Alkaline basalt is basicAlkaline volcanic rockThe general name of.It is more alkali than ordinary basalt, andsilicon dioxideCalcium oxide is low.The main minerals are plagioclase (plagioclase feldspar), olivine, calcium rich pyroxene and titanopyroxene. There is no reaction edge between olivine and pyroxene, and there is noPerilla pyroxeneAnd contains potash feldspar, anorthite, leucite, nepheline, sodalite, etcFeldspar mineral。
It is mainly distributed in continental areas and oceanic volcanic islands.In AsiaCircum Pacific volcanic beltAlkaline basalt is distributed near the mainland.In addition, in the Hawaiian Islands, basalt containing more than 5% of the actual mineral olivine and less than 5% of the standard mineral nepheline is also called alkaline basalt.Alkaline olivine basaltThe rock series is also called alkaline basalt rock series for short.[1]
features
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The mineral composition and chemical composition of alkaline basalt vary widely, with the prominent characteristics of rich alkali, KtwoO+NatwoO>5%，Up to 9%.General NatwoO>KtwoO，Very few KtwoO>NatwoO's.AndTholeiiteIn contrast, alkaline basalt contains a large amount ofAlkali feldspar, alkaline dark mineral and titanium rich pyroxene, olivine but no quartz appear in the matrix.If the alkalinity of rock is higher, feldspar like will appear.Compared with calc alkaline basalt, it is rich in TiOtwo(>2%), high alkali.Rocks often have glassy porphyritic texturehyalopilitic texture 。[1]
Species
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The common species of alkaline basalt series are as follows:
（1）Alkaline olivine basalt: Alkaline olivine basalt is the representative of alkaline basalt, containing nepheline, etcFeldspathic mineral。In alkaline basaltAugiteUnder the microscope, most of them are lavender titaniferous pyroxenes, with obvious polychrome and zonal development.Ordinary pyroxene phenocrysts are usually earlier thanplagioclaseThe phenocryst minerals occur in the order of olivine → augite → plagioclase.Peridot is common,Phenocryst、stromaBoth in.The formation pressure is large and the source depth is large.[1]
(2) Tephrite: SiOtwoBasaltic rocks with obvious unsaturated and high alkali content.It is composed of basic plagioclase, clinopyroxene and feldspathic, and can contain a small amount of olivine. The clinopyroxene is mainly titanium bearing pyroxene, and the feldspathic is mainly nepheline and leucite.According to the different types of feldspathic, they are named nepheline tephrite, leucite tephrite, etc.[1]
(3) Basanite: also SiOtwoAlkaline basalt with obvious unsaturated, high alkali content and rich feldspathic.The main minerals are basic plagioclase, olivine, clinopyroxene and feldspar like.Different from tephrite, olivine is rich, up to 25%.The phenocrysts are olivine and pyroxene, and feldspar like crystals mainly exist in the matrix.According to different types of feldspathic, it can be divided intonepheline basanite And leucite basanite, etc.[1]
significance
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Alkaline basalts usually contain different amounts of mantle derived xenoliths and xenocrysts, which is a window to study the composition of mantle materials.It contains blue corundum, zircon, garnet and other megacryst minerals, which are important gem materials.[1]
