米仓山碱性岩中的主要矿物研究及其成因信息

Studies of Essential Minerals in Alkaline Rocks of Micangshan Area and Their Genetic Information

米仓山碱性岩中的主要矿物为富铝的(次硅)铝铁质透辉石、钙质角闪石、霞石等,且大量出现霞石,但无或极少有碱性暗色矿物。研究表明:氧逸度低是碱性岩浆不能结晶出碱性暗色矿物的原因。本区这种无碱性暗色矿物的碱性岩是在封闭体系及深成条件下结晶的,而其它地区含有碱性暗色矿物的岩石则是在开放体系中形成的。

Located in Nanjiang County and Wangcang County of northern Sichuan,alkaline rocks in Micangshan area might be divided into ijolite--essexite series(I) and syenite series (Ⅱ). Nephelines are Present in large quantities, whe--reas there are no or very few alkaline dark minerals. Clinopyroxenes, amphi--boles and nephelines in alkaline rocks are richer in aluminium than thoseminerals from the same sort of rocks in otber areas, with alumoferro--diopside(fassaite) exhibiting this feature most obviously. Pyroxenes from Series I aredifferent from those from Series Ⅱ in composition and evolution trend. Rooksof Series I were crystallized in the temperature range of 775--1023℃, whereasthose of Series Ⅱ, 700--950℃, indicating that the depth for their crystalliza-tion was larger than 10 km. The richness of aluminium in pyroxenes should beattributed to the abundant aluminium in rocks and the crystallization ofminerals at fairly great depth. In other areas, alkaline rocks often have alka--line dark minerals, which contain very high Fe^(3+). In this area, however, thelow oxygen fugacity hindered Fe^(2+) from converting into sufficient Fe^(3+), andhence it was quite impossible for alkaline dark minerals to crystallize frommagma. The low--fO_2 alkaline rocks without alkaline dark minerals in this areawere crystallized from alkaline magma in a closed environment, whereas high--fO_2 alkaline rocks that contain alkaline dark minerals in other areas wereformed by crystallization of alk??Ine magma in an open system.