Genesis Of Clinopyroxene Megacrysts In Cenozoic Basalts Of Penglai And Evolution Of Their Host Magma

Clinopyroxene megacrysts are commonly found in Cenozoic basalts in Penglai.The relationship between megacrysts and host basalts(homologous or heterogenous)is still controversial.The estimation of megacrysts'temperature and pressure is very important to demonstrate the megacrysts'genesis.In this paper,the structure and composition of megacrysts,sulfide inclusions and host basalts were determined by optical microscope,EPMA,LA-ICP-MS and SEM to explore the genesis of megacrysts and the evolution of host basalts.The results show that the clinopyroxene megacrysts can be divided into two types:type I is augite,with homogeneous composition,without zoning,sulfide inclusions,low Si O₂,Ca O,Ti O₂ and MgO,high Al₂O₃,Na₂O and Fe O.The MgO has obvious correlation with Ti O₂,Al₂O₃ and Na₂O.The MgO has negative correlation with nd,Sm/Yb ratio and LA/Yb ratio,and the MgO has positive correlation with Ni,It is indicated that they were formed by fractional crystallization of magma with different evolution degrees;type II is augite/diopside with uneven composition and a large number of olivine inclusions.The contents of Si O₂,Ca O,Ti O₂ and MgO are higher,but the contents of Al₂O₃,Na₂O and Fe O are lower.The MgO of type II is related to major elements and has a similar trend with clinopyroxene phenocrysts.The composition of sulfide inclusions in type I indicates the lithology of pyroxene source rock and the melting recrystallization of sulfide inclusions,it indicates that magma may be mixed.The negative correlation between MgO and K₂O+Na₂O,Ti O₂,P₂O₅,Sm/Yb,and the positive correlation between MgO and Ca O/Al₂O₃,La/Sm,Ni indicate that MgO was formed by magma with different evolution degree;They were divided into high mg group(Mg^#=57.9-60.0),medium mg group(Mg^#=48.9-50.9)and low mg group(Mg^#=45.9).Type I is in equilibrium with medium Mg^#and low Mg^#basalts.The crystallization temperature and pressure are(T=1270-1278?,P=16.4-21.4kbar)and the crystallization depth is 55-70km,Type I is formed by high pressure fractional crystallization of melt;The temperature and pressure of crystallization are(T=1177-1235?,P=5.1-10.3 kbar),and the depth of crystallization is 17-35km,Type II is the product of low pressure rapid unbalanced crystallization after magma mixing,The accretion edge of type I megacrysts is the same as its origin.Magmatic evolution model:in the first stage,the high-pressure and slow crystallization in the giant magma chamber near the lithospheric asthenosphere boundary formed type I megacrysts.With the melt crystallization,the residual melt Mg^#also decreased,and sulfide in the melt is trapped by type I megacrysts,forming medium Mg^#and low Mg^#melts;In the second stage,high Mg^#melts were supplied,mixed with medium Mg^#and low Mg^#melts,wrapped type I megacrysts,and rapidly rose to the surface.During this period,type II megacrysts and type I megacrysts crystallized in magma pipes at the depth of the crust.