基性岩、中性岩.ppt

(5)玄武岩类型划分
(a)按斑晶成分划分
橄榄玄武岩——斑晶为Ol,
若伊丁石化一伊丁石玄武岩
辉石玄武岩—斑晶为P
斜长玄武岩—斑晶为P
(b)按结构构造划分:玻基斑状玄武岩,玻璃质玄武岩,
气孔玄武岩,杏仁玄武岩
(c)按岩石化学成分划分:
拉斑玄武岩:
高铝玄武岩
碱性玄武岩
橄榄玄武岩
O/+Cpx+PI E3
( MacKenzie等,1982)
Basalts: Basics
Basalts are most common primary melt of mantle and are
very common on Earth and terrestrial planets, hence its
important to have a good grasp on basalts
Basalts occur in every tectonic environment
5 tectonic settings commonly used to subdivide basalts
Each has a distinct chemistry:
MORB(Mid-Ocean Ridge Basalt
IAB (Island Arc Basalt)
CAB(Continental ArcBasalts, but CAB also used for Calc
Alkaline Basalts)
OIB ( Ocean Island Basalt)
CFB(Continental Flood Basalt).
Alkalic. Tholeiitic and Calc-Alkaline
Common three-fold geochemical classification of
basalts (and other igneous rocks)
Igneous rocks can be alkalic (alkaline) or sub-alkalic
depending on ratio of total alkalis to silica
OIB and some within-continent basalts are commonl
alkalic. May have nepheline crystals
Sub-alkalic rocks are much more common than alkalic
Alkalic. Tholeiitic and Calc-Alkaline
Sub-alkalic rocks can be subdivided into tholeiitic and calc-alkaline
Tholeiitic and calc-alkaline rocks plot as distinct fields on AFM plot
Tholeiitic refers to igneous
lotal Fe as Feo
rocks(usually basalts)that
are low in K low in
incompatible elements, low
THOLEIITT
Fe203/FeO ratio and displays
pronounced Fe-enrichment
on the AFM plot
Calc-alkaline rocks do not
CALC-ALKALINE
display Fe enrichment on
AFM plot and have higher
Na o+K o
incompatible elements
Maior Element Characteristics of basalt Types
MORB basalts are of typical tholeiite character: ie
very low in incompatible elements(K, Ba, P, Sr, U, Th
Zr), low Fe203/FeO etc
IAB and continental arc basalts tholeiite to calc
alkaline, but some alkaline also. Compared to MORB all
of these basalts are high in K20 and low in Mgo and
Cao
OIB basalts:Most commonly alkalic, but tholeiitic
(not as