Where is Porphyritic Granite found?
Granite (Porphyritic texture)
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Open the appPorphyritic Granite occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Porphyritic Granite
Sierra Nevada Batholith, USA
Contains extensive exposures of various granitic rocks, including porphyritic varieties, formed during Mesozoic subduction.
Cornwall, England
Known for its Hercynian (Variscan) granites, some of which exhibit porphyritic textures, particularly the Carnmenellis and Bodmin Moor granites.
Scandinavia (e.g., Sweden, Norway)
Numerous Precambrian and Paleozoic porphyritic granites are found in the Baltic Shield, often associated with ancient cratons.
Appalachian Mountains, USA
Various granitic intrusions, including porphyritic types, are found throughout the Appalachian orogen, dating from the Paleozoic.
Eastern Australia
Granitic intrusions with porphyritic textures are common in the Lachlan Fold Belt and New England Orogen.
Collecting tips
- Look for Outcrops. Porphyritic granite is an intrusive rock, so look for large exposures in mountainous regions, eroded batholiths, or areas where overlying rocks have been removed.
- Examine Texture Closely. The key identifying feature is the porphyritic texture. Look for distinct, larger crystals (phenocrysts) set within a finer-grained matrix. These phenocrysts are often feldspar and can be several centimeters long.
- Observe Mineral Composition. Identify the characteristic minerals of granite: quartz (glassy, no cleavage), feldspar (blocky, two cleavages, often white, pink, or gray), and mica (flaky, dark biotite or silvery muscovite). The phenocrysts will typically be feldspar.
- Check for Weathering Patterns. Granite is generally resistant to weathering, but feldspars can alter to clay minerals over time. Look for fresh surfaces to best observe the texture and mineralogy.
Quick Check
- Color:
- Light-colored (white, gray, pink, red)
- Luster:
- Vitreous to sub-vitreous (quartz, feldspar), pearly (mica)
- Streak:
- White (for constituent minerals)
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