monzosyenite - large hand/display specimen of an unusual igneous rock from the Laramie anorthosite complex, Wyoming

This monzosyenite is from the Sybille intrusion that wraps around the western rim of the Poe Mountain anorthosite of the Laramie anorthosite complex in Sybille Canyon, Albany County, Wyoming. It was collected along Wyoming Highway 34 at Morton Pass, near its contact with underlying Archean gneiss.

Syenites commonly occur as small independent intrusions or as satellite bodies comagmatic with larger intrusions that have different compositions and which are often granitic. There are two schools of thought: that the syenites evolved from the larger granitic intrusions, or that they formed by fractional crystallization from basaltic magmas.

Monzosyenite is a synonym of monzonite. It is a coarse grained igneous rock intermediate between diorite and syenite, and is composed of almost equal amounts of alkali feldspar (orthoclase) and plagioclase. It has very little quartz, if any at all. Notice its position on the QAPF diagram. Click once to select, click again to enlarge.

Why is this rock monzosyenite instead of syenite? Syenite is defined as a potassium feldspar (orthoclase)-rich rock with minor amounts of albite, a plagioclase feldspar. If a rock is hot enough, the plagioclase may have a considerable amount of Ca, and then it be comes oligoclase instead of albite. See the feldspar compositional phase diagram. A K-feldspar-rich rock with minor oligoclase is monzosyenite. The Sybille intrusion reached temperatures greater than 1000˚C, more than hot enough for the formation of oligoclase.

The Laramie anorthosite complex was emplaced roughly 1.43 billion years ago along the suture that was formed by the1.78 billion-years-ago collision of the continent with an island arc terrain during the Medicine Bow orogeny.

The pencil is 5.5" long for scale.

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