Origin of Magma

Origin of Magma
Useful Terms

Basement or basement rock - the igneous and/or metamorphic rock underlying sediment and sedimentary cover
Bedrock - solid rock underlying soil or unconsolidated sediments
Parent rock - original rock before it its changed by metamorphism or other process
Country rock - rock enclosing an igneous intrusion or vein

Origin of Magmas
Melting and Partial Melting

Melting

silica-rich minerals first to melt (lower melting T) melts first, crystallizes last
Mg- and Fe-rich, SiO2-poor minerals last to melt (first to crystallize, last to melt)

Partial melting

minerals with lowest melting T melt first
partial melt rich in silica
magma of partial melt may migrate to magma chamber

Why and where melts form

Heating

T increases with depth: Geothermal gradient
Igneous minerals melt at T’s of 700° to 1100°C
Subducting slab gets heated and minerals begin to melt
Ex: Convergent boundary (Fig. 2-5)

Pressure release

hot, rising material melts as pressure decreases
Ex: Divergent boundary and hot spots

Water

subducted crust carries water which enhances melting of minerals
Ex: Convergent boundaries

Origin of Magmas
Magmatic Differentiation (p. 81-82)

Magmas and crystal mineralogy evolve during crystallization
Continuous reaction series (Bowen's Reaction Series)

Ca to Na plagioclase
no structural change

Igneous Rocks
Magmatic Differentiation (cont.)

Discontinuous reaction series (Bowen's Reaction Series)

start with basaltic magma
mineral structure changes at each
step - “discontinuous”

Igneous Rocks
Magmatic Differentiation (summary figure)

Igneous Rocks
Fractional Crystallization

Reequilibration vs. fractional crystallization

If crystals are not removed, magma (melt) and crystals reequilibrate at lower T

Implications for mineral composition?

If early-formed crystals are separated from magma, overall composition of melt changes - fractional crystallization (p. 81)

gravity settling (Palisades Sill)
filter pressing

Origin of Magmas
Other factors

Magmatic differentiation
Partial melting
Assimilation (melting) of country rock

Subduction zones, hot spots under continent

Mixing of magmas - miscible melts - able to mix
Immiscible melts - can’t mix
Temperature differences cause chemical differences in magma chamber