L6 - The Granite Problem

Magmatic Processes: The Granite Problem

Introduction

• Magmatic processes, particularly the formation of granites, are complex and not fully understood.

• The lecture explores the granite problem, addressing how large volumes of felsic melt are generated, considering observations like flood basalts and the association of explosive eruptions with subduction zones.

Magma Types and Volcanic Settings

• Different magma types are associated with various tectonic settings:

  • Mid-ocean ridges and subduction zones: Basaltic and basaltic/andesitic melts.

  • Rhyolitic volcanic eruptions: Yellowstone.

Yellowstone as a Case Study

• Yellowstone Caldera is a complex system involving:

  • Rim boundary fault, geysers, resurgent dome, earthquakes, and crustal stretching.

  • Brittle and plastic deformation zones.

  • Shallow hot water reservoirs (freshwater and brine).

  • Granitic magma (partial melt) and basaltic magma intrusions.

  • Rising basaltic magma from the Yellowstone hotspot, which is sourced from the upper mantle.

Obsidian

• Obsidian Cliff in Yellowstone National Park exemplifies volcanic glass formation due to rapid cooling.

AFM Diagram

• The AFM diagram (Alkali-Ferric-Magnesium) is used to classify igneous rocks.

• The corners of the triangle represent: F (FeO + Fe₂O₃), A (Na₂O + K₂O), and M (MgO).

• Partial melting typically yields basalt.

• Minerals like forsterite (Mg-rich olivine) have compositions with 87-85% Mg.

Trends in Volcanic Rocks

• Rift and plume volcanism: More Fe-rich, following a Tholeiitic trend.

• Subduction zone volcanism: Depleted in FeO, following a Calc-Alkaline trend.

Conditions for Magma Formation

• Tholeiitic magmas: Created in reduced conditions.

• Calc-Alkaline magmas: Created in oxidized conditions, as melting involves water.

• Magnetite ($Fe_3O_4$) readily crystallizes out of Calc-Alkaline melts, further depleting them in Fe.

Early Iron Enrichment

• Melts can become enriched in FeO and depleted in MgO as Mg-rich endmembers of olivine and pyroxene crystallize and are removed.

Crystallization and Melt Composition

• As Mg-rich phases (forsterite, clinopyroxene, orthopyroxene) crystallize out, the remaining melt becomes enriched in Fe.

• Solid solutions form between Fe and Mg endmembers in minerals like olivine and pyroxene.

Harker Diagrams

• Harker diagrams plot major element oxides against $SiO_2$.

  • Partial melting: $SiO_2$ decreases.

  • Fractional crystallization: $SiO_2$ increases.

• This is a useful trend to look at because $SiO_2$ increases with the degree of differentiation.

• MgO and FeO are also useful trends because they can be combined to a Mg number which can decrease with the degree of differentiation

Magmatic Differentiation

• Magmatic differentiation of lavas leads to variations in major element oxides (MgO, FeO, CaO, Na₂O, Fe₂O₃, Al₂O₃, K₂O, $SiO_2$, H₂O).

• Note that Mg-rich forms of olivine, cpx and opx crystallise out before their Fe rich counterparts.

TAS Diagram

• The Total Alkali-Silica (TAS) diagram classifies igneous rocks based on their major element chemistry (Total Alkali = $Na_2O + K_2O$ vs. Silica = $SiO_2$).

TAS Diagram Applications

• The TAS plot illustrates how erupted lava compositions vary in different tectonic settings like East African Rift, Hawaii, and the Cascades.

• Subduction volcanism takes a low, shallow path on the TAS diagram.

• Ocean island volcanism (hot spot volcanism) takes a middling path on the TAS diagram.

• Rift volcanism takes a steep path on the TAS diagram

The Felsic Melt Question

• How are huge amounts of felsic melt formed, as seen in places like Half Dome, Yosemite National Park, and Mt St Helens, Washington?

Questions for Consideration

• Why are there flood basalts but not flood rhyolites?

• Why are explosive eruptions typically associated with subduction zones?

• How does melting differ in various tectonic settings?

• Why is oceanic crust basaltic?

Flood Basalts

• Flood basalts can produce extensive lava flows (exceeding 2000 km³).

• The largest volcanic eruption in the last 10,000 years (Mt Tambora in 1815) erupted only 50 km³ of silicic material.

• Flood basalts have extremely consistent compositions, mostly quartz tholeiitic (reduced conditions) basalts.

Flood Basalts Formation

• Linked to both rifting and hot spot volcanism.

• Thought to form from extensive eutectic melting of the mantle.

Eutectic Melting

• Composition suggests they form from extensive eutectic melting of the mantle because of their consistent composition.

  • Up to ~60% melting at the eutectic.

Silica Saturation

• Critical plane of silica saturation/undersaturation determines the type of basalt.

  • $SiO_2$ undersaturated: Alkali Basalt, Basanite, Nephelinite.

  • $SiO_2$ oversaturated: Quartz Tholeiite.

  • $SiO_2$ saturated: Olivine Tholeiite.

The Granite Problem

• Silicic igneous rocks are less common than basalts, raising the question of how large granitic intrusions form.

• It is difficult to produce large volumes of granitic magma through fractional crystallization of basaltic melts alone.

Solution to the Granite Problem

• Involves adding crustal material to basaltic melt, providing additional sources of $SiO_2$, $Al_2O_3$, $Na_2O$, and $K_2O$.

Crustal Composition

• Crustal compositions differ between continental and oceanic crust, influencing the resulting melt composition.

Melting the Crust

• Mechanisms for melting the crust:

  • Yellowstone: Mantle plume melts the mantle, generating basaltic melt (MP = 1200°C), which rises and melts the continental crust (MP = 700°C).

  • Yosemite & Mt St Helens: Subduction releases water, causing mantle melting and generating basaltic melt (MP = 1200 °C), which rises and melts the continental crust (MP = 700 °C).

  • Mountain-building: Extreme heating and burial of crustal material leads to melting, forming granite cores.

Types of Granite

• S-type: Formed by melting sediments in cores of mountain belts.

• I-type: Formed by melting igneous material, found in subduction zones and mountain belts.

• A-type: Anorogenic (and anhydrous), formed in extension settings, contain Fe-rich material including fayalitic olivine.

• M-type: Mantle-derived granites formed via fractional crystallization.

S-type Granites Details

• Sediments are melted during orogenic events, such as mountain-building in southwest England during the Variscan orogeny. Common metamorphic minerals are present: biotite, muscovite, cordierite, andalusite, garnet.

I-type Granites Details

• Formed by melting igneous material, with mantle melts at subduction zones enriched in Na and K which are known as the calc-alkaline trend.

• Formed in more oxidizing conditions, with melt depleted in Fe due to magnetite ($Fe_3O_4$) crystallization.

  Contains biotite, pyroxene, hornblende.

A-type Granites Details

• Anorogenic and anhydrous, such as Ailsa Craig (formed by melting due to the Iceland plume).

  Contains aegirine, riebeckite, arfvedsonite

M-type Granites Details

• Plagiogranites in oceanic crust are thought to be the result of extreme crystal fractionation of mantle melts - prime example of fractional crystallisation.

• Volume reduction because of mass loss from fractional crystallisation.

  Contains plagioclase, hornblende, biotite

Granite Emplacement

• Granitic magmas are highly viscous and buoyant, moving through the crust as large domes and solidifying into batholiths.

Pluton Spacing

• Granite plutons rise with regular spacing, dependent on the viscosity contrast between the granite and the crust.