The Hidden Adakitic Magma Reservoir In The Lower Crust Revealed By Amphibole Antecrysts

The late Jurassic volcanic rocks are widely distributed in the Yanshan orogenic belt,which preserve the records of Yanshanian deep process and magmatic evolution in eastern China.In order to explore the source attributes of the volcanic rocks and the deep dynamic information reflected in the volcanic rocks,the volcanic rocks of the Tiaojishan formation in shangzhuangtuo area of Qinhuangdao are selected as the research object,and the systematic mineralogical analysis is carried out.Petrographic observation and whole rock chemical analysis show that the phenocrysts of the andesite are mainly amphibole and clinopyroxene.This suite of andesite is characterized by high Sr/Y and La/Yb ratio in chemical composition,and belongs to adakite like rocks.The EPMA show that the amphibole in the andesite of the Tiaojishan formation has an obvious zonal structure,in which the core has a significantly high Al2O3 content,while the corresponding edge Al2O3 content is significantly reduced,which suggests that the pressure of the amphibole core and edge is quite different.In situ trace element data provide two constraints for revealing the evolution of magma:(1)The equilibrium melt of amphibole has adakitic characteristics,trace element content characteristics(such as Zr,Sm,Nb,Ta,etc.)show that the formation of adakitic is not directly related to the partial melting caused by the thickening of the lower crust.(2)Some trace elements of amphibole have obvious core mantle structure.For example,an obvious high Ni ring formed in the mantle indicates the existence of reabsorption boundary.The existence of high Ni ring indicates that amphibole is likely to be replenished by basic magma in the growth process of shallow magma chamber,which is accompanied by wall rock contamination.(3)The difference between the trace elements of clinopyroxene and amphibole indicates that there are multiple magma chambers,and the corresponding equilibrium melt of clinopyroxene has no adakitic characteristics.Based on the previous research results in this area,this paper holds that the continuous evolution of basic magma occurred in the late Jurassic in the study area.On the one hand,the separation crystallization occurred in the basic magma chamber,forming magnesium rich heap crystals(hornblende),leading to the continuous neutralization of the evolved magma;on the other hand,this process is accompanied by the decrease of magma density,and regional tectonic activity is also conducive to the occurrence direction of the evolved magma The upper part migrated to the shallow magma chamber.The early amphibole and clinopyroxene,which were brought in by multiple magma supply,were preserved in the shallow magma chamber.These minerals grew further in the shallow magma chamber,which was accompanied by the resupply of basic magma and the contamination of the crust.The above research shows that the minerals in the magma chamber under the volcanic rock record a more complex growth history,which also promotes us to further use in-situ analysis to identify the generation and growth history of minerals in the volcanic rock,and constantly approach the true face of the magma evolution process.