Characteristics And Genesis Of Iddingsite Alteration From Penglai Basalt,Shandong

Penglai basalt belongs to Cenozoic alkaline basalt in Shandong Province.After microscopic observation of the rock samples collected in Penglai,it is found that almost all the olivine in the samples have varying degrees of iddingsitization.Olivine is easily altered by weathering,hydrothermal or metamorphism.Many alteration products of olivine are called"iddingsite",and this term is limited to various reddish brown directional products.Iddingsite is formed by the initial olivine through many stages of structural and compositional changes,and the previous scholars hold different views on its formation environment.It is generally believed that the iddingsite was formed by weathering after magmatism stopped and with the participation of water.In fact,in the magma chamber of very high temperature,or in the later alteration process of medium low temperature magma,as well as the weathering at room temperature,it may lead to the formation of iddingsite.One of the most powerful evidences for the formation of iddingsite in high temperature magma is that iddingsite is surrounded by fresh olivine or wrapped in pyroxene.Because the formation temperature of olivine and pyroxene crystallized outside is above 1000?,it is impossible to form this kind of iddingsite at low temperature.In Penglai basalts,fresh olivine or pyroxene wrapped iddingsite is very common,which deserves special attention.According to petrography and composition characteristics,the iddingsite in Penglai basalt is classified into four types:type 1:reddish brown+dark-red double-layer iddingsite;type 2:dark-red single-layer iddingsite;type 3:iddingsite wrapped in pyroxene;type 4:iddingsite xenocrysts.Electron probe micro analysis(EPMA)of reddish brown+dark-red double-layer iddingsite and single-layer dark-red samples was carried out,and the composition of fresh olivine in the core and edge was compared.It can be seen that in the region of reddish brown and dark-red ring,with the increase of Ca and Al,with the decrease of Si,Mg,Mn and total content.The Fo of fresh olivine in the core is higher than that in the edge,which is similar to groundmass olivine.According to the deficiency of 2.5%?9.6%of the total amount of electron microprobes,considering that some of them are caused by Fe³⁺,the mass fraction of Fe³⁺can be estimated by the constant ratio of Fe²⁺/Mn in olivine,and the mass fraction of OH^-can make up for the deficiency of the remaining amount of electron microprobes.From this we can get the chemical formula range of HTI in Penglai area:(Mg_1.3 Fe₀².⁺_28⁰.62 Fe³⁺_0.06⁰.28)[Si O_3.56³.91(OH)_0.19⁰.89],which satisfies the charge conservation.By calculating the value of Si/(Si+Fe+Mg),the dark-red iddingsite,reddish-brown iddingsite and fresh olivine can be distinguished.According to the previous literature,the Si/(Si+Fe+Mg)ratio of laihunite ranges from 0.36 to 0.40,while that of standard olivine is about 0.33.The Si/(Si+Fe+Mg)values of fresh olivine in Penglai area range from 0.332 to 0.343.The Si/(Si+Fe+Mg)values of dark-red ring are generally higher than 0.360,ranging from 0.356 to0.408.The reddish-brown ring is an excessive component,and the Si/(Si+Fe+Mg)values range from 0.340 to 0.362.Therefore,it can be inferred that the two-layer structure is the transitional type of single-layer structure.By calculating the composition of pyroxene,the temperature,pressure and crystallization depth of the formation of iddingsite can be limited.After calculation,the formation temperature of this kind of pyroxene is about 1040?1156?,the pressure is about 7.2?11.3kbar,and it is formed at the bottom of the lower crust to the top of the upper mantle 26.3?41.2km.It is speculated that the temperature and pressure of the formation of Penglai iddingsite is not lower than this range,and it is formed at a deeper depth.Through the quantitative analysis of the structure of the two types of iddingsite,it can be seen that the double-layer iddingsite is the S-CSD curve of natural crystallization,and the single-layer iddingsite is the L-CSD curve caused by coarsening.Therefore,the origin of double-layer iddingsite is due to the high-temperature iddingsite developing in the water rich magma chamber.When the precipitation evaporates under oxygen fugacity,the process of high-temperature iddingsite stop to grow,and olivine continues to grow in the magma chamber or magma pipe,thus forming fresh thin olivine accretionary edge at the edge of olivine iddingsite.However,the overgrowth edge of single-layer iddingsite is generally thick,and the CSD curve has obvious"bow"phenomenon.It is speculated that after the end of iddingsite,new magma may have been injected and magma mixing occurred,resulting in the coarsening of the structure.