Granites

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Granites

• Petrography
• Field relations
• Origin
• Time-space patterns in plutons
• S-Type and I-Type compositions

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Petrography

• Fabric
• Classification
• Alteration

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Fabric

• Hypidiomorphic granular
• Equigranular
• Porphyritic
• Pegmatitic
• Graphic

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Anisotropy

• Foliatiation
• Lineation
• Segregation layers

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Compositional Classification

• Q-Ab-Or diagram
• Chemical classification is useful
• Quartz, K-spar, Plagioclase diagram
• Modal classification also is good

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Textural classification

• Porphyry
• Large crystals in a fine-grained matrix
• Aplite
• Leucocratic, fine sugary texture
• Pegmatite
• Extremely coarse-grained with rare euhedral
  minerals
• Li, Be, Zr, Nb, Mo, Sn, Ta, W,Th, U concentrated
• Granophyre
• Micrographic texture

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Alteration

• Most granites show some degree of alteration
• High temperature minerals replaced by low T ones

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Field relations

• General distribution
• External contacts
• From of intrusions
• Internal features
• Time and depth of emplacement

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General distribution of granites

• Concentrated in orogenic belts
• Located at the margins of continents
• Also in deeply eroded Precambrian shields
• Alkaline granites occur in rift zones

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Origin

• Magmas vs. granitization
• Room problem
• Form of large batholiths
• Zoned plutons
• Origin of magmas

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Time-space patterns in plutons

• S-Type granites
• I-Type granites
• Mesozoic Andean granites
• Mesozoic Batholiths of western USA