If the fine-grained part shows under the microscope as crystals of feldspar, hornblende, and mica, the rock is called syenite-porphyry. Both are light-colored rocks, corresponding to syenite.
These porphyries are characteristic rocks of the Northern Ontario goldfields. Feldspar porphyry is closely associated with the Kirkland Lake, Ontario, gold ore. A very large proportion of the productive gold mines of the world are found in the near neighborhood of acid porphyries. Lamprophyre , is a dark-coloured, fine-grained rock, with phenocrysts of black mica, hornblende, or pyroxene. It occurs in small dikes and sills, usually accompanied by aplite dikes.
Mica-lamprophyre is called minette. Quartz-porphyry Quartz-porphyry has phenocrysts of quartz and usually feldspar, in a fine-grained ground mass. This interpretation, however, is unable to give an adequate explanation to the question why are deep-seated plutonic rocks sometime porphyritic or porphyraceous.
They definitely cooled slowly and it is highly unlikely that there were noticeable changes in the cooling rate. These rock probably exhibit porphyritic texture because some crystals started to form before others and had therefore more time and room to grow. The role of volatile components in magma is probably important as well.
Hence, we do have a reason to believe that there are several different mechanisms involved and in many cases it may be a complicated task to unravel the cooling history of a particular igneous rock. Porphyry contains large crystals in the fine-grained matrix.
Rhomb porphyry latite from Norway is a rock type associated with continental rifts. Intrusive equivalent of rhomb porphyry is larvikite monzonite. Width of sample 13 cm. Rhomb porphyry sample from the Oslo Rift. Another rhomb porphyry sample from the Oslo Rift. Basalt porphyrite, plagioclase porphyrite or diabase? Probably all of them, it is mostly a matter of preference and depends on local traditions. White phenocrysts are plagioclase crystals.
The Isle of Mull, Scotland. The rock is 8 cm in length. Basaltic rock from Tenerife. Phenocrysts are plagioclase white and pyroxene black. Width of sample 14 cm. This one from Oahu is clearly not the most classic version of porphyry because it is mafic, it is extrusive, and the phenocrysts are mafic. It is often assumed that phenocrysts that form the porphyritic texture must be felsic feldspars preferably alkali feldspars or quartz.
The green mineral here is olivine. But it does meet the most important and universal characteristics of porphyritic rocks — it is igneous and some minerals are clearly larger than the groundmass. Width of sample 6 cm. Porphyry from Scotland with K-feldspar and quartz phenocrysts. Width of sample 8 cm. Andesite porphyrite with plagioclase phenocrysts from Santorini.
Andesite is an extrusive equivalent of diorite. Width of sample 7 cm. Porphyritic rhyolite from Estonia. Rocks like that crop out in the Baltic Sea. They were brought to Saaremaa by the advancing glacier during the ice age. It is locally known as quartz porphyry. Width of sample 9 cm. Best, Myron G. Igneous and Metamorphic Petrology , 2nd Edition. Jackson, J. Glossary of Geology , 4th Edition.
American Geological Institute.
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