IGNEOUS PETROLOGY (BASALT) Flashcards
Etymology of Basalt
Basaltes - misspelling of basanites
Basanites - very hard stone
Basanos-touchstone
Bauhun - Slate
Composition of Basalt
45-52% SiO2
Ca- Plag
Augite -CaPyx
Alkali <5%
3 Principal Basalt Types and their Environment of Formation
1) Tholeiitic - MOR and Ocean Island (Hotspots)
2) Alkali - Ocean Islands and Continental Rifts
3) Calc Alkaline - Convergent Plate Boundaries
Groundmass
Tholeiitic vs Alkali
Tholeiitic Basalt
?Fine grained
?No Olivine
?Cpx - Augite and/or Pigeonite
?Opx Hypersthene may rim Olviine
?No Alkali Feldspar
?Intersitital Glass
Phenocrysts
Tholeiitic vs Alkali Basalt
?Unzoned Olivine sometimes w/ Opx rim
?Opx uncommon
?Early Plag Common
?Cpx Pale Brown Augite
Basalt used in place of Nepheline Olivine Basalt
Alkali Basalt
Basalt containing olivine
Olivine Phyric Basalt
Basalt visibly enriched in olivine crystals ofte as phenocrysts
Picrite
Basalt with Olivine and Augite (Cpx) Phenocrysts which may have been enhanced due to gravitational accumulation
Ankaremite
Rocks that have lower SiO2 than Basalt and in the lower left of TAS <45% SiO2 and more olivine-rich and contain little plagioclaise
PicroBasalt
Have Mafic Mineral similar to basalt but contain plag of more sodic composition and silca of 52-57%
Basaltic Andesite
Basalts which recognizable alkali feldspars or feldsphathoids
Trachybasalt
Basanites (>10% Ol)
Tephrites (<10% Ol)
Trachybasalt that has Na>K content
Hawaiite
Trachybasalt that has Na
Potassic Trachybasalt
High Mg Basalt erupted from Back arc Basins and distinguished by its low Titanium Content and trace elements composition
Boninites
Tectonic Settings where Basalt Occurs
1)MORBs
2) OIBs
3) LIPs: Ocenic Plateaus and CFBs
4) Intra Continental Rifts
5) Subudction Related Basalts
Type of Basalt in MORBs
Olivine Tholeiitic Basalt
Distinctive of MORBs
Low K2O and Depleted of Incompatible Elements
Subdivisions of MORBs
1) N-MORB - Extensive Partial Melting (20-30%)
deplete of Incompatible Elements
2) E-MORB - Small degree of Partial Melting of a depleted Mantle and enriched in Incomatible Elements
Localized source of magma supply whose output thickens the basaltic crust and its immediate neighbourhood
Hotspots
Basalts in OIBs are
Alkali
Phases of OIBs
1) Development of Shied Volcano
2) Hiatus in which Erosion Occurs
3) Renewed Magmatism but this time more explosive and Alkali
Basalts in LIPs are
Ol-Pl-phyric Low-K tholeiites
Basalts in Intra continental Rifts are
Alkali (Nephelinnite and Mellilite)
Bimodal (Trachyte and Phonolite)
Subduction Related Basalts
1) Low K Arc Tholeiites - High Al compared to MORB Tholeiite and occurs in young ocean island arcs
2) Medium K Arc Basalts - Mature Volcanic Island Arc
high alumina often Porphyritic w/ phenocrysts of Pl-Ol-Augite and Magnette
3) High K Arc Basalts - Relatively Scarce exceed by high K andesite (Shoshonites)
Seismic Moho vs Petro Moho
Sesmic Moho occurs in between Layered Gabbro and Layered Ultramafic in between Layer 3 and Layer 4 while petrologic Moho coccurs in between Ultramafic cumulates and Residual Mantle Rocks
Dominant rock in the Upper Mantle
Peridotites
Fertile unaltered Mantle coposed of Oli+cpx+opx
Lherzolite
Mostly Oli
Dunite
Oli + Opx
Harzburgite
Oli + Cpx
Wehrlite