lab4 - 1 Lab 4 Dolerites and basalts Part A Dolerites...

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1 Lab 4 Dolerites and basalts Part A Dolerites Dolerites are fine-grained gabbroic rocks that occur as intrusives. As such, dolerites can be called "microgabbros". Another name for a dolerite is "diabase", which is commonly used in the US, although many British petrologists use this term for dolerites that are strongly altered. In terms of variety, there are as many "types" of dolerites as there are of gabbros. A dolerite can therefore be described as a "quartz dolerite" if it is quartz bearing, or an "olivine dolerite" if it is olivine bearing, . ... etc. Texturally, dolerites are simply finer- grained than gabbros, and the most commonly observed texture is the "ophitic" one. Vesicular, amygdaloidal or porphyritic textures can also occur in some dolerites, although they are not very common. Because they occur as intrusions in dykes or sills, dolerites may develop "chilled margins", which are much finer grained than the main intrusion, and which result from the rapid cooling of the intrusion along its margins where it comes in contact with the "colder" country rocks. Dolerites are commonly altered by fluids which have circulated through the intruded country rocks. Accordingly, the products of alteration will depend on the composition of the fluids, which will in turn be affected by the composition of the country rocks. SF-3: Olivine dolerite from St. Francois Mts. This is an example of a dolerite with ophitic texture that has been affected by some alteration. Note that “alteration” has resulted in the formation of several secondary minerals which include tremolite. 1- List the minerals occurring in this sample. 2- Which of these minerals are secondary? 3- What is the order of crystallization of these minerals? 4- The olivine in this sample has been partially replaced by a bunch of minerals that
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lab4 - 1 Lab 4 Dolerites and basalts Part A Dolerites...

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