5.5 Basalt

basaltes

basaltes

misspelling of L. basanites - very hard stone imported from ancient Greek

Basalt

fine grained mafic igneous rock consisting essentially of augite and calcic plagioclase

aphyric-devoid of phenocrysts, more commonly contain phenocrysts or microphenocrysts of olivine and of pyroxene

texture of basalt

Tholeiitic, Alkaline, Calc-alkaline

3 principal types of basaltic magma

mid-oceanic ridges and intraplate

setting of tholeiitic basalt

intraplate

setting of alkaline basalt

convergent plate boundaries

setting of calc-alkaline basalt

tholeiitic basalt is usually fine-grained, intergranular, no olivine, contains cpx and opx, no alkali feldspar, has interstitial glass while alkaline basalt is usually fairly, coarse intergranular to ophitic, has olivine, titaniferous augite, no opx, interstitial alkali feldspar or feldspathoid may occur, and interstitial glass are are and quartz absent

How to differentiate tholeiitic basalt and alkaline basalt

Alkali basalt

used in place of nepheline olivine basalt

Olivine phyric basalt

Basalt containing olivine

Picrite

a basaltic rock visibly enriched in olivine crystals, often as phenocrysts

Ankaramite

basaltic rock rich in olivine and augite phenocrysts; on some the abundance of mafic phenocrysts may have been enhanced by gravitational accumulation

Picrobasalts

having lower SiO2 content than basalt (ultrabasic in composition), more olivine-rich and contain little plagioclase

Basaltic andesite

have mafic mineral similar to basalt but contain plagioclase of more sodic composition (typically andesite)

Trachybasalt, basanites, and tephrites

usually contain recognizable alkali feldspar or feldpathoids

Boninite

a high-Mg form of basalt that is erupted generally in back-arc basins, distinguished by its low Titanium content and trace element composition.

MORBs, OIB, LIPs, CFBs, intra-continental rift basalts, subduction-related bsaalts

Where basalts occur

Mid-oceanic ridge basalt

MORBs, contains olivine tholeiitic basalts

Normal MORB

strongly depleted in highly incompatible elements

Enriched MORB

represent 20-30% partial melting of a well-mixed depleted mantle

Ocean Island Basalt

OIB, intraplate volcanic island and sea mounts

Hot spot

localized source of magma supply whose output thickens the basaltic crust in its immediate neighborhoods

Large Igneous provine oceanic plateaus and continental flood basalts

LIPs and CFBs, basalts dominantly ol-pl-phyric low-K tholeiites

Inta-continental rift basalts

in regions of post-subduction extension

Low-K island tholeiite (IAT) arc basalts

immature oceanic island arcs; ol-pl and augite, sparse opx, and/or magnetite

Medium-K arc basalts

more characteristic product of

mature island arc volcanism; high alumina basalts, often notably porphyritic containing phenocrysts of pl-ol-augite and magnetite occasionally accompanied by hb

High-K arc basalts

high-K calc-alkali association relatively scarce; exceeded in volume by high-K

andesites more evolved member contain phenocrysts of ol, augite accompanied by hb, mg/pl

Deep-sea sediments

Basaltc pillow lava

Diabase sheeted dikes

Plagiogranite

Gabbro

Ultramafc cumulates

Residual mantle pendotite

Ophiolite succession of oceanic crust

Peridotite

dominant rock type of the Earth's upper mantle

Lherzolite

Fertile unaltered mantle, composed of ol, opx, cpx

Dunite

mostly olivine; refractory residuum after basalt has been extracted by partial melting.

Harzburgite

ol+opx, refractory residuum after basalt has been extracted by partial melting.

Wehrlite

mostly composed of olivine plus clinopyroxene.