonic settings are important for understanding the mechanisms of the continental formation. Removal of the deep lithosphere of the North China Craton has attracted great international attentions. The thinning process was accompanied by extensive magmatism in Mesozoic and Cenzoic. Volcanic rocks and their xenoliths are an important probe to study the crust-mantle interaction. However, xenoliths are relatively rare. In contrast, the volcanic rocks are widespread in North China Craton. Studies of volcanic rocks provide an important approach to study the lithospheric evolution of the North China Craton.Many works have been done on the Mesozoic lavas in North China Craton. This paper reports age and geochemical compositions of the Daohugou lavas in Ningcheng, Inner Mongolia. We aim at discussing the petrogenesis and providing additional information on the thinning process of the North China Craton. Detailed geochronological and geochemical studies of the 45 Daohugou volcanic rocks lead to the following conclusions:(1) One rhyolite sample from the upper unit of the Daohugou sequence was dated by LA-ICP-MS, which gives the age of 159.7±2.1Ma (MSWD=3.2) . This sample contains no inherited zircons. The age is similar to those reported by the previous studies. The results indicate that Daohugou volcanic rocks are correlative tothe Tiaojishan Formation in northern Hebei and western Liaoning.(2) The Daohugou volcanic rocks consist of tranchy-andesite, trachyte, basaltic tranchy-andesite and rhyolite, with the domaince of high Mg (Mg^#>45) andesite.(3) The Daohugou volcanic rocks show many characteristics of adakites such as depletion in heavy rare-earth elements and Y and high Sr content and Sr/Y ratio (Sr=21-1459μg/g, Y=11-33μg/g, Yb=0.74-2.43μg/g) . The fractionation of heavy to light rare-earth elements is obvious, with high La_N/Yb_N (La_N/Yb_N=12-50) . The Daohugou andesites also exhibit depletion in Nb-Ta and enrichment in Pb-Sr and have negiligble Eu anomalies(Eu/Eu^*=0.92-1.17). In addition, the high-Mg andesites have especially high Mg, Cr and Ni contents (Cr mostly>200μg/g, Ni =73-148μg/g, mostly>100μg/g) . The initial Sr-Nd isotopic compositions of Daohugou lavas are(⁸⁷Sr/⁸⁶Sr)_i=0.705230-0.706402; ¹⁴³Nd/¹⁴⁴Nd (160Ma)= 0.511707-0.511980;εNd(160Ma)= -10.7 - -16.1.The high-Mg andesites from the Daohugou area have similar geochemical characteristics to high-Mg andesites from the Xinglonggou Formation from Western Liaonin but with much lower Nd isotopic ratios.(4) The fractional crystallization has played an important role in the evolution of the Daohugou magma. Sr content and Sr/Y ratio of Daohugou lavas were affected by fractional crystallization of plagioclase, which is documented by their significant negative correlations with SiO₂. The least fractionated samples with the lowest SiO₂ content possess the high Sr content and Sr/Y ratio.(5) The Daohugou high-Mg andesites have similar geochemical and isotopic characteristics to high-Mg andesites from the Xinglonggou Formation. Accordingly they may share a similar petrogenesis that the thickened lower crust foundered into the convecting mantle and melted and interacted with peridoite. Compared to the Xinglonggou lavas, Dahugou lavas lack the 2.5Ga inherited zircons which are widespread in North China Craton. We consider that Daohugou high-Mg andesites might be the new crust formed by thickening and subsequent foundering of basaltic underplates, which then melted and interacted with the peridoite. The age of the Dahugou lavas is similar to the age of the Xilonggou Formation. Combined with the -125Ma Zhangwu and Yixian high-Mg andesites, these results implies that the delamination lasted from the late Jurassic to the early Cretaceous.