Syenitic rocks are usually associated with other plutonic rocks. - TopicsExpress

Syenitic rocks are usually associated with other plutonic rocks. They form relatively small intrusive bodies or parts of larger intrusions. Most syenites seem to be associated with extensional tectonic regime (rifting continents). At least some syenites are believed to be fractionation products of alkali-rich basaltic magmas but there are several different mechanisms responsible for the genesis of syenitic magma. In this sense syenite is again similar to granite which is also defined solely by its mineralogical composition although there are very different possible ways how granitic magma can evolve. It is perhaps inevitable because minerals are something we can determine, unlike genesis which in many cases is still a mystery. An immense number of terms describing many varieties of syenite have been used in the past. Some of them are mentioned (foyaite, canadite, durbachite, umptekite) to describe the rocks shown below. In addition to these, nordmarkite (quartz syenite) and perthosite (alkali feldspar syenite) are perhaps the best known unofficial names of syenite varieties. Shonkinite is a dark-colored variety of foid syenite which generally contains more pyroxene than alkali feldspar. Larvikite is a famous decorative stone from Norway which is often named syenite. It exhibits a characteristic schiller but larvikite contains more plagioclase than alkali feldspar and is therefore monzonite, not syenite. Many so-called syenites are actually monzonites. For some reason the term “monzonite” seem to be little known and rarely used.