the geology of ireland part two. main tectonic zones & granites galway granite leinster granite...
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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
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
The Caledonian orogeny
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