Magma immiscibility in the Shonkin Sag and Square Butte laccoliths

Abstract

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.