The Shonkin Sag and Square Butte laccoliths of central Montana differentiated in place from single alkalic magmas to produce syenite and shonkinite through a gravity-controlled differentiation mechanism. The absence of rhythmic and cryptic layering and a lack of definite cumulate textures suggest that crystal settling did not produce syenite from shonkinite in either laccolith. Field relations strongly suggest coexistence of a mafic and a felsic magma. Both field relations and chemical trends, which are incompatible with a fractional crystallization model, can, on the other hand, be explained by magma immiscibility. During or following intrusion, immiscible syenite globules separated from a mafic shonkinite magma, coalesced, and rose to form a homogeneous upper layer of syenite. An increase in host viscosity, due to cooling, trapped late-rising syenite globules. These are preserved as isolated syenite segregations as much as 10 m in diameter enclosed in the lower shonkinite.

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