Magma immiscibility in the Square Butte laccolith of central Montana

Square Butte, an alkalic laccolith in the Highwood Mountains of Central Montana, has long been considered a classic example of differ entiation in place through fractional crystallization.My recent field work, chemical analyses, and pétrographie studies provide evidence for a differentiation mechanism involving magma immiscibility.Rock tex tures and field relationships inconsistent with a fractional crystal lization model are found at Square Butte, including a zone of isolated felsic globules up to 10 meters in diameter suspended in a mafic-rich matrix rock.This globule zone has a horizontal base, suggestive of a gravitationally-controlled separation process.Globules contain the same minerals as their host, in different proportions.Fe/Mg ratios are nearly identical for globules and their matrix rocks, indicating equilibrium.Major-element partitioning is consistent with liquid immiscibility theory.Although no phase diagrams exist for an orthoclase-augite system such as that at Square Butte, the Square Butte magma probably had the most favorable characteristics for immiscible separation: high KzO/total alkalis; moderate Ti and P; moderate oxygen fugacity; low pressure environment.Minor crystal settling may have occurred during differentiation but the primary mechanism of differentiation appears to have been a gravitational separation of immiscible felsic globules from a mafic-rich shonkinite host, result ing in an upper felsic unit and a lower mafic unit.An increase in host viscosity due to cooling trapped felsic globules rising from lower levels and produced isolated patches of light-colored rock sur rounded by shonkinite.Partial re-equilibration between the smaller trapped globules and their matrix produced a mottled rock of inter mediate composition.o»"'^ Swee tgrass Hills ^0