The Geology of Ireland Part Two. Main Tectonic Zones & Granites Galway Granite Leinster Granite...

The Geology of Ireland

Part Two

Main Tectonic Zones & Granites

• Galway Granite

• Leinster Granite

• Donegal Granite

Tectonic Environments of Granites• Ocean Ridge Granites (ORG)

• Volcanic-Arc Granites

• Collisional Granites (COLG)– Syn-tectonic granites associated with

continent-continent collision– Post-tectonic granites associated with

continent-continent collision– Syn-tectonic granites associated with

continent-arc collision

• Within-Plate Granites (WPG)

The Caledonian orogeny

Crustal Melting & Granite Magmas

• Partial Melting

• Segregation

• Aggregation

• Ascent

• Emplacement

Protoliths & Partial Melting

• Typical geothermal gradients of 20°C/km do not generate temps >800°C at 35 km depth required to melt most crustal rocks (Thompson 1999).

• Three main factors in inducing partial melting:– Increase in temperature– Decrease in pressure (adiabatic decompression)– Introduce H2O-rich volatiles– One or more of these may be met by the influence of

proximal mantle-derived basaltic magmas

Partial Melting & Melt Segregation

Image source: www.indiana.edu

• Melt segregation is the separation of the melt fraction from its restite and source during partial melting

• Melt segregation depends on the permeability of the source.

• Melts first forms at grain boundaries between mineral phases

Protoliths & Melt Composition

• Small degrees of partial melting of an amphibolite will produce a Si-rich melt.

• Granitic melts can be produced from a mafic protolith

Granite Mineralogy

Mafic Enclaves

Close-up view of the Drogheda Granite with mafic enclaves (www.gsi.ie)

Mafic Enclaves

Magma Mixing & Mingling

Galway Granite (www.gsi.ie)

Mafic Felsic Magma Interaction

Magma Mixing and Mingling

Porphyry Deposits

• Mineralisation associated with porphyritic intrusive rocks

• Ore occurs as disseminations along hairline fractures as well as within larger veins, which often form a stockwork

• The orebodies typically contain between 0.4 and 1 % copper with smaller amounts of other metals such as molybdenum, silver and gold

• They are formed when large quantities of hydrothermal fluids carrying small quantities of metals pass through fractured rock within and around the intrusive and deposit the metals

Mineralisation

Fluorite mineralisation

Molybdenite mineralisation

The Geology of Ireland Part Two. Main Tectonic Zones & Granites Galway Granite Leinster Granite...

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