IGNEOUS PETROLOGY (BASALT)

Question 1

Etymology of Basalt

Answer 1

Basaltes - misspelling of basanites
Basanites - very hard stone
Basanos-touchstone
Bauhun - Slate

Question 2

Composition of Basalt

Answer 2

45-52% SiO2
Ca- Plag
Augite -CaPyx
Alkali <5%

Question 3

3 Principal Basalt Types and their Environment of Formation

Answer 3

1) Tholeiitic - MOR and Ocean Island (Hotspots)
2) Alkali - Ocean Islands and Continental Rifts
3) Calc Alkaline - Convergent Plate Boundaries

Question 4

Groundmass
Tholeiitic vs Alkali

Answer 4

Tholeiitic Basalt
?Fine grained
?No Olivine
?Cpx - Augite and/or Pigeonite
?Opx Hypersthene may rim Olviine
?No Alkali Feldspar
?Intersitital Glass

Question 5

Phenocrysts
Tholeiitic vs Alkali Basalt

Answer 5

?Unzoned Olivine sometimes w/ Opx rim
?Opx uncommon
?Early Plag Common
?Cpx Pale Brown Augite

Question 6

Basalt used in place of Nepheline Olivine Basalt

Answer 6

Alkali Basalt

Question 7

Basalt containing olivine

Answer 7

Olivine Phyric Basalt

Question 8

Basalt visibly enriched in olivine crystals ofte as phenocrysts

Answer 8

Picrite

Question 9

Basalt with Olivine and Augite (Cpx) Phenocrysts which may have been enhanced due to gravitational accumulation

Answer 9

Ankaremite

Question 10

Rocks that have lower SiO2 than Basalt and in the lower left of TAS <45% SiO2 and more olivine-rich and contain little plagioclaise

Answer 10

PicroBasalt

Question 11

Have Mafic Mineral similar to basalt but contain plag of more sodic composition and silca of 52-57%

Answer 11

Basaltic Andesite

Question 12

Basalts which recognizable alkali feldspars or feldsphathoids

Answer 12

Trachybasalt
Basanites (>10% Ol)
Tephrites (<10% Ol)

Question 13

Trachybasalt that has Na>K content

Answer 13

Hawaiite

Question 14

Trachybasalt that has Na

Answer 14

Potassic Trachybasalt

Question 15

High Mg Basalt erupted from Back arc Basins and distinguished by its low Titanium Content and trace elements composition

Answer 15

Boninites

Question 16

Tectonic Settings where Basalt Occurs

Answer 16

1)MORBs
2) OIBs
3) LIPs: Ocenic Plateaus and CFBs
4) Intra Continental Rifts
5) Subudction Related Basalts

Question 17

Type of Basalt in MORBs

Answer 17

Olivine Tholeiitic Basalt

Question 18

Distinctive of MORBs

Answer 18

Low K2O and Depleted of Incompatible Elements

Question 19

Subdivisions of MORBs

Answer 19

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

Question 20

Localized source of magma supply whose output thickens the basaltic crust and its immediate neighbourhood

Answer 20

Hotspots

Question 21

Basalts in OIBs are

Answer 21

Alkali

Question 22

Phases of OIBs

Answer 22

1) Development of Shied Volcano
2) Hiatus in which Erosion Occurs
3) Renewed Magmatism but this time more explosive and Alkali

Question 23

Basalts in LIPs are

Answer 23

Ol-Pl-phyric Low-K tholeiites

Question 24

Basalts in Intra continental Rifts are

Answer 24

Alkali (Nephelinnite and Mellilite)
Bimodal (Trachyte and Phonolite)

Question 25

Subduction Related Basalts

Answer 25

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)

Question 26

Seismic Moho vs Petro Moho

Answer 26

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

Question 27

Dominant rock in the Upper Mantle

Answer 27

Peridotites

Question 28

Fertile unaltered Mantle coposed of Oli+cpx+opx

Answer 28

Lherzolite

Question 29

Mostly Oli

Answer 29

Dunite

Question 30

Oli + Opx

Answer 30

Harzburgite

Question 31

Oli + Cpx

Answer 31

Wehrlite